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Geology 130F

Lecture Sixteen


Mars

(Chp. 6 in Christiansen and Hamblin; Chp. 5 in Beatty and Chaikin)

Physical Characteristics

Rotational Period 24h37m (similar to Earth)
Obliquity 24 degrees (similar to Earth)
Atmosphere 0.007 bar (surface pressure 0.7% of Earth's)
Mostly CO2
No free oxygen
Traces of water vapor
Surface Temperature 150-300K
Magnetic Field Negligible
Bulk Density 3.9g/cm3
Between that of Earth and Moon (FeS core?)

Because of these generally Earth-like characteristic, Mars has been the most heavly explored planet.

Mars has two satellites, Phobos, and Deimos. Both are probably captured bodies, possibly from the asteroid belt. Phobos is tidally locked so that it keeps the same face toward Mars at all times. It also has an orbital period of 0.32 days, less than the rotation period of Mars. Hence it is tidally unstable; it is falling toward Mars. The tidal lifetime of Phobos is about 50 million years, so that there may be a new impact feature on Mars in the future. However, before it hits the surface of Mars, it may possibly be torn apart by tidal stresses. If so, Mars will join the gas giants in having a planetary ring.

Major Features

  1. Mars has two major geologic provinces, southern highlands, and northern plains. The highlands are heavily cratered, while the lowlands or plains are relatively crater-free.
  2. The two provinces are separated by a global escarpment
  3. Large volcanic fields
  4. Enormous rift valleys (Valles Marineris for example).

Geologic provinces

Southern Highlands

The southern highlands are characterised by high crater densities , indicating that they are old terrain similar to the lunar highlands. There are several multiring basins, including Hellas , which at nearly 2000km diameter is larger than the Caloris basin on Mercury or the Imbrium basin on the Moon. There are a number of volcanoes associated with Hellas. Like Mercury, the highlands on Mars have plains with lower crater densities; some of the plains are volcanic in origin. Unlike Mercury the highlands do contain shield volcanoes, and more unusual, large channels that appear to have been carved by flowing water. There are even dendritic systems similar to those seen on Earth.

Northern Plains

The northern plains are similar to the ocean basins on Earth. Some, near the Tharsis volcanic regions, consist of volcanic flows similar to maria on the moon. They are lightly cratered and hence probably young. Other regions are more heavily cratered, but the crater densities are generally much lower than in the highlands. There are indications that standing water once existed on portions of the lowlands. In Candor Chasma, in the Valles Marineris, one finds material that resembles sediment. The Chryse Planitia, where Viking 1 landed, has been shaped by catastrophic floods flowing down from the highlands to the south. There are large volcanic structures in the lowlands, some the size of continents on Earth. The most prominent is the Tharsis region, where the largest volcano in the solar system, Olympus Mons (seen here in perspective) is found.

Other Features

Volcanic Fields

Mars exhibits a number of large volcanic areas, including the Hellas region, associated with a giant impact, the Elysium Planitia, and the Tharsis region, the youngest of the three. The volcanic features are usually divided into three types:

  1. Shield Volcanoes, similiar to Hawaii, but often larger. They are the result of flows of fairly fluid lava; on Earth, they are formed of basalt. The sides slope at about 3 to 4 degrees, so the height is about 5% of the diameter of the base. The caldera, or crater, at the top of the volcano is about a tenth the diameter of the base. Olympus Mons is an example.
  2. Patera, which are found only on Mars. They have a much lower profile than do shield volcanoes, large bases (of order 1000km) and large calderas, up to half the size of the base.
  3. Volcanic flood plains. Some are similar to the intercrater plains on Mercury, others to the ocean floors of Earth or the maria of the Moon.
Mars has the largest volcanoes in the solar system. Why might this be? There are several possible factors.
  1. Size.
    1. Mars is larger than Mercury or the Moon, so it is likely to remain hotter for longer periods of time. Thus there is more time to accumulate volcanic material. On the other hand, the largest volcanoes on Mars are young, less than a billion years; clearly this is shorter than the period of time Mercury and the Moon were hot enough to produce volcanoes.
    2. Mars is smaller than Earth or Venus, and less dense, so it has a low surface gravity. This makes it easier to build large volcanic mountains. It also means that the lithosphere on Mars is thicker, and so able to support large mountains. There is some evidence that the lithosphere on Earth is being strained by Mauna Loa, the name of the volcano on the big island of Hawaii. There is a bit of a moat around the base of the island on the ocean floor. The lithosphere on Mars is also stationary (there is no evidence of plate tectonics). Olympus Mons is likely to have been produced by a mantle plume, as is Hawaii. The lack of plate movement on Mars means that the plume piled up material in one spot; on Earth the plume produced a series of volcanoes stretching across the central and eastern Pacific.
  2. As noted above, Mars has a lower bulk density, probably because it is composed of a higher fraction of volatile material. If so, the melting temperature of material in the mantle will be lower than on Venus or Earth, which tends to promote convection and volcanic activity.
Valles Marineris

Valles Marineris is a continent sized canyon extending eastward from the Tharsis volcanic field to the global escarpment marking the transition to the older highlands. It follows a series of faults believed to have been produced by the upwelling that formed the Tharsis volcanoes. The canyon walls clearly show the effects of massive landslides.

Martian Atmosphere

The terrestrial planets Venus, Earth, and Mars (and possibly Mercury) most likely all started with dense H and He atmospheres collected from the solar nebula. Because their surface gravities are low, they lost this primary atmosphere. The atmospheres we see around the larger terrestrial planets today are called secondary atmospheres. They consist of less volatile gasses such as CO2 or N2, which have molecular weights substantially greater than those of H and He. This gas probably was outgassed by volcanoes from the interiors of the planets, but some fraction may have come from accretion of comets. The initial atmospheric pressure on the terrestrial planets is estimated to be about 80 times that of the Earth's current atmosphere.

Terrestrial Planet Atmospheres
Mercury Venus Earth Mars
Pressure 0 90 1 0.01
CO2 0 96% 0.03% 95%
H2 0 0 1% trace
N2 0 3.5% 77% 2.7%
O2 0 <0.5% 21% 0
Trace gasses (Ar) 100% 90 <1% 1.6%

What happened on Mars?

  1. Weaker gravity
  2. Low temperature, leading to freezing out of less volatile gasses.
  3. Incorporation into rocks


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