The Mariner series of planetary probes launched in the 1960s and 1970s revealed a startling difference between the southern and northern hemispheres of Mars. The southern hemisphere is far more cratered than the northern hemisphere, which is covered with wind-blown material as well as volcanic lava. There have even been recent proposals that the smooth northern hemisphere hides a frozen ocean.
Volcanoes and lava plains from ancient volcanic activity abound on Mars. Because the planet’s surface gravity is low (0.38 that of the earth), the volcanoes can rise to spectacular heights. Like Venus, Mars lacks a strong magnetic field, but, in contrast to Venus, it rotates rapidly; therefore, astronomers conclude that the core of Mars is nonmetallic, nonliquid, or both. Astronomers believe that the core of the smaller Mars has cooled and is likely solid, consisting largely of iron sulfide.
Unlike the earth, Mars failed to develop much tectonic activity (instability of the crust), probably because its smaller size meant that the outer layers of the planet cooled rapidly. Instead, volcanic activity was probably quite intense some 2 billion years ago.
Also impressive are Martian canyons, including Valles Marineris, the “Mariner
Valley,” which runs some 2,500 miles (4,025 km) along the Martian equator and is as
much as 75 miles (120 km) wide and, in some places, more than four miles (6.5 km)
deep. The Valles Marineris is not a canyon in the earthly sense, since it was not cut
by flowing water, but is a geological fault feature.
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