Of all the Sun's other planets, Mars is the most Earth-like. Its polar axis tilts 25 degrees to its orbit (Earth's tilts 23 1/2 degrees), the Martian day lasts just 41 minutes longer than ours, and it has clouds, seasons, and polar ice caps. Despite these similarities, Mars is a frigid desert world, geologically inactive and hostile to life as we know it. Its atmosphere is too thin to allow liquid water at the planet's surface and offers no barrier to ultraviolet light from the Sun.
Mars has two distinct types of terrain, each occupying about half of the planet: in the south are the older highlands with many craters, while in the north, a relatively uncratered plain lies a few miles lower. How half of Mars could lie so much lower than the rest of the planet mystifies scientists. What causes the difference and where did all that crustal rock go?
The southern highlands have geological puzzles of their own. They contain the largest volcanic region on Mars—Tharsis—with its four immense volcanoes. Tharsis is about the size of North America and appears to have been built by episodes of crustal uplift followed by intense volcanism, which deposited lava on top of the uplift. Tharsis is topped by Olympus Mons, which is 16 miles (25 km) high, and 300 miles (500 km) wide.
From the number of craters on the highlands, scientists have calculated that this ter-rain is at least three billion years old. However, some portions, such as the slopes of Olympus Mons, appear quite young in geological terms. They have no craters and may be only a few million years old.
Radiating east from Tharsis is a huge crack called Valles Marineris—2 1/2 miles (4 km) deep and long enough to reach across the United States from coast to coast. It seems to have started as a tectonic fault, then, as the fault tapped sources of groundwater, the walls collapsed and eroded, opening up the valley. Parts of it are now 60 miles (100 km) wide.
Other regions flanking Tharsis show evidence of actual rivers. The largest channels drain from the highlands across the crustal boundary and onto the northern lowlands in the region of Chryse, where the Viking 1 spacecraft landed. (This site was chosen in the hope of detecting life that might have fed on the water.) Scientists believe that these flows were catastrophic and that they occurred when internal heat or meteorite impacts released groundwater in sudden floods. Although the volume of water in these flows would have been enough to fill Earth's Amazon River 100 times, scientists think the flows were brief, because Mars probably did not have enough water to sustain them continuously.