Mars
DISTANCE FROM SUN: 141,525,900 miles (227,900,000 kilometers)
REVOLUTION AROUND SUN: 687 days
ROTATION: 24.6 hours
DIAMETER: 4,215 mile (6,787 kilometers)
DENSITY: 3.9 x that of water
MASS: 0.1 x that of Earth
SURFACE TEMPERATURE: variable, averages minus 122° F (50° C)
Satellites (closest to most distant):
Phobos
Deimos
The first planet beyond Earth from the sun, Mars is just over half the size of our planet. Despite long-held hopes that it might have some primitive forms of life, it appears to be a dead world, a dry, wind-blown sphere covered with iron oxide dust. It is the dust that gives Mars its blood-red appearance, which inspired the ancient Sumerians, Greeks, and Romans to associate it with their gods of war.
Mars’s atmosphere, which extends some 100 miles (161 kilometers) up, is 95 percent carbon dioxide and 3 percent nitrogen, with trace amounts of oxygen, argon, carbon monoxide, and water. Predawn clouds, possibly of water vapor, form six miles (9.7 kilometers) or higher, but they usually dissipate by late morning. It is at the north pole that Mars has its most abundant water supply, in the form of a thick ice cap that grows and shrinks with the seasons.
There may also be huge reserves of water locked frozen beneath Mars’s surface. One thing is almost certain: The vast channels and canyons that spread across the Martian surface were dug by flowing water at a time when the planet was, for reasons still not understood, warmer and wetter.
Temperatures vary wildly on the planet with time of day and elevation. The daytime high at the surface averages 65°F (18°C). Just five feet (1.5 meters) up, however, the thinner atmosphere holds almost no heat, so the temperature is only 15°F (-9°C). It’s much like comparing the difference between temperature measurements taken at the base and summit of a large Earth mountain.
Mars’s core, which is between 1,600 and 2,500 miles (2,575-4,023 kilometers) in diameter, may be liquid, like Earth’s. Unlike Earth, Mars has no planet-wide magnetic field. Recent measurements have detected small, localized magnetic fields which may be “fossil” remnants of a long-gone planetary field. As a result, future human explorers on Mars will not be able to use magnetic compasses, which would either spin uselessly or point at the nearest local magnetic “pole.”

POLAR CAP Mars is a very dry place. The land is a dusty desert, the humidity is zero. Yet at its north pole, Mars wears a cap of frozen water which, if melted, could possibly irrigate the planet. In the past, as astronomers watched the cap spread and retreat with the seasons, scientists estimated the cap’s depth in yards. But the latest data from the orbiting Mars Global Surveyor shows that the cap is much, much thicker than previously thought; perhaps reaching a mile above the relatively flat, sandy regions that surround it. Those areas have recently been discovered to be covered with a procession of 50-to 150-foot-high (15-to 46-meter) sand dunes, spaced about a mile a part.
In some places the polar cap is punctured by bizarre spiral troughs which cut through the ice to depths as great as 3,600 feet (1,097 meters) below the ice surface. The trough walls often show a “staircase” structure, which indicates the polar ice cap’s ice has been laid down in layers through seasonal weather. Other vast areas of the cap are extremely smooth, with elevations that vary by only a few feet over many miles.
By studying the cap’s structure, scientists hope to understand the history of Mars’s climate. At one time, some astronomers think, its northern hemisphere was covered with a great ocean. That surface water is all gone. Some may be frozen below the surface; much is clearly frozen at the north pole.
Tapping Mars’s limited water supply is a key to the possibility of human exploration of the planet. Because rocket fuel weighs so much, many experts believe it would be more practical to send a robotic fuel processing plant to Mars ahead of any humans. In the year or so before their arrival, the plant would drill into the surface, extract water, and use it to produce the fuel explorers would need to return to Earth.

OLYMPUS MONS The largest known volcano in the solar system sits like a giant welt near Mars’s equator, the impact of its towering 15-mile (24-kilometer) height somewhat diffused by its awesome 342-mile (550-kilometer) diameter. Earth’s largest volcano, Mauna Loa in Hawaii—six miles (9.7 kilometers) high and 75 miles (121 kilometers) across—is dwarfed even by the three smaller neighbor volcanoes to Olympus Mons, which sprawl up to 280 miles (451 kilometers) and reach nine miles (14.5 kilometers) in height.
The edges of Olympus Mons don’t flatten out into gentle plains. Circling the mountain like a castle wall is a cliff several miles high. Beyond that is a moat filled with hardened lava.
Like the Hawaiian volcanoes, Olympus Mons and its companions are most likely shield volcanoes, built up when “hot spots” beneath them—areas where molten rock push close to the surface—slowly pushed lava upward through their centers. The Martian volcanoes got so large because unlike on Earth, where shifting plates slowly move the crust across hot spots, the Martian surface remains relatively stationary, giving mountains like Olympus Mons plenty of time to build themselves up.
Olympus Mons and its neighbors grew slowly over hundreds of millions of years. Other Martian volcanoes have more explosive histories. To the north of Olympus Mons is a volcano named Alba Patera, which is as wide as Olympus Mons but less than four and a half miles (seven kilometers) high. It appears to have blown itself apart in violent, ash-filled eruptions, like Mount St. Helens did in 1980.

PATHFINDER LANDING SITE “I spent two years worrying about the landing site,” says Matthew Golombek, the project scientist who headed up selecting the touchdown spot for NASA’s Mars Pathfinder. “We knew from old Viking images that there were areas of Mars that looked like they had been formed by catastrophic floods. On Earth places like that are where you can get a variety of rocks, so that was where we wanted to go.”
Golombek and his team got what they wanted: rocks and lots of them. The mission’s rover, Sojourner—which was equipped with an alpha proton x-ray spectrometer (APXS) to study the rocks—found that silicon levels were more earthlike than expected. Sojourner’s wheels were used to dig little trenches in some nearby sand dunes, uncovering a white rocky layer beneath the sand. Many larger rocks in the area tilt in a generally northwest direction, a hint that a flood coming from the opposite direction may have carried them here some two billion years ago.
Dust kicked up by the rover settled slowly back to the ground, pulled gently by Mars’s gravity, which, due to the planet’s smaller size, is just one-fourth that of Earth’s.