Mars: the Features Mars is a rocky planet like the Earth. It has a radius approximately half, and a mass around one tenth that of the Earth; in consequence, gravity on Mars is only about 40% that of the Earth. The surface is desert-like and shows a great wealth of different features, cf. the Mars Global Surveyor Mars Orbiter Camera Image Gallery that contains a very large collection of images obtained with this NASA spacecraft, now in orbit around Mars. A thin atmosphere Mars' atmosphere is also different from the Earth's: it is much thinner, the surface pressure is around 6 mbar (0.006 atms), compared with 1000 mbar (1 atms), and it is mainly carbon dioxide (~ 95%) rather than nitrogen. The thin atmosphere provides the martian surface with little protection from heat loss, thus the average daily temperature is around -60°C. Temperatures may reach +30°C at the equator in summer, and fall to -130°C at the poles in winter. A structure almost like that of the Earth Mars has a core-mantle structure similar to the Earth, but seems to have a rigid crust rather than the more flexible plate structure of the Earth, although recent results from the magnetometer on NASA's Mars Global Surveyor indicate that there might be evidence for limited tectonic spreading. Mars exhibits extreme examples of the features shown by the Earth. For example, the biggest volcano in the Solar System occurs on Mars: Olympus Mons is a shield volcano almost three times as high as Mount Everest. Shield volcanoes on Earth are formed as piles of magma that build up when the crust is above a hot spot in the mantle. Plate movement over the mantle hot spot prevents continuous accumulation of a magma pile in a single location. In contrast, on Mars where there is little or no plate motion, the volcanoes simply increase in size.

Olympus Mons on Mars, obtained by NASA/MOLA (Mars Orbiter Laser Altimeter)