Physics
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p41a0893z&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P41A-0893
Physics
5415 Erosion And Weathering, 5420 Impact Phenomena (Includes Cratering), 6225 Mars, 3344 Paleoclimatology, 1869 Stochastic Processes
Scientific paper
Williams et al (LPSC 2004; see also Kerr, Science vol. 304, 196 ) recently reported the discovery of a spectacularly eroded, apparently water-carved impact crater on Mars. The ~60 km diameter crater, dubbed "Mojave Crater" for the resemblance of its alluvial fans to alluvial fans seen on Earth, is located on Xanthe Terra, Mars. The crater is rated as late Hesperian or Amazonian, meaning that the impact took place some time in the past 3.5 billion years; there is some evidence that it is not extraordinarily young. Yet the crater seems unique for its size. This crater, and to present knowledge this crater alone, is the obvious source of its own precipitation. Impact by a live, ice-rich comet may provide a unique origin for this unique crater. Impacts by live comets are surprisingly rare in the inner solar system. To make a 60 km crater on Mars requires either a ~6 km diameter nearly isotropic comet (NIC), or an ~10 km diameter Jupiter-family comet (JFC). NICs strike Mars about 3 times in 10 billion if they pass its orbit. The latter happens about 6 times per decade for NICs bigger than 6 km, so that at current rates a NIC makes a 60 km crater on Mars about once every 5 billion years. The typical Mars-crossing JFCs actually hits Mars at a rate of about 8e-11 per comet per year. There are currently 3 known Mars-crossing JFCs that may be big enough to make 60 km craters on Mars (the nearly extinct P/28 Neujmin 1, P/10 Tempel 2, and perhaps P/49 Arend-Rigaux). The rate is about once per 4 billion years. Thus we expect on the order of one 60 km martian impact crater in the past 3.5 Gyr made by a comet. There would be on the order of ten 30 km craters. Preliminary GCM simulations of comet impacts indicate that, while very large impact events generate global climate effects that can lead to precipitation in topographically favored locations anywhere on Mars, on the scale of 60 km craters the impact-generated rainfall becomes localized, such that, to first approximation, a uniquely wet impact might be expected to create uniquely heavy local precipitation on scales comparable to the impact crater.
Colaprete Anthony
Zahnle Kevin
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