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6.17 Geology of Mars

Probes that went into orbit around Mars in the 1970s discovered many intriguing geological features. Photos showed not only ancient, heavily cratered areas, but also younger, sparsely cratered regions. People saw an alien landscape that included canyons and landslides, vast fields of sand dunes, and enormous young volcanoes. Many of the features were strikingly Earth-like, but Mars was missing the ingredient that makes the Earth special: water.

One of the most obvious features on the Martian globe is an enormous trench scarring an entire hemisphere. Valles Marineris, named after the Mariner spacecraft that discovered it, is 4500 km (2800 miles) long — it would stretch across the entire United States! The valley is 8 km deep in some places, five times as deep as Earth’s Grand Canyon. Valles Marineris probably has a tectonic origin. Without plate tectonics to relieve the stress, convection in the mantle could stretch the crust, forming a giant crack. Weathering by wind, and perhaps water, would have widened this crack into the chasm we see today. The formation of Valles Marineris may also be related to the Tharsis bulge, a large up welling in the Martian crust nearby.

Sitting on the Tharsis bulge are several of the solar system’s largest volcanoes. These shield volcanoes are relatively young, only about 200 million years old. The largest of them is Olympus Mons, rising 24 kilometers (78,000 feet) above Mars. In comparison, Mount Everest on Earth rises only 9 kilometers above sea level, and 13 kilometers above the deepest ocean floor. Mountains on Earth don’t grow as large as on Mars for several reasons.  Tectonic plates slide over volcanic “hot spots,” so instead of one large volcanic mountain, we get a chain of smaller ones. Secondly, the thicker lithosphere of Mars is also better able to support the mass of a large mountain. Earth’s thinner lithosphere and hotter interior causes slumping under the weight of a large mountain.

The only samples we have of Martian rocks are meteorites, and we don’t know where on Mars they originated. So scientists have to use craters to estimate the ages of different areas on the surface of Mars. The principle of crater counting is simple. We assume that a new surface forms with no craters and they are added at a constant rate after that. The method only works on planets or moon with thin or absent atmospheres, since the atmosphere would "rub out" many of the craters. We can calibrate crater counting using the Moon, since we have samples from Apollo with ages from radioactive decay. The most obvious age difference is between the northern and southern hemispheres. The northern hemisphere is smoother, with fewer craters, and is also lower in elevation than the southern hemisphere. Perhaps this area was flooded by lava, or it may be the floor of an ancient ocean.

Mars is occasionally covered by planet-wide dust storms that can last for months. Billions of years of erosion by wind, meteorite impacts, and water have reduced surface rocks on Mars to a layer of fine particles. During global storms, the entire planet’s surface is veiled by this red dust. The storms are so big, they can even be seen from Earth. Wind speeds at the surface during a dust storm can reach 30 meters per second (68 mph)! With this kind of speed, even very fine dust has enough power to sculpt the Martian surface. Changing dune fields and wind streaks are evidence of that influence. Wind will slow down when it passes over a raised feature like a crater or a hill, and then it will deposit material in a streak behind the feature. The streaks can be dark or light, depending on the color of the material and the underlying rock.

Every Solar System body has craters, but one type of crater is found only on Mars. Rampart craters are surrounded by ejecta that looks like it flowed like mud. When an asteroid hits the surface of Mars, part of its kinetic energy is transformed into heat. This would melt any ice mixed in with the soil, or any permafrost beneath the surface. The resulting water would liquefy the rocks and soil thrown out of the crater, forming the characteristic rampart ejecta blanket.  These distinctive craters are one of the indications that ice is present under the Martian surface.


Tharsis bulge region on Mars with Olympus Mons in the top left. Click here for original source URL.