Mars is the fourth planet from the Sun. It has such an elliptical orbit that there is a temperature variation due to this alone of about 30 C between perihelion and aphelion at the subsolar point. So that while the average temperature on Mars is about 218 K (-55 C, -67 F), surface temperatures can range from 140 K (-133 C, -207 F) at the pole in winter to 300 K (27 C, 80 F) at lower latitudes on a summer day.
Mars has a varied terrain including Olympus Mons, the largest mountain in the Solar System, Tharsis (a huge bulge) , the Valles Marineris canyons and Hellas Planitia (a huge crater). These are truely spectacular, Olympus towers 24 km above the surrounding plain, has a base more than 500 km in diameter with a 6 km cliff at the perimeter. The Valles Marineris canyons are about 4000 km long and up to 7 km deep. Tharsis is about 4000 km across and 10 km high. Hellas Planitia 2000 km in diameter and over 6 km deep. All this on a planet much smaller than Earth.
There seems to be an abrupt elevation change (of several kilometers) to occur at the boundary between the northern and southern hemisphere. The south, which is the higher, is older, the north is much younger and shows signs of a more tortured history. It has been speculated that this could be due to a massive impact on the planet. The Mars Global Surveyor indicates that the crust is about 80 km thick in the southern hemisphere and only about 35 km in the north.
Although there is evidence of past activity, there is no evidence of current volcanic activity and none of recent plate tectonics. There is however, clear evidence of erosion on Mars, including floods and river systems but it occurred a long time ago: up to 4 billion years ago.
Mars' atmosphere is very thin composed of carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure is between only about 9 millibars at the lowest to about 1 millibar at the top of Olympus Mons. There are however very strong winds and huge dust storms that can engulf the entire planet for months. The small greenhouse effect of Mars' thin atmosphere only raises the surface temperature by 5 degrees (K).
The permanent ice caps are composed mostly of solid carbon dioxide, in the northern summer the carbon dioxide clears exposing a layer of water ice. It is not known if is also water ice hidden below ground elsewhere. Large weak magnetic fields exist in regions of Mars, they are prossibly remnants of a global field.
Mars has two tiny moons which orbit very close to the surface, Phobos with a 11km radius and Diemos of 6km radius.