MARS

When considering the surfaces of planets, one is naturally led to consider analogues of the oceanic and continental regions of the Earth. In this spirit, the surface of Mars can be divided up into two types of terrain:

• Rough, cratered terrain (which superficially resemble the lunar highlands) which covers most of the southern hemisphere and on average, is greater than 3 km above sea level. The cratered regions are thought to be on the order of 3.9 billion years old.

• Plains (sites of extensive lava flows) dominate the northern hemisphere and which typically lie below sea level. On the plains the densities of large craters are 10 - 100 times lower than in the southern hemisphere.The plains are thought to be on the order of 3 billion years old.

The transition between the northern and southern hemispheres can be abrupt with elevation changes of 4 km occuring over the span of a few hundred kilometers.

An interesting region which straddles the two zones is the Tharsis Bulge. The Tharsis Bulge was volcanically active, is roughly the size of North America, and rises to a height of about 10 km above sea level.

The Tharsis Bulge is the least cratered and so is the youngest part of the planet's surface. Tharsis probably started to form ~3.9 billion years ago, but has been active until fairly recently. Near the top of the Tharsis Bulge, there are three great shield volcanoes, each rising 18 km to summits at elevations of 27 km with bases on size 400 kilometers. An even larger volcano, Olympus Mons rises 25 km above the northwestern slope of Tharsis to reach the same height as the Tharsis volcanoes of 27 km above sea level. Olympus Mons is nearly 700 km across its base, roughly the size of France and may have been active as recently as 100 - 300 million years ago. Note that:
• the tallest Hawaiian island is only 100 km across its base and is around 9 km high. If all of the Tharsis volcanoes are shield volcanoes, then this suggests that at the time the volcanoes formed there was not large scale plate tectonic activity going on.

• the slopes of the volcanoes are not heavily cratered indicating that the volcanoes were active within the last few hundred million years

Near the Tharsis Bulge is Valles Marineris, part of an interconnected system of east-west running canyons approximately 4,000 km long. The individual canyons are 3 km deep, but in the center of the canyon, the depression is nearly 7 km and more than 500 km wide. The Grand Canyon would fit easily into one of the tributaries of the Valles Marineris. These features likely started to formed around 3.5 - 3.9 billion years ago!

The Tharsis Bulge was thought to have formed through an uplift (vertical rise) of the crust of Mars. The Valles Marineris was also probably due to some sort of crust cracking driven by crustal motion. It was not cut by flowing water (as was the Grand Canyon).

There is ample evidence of geological activity on Mars, however, there is no clear evidence of plate tectonics as found on the Earth.