Mars

Imagine yourself standing on the surface of Phobos, one of the two Martian moons, the closer to Mars, only about 5000 km away. From this distance you would see Mars as a ball for basketball at a distance of the arm's length.

The atmosphere of Mars is about 200 times less dense than the atmosphere of the Earth. But it is enough for formation of Martian weather, like winds, storms and tornados. Sometimes sandstorms on Mars are so strong that the whole planet becomes featureless ball of dust, and only four darker points stick above the dust cloud. They are tops of tall mountains.

One of the Martian mountains is the tallest planetary mountain in the whole Solar System, the Olympus Mons volcano, nearly 22 km tall (almost three times the height of Mount Everest) and about 600 km wide. It was active for most of Martian history, and scientists believe it was still active just a few millions years ago. It is so wide that standing on the top of it, you would not recognize that this is a mountain. Its slope is just about 5%. But with the thin Martian atmosphere, at this height you would see black sky and bright stars in daytime, like in open space. Eruptions of Martian volcanoes were sometimes so strong that with the planet low gravity they could throw pieces of lava into the interplanetary space. Some of the meteorites that fell on the Earth were identified as Martian lava.

Valles Marineris, the largest canyon of Mars, is over 4000 km long, about 100 times bigger than the Grand Canyon.

Martian landscape developed during a couple of billion years after its formation about 4.5 billion years ago, and then it was frozen. There is no plate tectonics remained there. Specialists in Martian geology believe that this is the main cause why geological shapes on Mars are by one or two orders of magnitude as large as similar shapes on the Earth, where plate tectonics is still shaping its surface.

Geology of the planet is substantially different in two hemispheres. Mountains and canyons are in the South, huge plains with sand dunes are in the North. The southern highlands are geologically much older than the northern plains. A huge catastrophe that happened about 2 billion years ago caused the geological dichotomy of the planet. A meteorite about 2000 km across hit the northern hemisphere of the planet. Initially the Martian plains were just one huge crater, where the crust was completely disrupted, and a new crust formed, then it was gradually filled in with debris, sediments and sands.

Mars does not have a magnetic field; a compass would be useless there. However, some older mountains preserve some magnetization, which proves that the planet used to have a magnetic field in its earlier years.

And, of course, craters are a part of the landscape, especially in the South, where they have been preserved since the earlier years of the planet. They are everywhere, sometimes, in a large crater there are several smaller craters, sometimes craters overlap, sometimes they form groups. Some of them are more than a billion years old, others are very young. They are results of impacts of meteorites. Even small meteorites leave noticeable craters, because they are only slightly decelerated by the thin Martian atmosphere.

This text is based on the information presented in the BBC popular science radio programmes.