Impact craters generally have a rim with ejecta around them, in contrast volcanic craters usually do not have a rim or ejecta deposits. As craters get larger (greater than 10 km in diameter) they usually have a central peak. The peak is caused by a rebound of the crater floor following the impact.
Baltisk Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Dark areas are dunes. Fans are visible in the lower right.
Many craters once contained lakes. Because some crater floors show deltas, we know that water had to be present for some time. Dozens of deltas have been spotted on Mars. Deltas form when sediment is washed in from a stream entering a quiet body of water. It takes a bit of time to form a delta, so the presence of a delta is exciting; it means water was there for a time, maybe for many years. Primitive organisms may have developed in such lakes; hence, some craters may be prime targets for the search for evidence of life on the Red Planet.
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- Hugh H. Kieffer (1992). Mars. University of Arizona Press. ISBN 978-0-8165-1257-7. Retrieved 7 March 2011.
- Cabrol, N. and E. Grin. 2001. The Evolution of Lacustrine Environments on Mars: Is Mars Only Hydrologically Dormant? Icarus: 149, 291-328.
- Fassett, C. and J. Head. 2008. Open-basin lakes on Mars: Distribution and implications for Noachian surface and subsurface hydrology. Icarus: 198, 37-56.
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- Wilson, J. A. Grant and A. Howard. 2013. Inventory of Equatorial Alluvial Fans and Deltas on Mars. 44th Lunar and Planetary Science Conference.
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