Surface Features
of Mars

Mars has many interesting geological features on its surface that first became apparent with Mariner 9, were subsequently studied by the Viking missions, and many of which now are visible from the Hubble Space Telescope.

Mars seems to be divided into two parts, the heavily cratered southern highlands, and the northern smooth lowlands. The cratered south is more than 3.5 billion years old, and the north was resurfaced 1 billion years ago. This is probably due to more volcanism in the north and some suggest the north was actually filled by a water ocean. Alternatively, maybe a large impact broke the crust in the north. The answer is not exactly known.

Enormous Shield Volcanoes

In the previous section we saw an image of Mars with 3 large volcanoes on the left limb of the image. The volcanoes on Mars are now extinct, but they indicate a preceding period of significant Martian volcanism. Such volcanoes are called shield volcanoes, because they look like shields. The largest volcano on Mars is not one of the three shown previously. It is called Olympus Mons, and is illustrated below (Ref).

Olympus Mons is 600 km across its base and about 25 km above the surrounding plain. A perspective model is shown below

For reference, the largest shield volcano on the Earth is Mauna Loa, which is only about 200 km across its base, and the 25 km height of Olympus Mons is about 2 1/2 times the height of Mount Everest. This raises the interesting issue of why Mars in its past developed a few very large shield volcanoes, while on the Earth the more normal pattern is for volcanoes to develop in strings of smaller volcanoes. As we shall see, the answer is thought to lie in plate tectonics.

Absence of Plate Tectonics

There is no evidence on Mars for large-scale plate tectonics as we find on Earth. This is believed to be responsible for the different character of Martian and Terrestrial volcanoes, as illustrated in the following animation.

On the Earth, as crustal plates move over subsurface chambers of molten rock the lava tends to come to the surface in a line of places, producing strings of volcanoes (for example, volcanic island chains like the Hawaiian Islands). On Mars, with no horizontal motion of crustal plates the same point in the crust sits over subsurface chambers of molten rock and a few very large volcanoes are built rather than many small volcanos. Here is a more extensive discussion of volcanism on Mars.

Large Canyon Systems

The Martian surface has some large canyon systems. The largest is Valles Marineris, which extends for about 5000 km, is 500 km wide in the widest portions, and as much as 6km deep. The adjacent image shows a portion of the Valles Marineris (the full system is shown in the preceding section). This enormous system of connecting canyons appears to have been formed mostly by local tectonic activity (local motion of surface) rather than by erosion, though as we will discuss below there is some evidence for fluid erosion in portions of it.

The Valles Marineris faults are linked to a large volcanic bulge called Tharsis bulge. This has many smaller volcanos but Olympus Mons is just off of its northwest side. It is thought that a plume of rising magma underneath the crust has pushed up this bulge, and broken through in places to form volcanos and that the faults and the bulge exemplify that this is a thin region of the martian crust.

Running Water Erosion

There are channels on Mars as much as 1500 km long and 200 km wide that appear to have been cut by running water. Under present atmospheric conditions on Mars (low pressure), water cannot exist as a free liquid on the surface (it must be gas or solid). Thus, evidence for water erosion suggests that the Martian atmosphere may have been more dense in the past. The following two images show portions of the Martian surface where the erosion patterns have regions that are very similar to those found for erosion by surface water on the Earth.

Regions with possible evidence for water erosion on the Martian surface

Wind Erosion

The atmosphere of Mars is thin (about 1/200 of the pressure of the Earth's atmosphere), but this atmosphere supports high velocity seasonal winds that are correlated with solar heating of the surface and that produce duststorms that lead to a lot of surface erosion. The following image shows a local dust storm on the Martian surface (lower left).

A local dust storm on the Martian Surface (source)

Here is another example of a Martian dust storm. At times, such local dust storms grow and merge until essentially the entire surface of the planet is covered by a dust storm. These periods are correlated with times of maximum solar heating of the Martian surface.

Polar Caps

Mars has polar caps that wax and wane with the Martian seasons. The following image and animation show the North Polar Cap.

The Martian polar caps (source)

These polar caps appear to be partially composed of frozen carbon dioxide ("dry ice") and partially composed of frozen water. A movie (775 kB) of the rotating Mars animation is also available.