The southern hemisphere of Mars is quite heavily cratered and resembles the surface of our Moon. The surface is old and has been relatively unchanged for some time. The northern hemisphere is quite different. It has few craters and these appear far less eroded indicating they are much younger than those of the southern hemisphere. The northern hemisphere shows much geologic activity. The surface has been smoothed by repeated lava flows indicating a great deal of volcanic activity. There is also a lot of deformed uplifted crustal sections along with collapsed depressions.

  Martian volcanoes are the shield type like the Hawaiian islands on Earth. In these type, the lava flows freely and the lava flow can cover a tremendous area. The largest volcano on Mars is Olympus Mons. This volcano is a massive structure, with a base 600Km (370 mi.) across and towering 25km (16 mi.) high. This dwarfs Mauna Loa in Hawaii, the largest volcano on Earth which is only 10 km (6 mi.) high and 225 km (140 mi.) in diameter. Mauna Loa is so heavy it has actually sunk into the Earth's crust and formed a moat around itself. Olympus Mons does not show any sign of sinking and leads to the conclusion that the Martian crust is much thicker than Earth's.

      The entire region around Olympus Mons appears to be an uplifted volcanic plain, called the Tharsis region. There is a similar uplifted region called Elysium region which appears to be older than the Tharsis region. The Elysium region is cratered and eroded and has probably not been active for about 1.5 billion years. The Tharsis region may have been active as recently as 0.2 billion years ago. 

The Tharsis region contains another interesting geologic feature. There are numerous faults and one incredibly huge valley known as Valles Marineris (named after the Mariner spacecraft which discovered it.) The valley is nearly 4000 km (2500 mi.) long, and at its widest nearly 220 km (120 mi.) wide. If the valley were placed on Earth, it would stretch from New York City to Los Angeles. At its deepest is reaches 5 km (4 mi.) over four times the depth of the Grand Canyon! Valles Marineris appears to be the beginnings of what would have been crustal plate boundaries. Apparently the crust thickened and solidified before the plates could fully form. 

    What is truly amazing is that Valles Marineris stretches nearly two-thirds the way across the planet. Such large geologic features like Marineris and Olympus Mons would not be possible on a larger world with higher gravity and thinner crust like the Earth. 

The Lure of Mars

The Martian Atmosphere

The Surface of Mars

Seasons and Climate on Mars

Water on Mars

The Moons of Mars

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Kathy Miles, Author, and Chuck Peters, Systems Administrator
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