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Geology of Mars
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The geology of Mars, also known as areology (from Greek Ares and -logia), refers to the study of the composition, structure, physical properties, history, and the processes that shape the planet Mars.
ents present on Mars include among others oxygen (O), iron (Fe), silicon (Si) and sulfur (S).
ies of impact crater densities on the Martian surface allow us to identify 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.
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Encyclopedia
The geology of Mars, also known as areology (from Greek Ares and -logia), refers to the study of the composition, structure, physical properties, history, and the processes that shape the planet Mars.
Elemental composition
Elements present on Mars include among others oxygen (O), iron (Fe), silicon (Si) and sulfur (S).
Timeline
Crater density timeline
Studies of impact crater densities on the Martian surface allow us to identify 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 4.6 and 3.5 billion 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): 3.5 to 1.8 Ga BP. The Hesperian epoch is marked by the formation of extensive lava plains.
- Amazonian epoch (named after Amazonis Planitia): 1.8 Ga BP 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 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, and data from landers and orbital observations. The lava flows from Martian volcanos show 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 (Fe2O3) (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 1,800 km 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. While the possibility of carbonates on Mars has been of great interest to exobiologists and geochemists alike, there is little evidence for significant quantities of carbonate deposits on the surface.
One of the goals of potential NASA missions to the planet is to grow plants such as asparagus, green beans and turnips in the Martian soil, which, after some testing, had suggested Earth-like soil. These tests determined the soil was slightly alkaline and contained vital nutrients such as magnesium, sodium, potassium and chloride, all of which are necessary for living things (as we know them) to grow. In fact, NASA previously reported that the soil near Mars' north pole was similar that found in backyard gardens on Earth where plants could potentially grow. However, in August, 2008, the Phoenix Lander conducted simple chemistry experiments, mixing Earth-water with Martian soil in an attempt to test it's pH, and discovered traces of perchlorate, which is the oxidizing ion ClO4. Preliminary results from this second lab test suggest that produce planted in the soil may have to overcome a very harsh environment, one much less friendly to life than once believed. Further testing is necessary to determine how much perclorate exists in the Martian soil, how it formed, or if perhaps the soil sample was simply contaminated by emissions from Phoenix's burning fuel during landing.
Magnetic field and internal structure
Although Mars today has no global-scale intrinsic magnetic field, observations have been interpreted as showing that parts of the planet's crust have been magnetized and that polarity reversal of its dipole field occurred when the central dynamo ceased, leaving only residual permanent crustal fields. 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 Ga 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. It is possible to date the time when Mars' dynamo turned off. The large impact basins Hellas and Argyre, aged 4 Ga, are unmagnetised, so the dynamo would have to have turned off before then otherwise the molten rock would have remagnetised. An alternative theory advanced by Benoit Langlois is that a lunar-scale object struck the northern hemisphere at a shallow angle and high latitude at about 4.4 Ga. Computer models by Sabine Stanley show that this would have created a convection current powered dynamo in the southern hemisphere.
Gravity
Mars has approximately half the radius of Earth and only one-tenth the mass, which generates a surface gravity of 0.376 g, that is only about 38% of the surface gravity on Earth.
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 or completely fluid, with twice the concentration of light elements that exists at 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 planet's history.
Crust and mantle
The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet. 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 Earth's crust which varies between 5 km and 70 km. A recent radar map of the south polar ice cap showed that it does not deform the crust despite being about 3 km thick.
Tectonics
As a result of 1999 observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft, it was proposed that during the first half billion years after Mars was formed, the mechanisms of plate tectonics may have been active, with the Northern Lowlands equivalent to an ocean basin on Earth. Further data from the Mars Express orbiter's High Resolution Stereo Camera in 2007 clearly showed the 'global crustal dichotomy boundary’ in the Aeolis Mensae region.
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 of 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 .
Liquid water
Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods. Recently, there has been evidence to suggest that liquid water flowed on the surface in the recent past, with the discovery of gully deposits that were not seen ten years ago .
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 millimetres 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".
On July 31, 2008, NASA announced that the Phoenix lander confirmed the presence of water ice on Mars,
as predicted on 2002 by the Mars Odyssey orbiter.
Polar ice caps
Mars has polar ice caps that contain 85% highly carbon dioxide (CO2) ice and 15% water ice 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 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.
NASA scientists calculate that the volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 metres. Additionally, an ice permafrost mantle stretches from the poles to latitudes of about 60°.
Ice patches
On July 28 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 wide and about 2 km 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 southern Elysium Planitia. What appear to be plates of broken ice, ranging in size from 30 m to 30 km, 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 aged some 2 to 10 million years.
Ancient coastline
A striking feature of the topography of Mars is the flat plains of the northern hemisphere. With the increasing amounts of data returning from the current set of orbiting probes, what seems to be an ancient shoreline several thousands of kilometres long has been discovered. One major problem with the conjectured 2 Ga old shoreline is that it is not flat — i.e. does not follow a line of constant graviational potential. However a 2007 Nature article points out that this could be due to a change in distribution in Mars' mass, perhaps due to volcanic eruption or meteor impact — the Elysium volcanic province or the massive Utopia basin that is buried beneath the northern plains have been put forward as the most likely causes. The Mars Ocean Hypothesis conjectures that the Vastitas Borealis basin was the site of a primordial ocean of liquid water 3.8 billion years ago.
Glacial phases
A 2008 study provided evidence for multiple glacial phases during Late Amazonian glaciation at the dichotomy boundary on Mars.
Olivine
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 is associated with volcanic activity, is very susceptible to weathering by 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 region dating from >3.5 Ga (the Noachian epoch). Another outcrop is in the Ganges Chasma, an eastern side chasm of the Valles Marineris (pictured).
Impact crater morphology
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.
The largest unambiguous impact crater is the Hellas Basin in the southern hemisphere. However, it appears that the Borealis Basin, covering most of the low-lying northern hemisphere, is also an impact crater.
Major Geological Events
On February 19, 2008 an amazing geologic event was captured by the HiRISE camera on the Mars Reconnaissance Orbiter. Images which captured a spectacular avalanche thought to be fine grained ice, dust and large blocks are shown to have fallen from a high cliff. Evidence of the avalanche are shown by the dust clouds rising from the cliff afterwards. Such geological events are theorized to be the cause of geologic patterns known as slope streaks.
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 metres 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.
Mars Avalanche Photo Gallery
See also
External links
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