Chasma is a term used in astrogeology to refer to "a deep, elongated, steep-sided depression". The plural is chasmata. An example is Eos Chasma on Mars. Below are images of some of the major Chasma of Mars. The map shows their relative locations. Some of the details aboput Chasma are described below.

Interior layered deposits and sulfate

Parts of the floor of Candor Chasma contains layered deposits that have been termed interior layered deposits (ILD's). These layers may have formed when the whole area was a giant lake.Some places on Mars contain hydrated sulfate deposits. Sulfate formation involves the presence of water. The European Space Agency's Mars Express found evidence of perhaps epsomite and kieserite. Scientists want to visit these areas with robotic rovers.

Layers

Images of rocks in the canyon walls almost always show layers. Some layers appear tougher than others. In the image below of Ganges Chasma Layers, as seen by HiRISE, one can see that the upper, light-toned deposits are eroding much faster than the lower darker layers. Some cliffs on Mars show a few darker layers standing out and often breaking into large pieces; these are thought to be hard volcanic rock instead of soft ash deposits. An example of hard layers is shown below in the picture of layers in the canyon wall in Coprates, as seen by Mars Global Surveyor. Because of its closeness to the Tharsis volcanic region, the rock layers may be made of layer after layer of lava flows, probably mixed with deposits of volcanic ash that fell out of the air following big eruptions. It is likely the rock strata in the walls preserve a long geological history of Mars. Dark layers may be due to dark lava flows. The dark volcanic rock basalt is common on Mars. However, light-toned deposits may have resulted from rivers, lakes, volcanic ash, or wind blown deposits of sand or dust. The Mars Rovers found light-toned rocks to contain sulfates. Probably having been formed in water, sulfate deposits are of great interest to scientists because they may contain traces of ancient life.

Hebes Chasma and hydrated deposits

Hebes Chasma, a large enclosed valley, may have once held water. Hydrated minerals have been found there. It is thought that large-scale underground springs of groundwater at different times burst to the sufrace to form deposits called Light Toned Deposits (LTD's). Some suggest present or fossilized life forms may be found there because the deposits are relatively young.

Image:Coprates map.JPG|Map of Coprates quadrangle showing details of Valles Marineris, the largest canyon system in the solar system. Some of the canyons may have once been filled with water. The map shows the locations of a number of major Chasma.

Image:Candor Chasm in Coprates.JPG|False color image of Candor Chasma showing locations of hydrated sulfate deposits, as seen by THEMIS. Red colors show rocky places. Greens and blues show sandy, dusty areas.

Image:Melas Chasma.JPG|Melas Chasma, as seen by THEMIS. Click on image to see relationship of Melas to other features.

Image:Cliff in Candor Chasma.JPG|Cliff in Candor Chasma Plateau, as seen by THEMIS. Click on image to see relationship with other features in Coprates quadrangle.

Image:Ganges Chasma.JPG|Cliff in northern wall of Ganges Chasma, as seen by THEMIS. Click on image to see realtionship with other features in the Coprates quadrangle.

Image:Ophir Chasma Wall.JPG|Ophir Chasma Wall, as seen by HiRISE.

Image:Ius Chasma.JPG|Ius Chasma, as seen by HiRISE. Click on image to see layers.

Image:Tithonium Chasma Layers.JPG|Tithonium Chasma Layers, as seen by HiRISE.

Image:Ganges Chasma Layers.JPG|Ganges Chasma Layers, as seen by HiRISE.

References

1. USGS Astrogeology: Gazetteer of Planetary Nomenclature - Feature Types
2. http://themis.asu.edu/features/candorchasma
3. http://themis.asu.edu/features/coprateschasma
4. http://hirise.lpl.arizona.edu/PSP_005385_1640
5. http://hirise,lpl.arizona.edu/PSP_007430_1725
6. http:www.universetoday.com/2008/12/11/Groundwater May Have Played Important Role in Shaping Mars