Mathematics – Probability
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p43d1455k&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P43D-1455
Mathematics
Probability
[5420] Planetary Sciences: Solid Surface Planets / Impact Phenomena, Cratering, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
Recent observations constrain the formation of 100 new impact sites on Mars over the past decade; 19 of these were found using the Mars Global Surveyor Mars Orbiter Camera (MOC), and the other 81 have been identified since 2006 using the Mars Reconnaissance Orbiter Context Camera (CTX). Every 6 meter/pixel CTX image is examined upon receipt and, where they overlap images of 0.3-240 m/pixel scale acquired by the same or other Mars-orbiting spacecraft, we look for features that may have changed. New impact sites are initially identified by the presence of a new dark spot or cluster of dark spots in a CTX image. Such spots may be new impact craters, or result from the effect of impact blasts on the dusty surface. In some (generally rare) cases, the crater is sufficiently large to be resolved in the CTX image. In most cases, however, the crater(s) cannot be seen. These are tentatively designated as “candidate” new impact sites, and the CTX team then creates an opportunity for the MRO spacecraft to point its cameras off-nadir and requests that the High Resolution Imaging Science Experiment (HiRISE) team obtain an image of ~0.3 m/pixel to confirm whether a crater or crater cluster is present. It is clear even from cursory examination that the CTX observations are areographically biased to dusty, higher albedo areas on Mars. All but 3 of the 100 new impact sites occur on surfaces with Lambert albedo values in excess of 23.5%. Our initial study of MOC images greatly benefited from the initial global observations made in one month in 1999, creating a baseline date from which we could start counting new craters. The global coverage by MRO Mars Color Imager is more than a factor of 4 poorer in resolution than the MOC Wide Angle camera and does not offer the opportunity for global analysis. Instead, we must rely on partial global coverage and global coverage that has taken years to accumulate; thus we can only treat impact rates statistically. We subdivide the total data set of 100 sites into 3 sets of observations: the original 19 MOC observations found in a survey of 15% of the planet, craters found only in CTX repeat coverage of 7% of Mars, and the remaining 69 craters found in a data set covering 40% of the planet. Using the mean interval between the latest observation preceding the impact and the first observation showing the impact for these groups of craters, we determine that the cratering rate is roughly 8 ± 6 x 10-7 craters/km2/yr for craters greater than ~1 m diameter. The cratering rate on Mars is sufficiently high to warrant consideration both for scientific studies and as a hazard to future exploration. Impacts are sufficiently frequent to act as seismic sources for studies of shallow crustal structure, if a seismic network is sufficiently dispersed and long-lived. Impacts large enough to provide information about deep interior structure are rare but probably occur on a decadal timescale. As recently noted in Science, new craters can be used to probe the distribution of subsurface ice and to provide samples from shallow depths that otherwise require meter-scale drilling systems. There is a finite probability that visitors to Mars for more than a month or two will hear or feel the effects of a nearby impact.
Kennedy Robert M.
Malin Michael C.
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