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.
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 .
Olympus Mons is 600 km across its base and
about 25 km above the surrounding plain. A perspective model is shown
below
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.
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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. 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. 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.
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| 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).
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| A local dust storm on the Martian Surface |
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.
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| The Martian polar caps | |
These polar caps appear to be partially composed of frozen carbon dioxide ("dry ice") and partially composed of frozen water. An MPEG movie (775 kB) of the rotating Mars animation is also available.
Information supplied by: http://csep10.phys.utk.edu