| Martian Geologic Timescale |
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TIMELINE The Martian Geologic Timescale (or perhaps more properly the ''areologic timescale'') has three broad Epoch s defined by the number of Impact Crater s 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:
ImageSize = width:800 height:100 PlotArea = left:15 right:15 bottom:20 top:5 AlignBars = early Period = from:-3800 till:0 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:500 start:-3500 ScaleMinor = unit:year increment:100 start:-3800 PlotData= align:center textcolor:black fontsize:8 mark:(line,black) width:25 shift:(0,-5) text:Amazonian from:-1800 till:0 text:Hesperian from:-3500 till:-1800 text:Noachian from:start till:-3500 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. Much of the surface is deeply covered by dust as fine as talcum powder. MAGNETIC FIELD Observations of the Press release. URL accessed March 17, 2006. WATER Liquid water partly embedded.]] Amongst 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". '']] Polar ice caps Mars has polar ice caps that contain frozen water "Water at Martian south pole" - March 17, 2004 Press release. URL accessed March 17, 2006. Ice patches On story. URL accessed March 17, 2006.. 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. SEE ALSO REFERENCES |