Structure and Atmosphere
Mars' diameter is only a little more than half Earth's, but its surface area is
roughly equal to the area of land surface of our own planet. Mars is less dense
than the other terrestrial planets, so this suggests that its 1700 km (1000 mi)
diameter core is likely to contain more sulfur and less iron than its solar
neighbours. This is surrounded by a molten rocky mantle and a thin crust.
The martian surface is very old and cratered like our Moon's, but younger features such as rift valleys, ridges, and plains spot the surface. Olympus Mons, the largest mountain in the Solar System is a shield volcano that rises 27 km (17 mi) from the surrounding plain and its 600 km (400 mi) diameter base is set off from the plains by 6 km (4 mi) of cliff. A very prominent feature, Valles Marineris, is a series of canyons cutting 2 to 7 km (1 to 4 mi) into the crust and running 4000 km (2500 mi) along the terrain.
Except for Earth, Mars' surface is the most varied of the terrestrial planets. The
surface features suggest that water once flowed there. Its canyons are believed to
be carved out by gigantic rivers and its plains of rubble are believed to have been
created by floods. Other features resemble shorelines and river beds. The existence
of huge shield volcanoes suggests active volcanism similar to Earth's, but there is
little evidence of any "plate tectonics" that are associated with our own planet.
Recent evidence suggests that Mars once or currently hosted life, but this is still
a very controversial topic. Many scientists believe that current conditions on the
planet, including ultraviolet solar radiation, very low levels of moisture and its
specific soil chemistry make life there impossible.
The density of the Martian atmosphere is less than one percent of Earth's, averaging about 7 millibars. Clouds, fog and frost do exist on Mars, but the overall water content of Mars' atmosphere is only 1/1000th that on Earth and other chemicals appear in very different quantities. Carbon dioxide predominates at 95%, with nitrogen and argon also present in measurable amounts. Less than one percent is oxygen and carbon monoxide. Mars' climate varies significantly throughout its year, influenced largely by its elliptical orbit. Surface temperatures can vary from -130°C to 30°C (-200°F to 85°F). Ice caps at each pole are composed mostly of frozen carbon dioxide or dry ice. The northern pole has been observed to vaporize in summer to expose trace amounts of solid water ice beneath.
Throughout its year, Mars is host to numerous localized dust storms, and as Mars nears tht sun at perigee, its temperature and wind speed rise to extremes and the whole planet can become enveloped in flying dust.
