Mars

The surface features of Mars show that the planet has had an exciting history. Long ago, the surface was volcanically active. Early in the planet's history, it probably had crustal plates moving about as is the case on Earth, but as Mars cooled and its crust thickened, the tectonic activity ceased. The enormous size of Olympus Mons supports this idea. The crust slides over a hot spot, and lava coming up forms a series of mountains. On Mars, with no plate motion, the lava simply piles up in one spot. There are several volcanoes on Mars larger than any on Earth, suggesting the planet has a thick, inactive crust.

The eroded channels on Mars's surface show that the planet once had running water. Water boils at progressively lower temperatures as one goes to higher altitudes because the atmospheric pressure is lower. (At lower pressures it is easier for molecules to escape the surface of a liquid.) On Mars today, water would boil immediately even at the low Martian temperatures, because the atmospheric pressure is so low. This suggests that the Martian atmosphere was once much denser than it is now. Otherwise, water could never have flowed on the planet's surface.

Some of the eroded channels on Mars resemble terrestrial riverbeds, but some show evidence of a violent past. They seem to have been formed by enormous flash floods, perhaps caused when a Martian lake broke through a collapsing natural feature such as a rock wall and cascaded across the land. Several such incidents are documented in the geologic record on Earth.

Mars, as seen from space by Viking 1. The planet is slightly more than one-tenth as massive as the Earth. U.S. National Aeronautics and Space Administration (NASA).

Many scientists theorize that Mars's atmosphere thinned, and, as the planet cooled, the water boiled away. Some of the water may still remain on the planet, permanently frozen in the ice caps or in the soil. Much of it was probably lost when the Sun's ultraviolet radiation dissociated the water molecules into their hydrogen and oxygen atoms.

Although Martian tectonic activity has ceased and the atmosphere has largely dissipated, storms still rage across its surface. The Viking orbiters observed giant dust storms sweeping across the Martian land. The largest of these storms can sweep dust particles around the entire planet, rushing past the streambeds, ancient craters, volcanoes, and canyons, obscuring everything in their path. One of the greatest dust storms ever observed on Mars occurred in 1971, when the entire planet was shrouded just as one of the earliest Mars orbiters, Mariner 9, arrived to take pictures. No pictures could be obtained until the end of the storm in 1972.