Global InformationRampart crater information
Rampart craters are a specific type of impact crater which are accompanied by distinctive fluidized ejecta features found mainly on Mars. Only one example is known on Earth, the Nördlinger Ries impact structure in Germany.[1] A rampart crater displays an ejecta with a low ridge along its edge. Usually, rampart craters show a lobate outer margin, as if material moved along the surface, rather than flying up and down in a ballistic trajectory. The flows sometimes are diverted around small obstacles, instead of falling on them. The ejecta look as if they move as a mudflow. Some of the shapes of rampart craters can be duplicated by shooting projectiles into mud. Although rampart craters can be found all over Mars, the smaller ones are only found in the high latitudes where ice is predicted to be close to the surface. It seems that the impact has to be powerful enough to penetrate to the level of the subsurface ice. Since ice is thought to be close to the surface in latitudes far from the equator, it does not take a large impact to reach the ice level.[2] Based on images from the Viking program in the 1970s, it is generally accepted that rampart craters are evidence of ice or liquid water beneath the surface of Mars. The impact melts or boils the water in the subsurface producing a distinctive pattern of material surrounding the crater.
Ryan Schwegman described double layered ejecta (DLE) craters as showing two distinct layers of ejecta that appear to have been put in place as a mobile, ground-hugging flow. His measurements suggest that ejecta mobility (the distance ejecta travels from the crater rim) typically goes up with increasing latitude and may reflect ice concentration. That is the higher the latitude, the greater the ice content. The lobateness (curved shape of the perimeter of ejecta) usually goes down with increasing latitude. Furthermore, DLEs on sedimentary ground seem to display higher ejecta mobility than those on volcanic surfaces.[3]
A detailed discussion of various kinds of Martian craters, including double-layer ejecta craters (rampart craters) can be found in a 2014 paper by David Weiss and James Head.[4]
- ^ Sturm, Sebastian; Wulf, Gerwin; Jung, Dietmar; Kenkmann, Thomas (2013). "The Ries impact, a double-layer rampart crater on Earth". Geology. 41 (5): 531–534. Bibcode:2013Geo....41..531S. doi:10.1130/G33934.1.
- ^ Hugh H. Kieffer (1992). Mars. University of Arizona Press. ISBN 978-0-8165-1257-7. Retrieved 7 March 2011.
- ^ Schwegman, R. 2015. MORPHOLOGY AND MORPHOMETRY OF DOUBLE LAYERED EJECTA CRATERS ON MARS. The School of Graduate and Postdoctoral Studies The University of Western Ontario London, Ontario, Canada.
- ^ Weiss, D., J. Head. 2014. Ejecta mobility of layered ejecta craters on Mars: Assessing the influence of snow and ice deposits. Icarus: 233, 131–146.