Roughly half of Mars' surface is heavily cratered, like the lunar highlands. The cratered areas are located primarily in the southern hemisphere. The northern hemisphere contains younger, lightly cratered volcanic plains (like the lunar maria!). There is a large uplifted continent located across the equator, and this area contains a lot of interesting geologic features.
Most of the volcanism on Mars took place in the Tharsis region (the large, uplifted continent). Mars contains the largest volcano in the solar system: Olympus Mons. It is much larger than the largest volcano on Earth, both in height and diameter. Virtually no impact craters are found along the slopes of Olympus Mons, suggesting that the lava flows are VERY young (less than 100 million years old -- young compared to the age of the solar system, which is 4.5 billion years old).
Taken from the Viking Orbiter
Image courtesy of Malin Space Science Systems/NASA, from Mars Global Surveyor in 1998
Mars' largest tectonic feature is Valles Marineris, a large tectonic crack in the Martian crust that occured when the surface bulged outward. This canyon is larger than the continental United States, and much deeper than the Grand Canyon!
The evolution of Valles Marineris has been a function of its environment:
In general, we see less evidence for tectonic activity on Mars than we do on Venus (recall the wrinkled plains, volcanic domes, continents vs. lowlands). This is what we would expect for a smaller planet that would have cooled more rapidly, unless...... the evidence for a lot more tectonic activity on Mars has been covered with dust.
We have learned the most about the Martian soil from the spacecraft that landed on the surface: Vikings 1 and 2, Mars Pathfinder, and the two Mars Exploration Rovers (Spirit and Opportunity) that are there right now. The soil consists of oxidized clays and iron, which is consistent with the reddish color.
Last week there were two press releases that announced evidence for hints of liquid water that existed earlier on Mars' surface. Spirit, which landed in Gusev Crater, drilled into a dark volcanic rock and found bright material in interior crevices and cracks that looks like minerals crystallized out of water (more details can be found here). Opportunity, which landed in Meridiani Planum, also found evidence for water in the rocks it studied (details here). Clues from the rocks' composition, such as the presence of sulfates, and the rocks' physical appearance, such as niches where crystals grew, helped make the case for a watery history.
Left: rock drilled by Spirit (from http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040305a.html). Right: Opportunity spectrum of Mars rock showing sulfates and iron oxides, which suggest past water (from http://marsrovers.jpl.nasa.gov/newsroom/pressreleases/20040302a.html).
The temperatures get cold enough for frosts to condense out onto the surface (both water frost and carbon dioxide frost).
Samples of Mars have come to Earth as meteorites, and so far these are our only samples of the Martian surface. The rocks are volcanic basalts, and are approximately 1.3 billion years old. Here is a picture of the controversial meteorite that may or may not contain evidence of fossilized life:
Mars has a very thin atmosphere compared to that of the Earth; the surface pressure on Mars is 0.7% that of the Earth. It is composed primarily of CO2 (94%), making it more like Venus than the Earth, and has trace amounts of Nitrogen (3%) and water (less than 1%). Mars' atmosphere is thick enough to generate strong winds, and occasionally the winds are strong enough to generate global dust storms that encircle the entire planet.
Mars' atmosphere is thick enough to produce a greenhouse effect, but the surface temperature is only increased by about 5 degrees, a much smaller effect than for the Earth or Venus.
There are clouds in the Mars atmosphere (as you saw in some of the Hubble images last class), and they can be composed of:
