Geology of Mars

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False colour view of a landslide in Zunil crater

The geology of Mars, also known as areology (from Greek: Ἂρης, Arēs, "Ares"; and λόγος, logos, "knowledge"), refers to the study of the composition, structure, physical properties, history and the processes that shape the planet Mars.

Timeline

File:Mars surface ages.jpg

Surface age map of Mars (NASA).

Crater density timeline

Studies of impact crater densities on the Martian surface allow us to identify has three broad epochs in the planet's geological timescale, as older surfaces have more craters and younger ones less . The epochs were named after places on Mars that belong to those time periods. The precise timing of these periods is not known because there are several competing models describing the rate of meteor fall on Mars, so the dates given here are approximate. From oldest to youngest, the time periods are:

• Noachian epoch (named after Noachis Terra): Formation of the oldest extant surfaces of Mars between 3800 and 3500 million years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge is thought to have formed during this period, with extensive flooding by liquid water late in the epoch.
• Hesperian epoch (named after Hesperia Planum): 3500 million years ago to 1800 million years ago. The Hesperian epoch is marked by the formation of extensive lava plains.
• Amazonian epoch (named after Amazonis Planitia): 1800 million years ago to present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Olympus Mons formed during this period along with lava flows elsewhere on Mars.

The studying of craters is based upon the assumption that crater-forming impactors have hit the planet all throughout history at regular intervals, and there is no way to exactly date an area just based upon the number of impacts, only to guess that areas with more impacts must be older than areas with fewer impacts. For example this system of logic breaks down if a large number of asteroids had hit at once, or if there were long periods where few asteroids hit.

Mineralogical timeline

Based on recent observations made by the OMEGA Visible and Infrared Mineralogical Mapping Spectrometer on board the Mars Express orbiter, the principal investigator of the OMEGA spectrometer has proposed an alternative timeline based upon the correlation between the mineralogy and geology of the planet. This proposed timeline divides the history of the planet into 3 epochs; the Phyllocian, Theiikian and Siderikan.

• Phyllocian (named after the clay-rich phyllosilicate minerals that characterize the epoch) lasted from the formation of the planet until around 4000 million years ago. In order for the phyllosilicates to form an alkaline water environment would have been present. It is thought that deposits from this era are the best candidates to search for evidence of past life on the planet. The equivalent on earth is much of the hadean eon .

• Theiikian (named, in Greek, after the sulfate minerals that were formed), lasting until about 3500 million years ago, was a period of volcanic activity. In addition to lava, gasses - and in particular sulfur dioxide - were released, combining with water to create sulfates and an acidic environment . The equivalent on earth is the eoarchean era and the beginning of the paleoarchean era .

• Siderikan, from 3500 million years ago until the present. With the end of volcanism and the absence of liquid water, the most notable geological process has been the oxidation of the iron-rich rocks by atmospheric peroxides, leading to the red iron oxides that give the planet its familiar color . The equivalent on earth is most of the archean all of the proterozoic and up to now .

Surface chemistry

The surface of Mars is thought to be primarily composed of basalt, based upon the observed lava flows from volcanos, the Martian meteorite collection, data from landers and orbital observations. The lava flows from Martian volcanos show that that lava has a very low viscosity, typical of basalt. Analysis of the soil samples collected by the Viking landers in 1976 indicate iron-rich clays consistent with weathering of basaltic rocks. There is some evidence that some portion of the Martian surface might be more silica-rich than typical basalt, perhaps similar to andesitic rocks on Earth, though these observations may also be explained by silica glass, phyllosilicates, or opal. Much of the surface is deeply covered by dust as fine as talcum powder. The red/orange appearance of Mars' surface is caused by iron(III) oxide (rust). Mars has twice as much iron oxide in its outer layer as Earth does, despite their supposed similar origin. It is thought that Earth, being hotter, transported much of the iron downwards in the 1800km deep, 3,200 °C, lava seas of the early planet, while Mars, with a lower lava temperature of 2,200 °C was too cool for this to happen.

Magnetic field

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Bands in the crustal magnetic fileld of Mars (NASA).

Although Mars has no intrinsic magnetic field, observations have revealed that parts of the planet's crust have been magnetized. This Paleomagnetism of magnetically susceptible minerals has features very similar to the alternating bands found on the ocean floors of Earth. One theory, published in 1999 and re-examined in October 2005 with the help of the Mars Global Surveyor, is that these bands are evidence of the past operation of plate tectonics on Mars 4 billion years ago, before Mars' planetary dynamo ceased. The magnetization patterns in the crust also provide evidence of past polar wandering, the change in orientation of Mars' rotation axis.

As can be seen from the figure, Mars' magnetic field varies over its surface, and while it is mostly very small it can in places be locally as high as on Earth. In these places Mars' also has a detectable ionosphere. It is possible to date the time when Mars' dynamo turned off. Two large impact basins, Hellas and Argyre, dating from 4 billion years ago are unmagnetised, so the dynamo must have turned off before then otherwise the molten rock would have remagnetised.

Mars' core

Current models of the planet's interior suggest a core region approximately 1,480 km in radius (just under half the total radius), consisting primarily of iron with about 15-17% sulfur. This iron sulfide core is partially fluid, with twice the concentration of light elements that exists at the Earth's core. The high sulfur content of Mars' core gives it a very low viscosity, which in turn implies that Mars' core formed very early on in the planets history.

Mars' crust and mantle

The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but now appears to be inactive. The average thickness of the planet's crust is about 50 km, and it is no thicker than 125 km, which is much thicker than the Earth's crust which varies between 5km and 70km. As a result Mars' crust does not easily deform, as was shown by the recent radar map of the south polar ice cap which does not deform the crust despite being about 3km thick - . The high sulfur content of Mars' core gives it a very low viscosity, which in turn implies that Mars' core formed very early on in the planets history.

Hydrology

See also: Atmosphere of Mars

Ancient rivers - Modern gullies

The high resolution Mars Orbiter Camera on the Mars Global Surveyor has taken pictures which give much more detail about the history of liquid water on the surface Mars. Despite the many giant flood channels and associated tree-like network of tributaries found on Mars there are no smaller scale structures that would indicate the origin of the flood waters. It has been suggested that weathering processes have denuded these indicating the river valleys are old features. Higher resolution observations from spacecraft like Mars Global Surveyor also revealed at least a few hundred features along crater and canyon walls that appear similar to terrestrial seepage gullies. The gullies tended to be Equator facing and in the highlands of the southern hemisphere, and all poleward of 30° latitude. The researchers found no partially degraded (i.e. weathered) gullies and no superimposed impact craters, indicating that these are very young features.

Another theory about the formation of the ancient river valleys is that rather than floods, they were created by the slow seeping out of groundwater. This observation is supported by the sudden ending of the river networks in theatre shaped heads, rather than tapering ones. Also valleys are often discontinuous, small sections of uneroded land separating the parts of the river.

On the other hand, evidence in favor of heavy or even catastrophic flooding is found in the giant ripples in the Athabasca Vallis [1].

Liquid water

Mosaic shows some spherules partly embedded.

Among the findings from the Opportunity rover is the presence of hematite on Mars in the form of small spheres on the Meridiani Planum. The spheres are only a few millimeters in diameter and are believed to have formed as rock deposits under watery conditions billions of years ago. Other minerals have also been found containing forms of sulfur, iron or bromine such as jarosite. This and other evidence led a group of 50 scientists to conclude in the December 9, 2004 edition of the journal Science that "Liquid water was once intermittently present at the Martian surface at Meridiani, and at times it saturated the subsurface. Because liquid water is a key prerequisite for life, we infer conditions at Meridiani may have been habitable for some period of time in Martian history". Later studies suggested that this liquid water was actually acid because of the types of minerals found at the location. On the opposite side of the planet the mineral goethite, which (unlike hematite) forms only in the presence of water, along with other evidence of water, has also been found by the Spirit rover in the "Columbia Hills".

Photo of Microscopic rock forms indicating past signs of water, taken by Opportunity

Recently, there has been evidence to suggest that liquid water flowed on the surface of Mars much more recently than thought, with the discovery of gully deposits that were not seen ten years ago [2].

Polar ice caps

Mars has polar ice caps that contain frozen water and carbon dioxide that change with the Martian seasons. Each cap has surface deposits of carbon dioxide ice that form a polar "hood" during Martian winter, and then sublimate during the summer uncovering the underlying cap surface of layered water ice and dust. The southern polar cap (Planum Australe) differs from the northern polar cap (Planum Boreum) in that it appears to contain at least some permanent deposits of CO₂, which are changing on the time scale of years. The southern polar cap has recently been confirmed to be a 3km thick slab of about 80% water ice. An interesting finding of the radar study is the suspected existence of a small sheet of what looks like liquid water between the ice and Mars' crust.

Ice patches

On 28 July 2005, the European Space Agency announced the existence of a crater partially filled with frozen water; some then interpreted the discovery as an "ice lake". Images of the crater, taken by the High Resolution Stereo Camera on board the European Space Agency's Mars Express spacecraft, clearly show a broad sheet of ice in the bottom of an unnamed crater located on Vastitas Borealis, a broad plain that covers much of Mars' far northern latitudes, at approximately 70.5° North and 103° East. The crater is 35 km (23 mi) wide and about 2 km (1.2 mi) deep.

The height difference between the crater floor and the surface of the water ice is about 200 metres. ESA scientists have attributed most of this height difference to sand dunes beneath the water ice, which are partially visible. While scientists do not refer to the patch as a "lake", the water ice patch is remarkable for its size and for being present throughout the year. Deposits of water ice and layers of frost have been found in many different locations on the planet.

Equatorial frozen sea

Surface features consistent with pack ice have been discovered in the southeren Elysium Planitia. What appear to be plates of broken ice, ranging in size from 30m to 30km, are found in channels leading to a flooded area of approximately the same depth and width as the North Sea. The plates show signs of break up and rotation that clearly distinguish them from lava plates elswhere on the surface of Mars. The source for the flood is thought to be the nearby geological fault Cerberus Fossae which spewed water as well as lava some 2 to 10 million years ago .

Olivine

Olivine mineral (purple) in the Valles Marineris

Spectra from the NASA THEMIS probe have shown the possibility of the mineral olivine on Mars by looking for the characteristic infra-red radiation it emits. The discovery is interesting because the mineral, which form from volcanic activity, is very susceptible to weathering from water, and so its presence and distribution which can be obtained from satellite could tell us about the history of water on Mars

Olivine forms from magma and weathers into clays or iron oxide. The researchers found olivine all over the planet, but the largest exposure was in Nili Fossae, a >3.5 billion year old region dating from the Noachion epoch. Another outcrop is in the Ganges Chasma, an eastern side chasm of the Valles Marineris (pictured).

Impact crater morphology

Yuty impact crater with typical rampart ejecta

Crater morphology provides information about the physical structure and composition of the surface. Impact craters allow us to look deep below the surface and into Mars geological past. Lobate ejecta blankets (pictured left) and central pit craters are common on Mars but uncommon on the Moon, which may indicate the presence of near-surface volatiles (ice and water) on Mars. Degraded impact structures record variations in volcanic, fluvial, and eolian activity.

The Yuty crater is an example of a Rampart crater so called because of the rampart like edge of the ejecta. In the Yuty crater the ejecta competely covers an older crater at its side, showing that the ejected material is just a thin layer.

Slope streaks

A new phenomenon known as slope streaks has been uncovered by the HiRISE camera on the Mars Reconnaissance Orbiter. These features appear on crater walls and other slopes and are thin but many hundreds of meters long. The streaks have been observed to grow slowly over the course of a year or so, always beginning at a point source. Newly formed streaks are dark in colour but fade as they age until white. The cause is unknown, but theories range from dry dust avalanches (the favoured theory) to brine seepage.

See also

• Scientific information from the Mars Exploration Rover mission
• Life on Mars
• Geography of Mars
• Martian spiders
• Elysium Planitia
• Hecates Tholus

External Links

• Geologic Map of Mars