View source for Geology of Mars

The '''geology of Mars''', also known as '''areology''' (from [[Ancient Greek|Greek]] {{lang|grc|Ἂρης}} ''[[Ares|Arēs]]'' and {{lang|grc|-λογία}} ''[[-logy|-logia]]''), refers to the study of the composition, structure, physical properties, history, and the processes that shape the [[planet]] [[Mars]].

==Elemental composition==
{{Expand section|date=December 2008}}
Elements present on Mars include among others [[oxygen]] (O), [[iron]] (Fe), [[silicon]] (Si) and [[sulfur]] (S). The planet's distinctive red coloration is largely due to the [[oxidation]] of its surface iron. Recently Mars soil was found to contain ice trapped in minerals. It was also found to contain potassium, magnesium ans sodium.

==Timeline==
[[Image:Mars surface ages.jpg|thumb|left|Surface age map of Mars (NASA).]]
===Crater density timeline===
Studies of [[impact crater]] densities on the Martian surface allow us to identify three broad [[epoch (geology)|epoch]]s in the planet's [[geological timescale]], as older surfaces have many craters and younger ones have few.<ref>{{cite web
  | last = Caplinger
  | first = Mike
  | title = Determining the age of surfaces on Mars
  | url=http://www.msss.com/http://ps/age2.html
  | accessdate = 2007-03-02 }}</ref>
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:
*{{anchor|Noachian|Noachian epoch}}'''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.
*{{anchor|Hesperian|Hesperian 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. Formation of [[Olympus Mons]] likely began during this epoch<ref>Elizabeth R. Fuller, James W. Head III "Amazonis Planitia: The role of geologically recent volcanism and sedimentation in the formation of the smoothest plains on Mars"</ref>.
*{{anchor|Amazonian epoch}}'''Amazonian epoch''' (named after [[Amazonis Planitia]]): 1.8 Ga BP to present. Amazonian regions have few meteorite impact craters but are otherwise quite varied.  Lava flows on Mars continued during this period.

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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.<ref>{{cite web | last = Williams | first = Chris | title = Probe reveals three ages of Mars | url=http://www.theregister.co.uk/2006/04/21/three_mars_eras/ | accessdate = 2007-03-02 }}</ref><ref>{{cite journal |last=Bibring |first=Jean-Pierre |authorlink= |coauthors= |year=2006 |month= |title=Global Mineralogical and Aqueous Mars History Derived from OMEGA/Mars Express Data |journal=Science |volume=312 |issue=5772 |pages=400–404 |doi=10.1126/science.1122659 |url= |accessdate= |quote=|pmid=16627738 }}</ref>

*'''Phyllocian''' (named after the clay-rich [[:Category:Phyllosilicates|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 [[:Category:Sulfate minerals|sulfate minerals]] that were formed), lasting until about 3500 million years ago, was a period of [[vulcanism|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 [[peroxide]]s, leading to the red [[iron oxide]]s that give the planet its familiar color. The equivalent on Earth is most of the [[archean]] all of the [[proterozoic]] and up to now.

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==Surface chemistry==
[[Image:Sol1341B P2594 1 False L257 br.jpg|right|thumb|Mars' surface inside [[Victoria Crater]], thought to be original (predating the meteorite impact which created the crater). Photo taken by NASA's [[Opportunity rover]] on sol 1341 ([[Nov 13]] [[2007]]) Courtesy NASA/JPL-Caltech.]]
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.<ref name="basalt">{{cite web |url=http://erc.arc.nasa.gov/MarsVolc/basalt.htm |title=NASA Mars Page |work=Volcanology of Mars |accessdate = June 13 |accessyear = 2006 }}</ref> Analysis of the soil samples collected by the Viking landers in 1976 indicate iron-rich [[clay]]s consistent with weathering of basaltic rocks.<ref name="basalt"/> 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]] (Fe<sub>2</sub>O<sub>3</sub>) (rust).<ref name="Peplow2004">{{cite web |url=http://www.nature.com/news/2004/040503/full/news040503-6.html |doi=10.1038/news040503-6 |title=How Mars got its rust |accessdate=2006-04-18 |last=Peplow |first=Mark |coauthors= |date=2004-05-06 |work=Nature |publisher=}}</ref> 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&nbsp;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.<ref name="Peplow2004" /> 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 its [[pH]], and discovered traces of [[perchlorate]], which is the oxidizing ion ClO<sub>4</sub>.  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.<ref>{{cite web|url=http://news.yahoo.com/s/ap/20080805/ap_on_sc/phoenix_mars|title=Mars' soil may be toxic|accessdate=2008-08-05 | publisher = news.yahoo.com}}</ref><ref>{{cite web | title = NASA Spacecraft Analyzing Martian Soil Data | url = http://www.jpl.nasa.gov/news/phoenix/release.php?ArticleID=1816 | publisher = JPL|accessdate=2008-08-05}}</ref><ref>{{cite web |title=Phoenix Mars Team Opens Window on Scientific Process | date=2008-08-05 |accessdate=2008-08-06 | url=http://www.jpl.nasa.gov/news/phoenix/release.php?ArticleID=1819 }}</ref>

==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.<ref>[http://mgs-mager.gsfc.nasa.gov/ NASA Mars Global Surveyor]</ref><ref name="wander">
{{cite conference 
| author = Arkani-Hamed, Jafar; Boutin, Daniel 
| title = Polar Wander of Mars: Evidence from Magnetic Anomalies 
| booktitle = Sixth International Conference on Mars 
| publisher = Dordrecht, D. Reidel Publishing Co.
| date = July 20-25 2003 
| location = Pasadena, California 
| url = http://www.lpi.usra.edu/meetings/sixthmars2003/pdf/3051.pdf 
| format = PDF 
| accessdate = 2007-03-02 
}}</ref>  This [[Paleomagnetism]] of magnetically susceptible minerals has features very similar to the [[Plate tectonics#Explanation of magnetic striping|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&nbsp;Ga ago, before Mars' planetary [[Dynamo theory|dynamo]] ceased.<ref name="plates">[http://www.nasa.gov/centers/goddard/news/topstory/2005/mgs_plates.html "New Map Provides More Evidence Mars Once Like Earth"] - Oct. 12, 2005 [[Goddard Space Flight Center]] Press release. URL accessed March 17, 2006.</ref> The magnetization patterns in the crust also provide evidence of past polar wandering, the change in orientation of Mars' rotation axis.<ref name="wander"/><ref>
{{cite 
| author = Arkani-Hamed, Jafar; Boutin, Daniel 
| title = Paleomagnetic poles of Mars: Revisited 
| booktitle = Journal of Geophysical Research, Volume 109, Issue E3
| publisher = | date = March 2004 | location = Pasadena, California 
| url = doi:10.1029/2003JE002229 
| format =  
| accessdate = 
}}</ref>

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&nbsp;Ga, are unmagnetised, so the dynamo would have to have turned off before then otherwise the molten rock would have remagnetised.<ref>
{{cite web
  | last = 
  | first = 
  | authorlink =   | title =The Solar Wind at Mars  | work = 
  | publisher =NASA  | date =[[2001-01-13]]
  | url =http://science.nasa.gov/headlines/y2001/ast31jan_1.htm
  | accessdate = 2007-03-16 
}} </ref>  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&nbsp;Ga.<ref>David Shiga [http://space.newscientist.com/article/mg19826624.700 "Almighty smash left record crater on Mars"] ''New Scientist'' 25 June 2008</ref>  Computer models by Sabine Stanley show that this would have created a convection current powered dynamo in the southern hemisphere.<ref>Michael Brooks [http://space.newscientist.com/article/dn14827-giant-impact-explains-marss-wonky-magnetic-field.html "Giant impact explains Mars's wonky magnetic field"] ''New Scientist'' 26 September 2008</ref> 

===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&nbsp;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===
[[Image:Landslide on Mars.jpg|right|thumb|False colour view of a [[landslide]] in [[Zunil (crater)|Zunil crater]]]]
The core is surrounded by a silicate [[Mantle (geology)|mantle]] that formed many of the tectonic and volcanic features on the planet. The average thickness of the planet's crust is about 50&nbsp;km, and it is no thicker than 125&nbsp;km,<ref> 
{{cite news 
|author = Dave Jacqué 
|url = http://www.anl.gov/Media_Center/News/2003/030926mars.htm 
|title = APS X-rays reveal secrets of Mars' core 
| publisher = Argonne National Laboratory 
| date = 2003-09-26 
| accessdate = 2006-07-01 
}}</ref> which is much thicker than Earth's crust which varies between 5&nbsp;km and 70&nbsp;km. A recent radar map of the south polar ice cap showed that it does not deform the crust despite being about 3&nbsp;km thick.<ref name="radar">{{cite web
| last =Dunham  
| first =Will
| authorlink =Will Dunham   
| title =Immense ice deposits found at south pole of Mars   
| work =Yahoo! News   
| publisher =Yahoo!, Inc.
| date =[[2007-03-15]]
| url =http://news.yahoo.com/s/nm/20070315/sc_nm/mars_water_dc_2   
| accessdate = 2007-03-16 }}</ref>

===Tectonics===
As a result of 1999 observations of the [[magnetic field]]s 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 [[Martian dichotomy|crustal dichotomy]] boundary’ in the [[Aeolis (Mars)|Aeolis Mensae]] region.<ref name="aeolis">{{cite web
| last =  
| first =
| authorlink =   
| title =  Tectonic signatures at Aeolis Mensae
| work =ESA News   
| publisher =European Space Agency   
| date =[[2007-06-28]]
| url =http://www.esa.int/SPECIALS/Mars_Express/SEMF399OY2F_0.html   
| accessdate = 2007-06-28 }}</ref>

==Hydrology==

{{Main|Water on Mars}}
{{See|Atmosphere of Mars}}
[[Image:History of water on Mars.jpeg|thumb|right]]

===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.<ref>{{cite journal |last=Malin |first=Michael C. |authorlink= |coauthors=Edgett, Kenneth S. |year= 2000|month= |title=Evidence for Recent Groundwater Seepage and Surface Runoff on Mars |journal=Science |volume=288 |issue=5475 |pages=2330–2335 |doi=10.1126/science.288.5475.2330 |url= |accessdate= |quote=|pmid=10875910 }}</ref> 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.<ref name="Jakosky1999">{{cite journal |last=Jakosky |first=Bruce M. |authorlink= |coauthors= |year=1999 |month= |title=Water, Climate, and Life |journal=Science |volume=283 |issue=5402 |pages=648–649 |doi=10.1126/science.283.5402.648 |url= |accessdate= |quote=|pmid=9988657 }}</ref>

On the other hand, evidence in favor of heavy or even catastrophic flooding is found in the giant ripples in the [[Athabasca Vallis]] [http://www.msss.com/mars_images/moc/2004/09/27/].

===Liquid water===
[[Image:07-ml-3-soil-mosaic-B019R1 br.jpg|thumb|right|Mosaic shows some [[Martian spherules|spherules]] partly embedded.]]
Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods.<ref>{{Citation 
| journal = Journal of Geophysical Research
| date= 2005-05-07
| last= Heldmann et al.
| first= Jennifer L.
| title= Formation of Martian gullies by the action of liquid water flowing under current Martian environmental conditions
| url= http://daleandersen.seti.org/Dale%20Andersen/Articles_files/Heldmann%20et%20al.2005.pdf
| format= {{Dead link|date=June 2008}} &ndash; <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3AHeldmann+et+al.+intitle%3AFormation+of+Martian+gullies+by+the+action+of+liquid+water+flowing+under+current+Martian+environmental+conditions&as_publication=Journal+of+Geophysical+Research&as_ylo=&as_yhi=&btnG=Search Scholar search]</sup>| volume=110
| pages=Eo5004
| doi=10.1029/2004JE002261
| accessdate=2007-08-12}} 'conditions such as now occur on Mars, outside of the temperature-pressure stability regime of liquid water' … 'Liquid water is typically stable at the lowest elevations and at low latitudes on the planet because the atmospheric pressure is greater than the vapor pressure of water and surface temperatures in equatorial regions can reach 273 K for parts of the day [Haberle et al., 2001]'</ref><ref>{{Citation 
| journal=Geophysical Research Letters
| volume = 33
| pages = L11201
| date=[[June 3]], [[2006]]
| last=Kostama
| first=V.-P.
| last2=Kreslavsky
| first2=M. A.
| last3=Head
| first3=J. W.
| title=Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement
| url=http://www.agu.org/pubs/crossref/2006/2006GL025946.shtml
| doi=10.1029/2006GL025946
| accessdate=2007-08-12}} 'Martian high-latitude zones are covered with a smooth, layered ice-rich mantle'</ref> Recently, there has been evidence to support the popular belief that liquid water flowed on the surface in the recent past, with the discovery of gully deposits that were not seen ten years ago.<ref>[http://www.jpl.nasa.gov/news/news.cfm?release=2006-145 JPL news release 2006-145]</ref><ref> {{cite journal|title=Present-Day Impact Cratering Rate and Contemporary Gully Activity on Mars |journal=Science|date=8 December 2006|first=Michael C.|last=Malin|coauthors=Kenneth S. Edgett, Liliya V. Posiolova, Shawn M. McColley, Eldar Z. Noe Dobrea |volume=314|issue=5805|pages=1573 - 1577|id= {{doi|10.1126/science.1135156}}|url=http://www.sciencemag.org/cgi/content/abstract/314/5805/1573|format=|accessdate=2009-09-03 }}</ref>
 
Among the findings from the ''[[Opportunity rover|Opportunity]]'' rover is the presence of [[hematite]] on Mars in the form of [[Martian spherules|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 (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<ref>Benison, Kathleen C. and LaClair, Deidre, 2003, Astrobiology, v. 3, p. 609-618.</ref><ref>Benison, Kathleen C. and Bowen, Brenda B., 2006, Icarus, v. 183, p. 225-229.</ref>. 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 (Mars)|Columbia Hills]]".
[[Image:nasa mars opportunity rock water 150 eng 02mar04.jpg|thumb|left|Photo of Microscopic rock forms indicating past signs of water, taken by ''[[Opportunity rover|Opportunity]]'']]

On [[July 31]], [[2008]], NASA announced that the [[Phoenix (spacecraft)|Phoenix lander]] confirmed the presence of water ice on Mars,<ref>{{cite web
 | last = Johnson | first = John 
 | title = There's water on Mars, NASA confirms
 | work = Los Angeles Times
 | publisher = 
 | date = 2008-08-01
 | url = http://www.latimes.com/news/science/la-sci-phoenix1-2008aug01,0,3012423.story
 | accessdate = 2008-08-01}}</ref>
as predicted on 2002 by the [[2001 Mars Odyssey|Mars Odyssey]] orbiter.

===Polar ice caps===
[[Image:Martian north polar cap.jpg|thumb|The [[Mars Global Surveyor]] acquired this image of the Martian north polar ice cap in early northern summer.]]
Both the the northern polar cap ([[Planum Boreum]]) and the southern polar cap ([[Planum Australe]]) are believed to grow in thickness during the winter and partially [[sublimation|sublime]] during the summer.  Data obtained by the [[Mars Express]] satellite, made it possible in 2004 to confirm that the southern polar cap has an average of {{convert|3|km|mi}} thick slab of CO<sub>2</sub> ice<ref name=NASAwater >{{cite news | first= | last= | coauthors= |authorlink= | title=Mars' South Pole Ice Deep and Wide | date=March 15, 2007 | publisher=NASA | url =http://jpl.nasa.gov/news/news.cfm?release=2007-030 | work =Jet Propulsion Laboratory | pages = | accessdate = 2009-09-11 | language = }}</ref> with varying contents of frozen water, depending on its latitude; the polar cap is a mixture of 85% CO<sub>2</sub> ice and 15% water ice.<ref name=ESAwater > {{cite news | first= | last= | coauthors= |authorlink= | title=Water at Martian south pole | date=17 March 2004 | publisher=European Space Agency (ESA) | url =http://www.esa.int/SPECIALS/Mars_Express/SEMYKEX5WRD_0.html | work = | pages = | accessdate = 2009-09-11 | language = }}</ref>  The second part comprises steep slopes known as 'scarps', made almost entirely of water ice, that fall away from the polar cap to the surrounding plains.<ref name=ESAwater /> The third part encompasses the vast permafrost fields that stretch for tens of kilometres away from the scarps.<ref name=ESAwater /><ref>{{Citation 
| journal=Geophysical Research Letters
| volume = 33
| pages = L11201
| date=[[June 3]], [[2006]]
| last=Kostama
| first=V.-P.
| last2=Kreslavsky
| first2=M. A.
| last3=Head
| first3=J. W.
| title=Recent high-latitude icy mantle in the northern plains of Mars: Characteristics and ages of emplacement
| url=http://www.agu.org/pubs/crossref/2006/2006GL025946.shtml
| doi=10.1029/2006GL025946
| accessdate=2008-08-01}}</ref>  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.<ref>{{cite web
 | publisher = NASA
 | date=[[March 15]], [[2007]]
 | title = Mars' South Pole Ice Deep and Wide
 | url= http://jpl.nasa.gov/news/news.cfm?release=2007-030
 | accessdate = 2009-09-13 }}</ref>

===Ice patches===
On [[July 28]] [[2005]], the [[European Space Agency]] announced the existence of a crater partially filled with frozen water;<ref name="lake">[http://www.esa.int/SPECIALS/Mars_Express/SEMGKA808BE_0.html "Water ice in crater at Martian north pole"] - July 27, 2005 [[ESA]] Press release. URL accessed March 17, 2006.</ref> some then interpreted the discovery as an "ice lake".<ref name="BBClake">[http://news.bbc.co.uk/2/hi/science/nature/4727847.stm "Ice lake found on the Red Planet"] - July 29, 2005 [[BBC]] story. URL accessed March 17, 2006.</ref> 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&nbsp;km wide and about 2&nbsp;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&nbsp;m to 30&nbsp;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 elsewhere 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.<ref name="Murray2007">{{cite journal |last=Murray |first=John B. |authorlink= |coauthors=''et al.'' |year=2005 |month= |title=Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars' equator |journal=Nature |volume=434 |issue= |pages=352–356 |doi=10.1038/nature03379 |url= |accessdate= |quote= }}</ref>

===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&nbsp;Ga old shoreline is that it is not flat — i.e. does not follow a line of constant graviational potential. However a 2007 ''[[Nature (journal)|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.<ref name="Zuber2007">{{cite journal |last=Zuber |first=Maria T. |authorlink= |coauthors= |year=2007 |month= |title=Mars at the tipping point |journal=Nature |volume=447 |issue=7146 |pages=785–786 |doi=10.1038/447785a |url= |accessdate= |quote= }}</ref>  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. <ref name=Baker>Baker, V. R., R. G. Strom, V. C. Gulick, J. S. Kargel, G. Komatsu and V. S. Kale, 1991: Ancient oceans, ice sheets and the hydrological cycle on Mars, Nature, 352, 589-594.</ref>

=== Geysers on Mars===
{{main|Geysers on Mars}}
[[File:Geysers on Mars.jpg|thumb|200px|Artist concept showing sand-laden jets erupt from geysers on Mars. (publishd by NASA; artist: [[Ron Miller (artist and author)|Ron Miller]].]] 
The seasonal frosting and defrosting of the southern ice cap results in the formation of spider-like radial channels carved between the ground and 1 meter thick ice. Then, sublimed CO<sub>2</sub> -and probaby water- increase pressure in their interior producing geyser-like eruptions of cold fluids often mixed with dark basaltic sand or mud.<ref name=2006-100 > {{cite news | first= | last= | coauthors= |authorlink= | title=NASA Findings Suggest Jets Bursting From Martian Ice Cap | date=August 16, 2006 | publisher=NASA | url =http://www.jpl.nasa.gov/news/news.cfm?release=2006-100 | work =Jet Propulsion Laboratory | pages = | accessdate = 2009-08-11 | language = }}</ref><ref name=Kieffer2000 >{{Citation| first=H. H. | last=Kieffer| coauthors=| contribution=ANNUAL PUNCTUATED CO2 SLAB-ICE AND JETS ON MARS.| title=Mars Polar Science 2000| editor-first=| editor-last=| coeditors=| publisher=| place=| pages=| date=| year=2000| id= | contribution-url=http://www.lpi.usra.edu/meetings/polar2000/pdf/4095.pdf| format=PDF| accessdate=2009-09-06 }}</ref><ref name=Portyankina > {{Citation| first= | last=| coauthors=| contribution=SIMULATIONS OF GEYSER-TYPE ERUPTIONS IN CRYPTIC REGION OF MARTIAN SOUTH| title=Fourth Mars Polar Science Conference| editor-first=G. Portyankina| editor-last=| coeditors=| publisher=| place=| pages=| date=| year=2006| id= | contribution-url=http://www.lpi.usra.edu/meetings/polar2006/pdf/8040.pdf| format=PDF| accessdate=2009-08-11 }}</ref><ref name=Hugh2006 > {{cite journal|title=CO2 jets formed by sublimation beneath translucent slab ice in Mars' seasonal south polar ice cap|journal=Nature|date=30 May 2006|first=Hugh H.|last=Kieffer|coauthors=Philip R. Christensen and Timothy N. Titus|volume=442|issue=|pages=793-796 |id= {{doi|doi:10.1038/nature04945}}|url=http://www.nature.com/nature/journal/v442/n7104/abs/nature04945.html|format=|accessdate=2009-09-02 }}</ref>  This process is rapid, observed happening in the space of a few days, weeks or months, a growth rate rather unusual in geology - especially for Mars.


===Glacial phases===
[[Image:Mars glacial-like lobe deposit.jpg|thumb|left|Perspective view of a 5-km-wide, glacial-like lobe deposit sloping up into a box canyon along the crustal dichotomy boundary on Mars.]]
A 2008 study provided evidence for multiple glacial phases during Late Amazonian glaciation at the [[Martian dichotomy|dichotomy]] boundary on Mars.<ref name="Dickson2008">{{cite journal |last=Dickson |first=James L. |authorlink= |coauthors=Head, James W.; Marchant, David R. |year=2008 |month= |title=Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases |journal=[[Geology (journal)|Geology]] |volume=36 |issue=5 |pages=411–414 |doi=10.1130/G24382A.1 |url= |accessdate= |quote= }}</ref>

==Olivine==
[[Image:Olivine valles marineris.jpg|thumb|right|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 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&nbsp;Ga (the [[Noachian epoch]]). Another outcrop is in the [[Ganges Chasma]], an eastern side chasm of the [[Valles Marineris]] (pictured).<ref>
{{cite web 
| title = Pretty Green Mineral -- Pretty Dry Mars?
| author = Linda M.V. Martel
| publisher = psrd.hawaii.edu
| url = http://www.psrd.hawaii.edu/Nov03/olivine.html
| accessdate = 2007-02-23
}}</ref>

==Impact crater morphology==
[[Image:Mars rampart crater.jpg|thumb|left|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 crater]]s 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.<ref>{{cite web | title = Stones, Wind and Ice
               | author = Nadine Barlow
            | publisher = Lunar and Planetary Institute
                  | url = http://www.lpi.usra.edu/publications/slidesets/stones/
           | accessdate = 2007-03-15}}</ref>

The [[Yuty Crater|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.<ref>{{cite web | title = Viking Orbiter Views Of Mars 
               | author = 
            | publisher = NASA
                  | url = http://history.nasa.gov/SP-441/ch7.htm
           | accessdate = 2007-03-16}}</ref>

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.[http://web.mit.edu/newsoffice/2008/mars-basin-0625.html]

==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 {{convert|2300|ft|m|sing=on}} high cliff.  Evidence of the avalanche are shown by the dust clouds rising from the cliff afterwards.<ref>[http://www.discoverychannel.ca/reports/article.aspx?aid=7418 DiscoveryChannel.ca - Mars avalanche caught on camera<!-- Bot generated title -->]</ref>  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.<ref>
{{cite web 
| title = Newly-Formed Slope Streaks  
| author = 
| publisher = NASA
| url = http://hiroc.lpl.arizona.edu/images/PSP/diafotizo.php?ID=PSP_002396_1900 
| accessdate = 2007-03-16
}}</ref>

==Mars Avalanche Photo Gallery==
<gallery>
Image:Mars Avalanche 2.jpg |Image of the February 19, 2008 Mars avalanche captured by the Mars Reconnaissance Orbiter.
Image:Mars Avalanche Hirise.jpg|Closer shot of the avalanche.
Image:Mars Avalanche Dust Clouds.jpg |Dust clouds rise above the {{convert|2300|ft|m|sing=on}} deep cliff.
Image: Mars Avalanche with Scale.jpg |A photo with scale demonstrates the size of the avalanche. 
</gallery>

==See also==
*[[Dark dune spots]]
*[[Elysium Planitia]]
*[[Geography of Mars]]
*[[Hecates Tholus]]
*[[Life on Mars]]
*[[Martian dichotomy]]
*[[Scientific information from the Mars Exploration Rover mission]]

== References ==
{{reflist}}

== External links ==
*[http://www.lpi.usra.edu/resources/mars_maps/1083/index.html Geologic Map of Mars]
*[http://www.impacttectonics.org/SyriaPlanum.html Oblique-impact complex on Mars including Syria Planum and Sinai Planum]
{{Mars}}

{{DEFAULTSORT:Geology Of Mars}}
[[Category:Mars]]
[[Category:Planetary geology]]
[[Category:Science timelines]]
[[Category:Graphical timelines]]