Exploring the Planets1/18
Welcome Back! National Air and Space Museum Docent Training NMB Spotlight Training: Exploring the Planets 2/18
In this training... This lesson will introduce you to providing spotlights in the Exploring the Planets gallery at the National Air and Space Museum: National Mall Building. After completing this lesson you will have learned more about:
This lesson, and its corresponding quiz cover the very basic content that you will need to know for starting Exploring the Planets Spotlights at NASM's National Mall Building. 3/18
Exploring the Planets Exploration reveals that our solar system is filled with amazingly diverse places that transform our understanding of Earth and worlds beyond. The Kenneth C. Griffin Exploring the Planets Gallery probes the science and history of our exploration of planets, moons, and other objects in our solar system. This exhibit tells the stories of the diversity of worlds circling our Sun and how exploring those worlds helps enhance our own understanding of Earth. Exploring the Planets draws on research from scientists in the Museum’s Center for Earth and Planetary Studies, who are actively involved in current planetary missions. 4/18
Introduction The planets circling our Sun are as amazing as they are diverse. Since the turn of this century, we’ve learned there are many more planets beyond our solar system, circling their own stars. Come discover our planetary neighborhood—as if from interstellar space, moving in toward our Sun. This “outside-in” perspective is how we may someday explore other solar systems. 5/18
Planets Across the Galaxy Astronomers have discovered planets beyond our Sun that orbit other stars. They call these “exoplanets.” They have found thousands of them and continue to discover more. Our home galaxy, the Milky Way, may include millions—even billions—of planets. Stranger Than FictionFor centuries, people have made up stories about exploring exotic worlds teeming with strange forms of life. For decades, planetary scientists have explored the worlds of our own solar system. Recently, scientists have discovered many exoplanets—those beyond our solar system. They have not yet found life beyond Earth, but they have found real worlds that are as strange as science fiction. Some Exoplanets Compared to Earth Kepler-62 Kepler-62 is a system of five planets circling a star 1,200 light-years (the distance light travels in space in one year) from Earth. Two of the five orbit the star just close enough that oceans of liquid water could cover their surfaces. PSR B1620-26b The M4 star cluster [above] contains an exoplanet, PSR B1620-26b, that orbits not one, but two stars. It is nearly three times as old as Earth. 55 Cancri e Another exoplanet (55 Cancri e) is twice as big as Earth and is more dense—it has 18 times more mass. One year—the time it takes to orbit its star—lasts only 18 hours. One side always faces its star and is hotter than volcanic lava on Earth. Proxima b One world (Proxima b), not much bigger than Earth, orbits its small star, Proxima Centauri, in about 11 Earth days. Just over four light-years away, it is the closest exoplanet found so far that could have liquid water on its surface. How Do We Find Exoplanets?It’s not easy. Astronomers use several tools and methods. Some of their instruments measure subtle shifts in a star’s movement, which could be caused by the tug of a planet’s gravity. Other instruments watch for tiny, periodic drops in a star’s brightness as a planet moves across its face. Some exoplanets have even been captured in telescopic images. Stellar Wobble [Above] Astronomers discovered an exoplanet in 1995 by measuring small shifts in a star’s light waves –a stellar wobble. These shifts are caused by a star “wobbling” back and forth due to the gravitational pull of one or more planets orbiting it. The star’s light waves compress or stretch as the star wobbles slightly toward or away from us. By carefully measuring a star’s light, astronomers can spot repeated drops in brightness caused by a planet moving in front of the star. The larger the planet, the larger the decrease in light. The time between dips in brightness tells how long it takes for the planet to orbit the star. Astronomers captured this amazing image of the exoplanet CVSO 30c (arrow) near its parent star using a large telescope on Earth in 2016. Diverse Stories: Courtney Dressing Planet-Finding Astrophysicist Courtney Dressing is “intrigued by the possibility that life could exist elsewhere in the galaxy.” Growing up in the DC metro area, her fascination with space led to frequent trips to the National Air and Space Museum—and a high school internship with one of the Museum’s planetary geologists. Following her passion, Dressing received a PhD from Harvard for her work investigating small planets orbiting small stars. Dressing’s search for small, rocky exoplanets continued at the California Institute of Technology (Caltech). Using the Kepler space telescope she revealed that about one-fourth of low-mass stars (less mass than the Sun) have a potentially habitable Earth-sized planet. Object Highlight: Kepler Space Telescope The Kepler Space Telescope was the first spacecraft designed to look for exoplanets. Launched in 2009, it watched for subtle dips in starlight caused by planets moving in front of stars. During its primary mission, Kepler monitored the light from more than 100,000 stars in one region of the sky. It found several thousand possible exoplanets and confirmed the existence of thousands more. This apparatus was used to test the detector technology created for Kepler. It proved that—while operating in space—Kepler would be able to accurately measure the light from thousands of stars. Object Highlight: Searching for Goldilocks By Angela Palmer Illuminated etched glass plates, 2009 Artist Angela Palmer used sheets of engraved glass to portray the first “Goldilocks” worlds identified by the Kepler Space Telescope. Each of these planets orbits its star at just the right distance—not to too hot, not to cold (remember the Goldilocks fairy tale?)—so liquid water might form on its surface. The opaque bright white circles represent stars with Goldilocks planets. Each glass plate represents a distance 250 light-years farther from Earth. 6/18
Touring Our Solar System Scientists have sent spacecraft to all the planets and many of the moons in our solar system. Each time we explore a world, we learn more about how it is similar to—and different from—others. Studying this wondrous range of worlds helps us understand how our solar system formed and evolved. This artist’s concept shows a planet in front of its star. What is a Planet? IMAGE: This artist’s concept shows the planets in our solar system. Ages ago, people noticed that a few stars moved around the sky from night to night among all the countless nonmoving stars. They called them “planets,” which means “wanderers.” We now know that these planets are not stars at all. They are fairly nearby objects that orbit our Sun. Many objects orbit our Sun but not all of them are planets! Defining which ones are planets is trickier than you might think. The Planets and Some of Their Moons The globes above show the eight planets, the dwarf planet Pluto, and some major moons at the same scale. The look of each one was created from actual images. Here are all of the planets and moons included in the above globes:
Scientists once agreed that nine planets orbited the Sun, from Mercury to Pluto. But then they discovered objects orbiting beyond Pluto, some quite large. Which ones were planets? Scientists no longer agreed. The International Astronomical Union (IAU), comprised of scientists from nations around the world, debated the issue. In 2006 they voted on a new definition for a planet. Planet: The Official Definition The IAU Prague General Assembly adopted the following definition on August 24, 2006. A “planet” is a celestial body that
Now There Are Dwarf Planets! The IAU Prague General Assembly also approved a new term, dwarf planet. A “dwarf planet” is a celestial body that
Our Solar System from the Outside In Imagine entering our solar system from interstellar space. As you travel toward our Sun, you would move through three distinct regions. First you would pass countless icy worlds. Then you would enter the realm of the giant planets. Finally you would reach the rocky planets closest to the Sun. The Oort Cloud The Oort Cloud is vast. It starts between 2,000 and 5,000 AU from the Sun and extends out to 50,000 AU. (One AU, or astronomical unit, is the average distance between the Earth and Sun.) It may contain trillions of objects larger than two-thirds of a mile (one kilometer) in size. Icy Worlds A spherical “cloud” of comets (Oort Cloud) surrounds the outer reaches of our solar system. Closer to the Sun but still beyond Neptune is a doughnut-shaped region (Kuiper Belt) containing countless icy bodies. Some are quite large. Pluto is the largest member of the Kuiper Belt. Giant Planets Giant planets are much larger than Earth. They are made mostly of gases instead of solid materials. Neptune and Uranus likely have regions of ices beneath their atmospheres. Saturn and Jupiter are our largest planets. They are mostly made of hydrogen and helium, the Sun’s most abundant elements. Rocky Planets The planets closest to the Sun—Mars, Earth, Venus, and Mercury—are made mostly of rock. Earth is the largest of our rocky planets. It is the only planet we know of that has both abundant liquid water and living organisms. Planets Circle the Sun in Paths Called Orbits The planets move around the Sun in nearly a single flat plane. Dwarf planet Pluto and many other objects follow orbits tilted at much higher angles to this plane. Astronomers do not yet know how “typical” our solar system is compared with others. The Sun The largest and most massive object in our solar system. Object Highlight: Voyager Above is a full-scale replica of a Voyager spacecraft. From 1979–1989, Voyagers 1 and 2 explored Jupiter, Saturn, Uranus, and Neptune. They examined the atmosphere, magnetic field, and radiation belts of each planet. They also studied their ring systems and the surfaces and atmospheres of their many moons. Both spacecraft were traveling fast enough to escape the Sun’s gravity. Since leaving our solar system, they have returned data on what space is like between the stars. Parts of the Voyager displayed here were used for pre-launch engineering tests and testing for later spacecraft missions. Voyager Interstellar Mission Voyager 2 had its last planetary encounter in 1989, when it flew by Neptune. Both Voyager spacecraft went on to explore the space outside of the planetary orbits, looking for the outer boundary of our solar system. Scientists watched for information about where the effects of the Sun’s magnetic field and solar wind ends and interstellar space begins. Spacecraft Get Smaller and BetterNew Horizons [see full-size photo behind Voyager] was the first spacecraft to explore Pluto and its moons. It is much smaller than the Voyager, but its instruments are much improved because of technology advances. Object Highlight: Power to Explore By Pat Rawlings, 2011 Heat from nuclear material provides power to operate spacecraft far from the Sun. Fascinated by Planets People have long been fascinated with events in the sky and the objects that move across it. They recorded the movements of planets and the occurrence of solar and lunar eclipses. They told stories about gods and goddesses to explain these strange events. Astronomers later tried to recreate the movements of planets with mechanical devices. Ancient Astronomers People of many cultures learned to use carefully positioned stones to track and forecast seasons—to know when to plant crops—and eclipses—to understand that they were not so random and frightening. Once writing became widespread, it greatly improved astronomical record keeping. Some ancient records remain useful today. Stonehenge in England is one of the oldest astronomical observatories. Parts of it date back 4,500 years. Its stone monoliths and markers allowed observers to track the Sun and Moon in ways only recently understood. Long before telescopes, Chinese astronomers kept detailed written records of comets, eclipses, and even the presence of large sunspot groups on the Sun. The Mayan symbol for Venus appears on this temple wall in Mexico. Mayans kept excellent records of Venus. Their calendar accurately predicted when it would appear in the morning or evening sky. The Mysterious Mechanism from Antikythera Sponge divers working near the Greek island of Antikythera in 1900 discovered the remains of a Roman treasure ship. Among the objects recovered from it were pieces of a machine with metal gears, a perplexing discovery. The shipwreck dated back to more than 1,500 years before the widespread use of wooden gears! The geared bronze wheels of the Antikythera mechanism puzzled archeologists. The largest gear is only 5½ inches (14 centimeters) across. The number of teeth in several of the gears led some to suggest the machine had an astronomical purpose. Researchers carefully analyzed the original mechanism and constructed this full-size model of it. They determined it could have been used to calculate the positions of the planets among the stars, and the dates for solar and lunar eclipses. Mechanical Solar Systems Astronomers in the Middle Ages learned our solar system operated in predictable patterns, like a machine. In the 18th century, mechanical devices used gears or motors to reproduce these patterned movements of planets around the Sun. They showed the idea that our solar system ran like “clockwork” following Newton’s laws of gravity. Object Highlight: Orrery This physical model of the solar system is called an orrery [OR-er-ee]. Built sometime in the 19th century, it uses gears to reproduce the movements of the planets around the Sun. Floorrery Go see a floor version of an orrery on the opposite side of this gallery. 7/18
Icy Worlds Worlds in our outer solar system consist mostly of water ice, other ices, and some rock. Various processes have shaped their surfaces into strange landscapes. Because they are so far from Earth, we are just starting to learn about them, how they formed, and how they interact with the rest of our solar system. Scientists are especially interested in whether all this water in our outer solar system may contain life. The Rosetta spacecraft took this image of Comet 67P just before landing on its surface in 2014. After flying past Pluto, New Horizons took this image of its water-ice mountains (left) and smooth nitrogen-ice plains (right). Layers of haze can be seen in Pluto’s thin atmosphere. Where Are Icy Worlds Found? Icy worlds are found in two vast areas on the outskirts of our solar system. One is a doughnut-shaped region beyond the orbit of Neptune called the Kuiper (KYE-per) Belt. The other is called the Oort Cloud, a spherical cloud of material where comets are abundant. All these icy bodies are left over from when our solar system formed. They hold secrets to its early composition and to how all solar systems form. A thick ring of Kuiper Belt objects is beyond the orbits of the eight planets. Beyond this is a sphere of icy comets called the Oort Cloud. The Kuiper Belt: Where Icy Worlds Reside After Pluto was discovered, many astronomers began wondering if other objects like it could exist. In 1952, American astronomer Gerard Kuiper suggested that a region containing small bodies might exist beyond Neptune. This region became known as the Kuiper Belt in 1980, when it was verified by Uruguayan astronomer Julio Fernández. Diverse Stories: Gerard Kuiper Astronomer Gerard Kuiper proposed the idea of a disc of material lying beyond Neptune. Diverse Stories: Jane Luu Jane Luu, a Vietnamese American astronomer, was a graduate student when she helped co-discover the first large Kuiper Belt object after Pluto. Each green dot represents a Kuiper Belt object. In 1987, American astronomer David Jewitt and graduate student Jane Luu used sensitive digital cameras to discover one. Since then astronomers have found over a thousand. This Hubble Space Telescope image shows Eris, one of the largest Kuiper Belt objects, and its moon Dysnomia. Eris is almost as large as Pluto and even more massive. This artist’s concept shows oval-shaped Haumea and its two moons. Haumea is probably made mostly of rock with a thin ice coating. Haumea is named for the Hawaiian goddess of fertility. Its moons are named for her daughters Namaka and Hi’iaka. This artist’s concept depicts Quaoar and its tiny moon Weywot. Quaoar is almost half the size of Pluto and far more distant. Its reddish surface may be due to hydrocarbon compounds. Quaoar is named for the creation god of the Native American Tongva people, and Weywot is Quaoar’s son. The Oort Cloud: Where Comets Live During the early days of our solar system, the Sun’s energy swept lighter materials away from the inner planets. These ices and bits of rock and dust clumped together into countless small bodies resembling “dirty snowballs” called comets. Many inhabit a vast “cloud” beyond the Kuiper Belt. Diverse Stories: Jan Oort Estonian astronomer Ernst Öpik first suggested that a cloud of icy bodies surrounds our solar system. In 1932, Dutch astronomer Jan Oort [above] proposed that such a cloud could be the source of new comets. The Oort Cloud is named after him. The Oort Cloud is vast. It starts between 2,000 and 5,000 AU from the Sun and extends out to 50,000 AU. (One AU, or astronomical unit, is the average distance between the Earth and Sun.) It may contain trillions of objects larger than two-thirds of a mile (one kilometer) in size. Comets Comets Are Visitors from the Outer Limits The gravitational tug of a large planet or nearby star can disturb the orbit of an Oort Cloud object. Nudged onto a slow path toward the Sun, it becomes a “new” comet, one never observed before. It can become bright enough to be seen in the night sky. The comet may strike a planet or the Sun on its first visit to our inner solar system. Or it may settle into an elongated orbit and return decades or centuries later. “Comet” comes from a Greek word meaning “the hairy one,” because of its hazy appearance and long tail. This tail forms as a comet approaches the Sun and becomes warm enough for its ice to turn into vapor. The Rosetta orbiter studied activity on Comet 67P for months. It captured this dramatic image of gas and dust jetting from the comet nucleus. Comet Shoemaker-Levy 9 collided with Jupiter in 1994. The impact created a dramatic pattern of dark clouds that later faded. This was the first time that an impact on a planet was predicted beforehand. Astronomers Carolyn S. Shoemaker, Eugene Shoemaker, and David Levy discovered the comet. People once thought comets signaled a great event or disaster, such as the death of King Harold at the Battle of Hastings in England in 1066. Comet Halley appeared in the sky that year and was depicted (top center) in the Bayeux Tapestry. Exploring Comets Comets have changed little since the dawn of our solar system. For decades astronomers used telescopes and radio astronomy techniques to probe their chemical makeup and to understand the distant reaches from which they come. In recent times, spacecraft have visited and even landed on comets. The approach of Comet Halley in 1986 marked the first time a spacecraft flew close to a comet. The European Space Agency’s Giotto probe flew the closest and took this remarkable photo of the comet’s icy nucleus, which appears quite dark. Stardust Captures Dusty Traces of a Comet NASA’s Stardust probe flew through the tail of Comet Wild 2 in 2004. It collected tiny comet particles, and sent the capsule containing the samples back to Earth. Scientists discovered that the bits of rock in the comet had formed under high temperatures by the young Sun in our early solar system before being pushed out far beyond Neptune, where they combined with ices to form comets. Stardust photographed Comet Wild 2’s nucleus, which is about three miles (five kilometers) across. Object Highlight: Samples of Comet MaterialsThe composition of these particles, gathered from deep-sea sediments on Earth, suggests that they may be from comets. Object Highlight: Aerogel The Stuff Used to Capture Comet Dust Stardust used a porous, silicon-based material called aerogel to capture dust samples. Aerogel is the lightest solid ever made. It has a sponge-like structure that is 99% empty space. It is only slightly denser than air. The grid for capturing comet particles (at the end of the capsule’s arm) held 132 aerogel tiles. The grid for capturing interstellar particles (not included here) held 132 slightly thinner tiles. Embedded in this thin slice of aerogel is a dust particle from the tail of Comet Wild 2. It entered the sample from the bottom right and moved to the upper left. Object Highlight: Vega Spacecraft Instruments The Soviet Vega spacecraft intercepted Halley’s Comet in 1986 and passed through its tail. It used instruments like this dust counter and mass analyzer to measure the density and size of particles in the tail. Object Highlight: Stardust Microchips Stardust carried two identical sets of microchips etched with the names of over a million people who responded to a “Send Your Name to a Comet” campaign. One microchip set came back in the return capsule. Object Highlight: ICE Plasma Wave Detector The ICE spacecraft (International Cometary Explorer) was the first to investigate two comets. It measured radio waves from ionized gases within the comet’s tail. Object Highlight: Stardust Capsule Stardust was the first U.S. spacecraft to bring back material from beyond the Moon. Seven years after its 1999 launch, Stardust passed by Earth and released the capsule, which parachuted onto the Utah desert. The canister holding the samples was sealed within an outer shell that protected it from the heat of reentry. Join the search for comet dust! The “Stardust at Home” project allows people like you to assist with mission science. You can participate in the search for particles within the aerogel collectors from your home computer as a citizen scientist. To learn how, visit: stardustathome.ssl.berkeley.edu. Rosetta Follows a Comet and Lands a Probe Rosetta was the first spacecraft to orbit a comet’s nucleus, and the first to follow a comet heading toward the center of our solar system. Launched by the European Space Agency, it began orbiting Comet 67P/Churyumov-Gerasimenko in 2014 after a decade-long trip. The Rosetta orbiter studied the comet as it neared the Sun, and a lander named Philae descended onto its surface. The Rosetta orbiter studied activity on Comet 67P for months. It captured this dramatic image of gas and dust jetting from the comet’s nucleus. Rosetta traveled far from the Sun, beyond the orbit of Jupiter, to catch up with the comet. To generate enough power, it had huge solar panels 106 feet (32 meters) across. This composite image shows Philae during its descent to the comet’s surface in 2014. Philae had harpoons to attach itself to the comet, but they failed to deploy. Philae bounced off the surface twice before making a safe landing. Because of Philae’s bouncy landing, its final location was not known until nearly two years after it landed. Diverse Stories: Claudia Alexander 1959–2015 Space Scientist Claudia Alexander’s work spanned the fields of planetary science, plasma physics, comet science, and geophysics. She was the last Project Manager for the Galileo Mission to Jupiter, Project Manager for the U.S. involvement in the Rosetta mission to Comet 67P/Churyumov-Gerasimenko, and a member of the Cassini Science Team. Alexander combined her skills as a scientist with the ability to manage and guide teams to success. In addition to her scientific work, she wrote children’s books, promoted STEM education, and established a scholarship program at the University of Michigan. Pluto A World Like No Other Pluto resides in the cold, dark, distant Kuiper Belt. Discovered less than a century ago, astronomers have learned many remarkable things about Pluto:
Pluto as seen by the New Horizons spacecraft in 2015. The surface indicates it has had a very complicated history. Pluto’s large moon Charon has a surprising dark red north pole, caused by trapped hydrocarbons. Canyons are visible near the equator. Pluto's Discovery In the early 20th century, astronomer Percival Lowell began to search for a planet beyond Neptune. He thought (incorrectly) that a ninth planet was needed to account for unexplained motions in the orbits of Uranus and Neptune. Diverse Stories: Clyde Tombaugh Clyde Tombaugh, a young astronomer at the Lowell Observatory in Arizona, later took up the search. In 1930, he discovered the object later named Pluto. When the New Horizons spacecraft flew past Pluto in 2015, we got to see it close-up as a world. Before New Horizons arrived at Pluto, scientists carefully looked for orbiting debris that could pose a risk to the spacecraft. They discovered four tiny moons. The Moons of Pluto While studying Pluto with a telescope in 1978, astronomer James Christy noticed that Pluto’s shape seemed to change every 6.4 days. That change in shape turned out to be a large moon circling Pluto. The moon Charon is so close to Pluto that it makes a single orbit in that short time. A periodic change in Pluto’s shape turned out to be the moon Charon. Like our own Moon and Earth, Charon always shows the same face to Pluto. Unlike our Moon, it always faces one side of Pluto and never rises or sets in the sky. New Horizons Visits Pluto and the Kuiper Belt New Horizons was the first spacecraft to explore Pluto and its moons. It mapped their surfaces and composition and studied Pluto’s atmosphere. Since flying past Pluto in 2015, New Horizons has been heading toward another icy body in the Kuiper Belt. The disk of the Pluto system is tilted with respect to the rest of our solar system. The New Horizon’s spacecraft flew right through the system in just hours. An Atlas V rocket propelled New Horizons to about 36,000 mph (58,000 km/h), making it the fastest spacecraft ever. It passed the Moon’s orbit after only nine hours. (Apollo spacecraft took three days.) A gravity assist from Jupiter increased its speed even more. Pluto and Charon Revealed! New Horizons revealed that Pluto and moon Charon have dramatic landscapes and altered surfaces that are surprisingly young. Scientists had expected to find an old and heavily cratered surface. New Horizons captured this close-up image of landslides within a large canyon on Pluto’s moon Charon. This possible ice volcano on Pluto is about 100 miles (160 kilometers) wide. These mountains on Pluto are made of water ice and surrounded by nitrogen-ice plains. Varying crater shapes raise questions about the composition of Pluto’s surface crust. Why Are Icy Worlds Important? Icy worlds can help scientists learn more about how our solar system began. Icy worlds may also help us learn how life first took hold on Earth. Many of them contain the ingredients for simple forms of life. Scientists once thought a rain of comets laden with water ice and other compounds helped to create our oceans and provided the building blocks for life to evolve. But the Rosetta mission showed that our oceans might not have come entirely from comets. Triton Is Triton a Captured Moon? Triton is a large and very unusual moon. It orbits Neptune, the outermost planet, in the opposite direction from most other moons. It has active geysers that release nitrogen. Its icy surface appears fairly young, which means that an episode of warming may have erased earlier features. All these oddities could mean that Triton formed elsewhere—in the Kuiper Belt—and was captured by Neptune’s gravity. 8/18
Giant Planets Our solar system has four giant planets: Neptune, Uranus, Saturn, and Jupiter. They are unimaginably huge, stunningly beautiful, and sometimes a little weird. The largest is over 300 times more massive than Earth. Although they may have small rocky cores, they don’t have solid surfaces. They are made up mostly of gases and ice-like materials. A host of moons orbits each one. Ice Giants: Neptune and Uranus Neptune and Uranus reside in the cold, dark reaches of our solar system far from the Sun. These ice giants have little hydrogen and helium (compared to the two gas giants). They contain mostly fluids of elements known as ices, because they freeze at higher temperatures than gases. Methane gives them a blue or blue-green color. Large dark-colored storms swirl through their atmospheres. Neptune: The Blue Giant Neptune’s frigid winds rage at up to 1,200 mph (2,000 km/h). This cold planet is so far from the Sun that its year (one complete orbit) lasts 165 Earth years. Since its discovery in 1846, Neptune has made only one full trip around the Sun. The First "Discovered" Planets The five planets visible to the naked eye had been known since ancient times. Uranus and Neptune, however, were not discovered until the age of the telescope. British astronomer Sir William Herschel “discovered” Uranus in 1781. (Earlier observers had seen it many times but thought it was a star.) A German astronomer found Neptune in 1846 where a French mathematician predicted it would be. Diverse Stories: Sir William Herschel Sir William Herschel discovered Uranus accidentally while conducting a star survey. One “star” seemed different. Years of earlier observations, some by German astronomer Johann Bode, revealed that it was a planet. Diverse Stories: Joseph Le Verrier and John Couch Adams Irregularities in the orbit of Uranus led French mathematician Joseph Le Verrier [left] and British astronomer John Couch Adams [right] to independently calculate where an unknown body affecting Uranus might be. Based on Le Verrier’s prediction, Berlin Observatory astronomer Johann Galle found Neptune. Uranus: The Tilted Giant Spinning like a top laying on its side, Uranus completes a rotation (one Uranus day) in only 17 hours. Its 84-year orbit takes it more than 1.9 billion miles (3 billion kilometers) from the Sun. A Sideways World Uranus’s odd tilt makes it unique among our solar system’s giant worlds. A collision with a planet-sized object may have caused Uranus to tip over early in its history. With its axis tilted almost 98°, Uranus’s environment differs from other planets in our solar system. The Sun points directly at each pole for long stretches of the planet’s orbit. This produces years-long winters when half of the planet is in continuous darkness, shadowed from the Sun’s light. Uranus rotates in the opposite direction from most of the other planets. If you could view our solar system from above, you would see six planets rotating counter-clockwise.Only Uranus and Venus spin clockwise. Stormy Weather Large storms swirl in the atmospheres of the ice giants. Uranus looked mostly featureless to Voyager 2. But this infrared view collected in 2012 by the Keck Observatory in Hawaii shows the planet can have many stormy structures. While exploring Neptune in 1989, Voyager 2 observed a storm larger than the planet Earth. Astronomers called it the Great Dark Spot. By 1994, the Hubble Space Telescope revealed that the spot had disappeared, but another large storm soon developed. Just beneath the Great Dark Spot in this Voyager image is a bright cloud feature called the Scooter, because it sped around Neptune faster than the dark spot. To the south of the Scooter is a similar but smaller storm named Dark Spot 2. Gas Giants: Saturn and Jupiter The largest planets in our solar system, Saturn and Jupiter are made up mostly of hydrogen and helium. They rotate fast and have strong winds and storms. Because they are so massive, temperatures and pressures deep within them increase to extraordinary levels. Hydrogen takes on a liquid metallic form. The nature of their rocky cores remains a mystery. Saturn was imaged by Cassini when the spacecraft passed through the planet’s shadow. Saturn: The Ringed GiantI n this Cassini image of Saturn and its dazzling rings, some of Saturn’s moons are visible. The Cassini spacecraft took detailed images of Saturn’s oddly shaped north polar vortex through four different filters. Unlike on Earth, where winds blow around the poles in a circular pattern, Saturn’s north polar vortex is shaped like a hexagon! It has persisted for at least many decades. A huge storm appeared in Saturn’s northern hemisphere in late 2010. In this view from only 12 weeks later, the storm has extended all the way around the planet and overtaken itself. Jupiter: A Giant Among Giants Jupiter is wider than 11 Earths. It has more mass than all the other seven planets of our solar system combined. Jupiter’s Great Red Spot is a gigantic storm that has been observed for over three centuries. More than 10,000 miles (16,000 kilometers) across, it rotates counterclockwise. Jupiter’s colorful cloud bands spin around the planet at different speeds. The dark dot is the shadow of the moon Europa on Jupiter’s ammonia clouds. Moons of the Giants The Voyager spacecraft revealed the moons of the giant planets to be surprisingly diverse worlds in their own right. Further exploration has unveiled intricate surfaces both young and old, volcanoes and impact features, huge plumes and geysers, deep canyons, subsurface oceans, and even clues to possible environments that might be friendly to simple forms of life. Complex Terrains The moons of Jupiter and Saturn are not dead, unchanging worlds. Several show evidence of geologic activity throughout their history. Complex patterns, faulting, cliffs, and deep canyons tell stories of stresses and resurfacing that have shaped and reshaped the terrain. Ganymede’s two different types of terrain meet in this Galileo spacecraft image. Iapetus Saturn’s moon Iapetus has an odd six-mile (10-kilometer) high bulge circling its equator. Some scientists think its origin may relate to the moon’s early rotational history. Others think it may have formed from the collapse of a ring. Dione Made of rock and ice, Saturn’s moon Dione is very dense. Bright wispy features extend across its face, cutting through craters. These features are fractures—deep canyons with bright ice exposed along their walls—possibly formed by tidal forces. Ganymede Jupiter’s moon Ganymede is the largest moon in our solar system, even larger than the planet Mercury. Its surface is an icy layer overlying a rocky mantle and iron core. Salty water might exist within the outer crust as well. Ganymede has two broad types of terrain: dark cratered regions and brighter-colored deeply grooved areas. Fountain Worlds A few of the giant planets’ moons spew large geyser-like plumes from their icy surfaces. This “ice volcanism” involves water vapor, ices, gases, and dust—not hot lava. Enceladus An icy moon of Saturn, Enceladus has a bright surface and large areas with few craters. This smoother terrain is young, having been resurfaced in recent geologic time. Deep cracks near the south pole, often called “tiger stripes,” are vents for huge plumes of water vapor. The plumes of Enceladus are fed by a global ocean, 6 miles (10 kilometers) beneath the moon’s icy crust. The existence of this ocean makes Enceladus an intriguing site for the search for life in our solar system. Triton Voyager 2 discovered geyser-like features on Neptune’s moon Triton. Scientists think they are eruptions of nitrogen gas mixed with dark dust particles. In this image, arrows indicate a vertical plume about five miles (eight kilometers) high being blown westward in a cloud 90 miles (150 kilometers) long. Europa Scientists using the Hubble Space Telescope reported evidence of water vapor plumes (lower-left edge) erupting from the south pole of Jupiter’s moon Europa. These plumes could provide a sample of the deep ocean beneath the icy crust—an exciting target in the search for life. Europa Clipper Planned to arrive at Jupiter in the 2020s, the Europa Clipper spacecraft will perform a series of flybys of the icy moon to explore its surface, subsurface, and plumes, in order to assess its potential to harbor life. Impact! Strikes by meteorites, comets, and asteroids have shaped planetary surfaces throughout our solar system. The faces of the giant planets’ moons record their impact history. Some bear the scars of giant cataclysms. Large impact basins often exhibit a few concentric rings. But scientists were amazed when Voyager 1 revealed this “bulls-eye” of countless rings of Callisto’s huge Valhalla Basin. The basin is 2,500 miles (4,000 kilometers) across. Callisto Jupiter’s moon Callisto has one of the most heavily cratered surfaces in our solar system. This ancient terrain preserves a record of intense bombardment early in the moon’s history. Mimas Saturn’s water-ice moon Mimas was nicknamed “the Death Star moon” for its eerie resemblance to the weapon in the movie Star Wars. The huge Herschel Crater (William Herschel discovered Mimas) is three miles (five kilometers) deep and a third as wide as the entire moon. Miranda Uranus’s moon Miranda looks like a sloppy patchwork of unconnected parts. Made of ice and rock, it has huge chasms 12 times deeper than the Grand Canyon. Miranda’s strange appearance mystifies scientists. Perhaps the moon was shattered and came back together. Or maybe huge impacts broke up its icy surface, which then refroze. Titan Larger than the planet Mercury, Saturn’s moon Titan is the only moon in our solar system with a substantial atmosphere. Mostly nitrogen and methane, its foggy air blocks Titan’s surface from view. Beneath the haze, long river valleys have been carved by liquid hydrocarbons (methane and ethane) that fill large lakes. Radar can peer through Titan’s clouds to image the surface below. The dark areas in this Cassini radar image are lakes of hydrocarbons. The big one at the bottom is one-third larger in surface area than Lake Superior, the largest of the Great Lakes. Unique, Weird, and Wacky The moons of the giant planets have some unexpected and surprising features. Io Some scientists predicted that Jupiter’s moon Io would show signs of volcanism. But few expected the colorful beauty and ongoing volcanic activity revealed by the Voyager spacecraft in 1979. While processing a Voyager navigation image, engineer Linda Morabito at the Jet Propulsion Laboratory discovered a crescent-shaped feature extending from Io’s edge. It turned out to be a huge volcanic plume. Nearly 20 years later, this Galileo spacecraft image showed widespread surface changes due to continuing volcanism. Io is the most volcanically active object in our solar system. Three volcanoes were erupting as New Horizons flew by in 2007. The huge plume of a volcano gushes 180 miles (290 kilometers) upward near the north pole. Smaller plumes can be seen on the left horizon and as a bright spot near the south pole. Rings All four giant planets have rings. Jupiter, Uranus, and Neptune have only a few faint, narrow rings that are difficult to observe. Saturn has a massive ring system. Seen edge on, Saturn’s thin rings almost seem to disappear. But viewed from above or below, they present a scene of dramatic beauty unique in our solar system. Uranus' Rings The rings of Uranus were first observed by scientists using Earth-based telescopes and the Kuiper Airborne Observatory. The rings were discovered when they caused the light of a star to flicker as Uranus passed in front of it. Voyager 2 took the first images of the rings. This one shows five of the 13 rings identified around Uranus. Saturn's Rings Saturn’s rings are made of billions of fragments of mostly ice and some rock. They range in size from dust particles to large boulders. A few bodies may be as large as two-thirds of a mile (one kilometer) across. Cassini captures a view of Saturn’s rings from the perspective of the moon Enceladus. The rings are named with letters in the order they were discovered. They are separated in places by narrow gaps. Cassini even captured the distant Earth in this image. In one of Cassini’s last orbits, before plunging into Saturn at the end of its mission, the spacecraft approached the risky gap bordering the planet. The image shows the variety and intricate patterns of Saturn’s rings. The gravity of small nearby moons influences the appearance and structure of the rings. Called shepherding satellites, these tiny moons help form clumps, bends, and braids in the rings. Here, Pandora (orbiting outside the ring) and Prometheus (inside) shape Saturn’s F ring. The inset details the tiny moon Prometheus. Cassini’s last orbits were dramatic dives between the planet and its rings. This is a view of Saturn’s rings from the perspective of the moon Enceladus. In this mosaic of Cassini spacecraft images, Saturn was purposely overexposed to bring out the fine detail in its spectacular rings. Jupiter's Rings Voyager 2 made the surprising discovery that Jupiter had rings. They are so thin and faint that they had never been seen from Earth. The Galileo spacecraft took this image of the rings when Jupiter was between the spacecraft and the Sun. From this vantage point, the faint rings stand out clearly. On its way to Pluto, New Horizons captured this image of Jupiter’s rings. The innermost ring, known as the halo, consists of dust particles that rise above and below the ring plane. The main ring is flat. Two thin structures called the gossamer rings lie outside it but are too faint to be seen here. Neptune's Rings As with Uranus, Neptune’s rings were discovered when astronomers observed them blocking the light from a star. Six rings were later identified in Voyager 2 images. The outermost ring contains bright areas called arcs, where material is clumped together. These structures puzzle scientists, because the material should even out. Object Highlight: The Telegram Announcing the Uranus Rings Discovery This is the original draft of the message sent in 1977 to the Central Bureau for Astronomical Telegrams (CBAT) reporting the discovery of Uranus’s rings. CBAT is the official center for reporting newly observed astronomical objects and events. The message was sent by an astronomer aboard the Kuiper Airborne Observatory. Object Highlight: Uranus Ring Discovery Chart This graph of light from the star SAO 158687 was made from data recorded on board the Kuiper Airborne Observatory. The light intensity dips where the rings of Uranus passed in front of the star. The Asteroid Belt: A Planet That Never Came Together A belt of over a million rocky bodies called asteroids lies between Jupiter and Mars. They range in size from less than 33 feet (10 meters) to over 600 miles (1,000 kilometers). Though relatively small and often irregular in shape, some have their own moons. Most asteroids are debris left over from when our solar system formed. Giant Jupiter’s gravity may have prevented them from merging into a planet. Itokawa Japan’s Hayabusa spacecraft visited oddly-shaped Itokawa, which may have formed from two bodies clumping together. Hayabusa landed, collected a sample, and returned it to Earth. Analysis showed that the dust it collected had been on the asteroid’s surface for 8 million years. Vesta Vesta, the second most massive body in the Asteroid Belt, was visited by the Dawn spacecraft in 2011. The long grooves shown here, more than two-thirds of a mile (one kilometer) deep, resulted from two giant impacts at its south pole. Ceres In 1801, while making a star map, Italian astronomer Giuseppe Piazzi discovered a small object between the orbits of Mars and Jupiter. He named it Ceres. Like Pluto, Ceres is now considered a dwarf planet, the only one in the Asteroid Belt. This image is from the Dawn spacecraft, which has orbited Ceres since 2015. Data from Dawn indicate that the mysterious bright spots in Occator crater may contain magnesium sulfate salts and frozen ammonia. The largest spot flanks a fractured hill in the center of the crater. 9/18
Rocky Planets, Part 1 The rocky planets: Mercury, Venus, Earth, and Mars, all formed in our inner solar system. Their geological history is preserved on their surfaces. Their landscapes reveal the processes that shaped them: impacts, crustal movements, volcanic activity, and erosion. Gravity, temperature, air, and water all play leading roles in their geological stories. We have studied the rocky planets for decades. We now have so much data that we can compare geologic processes on different planets, rather than studying each one in isolation. Impacts: When Objects Smash Together Planets and moons across our solar system bear the scars of collisions. Impact craters form on their surfaces when a dust particle, rock, asteroid, or comet smashes into them. Impact craters come in all sizes and shapes, depending on the impacting object size, impact angle, and surface into which the object crashes. Craters range in size from microscopic—smaller than the width of a human hair (100 microns, or .0001 meter)—to as wide as the continental United States. A recent impact crater on Mars shows stunning “rays.” These formed when material ejected by the impact disturbed the surface. Progression of impact crater shape as they increase in size. Surface details change as crater size increases. Interactive: Samples of Our Solar System These kinds of Earth rocks can also occur on other rocky worlds. Breccia Breccia is rock made of pieces of other rocks. It can form when the high temperature and pressure created by an impact fuses pieces of local rock together. This breccia is from Sudbury, Canada, the site of a large impact crater. Basalt Basalt is the most common rock type on the rocky planets. It forms when iron-rich lava erupts onto a planetary surface. Gases released during eruption created the circular pits in this Hawaiian basalt. Anorthosite Anorthosite is made almost entirely of feldspar, a common mineral. It makes up the earliest crust of our Moon and may be prominent in early crusts of other rocky planets. This sample is from California. Sandstone When sand grains are buried deeply or long enough, they can form solid rock. Sandstone is common at some rover landing sites on Mars. This sample is from Utah. Granite Granite forms beneath Earth’s surface, where minerals in magma cool and crystalize slowly. Granite is common on Earth. But it is uncommon on other rocky planets, because they lack plate tectonics, which controls the processes that form granite. This sample is from California. What are some clues geologists use to help them identify the rocks they are studying?
Diverse Stories: Toshiko Mayeda 1923–2004 Leading Isotope Geochemist Born in Tacoma, Washington, Toshiko Mayeda was imprisoned during World War II at the Tule Lake internment camp along with other Japanese Americans. After her release, she moved to Chicago, where she studied chemistry and worked for Harold Urey, a Nobel Prizewinning chemist at the University of Chicago. At the University of Chicago, Mayeda used instruments called mass spectrometers to measure isotopes (forms of the same element with differing atomic mass). She became highly skilled in operating the custom-made spectrometers and a recognized expert in analyzing meteorites to understand their age and where they formed in our solar system. Scientists around the world sent her samples to analyze. When Apollo 11 brought back the first samples from our Moon, NASA sent lunar soil to the University of Chicago for analysis. Craters of Rocky Planets Parts of a Crater Impact craters have different features that vary with the size of the crater.
Some Craters Are Simple Simple craters are bowl-shaped depressions surrounded by a raised rim and a blanket of ejecta. They are deeper relative to their diameter and more bowl-shaped than complex craters. They lack features like wall terraces and central peaks. Piazzi Smyth Crater (Earth's Moon) This simple crater on our Moon (eight miles/13 kilometers across) has a classic bowl shape. Meteor Crater (Earth) Meteor crater in Arizona is one of the best preserved impact craters on Earth. It formed about 50,000 years ago when an ironnickel meteorite about 98 feet (30 meters) in size slammed into the desert. The impact left a crater three-fourths of a mile (1.2 kilometers) across. Some Craters Are Complex Complex craters are larger and often have more intricate features than simple craters. They have a central peak or pit, sections of their walls have collapsed to form terraces, and, like simple craters, have a raised rim and a blanket of ejecta. Addams Crater (Venus) The surface of Venus is extremely hot. When an impact creates a crater, some of the ejecta is hot and fluid. Flows of this molten material, called impact melt, can extend for long distances, as shown by this radar image of Addams crater (54 miles/87 kilometers across). Copernicus Crater (Earth's Moon) Copernicus crater (60 miles/90 kilometers across) is one of the younger craters on our Moon. Its rays are striking—radial streaks of ejecta that extend hundreds of miles. Copernicus is so pristine and bright that you can see it with binoculars. The Largest Impacts Creat Huge Basins The largest impacts have major effects.Our Moon likely formed when an object the size of Mars collided with Earth. No surface feature remains from that colossal impact. But huge basins created by impacts exist on other worlds. A large impact can spread a thick blanket of ejecta over a vast area and reshape the landscape. On a planet with an atmosphere, it can even affect the global climate. Orientale Basin (Earth's Moon) Orientale basin (560 miles/900 kilometers) on our Moon is the youngest and best-preserved impact basin in our inner solar system. Vredefort Basin (Earth) Vredefort basin in South Africa is the largest known impact basin on Earth. It is more than 185 miles (300 kilometers) across. What Affects the Size and Shape of a Crater? Many factors are involved. The bigger and faster a moving object is, the larger the crater it will make. The more gravity a planetary body has, the faster an object will be moving when it hits. Gravity, the presence of an atmosphere, the angle of impact, and the nature of the surface material also affect crater shape. Clusters of Impact Craters (Venus) An atmosphere can slow down an incoming object or melt it completely. The atmosphere of Venus is so dense that some objects break apart and form clustered impact craters. Schiller Crater (Earth's Moon) When an object strikes the surface at a very low angle, it creates an elongated, asymmetrical crater, like Schiller crater on our Moon, which is 110 miles (180 kilometers) long. Tooting Crater (Mars) The nature of the surface—solid rock, fragmented rock, or ice—affects crater size and shape. The harder the surface materials, the smaller the crater. The material ejected from Tooting crater on Mars formed distinct ejecta lobes. Water or ice in the surface material may have melted or vaporized during impact and caused the fluid-looking ejecta pattern. Tectonics: When Internal Forces Reshape Worlds Rocky worlds can also reshape themselves from internal forces that push and pull at their crustal materials, a process called tectonics. Contractional forces shove crustal material together to create cliff-like fault scarps, ridges, and mountains. Extensional forces stretch and pull the crust apart to form troughs and rift valleys. While impacts are sudden, tectonic forces operate over long periods of time. The landforms they create can take millions of years to form. Landforms Created by Contractional Forces When forces push a planetary surface together, it buckles and breaks. Parts of the crust are thrust or shoved upward along breaks in the rock called faults. They form ridges, fault scarps, and mountainous terrain. Cliff-like landforms called lobate scarps are found on Mercury, Mars, Earth, and the Moon. They are formed when contraction of the crustal rock forces the surface to break and be pushed upward along faults. Cedar Mountain in Wyoming is one example of a lobate scarp. The highlands of Venus contain large, broad landforms called tesserae. These complex, mountainous areas may have resulted from forces that repeatedly contracted and extended crustal rocks. Landforms Created by Extensional Forces When forces pull a planetary surface apart, the most common landforms created are called graben. These trough-like features form when rock is pulled apart, breaks, and drops down between parallel faults. When forces pull equally in all directions, fractures and graben develop in circular or polygonal patterns. Shown here are circular and polygonal graben in lava plains that buried an impact crater on Mercury. The circular graben outline the rim of the buried ‘ghost crater.’ Large valleys result when crustal rock is pulled apart by extensional forces. Valles Marineris on Mars is the largest such valley in our solar system—hundreds of miles wide, several thousand miles long, and up to six miles (10 kilometers) deep. The much smaller Great Rift Valley in East Africa formed the same way. Planetary Plates Earth’s “outer shell”—its crust and upper mantle, called the lithosphere—is broken into a mosaic of about 12 plates. They move around and forcibly interact in a process called plate tectonics. Earth appears unique in this regard. The other rocky planets and our Moon seem to have unbroken (one-plate) outer shells. As the Earth slowly cools, heat loss creates currents in the mantle that move the crustal plates. On Earth, most tectonic landforms are created by forces that push, pull, and slide tectonic plates. Mountain ranges, rift valleys, and long faults like the San Andreas in California mark the borders of plates. Venus is the same size and density as Earth, but it does not have plates. It has areas of higher and lower elevation, but none resemble Earth’s continents and ocean basins, the signature of plate tectonics. One prominent highlands region on Venus is Aphrodite Terra, shown in shades of orange and red in this topographic map. Shrinking Worlds The rocky planets and our Moon all have something in common: hot interior cores surrounded by hot mantles. As its interior cools, a planet or moon shrinks. On a planetary body with a continuous lithosphere (outer shell), this causes its crust to wrinkle, like the skin of an apple as the core dries and shrivels over time. Although it is the smallest planet, Mercury has some of the largest fault scarps in our solar system. The shrinking of its interior has forced its single-plate surface to contract, creating fault scarps all over the planet. The largest is 620 miles (1,000 kilometers) long, about the size of California’s San Andreas Fault. Small, very young fault scarps (arrows) on our Moon indicate that, like Mercury, it too is still slowly cooling and shrinking. Volcanism When Worlds Erupt One way rocky worlds release interior heat is through volcanic activity. This can involve molten rock, or magma, being forced into the crust. The magma may erupt onto the surface to become lava. Scientists have found evidence of volcanic activity on all the rocky planets and our Moon. Volcanism creates a variety of landforms, not just volcanoes, depending on the properties of the lava (such as viscosity and composition) and on the planetary environment (like gravity and presence of an atmosphere). Volcanoes form from the buildup of cooled lavas and ash on a planetary surface. Solids, liquids, and gases can erupt from a volcano. The amount and makeup of the erupting material affects the volcano’s size and shape. Venus displays the greatest diversity of volcanic features among the rocky worlds. This large volcano has dozens of lava flows extending from its central caldera (vent). Our Moon has a few volcanoes (circled in red), but they are not made from very fluid lava like most volcanoes. Instead they are made of “stickier” (more viscous) lava that built up into tall mounds in the low gravity of the Moon. Object Highlight: Pioneer-Venus Orbiter Scientists used data from the radar altimeter on the Pioneer-Venus Orbiter to make the first global topographic map of Venus in 1978–80. The spacecraft also acquired radar images of the surface near Venus’s equator. Object Highlight: Magellan Spacecraft Magellan’s single antenna (visible in this model) received radar data and relayed it back to Earth. Since the surface of Venus is completely hidden by clouds, the only way to view and map the landscape is with radar. From 1990 to 1994, Magellan collected radar images of about 98% of the surface of Venus from orbit. Diverse Stories: Alexander Basilevsky 1937–present Russian Planetary Scientist Alexander (Sasha) Basilevsky worked to promote science on Soviet lunar and planetary missions. He often traveled to international meetings to collaborate with colleagues from the United States and other nations. Basilevsky was involved in many planetary missions. He helped to select landing sites for the Luna and Lunokhod landers on the Moon in the 1970s, and the Venera landers on Venus in the 1970s and 1980s. He was a Soviet guest investigator on the Voyager Neptune encounter and NASA’s Magellan mission to Venus, as well as on the Mars Odyssey and Mars Express international missions to Mars. Gassy Eruptions Many of the rocky planets are covered in lavas and other types of volcanic deposits that have erupted from vents and fissures (long linear vents) that may or may not be associated with volcanoes. Many of these eruptions involve a hot mix of ash, gases, and lava, which scientists call pyroclastic. A pyroclastic flow is a mix of hot ash, gases, and lava fragments that moves rapidly downslope. This pyroclastic flow spilled down the flank of Popocatépetl in Mexico. If a large amount of gas is present, a fountain-like eruption occurs, in which lava is blown into pieces by the force of the gases. This vent on Mercury released such a pyroclastic eruption. The halo around the vent is formed from erupted glass fragments. This pyroclastic deposit of dark material (circled in red) surrounds a vent that erupted in Orientale basin on the Moon. 10/18
Rocky Planets, Part 2 Magma IntrusionsMolten rock does not always erupt onto the surface. Sometimes it stalls and cools just beneath the crust, forming what’s called an intrusion. Intrusions often push up and fracture the overlying surface, creating a system of cracks. Erosion sometimes wears away the overlying material, exposing the hardened intrusion. The floor of this crater on our Moon was uplifted when magma intruded into the fractured rock just beneath the crater floor. Shiprock in New Mexico is a preserved core of a volcanic vent exposed by erosion. The vent was active 30 million years ago. Several sheet-like lava intrusions called dikes, in the foreground, radiate from the core on the left. Lava Channels Channels and sinuous rilles (winding channels) are common landforms within volcanic terrains. They can form by either the buildup of lava along the side of a lava flow or when a lava flow carves into the surface. Our Moon has many sinuous rilles. Here, a small one lies within an older, larger one on the Aristarchus Plateau. Volcanic Plains Not all volcanic activity involves volcanoes. Many volcanic deposits on the rocky planets were produced by flood lavas, large-volume eruptions that covered vast areas. The lava usually poured out of vents now buried in low-lying areas, such as impact basins and craters. The dark lunar plains (maria) are an example of flood lavas. Most of these lie on the near side of our Moon [left]. The far side [right] has few and much smaller maria. Lava plains on Mercury also lie mainly in one hemisphere. Over 80% of Venus is covered by relatively young volcanic plains, less than 500 million years old. The abundance of these plains suggests that Venus may have experienced catastrophic volcanic events that caused lava to flood large parts of its surface. Some lava flows erupt from long cracks, called fissures. This fissure on Mars extends more than 620 miles (1,000 kilometers). Features like this formed from the upward movement of molten rock. Wind-blown sand forms ripples and dunes, like these seen by the Mars rover Curiosity. Dune fields are scattered over much of the Martian surface. Scientists have found only two similar dune fields on Venus, despite its windy surface. Although Mars has only a thin atmosphere, winds blow sand and dust around. At times, dust storms envelop the entire planet. Erosion When Rock Wears Down Gravity, wind, water, ice: all help break down solid rock and scatter the pieces. Wind and water transport dust and sand. Gravity and glacial ice can move large pieces. Erosion wears down landforms, but it also creates new ones. Planets with and without atmospheres experience erosion, but by different geologic processes. On Earth and Mars, water and wind are the main agents of erosion. On our Moon and Mercury, worlds without wind or liquid water, impact cratering erodes the landscape. Atmospheres and Winds Venus, Earth, and Mars are all covered by substantial atmospheres. Venus has a very thick, hot, carbon dioxide atmosphere. Its surface pressure is more than 90 times that of Earth’s. Mars has a thin, cold, carbon dioxide atmosphere. Its air pressure is less than 1% that of Earth’s. Life has greatly changed our own atmosphere over time, so it is now mostly composed of nitrogen and oxygen. Winds in the upper atmosphere of Venus travel 110–220 mph (180–360 km/h). Here, those powerful winds have blown small ejecta particles (the dark material) from Stuart crater over a vast area. Wind-blown sand has eroded parallel ridges into the bedrock in many places on Earth (shown here) and Mars. Scientists call these ridges yardangs. Object Highlight: Mars Meteorite This meteorite was recovered from Antarctica in 1979. The glassy portions contain trapped gases. Scientists compared these gases to those in the Martian atmosphere, which the Viking landers measured in 1976. The gases were nearly identical. Scientists also found that the rock is quite “young” for a meteorite—less than 1.5 billion years old. It must have formed on a large object, like a planet. All this led them to conclude that it must have come from Mars. The rock was blasted off the surface by an impact. It eventually drifted to Earth and landed as a meteorite. Diverse Stories: Ronald Greeley 1939–2011 Pioneering Planetary Scientist Ron Greeley helped establish the modern field of planetary science. He pioneered the practice of interpreting planetary images by comparing them with laboratory experiments and field studies of features on Earth. The results shed light on how geologic processes produced and shaped planetary surfaces. Greeley was especially known for using and designing wind tunnels to study wind-related processes on the planets. He was involved in spacecraft missions to Jupiter and Venus and almost every Mars mission. As a professor of planetary geology at Arizona State University, he taught and guided many students who became planetary scientists. Ice on Other Worlds Mars, Mercury, and the Moon have concentrations of ice around their north and south poles, much like Earth. Mars has thick ice caps at both poles. Sounding radar from orbiting satellites reveals their internal layers (horizontal light and dark stripes). On the Moon and Mercury, some impact crater floors near the poles are always in shadow. They stay very cold and have accumulated water ice. Areas on Mercury that may have ice deposits are shown in yellow. Mercury appears to have more ice in its permanently shadowed craters than our Moon. A glacier is ice that flows downslope due to its weight. Glaciers occur on nearly 10% of the surface of Earth and at all latitudes. Morteratsch Glacier is in the Swiss Alps. Water on Mars The surface of Mars is a cold, dry desert. But during its early history, it had rivers and lakes—and perhaps even a northern ocean. As on Earth, small streams on Mars join to form larger ones, creating tree-like networks of tributary valleys. During a wetter period on Mars over 3.5 billion years ago, some of this water also flowed into craters to form lakes. Eberswalde crater on Mars contains an ancient river delta of meandering stream channels that once emptied into a body of water (like a lake). Three Generations of Mars Rovers Object Highlight: Mars Science Laboratory Rover Full-Scale Model The Mars Science Laboratory rover, named Curiosity, is much larger and better equipped than previous Mars rovers. Weighing about a ton (1,000 kilograms), it landed in Gale crater in 2012 by means of a “sky crane”—it was safely lowered to the surface from a hovering platform. The nuclear-powered rover used 11 scientific instruments to study rock layers as it climbed a mountain inside Gale crater. Curiosity found sediments from former lakes and streams that could have supported bacterial life. Scientists still don’t know whether life ever existed on Mars. “Mars exploration by three generations of rovers, each far outlasting its expected lifetime, is providing fundamental information on the planet’s geologic history of water, and past habitability.” -John Grant, Geologist, NASM Can you read Morse code? The treads on Curiosity’s wheels leave tracks on Mars that help tell how far it travels. And they also spell the name of the lab that built the rover, JPL. Object Highlight: Mars Exploration Rover (MER) Surface System Test Bed This rover was used on Earth to troubleshoot problems that the rovers on Mars encountered. Two MER rovers, named Spirit and Opportunity, landed on opposite sides of Mars in 2004. Those robotic “field geologists” used instruments to examine rocks and soils. Both Spirit and Opportunity eventually found clear evidence that water once had been present at their landing sites. Spirit operated until 2010. Opportunity continued to explore until 2018. Object Highlight: Watch for Keeping Mars Time Some scientists working on the Mars Exploration Rover missions had watches like this one to stay on “Mars time” during initial rover operations. The watch keeps time based on the length of a Martian day: 24 hours and 39 minutes. Object Highlight: Test Rock for a Rover Tool The Mars Exploration Rovers Spirit and Opportunity each carried a Rock Abrasion Tool (RAT) to grind holes in rock. This piece of Cleveland Shale was used in laboratory tests of the RAT. Object Highlight: Marie Curie Rover Sojourner’s Flight Spare Sojourner was the first successful Mars rover. It landed aboard the Pathfinder spacecraft on July 4, 1997, and operated for 85 days. It was named for Sojourner Truth, the 19th century abolitionist who also fought for women’s rights. The rover displayed here is Marie Curie, Sojourner’s flight spare. It was named after the famous scientist who pioneered the study of radioactivity. The Pathfinder mission tested the airbag landing system, cameras, rover mobility, and other technology used for later Mars rovers. The solarpowered Sojourner weighed about 25 pounds (11.5 kilograms). It drove about 330 feet (100 meters) and measured the composition of rocks near the lander. Object Highlight: Viking Thermal Mapping Instrument Two Viking orbiters arrived at Mars in 1976, each carrying an InfraRed Thermal Mapper (IRTM). The IRTMs measured the atmospheric and surface temperatures on Mars. They were used to help select landing sites for the Viking landers. They mapped the thermal properties of the surface and atmosphere for three Mars years (six Earth years). Object Highlight: Meterology Sensor Assembly The Viking Lander Meteorology Sensor Assembly (MSA) measured the Mars atmospheric temperature, wind speed, and wind direction. Atmospheric pressure was also measured by a sensor in the lander. Object Highlight: Viking Lander Camera The Viking landers carried this type of line-scan camera. The camera makes a narrow vertical scan of a landscape, and then rotates slightly and makes another scan. An image is created by stacking all these vertical scans into a single picture. Object Highlight: Viking Gas Chromatograph Mass Spectrometer The Viking landers each had a mass spectrometer to measure the composition of the Martian atmosphere and search for the chemistry of life in the soil. Diverse Stories: People of Perseverance In 2021 the rover, Perseverance, landed on Mars. It looks similar to the Curiosity model here, but it has stronger wheels, uses new science instruments, and is able to collect rock samples to be returned to Earth at a later time. The rover also carried the first powered spacecraft to fly on another planet. It takes hard work and a team of hundreds of people across a vast spectrum of expertise to develop and manage Perseverance’s complex mission. Here are just of a few of those persevering people who worked on different remarkable aspects of the mission. Monitoring the Mars helicopter, Ingenuity, the first aircraft to fly on Mars (from right to left): Bob Balaram, robotics expert and Ingenuity Chief Engineer, proposed the idea for a lightweight craft to fly on Mars; Mimi Aung, Project Manager for Ingenuity, guided its development and operation; Teddy Tzanetos, Tactical Lead for cruise and surface operations, worked to plan Ingenuity’s tasks throughout the mission. Diverse Stories: Moogega Cooper Planetary Protection Moogega Cooper, Planetary Protection Lead Engineer, made sure the spacecraft and rover met strict cleanliness standards to prevent bringing Earth microbes to Mars. DIverse Stories: Roger Wiens Sounds of Mars Roger Wiens is Principal Investigator for the SuperCam, an instrument to collect data on the chemical composition of Martian rocks and soils. It also carries a microphone to record the windy sounds of the Martian environment. Diverse Stories: Jose Antonio Rodriguez-Manfredi Weather Jose Antonio Rodriguez-Manfredi is the Principal Investigator for the Mars Environmental Dynamics Analyzer (MEDA) that gives a weather report for Mars with data on temperature, humidity, pressure, wind, and more. Diverse Stories: Mariah Baker Smithsonian scientist Mariah Baker is one of a large team of scientists analysing data in a variety of disciplines. She studies wind-driven sand and dust with an eye to understanding risks to landed spacecraft and future human explorers. 11/18
Points of Light to Physical Worlds Planets were once thought of as simply lights in the sky that moved mysteriously from night to night. When people first saw planets through telescopes in the 1600s, they discovered they were not just lights, but places. Using increasingly powerful tools, we came to know them as actual worlds with unique natures. Later we sent spacecraft to visit them. Now we are finally getting to know and understand these worlds. Our Changing Views of the Planets Long before written history, people noticed that five “stars” seemed to wander among all the others from night to night. These points of light became known as Mercury, Venus, Mars, Jupiter, and Saturn. Because their movements were different and seemingly less predictable than the other stars, the ancient Greeks called them asteres planetai, or “wandering stars.” In time, telescopes and spaceflight would profoundly change our understanding of these planets. What Do Planets “Mean”? How would you explain the planets if you didn’t know what they were; if you didn’t even know you lived on one? People in ancient times observed these strange points of light that changed their positions from night to night and wondered about their meaning. They recorded their movements and later learned to predict them. Object Highlight: Babylonian Clay Tablet The ancient Babylonians created the oldest known written records of the planets. They recorded their observations on clay tablets. The inscriptions on this tablet (a reproduction) are of astronomical calculations made more than 2,000 years ago, related to the rising and setting of the Moon and planets. Planets Become Places Galileo Galilei was the first to focus a telescope on the planets. What he saw completely changed humanity’s understanding of them. Through his telescope, stars remained points of light. But planets looked like spheres with surfaces and features. Galileo drew these sketches showing Saturn’s perplexing shape. We now know he was the first to see Saturn’s rings, although he did not know it. The Dutch astronomer Christiaan Huygens first described the rings in 1659. In 1609, Galileo pointed his telescope at the night sky and saw for the first time that planets were not stars at all. He also discovered four moonsaround Jupiter and saw that Venus had phases like our Moon. He drew this sketch of Venus. Places Become Worlds As astronomical tools improved, our views of the planets sharpened and our understanding of them deepened. Two new planets, Uranus and Neptune, were discovered, along with more moons. The focus of astronomers shifted from how planets moved to what they were like as individual worlds. But good views of their surfaces still proved elusive. Diverse Stories: Asaph Hall In 1877, Asaph Hall, an astronomer at the U.S. Naval Observatory in Washington, DC, discovered two small moons around Mars that he named Phobos and Deimos. Object Highlight: Asaph Hall Eyepiece and Logbook The fog from the Potomac River made it hard to detect the moons. But Hall was persistent. He searched the edge of Mars with this telescope eyepiece until he found them. On the night of August 16, 1898, Harvard University astronomer William H. Pickering discovered a new moon of Saturn. Looking through a 24-inch telescope in Arequipa, Peru, he recorded his observations in his notebook, reproduced here. Pickering named the moon Phoebe. Eyes on Other Worlds Displayed here are several types of cameras that have been sent to explore the planets. The earliest are television cameras of the type sent to our Moon and Mars in the 1960s. Since then, TV cameras have been replaced by CCD (Charge-Coupled Device) imaging technology. Object Highlight: Surveyor 3 Camera, 1967 This television camera journeyed to our Moon on Surveyor 3 in 1967. The crew of Apollo 12 returned it to Earth 31 months later. Holes cut into the camera mark where NASA removed samples to study how the lunar environment affected the materials. Apollo 12 astronaut Pete Conrad touches the Surveyor 3 camera. Object Highlight: Mariner Television Camera, 1969 Before digital camera systems were developed, planetary spacecraft used technology first developed for television. This camera is identical to ones that flew on early Mariner missions to Mars. Object Highlight: HiRISE CCD, 2006 A CCD contains an array of tiny detectors that measure light to form a picture. This CCD is a flight spare for the ones used in the HiRISE camera on the Mars Reconnaissance Orbiter. Object Highlight: Wide Angle Camera, 2009 The Wide Angle Camera from the Lunar Reconnaissance Orbiter is a CCD camera equipped with a wide baffle instead of a telescope. Object Highlight: Narrow Angle Camera, 2009 The Narrow Angle Camera from the Lunar Reconnaissance Orbiter is a CCD camera attached to a powerful telescope with a 700mm focal length. Worlds Receive Visitors Despite amazing progress in space travel, humans have been unable to travel to the other planets. But by the 1980s, spacecraft had traveled to all seven and viewed them close up. Scientists could now study the geology and atmospheres of the planets. By comparing the similarities and differences among them, they have learned more about our own world. The first successful mission to Mars took place in 1965, when Mariner 4 flew by the planet and returned 21 images.As spacecraft imaging capabilities have improved, we have been able to view the martian surface with increasing clarity and detail. The Wide Angle Camera obtained images each month of nearly the entire surface of our Moon in seven wavelengths and in stereo. This mosaic of its images shows the Moon’s far side. The Narrow Angle Camera captured images of hardware left on our Moon during the Space Race in the 1960s. This image shows the Soviet Lunokhod 1, the first rover to operate successfully on another world. You can even make out some of the tracks it left. Modern CCD cameras have very high resolution. In this HiRISE camera image of Mars from 2007, you can even see the tracks left by the Opportunity rover as it traveled around the rim of Victoria crater. This mosaic showing a crater on the lunar surface was constructed from Surveyor 3 television images. From Observations to Missions Ideas about the planets based on observations from Earth are often overturned by close-up views. Spacecraft missions have revealed planets as dynamic worlds, each with its own distinct physical nature and history. Probing Mysterious Mars Mars has always been an especially puzzling planet. Earth’s blurring atmosphere makes observing Mars by telescope very difficult. Astronomers have taken advantage of oppositions—times about every two years when Mars and Earth are closest to each other—to observe, sketch, and photograph the planet. American astronomer Earl C. Slipher took these photographic plates of Mars in 1907. The polar ice caps are visible, but it is not clear what the other dark or light regions are. Greek astronomer E. M. Antoniadi created this master map of Mars at the end of the 19th century. He and other astronomers could see differences in the brightness of the planet’s surface. But they had trouble identifying features to associate with these differences, or with the seasonal changes in brightness they saw through their telescopes. This telescopic image of Mars was taken in 1956 at the Mount Wilson Observatory in California. Astronomers would not get clearer views of the red planet until Mariner 4 flew past it in 1965. Diverse Stories: Percival Lowell Beginning in 1895, American astronomer Percival Lowell published a series of popular books explaining why he thought there was life on Mars and what it would be like to live there. Object Highlight: Percival Lowell's Sketches of Mars These are prints of sketches he made in 1901 and 1902. Object Highlight: Percival Lowell's Mars Globe Percival Lowell made this globe in 1901. It summarized his observations of Mars during that year’s closest approach. Lowell claimed to have seen straight lines on the surface. He concluded (incorrectly) they were canals built by intelligent beings to supply water from melting polar ice caps to a desert world. For decades, astronomers debated whether Mars really harbored life, and what might account for its apparent seasonal changes in surface brightness. Mariners to Mars NASA launched several Mariner spacecraft to Mars in the 1960s. Three flew past Mars and took pictures along the way. A fourth went into orbit around it. The Mariner missions transformed our view of Mars. It turned out not to be an Earth-like planet with liquid water. It was a dry, cold, hostile world that may have been more Earth-like in the past. Object Highlight: Mariner 9 Photomosaic Globe of Mars Mariner 9 studied Mars from orbit for nearly a year in 1971–1972. It sent back more than 7,000 television images that covered 85% of the planet’s surface. It discovered huge volcanoes and a vast canyon system. None of this had been seen before, despite hundreds of years of looking at Mars through telescopes. This globe was produced from Mariner 9 images. Notice the great differences between it and Mars as depicted on Percival Lowell’s globe. Object Highlight: Deep Space Network Antenna NASA’s Deep Space Network consists of large antennas stationed in Australia, Spain, and California. The network allows continuous radio communication between interplanetary spacecraft and Earth. This model depicts one of the network’s 230-foot (70-meter) antennas. A spacecraft traveling across the solar system navigates by means of precisely timed radio signals sent back and forth between it and antennas like this. Navigators on the ground track its location and speed, transmit course adjustments, and receive data sent by the spacecraft. Object Highlight: Pluto Discovery Plate This photographic plate on which Pluto appears was taken on the night of January 23, 1930, by Clyde Tombaugh at the Lowell Observatory in Arizona. Object Highlight: Zeiss Blink Comparator Percival Lowell predicted a ninth planet beyond Neptune. In 1930, using a blink comparator identical to this one, American astronomer Clyde Tombaugh spotted a moving object that he named Pluto. A blink comparator allows the operator to switch back and forth between two images of the same section of sky taken days apart. A moving planet will appear to hop back and forth. Exploring Planets, Expanding Knowledge We now know our solar system is much more vast than we once thought. It contains countless objects—moons, dwarf planets, asteroids, comets, and other small bodies. Each contains clues to how our solar system formed and evolved. As we explore the planets, we also learn more about the universe and the laws that govern it. Putting Pluto in Context Pluto was discovered because astronomers believed another planet orbited beyond Neptune. They later learned that a whole collection of icy objects exists out there. Once thought of as a planet, Pluto is now considered the largest dwarf planet in what is called the Kuiper Belt. Diverse Stories: Clyde Tombaugh 1906–1997 The Man Who Found Pluto Clyde Tombaugh discovered Pluto in 1930 while working at the Lowell Observatory. The son of a farmer, he didn’t even have a college degree. But he loved astronomy and built several telescopes before going to work at the observatory. Tombaugh used a blink comparator to examine differences in thousands of photographic plates taken of the same sections of sky several nights apart. The work did not require advanced training, but it did require a sharp eye and an ability to spot tiny differences between plates. Tombaugh’s discovery of Pluto—considered the ninth planet for over 70 years, and the only one found in the 20th century—remains a remarkable achievement. Tombaugh and his granddaughter look through a telescope. Object Highlight: Mariner 10 This flight spare is identical to Mariner 10, the final spacecraft of the Mariner series. It was the first spacecraft to use the gravitational pull of one planet to reach another, and the first to visit two planets. Launched in 1973, Mariner 10 reached Venus three months later. Using a gravity assist from Venus, it flew past Mercury for the first time several weeks later, and then twice more within a year. Mariner 10 returned data about the atmospheres and surfaces of both planets. A large shade kept its electronics cool while operating so near the Sun. Mariner 10 measured planetary mass characteristics of Venus and Mercury, and used radio signals to test Einstein’s theory of relativity. New Points of Light The Kepler Space Telescope has helped revolutionize our knowledge of planets yet again. Scientists using Kepler have discovered that many of the stars we see in the night sky have planets of their own. Comparing our solar system to others discovered by Kepler is teaching us about how planets evolve. Beyond Kepler Astronomers and planetary scientists are excited to continue the hunt for exoplanets. They will use space-based, transit-detecting telescopes like Kepler that will look at previously unexplored parts of the galaxy, as well as technologies not yet developed or even imagined. Perhaps we may one day discover a planet that can support life as we know it. Astronomers can also study exoplanets and their formation from observatories on the ground. A radio telescope in Chile captured this image of a disk of dust and gas around a young star. Scientists believe it may show the early stage of planet formation. Such discoveries are helping us understand how our own solar system formed. How Do You Test New Technologies for Exploration? Every spacecraft sent out on a mission has to survive the extreme forces of launch. It also has to be able to function in the severe temperatures and radiation environment of space. Just because a technology works on Earth does not mean it will also work in space. In the case of the Kepler Space Telescope, scientists had to develop a technology that could detect very precise changes in the light emitted from a far-away star when a relatively small planet passed in front of it. Even more difficult, they had to prove it would work in the very “noisy” environment of space, where many wavelengths of radiation also bombard the telescope. We now know that our solar system may not be typical. Some solar systems have Jupiter-sized planets orbiting close to their stars, and others have “super-Earths”—rocky planets several times larger than our own. (Image is an artist’s concept of a “hot Jupiter” detected around star WASP-18.) 12/18
Hostile Worlds Walking on Other Worlds Planetary exploration is fraught with peril. Spacecraft face a dramatic range of conditions, from airless worlds bombarded by tiny meteorites to Venus’s torridly hot and crushingly dense atmosphere. Even space itself poses risks. Spacecraft must sometimes endure intense radiation or extreme solar heat. They must be carefully engineered to survive. The spinning vortex of Saturn’s north polar storm appears in this false-color image from the Cassini spacecraft. The Space Realm Before space travel, scientists knew little about what lay beyond Earth’s atmosphere. The hazards of radiation and speeding particles could only be estimated from ground-based or high-altitude observations. In 1962, Mariner 2 became the first spacecraft to travel to another planet. As it flew toward Venus, it recorded data showing that interplanetary space is relatively safe for spacecraft. However, the regions close to the Sun and around giant Jupiter pose great perils. Missions with much longer exploration lifetimes than Mariner 2 require many more protective features. Mariner 2 flew by Venus to measure its surface temperature. Object Highlight: Heater Units The deep cold of space is another challenge. Radioisotope heater units, which create heat through the decay of radioactive material, were used to warm critical parts of the Voyager spacecraft. (This display contains no radioactive material.) Object Highlight: Radiators Radiators cooled certain parts of the Voyager spacecraft that generated excess heat. Object Highlight: Thermal Blankets Temperatures on a spacecraft can rise rapidly in sunlight, especially in space or on an airless world. Thermal blankets like these were used to insulate the electronics on the two Voyager spacecraft, which explored the outer planets. Space is Not Just Space! We think of space as “empty,” having nothing in it. But electric and magnetic fields need no “material” to travel through, and they are present nearly everywhere. Particles from the Sun and from outside our solar system travel through space at enormous speeds. NASA’s Solar Dynamics Observatory recorded solar flares in 2014. Giant solar explosions send streams of particles far from the sun, which can disrupt radio communication on Earth and create hazards to astronauts. While orbiting Venus, the Magellan spacecraft “hid” its electronics behind its solar panels to limit heating by the intense sunlight. Jupiter’s powerful magnetic field creates brilliant aurora displays, like this one captured by the Hubble Space Telescope. High-altitude balloons carried some of the first instruments to measure the high-energy radiation called cosmic rays. Seen from the International Space Station, the aurora display known as the northern lights is a stunning sight. It occurs when particles from the Sun interact with Earth’s magnetic field and upper atmosphere. The two Voyager spacecraft traveled to the edge of our solar system. In 2012, Voyager 1 passed the heliopause, the limit of influence of the Sun’s particles and magnetic field. Meteorites and Their Impacts Dust grains left over from when our solar system formed, along with roving rocks called asteroids, move through interplanetary space. Most burn up when they fall through a planet’s atmosphere; if one lands, it’s called a meteorite. On airless worlds like the Moon, countless impacts by tiny meteorites slowly break down rock into fine-grained dust. Less frequent impacts by bigger objects create large craters and vast basins. The small white patches on this lunar rock are “zap pits,” where tiny meteorites have created tiny craters surrounded by circles of melted material. A piece of the rock broke off and shows the surface below with almost no zap pits. Large craters on icy moons of the outer planets have vast rings of cracks around them, but they are shallower than craters on rocky worlds. Valhalla crater on Callisto (a moon of Jupiter) is almost 2,500 miles (4,000 kilometers) across. An Apollo 16 astronaut rakes the upper layer of lunar dust to gather small rocks. The dust forms when micrometeorites break down rock. The layer may be as thin as 10 feet (three meters) or 10 times deeper in older surface areas. Extreme Temperature and Pressure The temperature on a planet depends not only on its distance from its star, but also on the composition and density of its atmosphere. Airless Mercury can be hot enough to melt lead during the day, but cold enough to turn gases to ice at night. Blanketed by a thick atmosphere, Venus holds in heat. Its day and night temperatures are almost the same. On worlds beyond Mars, extreme cold helps create rugged landscapes of exotic ices. This Opportunity rover image shows clouds in the Martian sky. The thin atmosphere of Mars allows temperatures on the planet to swing from 68°F (20°C) on the warmest days near the equator to –58°F (–50°C) at night. Inner Worlds Are All Unique Conditions on worlds closest to the Sun vary widely. Atmospheres range from dense to very thin. Earth and Mars rotate every 24 to 25 hours, while one day on Venus lasts 240 Earth days. Because of these factors and their distances from the Sun, each planet has a different average temperature and temperature range from day to night. Venus’s atmosphere presses down on its surface with 90 times the weight of Earth’s. Heat is trapped by dense clouds and haze. The average surface temperature of 864°F (462°C) is among the highest in our solar system. This infrared image from Japan’s Akatsuki spacecraft highlights the cloud structure. Because sunlight strikes at such a low angle at the Moon’s poles, the floors of many craters here are always in shadow. Some may be among the coldest places in our solar system at –413°F (–247°C). Extreme Exploration Probes that venture to the surface of a planet must be able to control their descent, sometimes withstand intense heat and pressure, and survive long enough to collect and send data. Scientists and engineers have developed many technologies to allow probes to explore extremely challenging environments. Surviving the Landing Early lunar probes simply crashed onto the Moon’s surface, destroying themselves in the process. Since then, a variety of techniques have been used for different planetary environments to allow spacecraft to make soft landings on other worlds. The photo is dated Dec. 22, 1961, and is from a group of images labeled “Ranger 3 still shots at Cape Canaveral.” Given the date and caption, it’s safe to say this is a photo of technicians preparing the ill-fated Ranger 3 spacecraft for launch to the Moon. Saturn’s moon Titan has a thick atmosphere. The European Space Agency’s Huygens probe deployed a parachute to land on the surface. This artist’s concept is based on photos taken by the lander. The ingenious sky crane system lowered the large rover Curiosity to the surface of Mars. It used a combination of parachutes at high altitude and then rockets to slow the craft as it approached the surface. Surviving Venus Venus may be the most hostile environment of all the planets. Just reaching the surface is challenging. The dense atmosphere slows a probe’s descent, but heat and pressure increase as it approaches the ground. Once on the surface, the intense heat destroys spacecraft electronics. These challenging conditions require innovative technologies. The balloon descent module, for exploring Venus’s atmosphere (engineering model shown) rode atop the Russian Vega spacecraft, with the balloon stored in the upper sphere. A lander (not shown) was also released to make compositional measurements of Venus’s surface. The atmospheric pressure on Venus is 90 times greater than on Earth—the same as being 3,000 feet (900 meters) below the ocean surface! That’s a depth out of reach to all but specialized submarines, like the deep-ocean research vessel Alvin. Object Highlight: Pioneer Venus Multi-Probe The Pioneer Venus Multi-Probe carried three small probes and one large probe. All measured atmospheric composition, temperature, and pressure as they descended through the dense atmosphere of Venus. Object Highlight: MESSENGER The MESSENGER spacecraft made three flybys of Mercury before entering into orbit around it in 2011. It was the first spacecraft to explore the entire surface of Mercury. MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) carried cameras, a laser altimeter to measure landform heights, and other instruments to determine the chemistry of the rocky surface. Since Mercury is the closest planet to the Sun, MESSENGER had to carry a shield-like sunshade to keep the spacecraft and its instruments cool. The material covering the sunshade is the same ceramic cloth used on the actual spacecraft. Debussy crater on Mercury (50 miles/81 kilometers across) is a relatively fresh complex crater. Shown here are fractures in lava plains that buried an impact crater on Mercury. When forces pull equally in all directions, fractures develop in circular or polygonal patterns. 13/18
Walking on Other Worlds No Place Like Home? Our solar system is the key to understanding planets being found around other stars. So far scientists have yet to find another world like Earth. But people have long imagined Earth-like planets and alien creatures living on them. They have shared their visions through books, movies, and popular culture. What would it take for people to live on another planet? How are scientists trying to find other habitable worlds? Earth, our oasis in space. Our solar system includes seven other planets and many moons and other small worlds. But only Earth has all the ingredients for life as we know it. There is no place like home—at least in our neighborhood. Props from the Original TV Series Star Trek Object Highlight: Spock Ears Ear tips helped actor Leonard Nimoy portray the half-alien Mr. Spock on television in Star Trek (NBC, 1966-69). Nimoy kept this set as a personal memento in his home, displayed in this handmade black box. Tribbles This television prop is a “tribble,” a fictional alien creature featured in the Star Trek episode “The Trouble with Tribbles.” A tribble was a living furry creature that did nothing but eat and multiply. Over-multiplying tribbles created havoc aboard the Starship Enterprise. Imagining Planets For hundreds of years people thought life could exist nearly everywhere in our solar system. Discoveries made by spacecraft missions to the planets changed that view. In 1835, the New York newspaper The Sun published a series of stories about astronomers discovering life on the Moon. People rushed to buy copies of the paper to read about giant winged people and other creatures. “The Great Moon Hoax” succeeded in selling a lot of newspapers. Percival Lowell’s idea that intelligent life might exist on Mars captivated the public. It inspired author H. G. Wells to write The War Of The Worlds, one of the first stories to depict the meeting of humans and aliens. Wells’s book helped popularize modern science fiction and the idea that aliens might exist. In his 1823 book, The Christian Philosopher, Thomas Dick calculated that our solar system contained exactly 21,891,974,404,480 inhabitants! Dick assumed every planet was a lot like ours. How Big Is 21,891,974,202,480? It’s over 21 trillion—a lot more than the 7+ billion people on Earth! Canali In 1877, Italian astronomer Giovanni Schiaparelli reported lines crisscrossing the surface of Mars and drew this map. He called these lines “canali,” meaning “channels” in Italian, but it was mistranslated into English as “canals.” Percival Lowell became fascinated by these “canals.” He built his own observatory to study them. Can Humans Live Anywhere Else in Our Solar System? Here’s what we’d need:
Gas giants Jupiter, Saturn [above], Uranus, and Neptune have no hard surfaces to walk on. It is also impossible to survive on many of their moons. Jupiter’s and Saturn’s moons are affected by the gas giants’ strong radiation fields, and the smaller moons have such low gravity that an explorer’s muscles would weaken over time. According to a NASA study, humans might make long-term visits to our own Moon, Mars, and two larger moons—Jupiter’s Callisto, and Saturn’s Titan. Mercury is too close to the Sun to support human life. Its surface temperature averages 800°F (430°C) and is bathed by solar radiation. Venus is even hotter than Mercury. The average surface temperature is hot enough to melt lead. The pressure of the atmosphere is crushing—90 times what we experience on Earth. Looking for Life ElsewhereT he question of whether life exists somewhere else in the universe has intrigued people for thousands of years. Finding the answer is a large part of what drives space exploration. The Changing Earth Earth today looks nothing like it did over four billion years ago when life began. The movement of the Earth’s surface, by plate tectonics, has created the continents we see today. The maps [at right] show continental movements from 210 million years ago to present. The atmosphere has changed from mostly carbon dioxide to mostly nitrogen and oxygen. Even the saltiness of the oceans has varied over time. These changes have wiped out clues to how life began. [Above] The basic building block of life is the molecule known as DNA (deoxyribonucleic acid). Early life began with a simpler molecule called RNA (ribonucleic acid). Both consist of carbon, oxygen, hydrogen, nitrogen, and phosphorus. On Earth these elements managed to assemble and use energy from their environment to grow, move, and make copies of themselves. 210 million years ago 140 million years ago 70 million years ago Present Don’t Take Granite for Granted! Because of its plate tectonics, Earth is the only place in our solar system that has large amounts of granite. Granite makes up most of Earth’s continents, but basalt makes up most of the ocean floor on Earth. Did you know that abundant granite is as unique to the Earth as life is? Protective Magnetic Field Conductive material in Earth’s core generates the magnetic field that surrounds our planet. The magnetic field protects us from the Sun’s solar wind, which would blow away our atmosphere. Some of the solar particles are deflected harmlessly toward the poles, where they form auroras. Early Life on Earth The characteristics that make Earth unique, or some combination of them, may be essential for creating life and for sustaining it long enough to become intelligent. Some of the earliest evidence for life are stromatolites—mounds with fossil material created by primitive organisms. These organisms consumed carbon dioxide and released oxygen as waste, building up oxygen in Earth’s atmosphere over billions of years. Life that thrived on oxygen evolved. Some stromatolites are still forming, like these in Australia. Earth lies within a “habitable zone” where it’s not too hot and not too cold. Diverse Stories: Carl Sagan 1934–1996 Astronomer with a Lust for Life Carl Sagan was a prominent planetary scientist at Cornell University during the early exploration of our solar system. He championed the search for life beyond Earth and supported early efforts to listen for radio signals from alien civilizations. Sagan was instrumental in placing messages on spacecraft that became the first human artifacts to leave our solar system. Pioneer 10 and 11 each carried a plaque illustrating humans and the spacecraft, plus astronomical information to identify Earth’s location. Voyager 1 and 2 each carried a metal record (plus a needle needed to read it) with sounds and images of life on Earth. Object Highlight: The Martian Chronicles, 1st ed., New York, 1950 Even before the first missions to Mars were planned, readers consumed science fiction stories of Mars exploration and settlement. Ray Bradbury’s The Martian Chronicles envisioned an interplanetary future. Carl Sagan and Ray Bradbury (center) at the Viking 1 landing press conference in 1976. Object Highlight: Voyager Record Cover Voyager 1 and 2 both carried a metal record, similar to a phonograph record, containing information about Earth and the life on it. The markings on its cover are intended to inform advanced alien beings how to decode the information on the record. This is a duplicate cover. 14/18
Exploration Continues Exploring other planets is an ongoing process. We are learning more all the time about other worlds. In doing so, we are learning more about our own world and ourselves. Scientists are conducting research in our Museum, NASA centers, and other laboratories worldwide. You can be a part of this effort! Learn about how to continue your own exploration from information shown on the screens. Planetary Art Planetary exploration has stirred artists to imagine what it might be like to visit other worlds. From Chesley Bonestell, inspired by telescopic observations in the 1950s, to Alan Bean, inspired by personally walking on the Moon, artists have given us a “feel” for what standing on another world might be like. Art can play an important role in conveying the thrill of planetary exploration. Examples here show an astronaut on the Moon confirming Galileo’s hypothesis about gravity, and an artist’s vision of a strange planetary environment. Object Highlight: The Exploration of Mars By Chesley Bonestell Oil on board, 1956 Interest in planetary visitation and exploration was spurred in the 1950s by the writings of Wernher von Braun, Willy Ley and others, and the fabulous illustrations by Chesley Bonestell. Object Highlight: Impact on the Night Side of Saturn’s Satellite, Rhea By William K. Hartmann, 1992 The flash of a meteor strike and the plume of ejected material from the forming crater are shown in an artist’s conception of an impact on Saturn’s icy moon, Rhea. Object Highlight: The Hammer and the Feather By Alan Bean, 1986 Apollo 12 astronaut and artist Alan Bean portrayed David Scott on Apollo 15 performing an experiment whose legacy goes back over four centuries to Galileo. Scott dropped a hammer and feather simultaneously to confirm that they accelerate downward at the same rate in the absence of air. Floorrery What Are All These Dots on the Floor? The rings of dots on the floor help you see how planets move around the Sun. It is a kind of orrery [OR-er-ee]—we call it a Floorrery! Dots in the Floorrery are spaced 10 Earthdays apart. The original orreries were mechanical models of our solar system. They used gears or motors to reproduce the movements of the planets around the Sun. You can see an historical orrery beneath the planet globes near the entrance to this gallery. Did You Know? In the 18th century, mechanical devices that reproduced the movement of planets around the Sun became popular. These devices were called orreries. Explore the “human-scale” orrery on the floor here to learn about planets and their orbits. 15/18
vSys Shifts: Signing Up For Spotlights You MUST sign up for Spotlight shifts using vSys. How To Sign Up For NMB Spotlight Shifts in vSys:
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Recording Your Hours in vSys 17/18
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