Geology of Mars

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The geology of Mars, sometimes called areology, is the study of its composition, structure, physical properties, history and the processes that shape it. It is a specific case of the more general astrogeology.

Contents 1 Timeline 1.1 Alternative timeline 2 Surface 3 Magnetic field 4 Water 4.1 Liquid water 4.2 Polar ice caps 4.3 Ice patches 5 See also 6 References

[edit] Timeline

The Martian geologic timescale (or perhaps more properly the areologic timescale) has three broad epochs defined by the number of impact craters on the surface; older surfaces have more craters. The epochs are 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, dates given here are approximate. From oldest to youngest, the time periods are:

• Noachian epoch (named after Noachis Terra): Formation of Mars to 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.

[edit] Alternative 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.^[1]

• Phyllocian (named after the clay-rich phyllosilicate minerals that characterise 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.

• 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 sulphates and an acidic environment.

• 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 colour.

[edit] Surface

The surface of Mars is thought to be primarily composed of basalt, based upon the Martian meteorite collection and orbital observations. 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.

[edit] Magnetic field

Observations of the magnetic fields on Mars by the Mars Global Surveyor spacecraft have revealed that parts of the planet's crust have been magnetized. This magnetization has been compared to alternating bands found on the ocean floors of Earth. One theory, published in 1999 and reexamined 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.^[2]

[edit] Water

[edit] Liquid water

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".

[edit] Polar ice caps

Mars has polar ice caps that contain frozen water^[3] 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 differs from the northern polar cap in that it appears to contain at least some permanent deposits of CO₂, which are changing on the time scale of years.^[4]

[edit] Ice patches

On 28 July 2005, the European Space Agency announced the existence of a crater partially filled with frozen water^[5]; some then interpreted the discovery as an "ice lake"^[6]. 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.

[edit] See also

• Life on Mars
• Areography
• Martian spiders

[edit] References

1. ^ http://www.theregister.co.uk/2006/04/21/three_mars_eras/
2. ^ "New Map Provides More Evidence Mars Once Like Earth" - Oct. 12, 2005 Goddard Space Flight Center Press release. URL accessed March 17, 2006.
3. ^ "Water at Martian south pole" - March 17, 2004 ESA Press release. URL accessed March 17, 2006.
4. ^ Orbiter's Long Life Helps Scientists Track Changes on Mars - Sept. 20, 2005 NASA Press release. URL accessed March 17, 2006.
5. ^ "Water ice in crater at Martian north pole" - July 27, 2005 ESA Press release. URL accessed March 17, 2006.
6. ^ "Ice lake found on the Red Planet" - July 29, 2005 BBC story. URL accessed March 17, 2006.