MARS IS A BITTERLY COLD DESERT WORLD, STAINED A RUSTY RED BYIRON-RICH DUST ON ITS SURFACE. THOUGH HALF THE DIAMETEROF EARTH AND MUCH FARTHER FROM THE SUN’S WARMTH, MARSSHOWS MANY STRIKING SIMILARITIES TO OUR HOME PLANET.
Images of Mars returned by NASA spacecraft reveal a world that looks eerily familiar, with rock-strewn deserts, rolling hills, spectacular canyons, and a hazy sky flecked by occasional white clouds. Mars has a 25-hour day, polar ice caps that wax and wane like Earth’s, an axis tilted only two degrees steeper than ours, and dry riverbeds that hint at the past presence of water. Volcanoes and rift valleys suggest tectonic forces were once generated by a hot interior. Yet despite the many parallels, Mars and Earth are worlds apart. With only a tenth of Earth’s mass, Mars lacks the gravity to hold on to a dense atmosphere, and its tenuous air is almost devoid of oxygen. While Earth’s large, molten core keeps the planet’s fractured crust in motion and generates a protective magnetic shield, Mars’s smaller core has cooled and at least partially solidified. Its crust has frozen solid, and its magnetism is too weak to deflect solar radiation. At one time Mars may have been warm and wet, but today it is an uninhabitable and barren wasteland.
MARS STRUCTURENO ONE KNOWS EXACTLY WHAT THE INTERIOR OF MARSIS LIKE, BUT THROUGH VARIOUS STUDIES, INCLUDINGUNCREWED SPACECRAFT MISSIONS, SCIENTISTS HAVE BUILTUP A THEORETICAL PICTURE OF THE PLANET’S STRUCTURE.
As a young planet, Mars cooled down more rapidly than Earth, because it is smaller and farther from the Sun, although the outer region of its iron core is thought to still be partially molten. A rocky crust of variable thickness forms the outermost layer of the planet. This is in one solid piece, rather than split into separate moving plates as on Earth. Beneath the crust is a deep mantle of silicate rock, once a fluid layer that was in constant motion. As the mantle shifted, it changed the face of Mars, causing great rifts in the crust and breaking through the surface to form gigantic volcanoes.
MARS IS A DRY WORLD. IT HAS WATER ABOVE, ON, AND UNDER ITS SURFACE, BUT THE WATER IS IN THE FORM OF VAPOR OR ICE. LIQUID WATER WAS ONCE ABUNDANT ON MARS, AND ITS EFFECT ON THE LANDSCAPE IS STILL EVIDENT.
Today, liquid water cannot exist on the Martian surface because of the low temperature and atmospheric pressure. However, sedimentary rocks built up by water-deposited material, minerals formed by standing water, and landscape features shaped by flowing water all point to the fact that Mars may once have had large volumes of liquid water. Ancient water Billions of years ago, when Mars was a warmer planet, riverbeds and channel-like valleys hundreds of miles long formed as fast-flowing water carved through the landscape, and catastrophic floods covered vast areas, leaving floodplains behind. Valleys such as Kasei Valles, the site of two giant waterfalls eight times the height of Earth’s Niagara Falls, are now dry. So too are Mars’s deltas, lakes, and shallow seas. Increasing our knowledge of the planet’s watery past helps in our search for life. Liquid water is essential for life— if it once existed on Mars, then perhaps life did too. Outflow channel The surface of Mars features outflow channels—vast swaths of water-scoured ground. The largest and longest of these is Kasei Valles, at over 1,500 miles (2,400 km) long. It was created by a huge outpouring of fast-flowing water. In this view, the water flowed toward the bottom left, and created an island in the center of the channel. Impact meltwater Some of the water that flowed on Mars was released by volcanic activity or asteroid impact. This false-color image shows Hephaestus Fossae, a region of impact craters and channels. The impact that created the large crater penetrated the surface and melted underground ice, apparently causing a catastrophic flood.
A WHITE CAP MADE PREDOMINANTLY OF FROZEN WATER SITS ON EACH OF MARS’S POLES. ALTHOUGH THESE ALMOST CIRCULAR CAPS ARE A PERMANENT FEATURE OF THE MARTIAN LANDSCAPE, BOTH CHANGE WITH THE SEASONS.
The polar caps are huge mounds of ice that stand above the land that surrounds them. Cliffs at their edges reveal that the caps are made of layer upon layer of ice, sand, and dust laid down over millions of years. In winter, the caps extend as they are covered with new deposits of carbon dioxide snow and ice. With rising temperatures in summer, the carbon dioxide returns to the atmosphere as gas and the caps shrink. North cap The northern cap, Planum Boreum (Northern Plain), is the larger of the two—about 620 miles (1,000 km) across and 1.2 miles (2 km) thick—and is 90 percent water ice. Data on the thickness and composition of the cap’s layers, collected by NASA’s Mars Reconnaissance Orbiter, is being used to study the planet’s history of climate change.
MARS HAS TWO MOONS, PHOBOS AND DEIMOS. THEY ARE IRREGULARLY SHAPED, ROCKY LUMPS WITH CRATERED SURFACES. TINY COMPARED TO EARTH’S MOON, THEY HURTLE AROUND MARS IN LESS THAN A DAY AND A HALF.
The pair were discovered within days of each other by American astronomer Asaph Hall, in 1877. Their names are from Greek mythology—Phobos and Deimos were the twin sons of Ares, the god of war. Phobos was the god of fear and Deimos of terror, and the brothers accompanied their father Ares into battle. In the solar system, the two moons accompany Mars, the Roman equivalent of Ares. The moons have been seen in detail only in relatively recent times. Phobos has been studied most closely; it was the subject of a series of flybys by Mars Express in 2010. The origin of the moons is uncertain; some astronomers think the pair are asteroids captured by Mars’s gravity, others that Phobos formed from debris left over from the formation of Mars.
MARS HAS BEEN KNOWN SINCE ANCIENT TIMES. EARLY ASTRONOMERS NOTED ITS COLOR AND MOVEMENT ACROSS THE SKY, AND TELESCOPES LATER CAPTURED SURFACE DETAIL.
The color of Mars led the ancient Greeks and Romans to associate the planet with blood and war. It wasn’t until much later that telescopes revealed more than just a reddish point of light. The mistaken sighting of channels led to the idea that Mars might be home to an advanced civilization, but when spacecraft visited, they found a dry, lifeless desert. Nevertheless, evidence suggests that Mars had a watery past. Several generations of rovers have now explored the surface, and Mars is the planet most likely to be visited by people in the future.
MISSIONS TO MARS IN THE PAST 60 YEARS, MORE THAN 40 MISSIONS HAVE BLASTED OFF FROM EARTH FOR MARS. THE PLANET HAS BEEN FLOWN BY, ORBITED, LANDED ON, AND ROVED OVER, AND WAS THE FIRST PLANET EVER SEEN IN CLOSE-UP.
Missions sent to Mars in the 21st century have been extraordinarily successful, sometimes far exceeding expectations. But success has been built on earlier disappointments, with more than half of all Mars missions either failing to get away from Earth or losing contact with their controllers as they closed in on their target. The first attempts at Martian exploration were undertaken by the USA and the then Soviet Union in the 1960s and 70s, after which there was little interest in Mars until the mid-1990s. Now, six countries have sent craft to Mars, more missions are planned, and a privately funded project is underway to develop a spaceflight system capable of taking a human crew to Mars.
ASTEROIDS ARE ROCKY BODIES THAT VARY IN SIZE FROM A FRACTION OF AN INCH TO HUNDREDS OF MILES WIDE. THEY EXIST THROUGHOUT THE SOLAR SYSTEM, BUT MOST ARE FOUND IN THE ASTEROID BELT BETWEEN MARS AND JUPITER.
Sometimes called minor planets, asteroids orbit the Sun in the same direction as planets, but only the very largest have sufficient mass to pull themselves into a regular, rounded shape. Asteroids were much more numerous in the solar system’s early years. As they orbited the Sun, they collided and sometimes joined through gravity, accumulating to form larger bodies. Some of these embryonic worlds were destined to become today’s terrestrial planets, but those near Jupiter’s orbit were disturbed by the giant planet’s powerful gravity, which caused them to crash violently and fragment. As a result, a ring of rocky debris has remained between the orbits of Mars and Jupiter ever since, forming the asteroid belt. Today, the asteroid belt is sparsely populated; the total mass of the main belt is equal to only 4 percent of the Moon’s mass. Collisions dominate this part of the solar system, and most asteroids are fragments of larger bodies that were destroyed.
MISSIONS TO ASTEROIDS MORE THAN A DOZEN ASTEROIDS HAVE BEEN VISITED BY SPACECRAFT, BUT ONLY FOUR MISSIONS HAVE BEEN DEDICATED TO STUDYING THESE ROCKY BODIES. THE MOST RECENT SUCCEEDED IN RETURNING A SAMPLE TO EARTH.
The first close-up image of an asteroid came in 1991 when the Galileo spacecraft sent back remarkable images of Gaspra— an 11.2-mile- (18-km-) long, crater-covered boulder—during the spacecraft’s journey to Jupiter. The first dedicated asteroid mission was NEAR Shoemaker, which landed on Eros in 2001. Nearly five years later, the Japanese spacecraft Hayabusa touched down on the half-mile- (1-km-) wide asteroid Itokawa, collected a sample, and brought it back. After orbiting Vesta, Dawn is on target for a 2015 encounter with the largest and first discovered asteroid, Ceres. Even more ambitious projects are under discussion, including a NASA mission to capture an asteroid and tow it into lunar orbit, where astronauts can visit it.