A WETTER PAST
Running water, lakes, possibly rain, a warmer climate, and a thicker atmosphere—ancient Mars might well have been remarkably Earth-like. So what happened? Two things are the keys to understanding present-day Mars.
First, being a small planet, Mars's geological engine seems
to have run down. If it has not stopped entirely, it is nowhere near as active as Earth's. It also seems that Mars has never had a plate-tectonics cycle—as one scientist put it, Mars appears to be a one-plate planet. A rigid crust could explain why Tharsis is such a large region. On a planet with active plate tectonics, such as Earth, the crust above an erupting hot spot would keep moving and volcano would never grow as big as Olympus Mons.
The second key to present-day Mars is its lack of a large satellite like our Moon. Mars has only two small moons, Phobos and Deimos, which are probably asteroids captured from the nearby main belt. Both are mere pebbles compared to Mars. Lacking the stabilizing influence of a large moon, Mars's axial tilt could change abruptly (in geological terms), throwing its climate from warm, wet conditions into a global ice age and back again, perhaps several times over in the course of a billion years.