Geology of the Solar System 7: Mars


Because the notes on Mars are also freaking long (see my notes on the Moon), I will also be glossing over whatever I can here.

Basically, it’s the fourth planet from the Sun, rotates on its axis once every 24.5 Earth days, and orbits the sun in 1.88 Earth years. It’s only a little bigger than half the size of Earth, and has a density of 3.93 g/cm3. It has a thin atmosphere comprised mostly of carbon dioxide, and wind speeds at its surface make our hurricanes look like a joke. There is a weak remnant magnetic field in the crust left over from a warmer period in Martian history, when it would have had an active one.

Internal Structure

It has a very low density, which is not surprising as it formed farther from the Sun and should contain lower temperature condensates.

Here is a picture to more or less describe the internal structure, so that I don’t have to.

Geologic Provinces

There is a clear division between the northern and southern hemispheres, and a few different geologic provinces to know about.

Densely cratered highlands: Makes up the southern hemisphere, and contains several large, multiring craters in addition to numerous smaller ones.  The ejecta from Martian craters is odd, in that it sometimes appears to flow, suggesting the presence of water. The current theory is that the impact melts ice into water, which then mixes with the ejecta to produce flows.

Northern plains: Makes up the northern (duh) hemisphere, and are separated from the south by an escarpment.  There are basalts and other volcanic features, as well as stream-borne sediments and evidence of wind erosion.

Global escarpment: It encircles Mars with a few interruptions, and is thought to have been caused by a major impact. Since its creation, it has been dissected by stream erosion and mass movement or slumping, and there are some isolated blocks or mesas just north of it.

Crustal upwarps: There are two continent-sized upwarps in the northern plains, and they are covered in volcanoes and basalts.  They were likely produced by hot mantle plumes pushing up on the crust. The largest volcano in the solar system, Olympus Mons, is located here.

These volcanoes are no longer active.

Polar regions: Mars has polar ice caps like Earth does, comprised of water ice as well as dry ice in the winter. Drops in atmospheric pressure resulting from the freezing of CO2 creates a single large Hadley cell in which air is moved from the summer hemisphere to the winter one.

Volcanic and Tectonic Features

There are three major features related to volcanism on Mars: shield volcanoes, volcanic plains with basalts that are more enriched in volatiles than Earth’s basalts, and patera, which are large, low relief structures with central volcanoes, extensive flows, pyroclastics, and extensional tectonic features.

There are no plate tectonics on Mars.  Ridges are caused by the buckling of cooling lava in volcanic plains, and canyons were created by crustal expansion upward when upwelling occurred.

Water and Life on Mars

There is plenty of evidence for water on Mars, and there is no question of its presence.  The water is found in ice caps, as ice crystals in the rocks and sediments, and in the atmosphere.  There is also evidence of there once being liquid water on the surface in the form of dendritic drainage systems, outflow channels, and boulders eroded and stacked so they are tilting in the same direction, indicating a very strong flow of water.

Life, on the other hand, is a little more questionable.  For every bit of possible evidence of life, there is also something to suggest that the evidence was not created by organic processes but inorganic ones. The possibility hasn’t been rules out, but definitive evidence hasn’t yet been found either.


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