Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.u21a0013h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #U21A-0013
Physics
5420 Impact Phenomena, Cratering (6022, 8136), 6235 Mercury
Scientific paper
For small (D < 10 km) craters on Mercury, shadow measurements and photoclinometry are the only viable methods for assessing crater rim-floor depth (d) with the Mariner and MESSENGER Flyby data. A limitation of past shadow measurements was the requirement for simple craters that the shadow pass through crater center in order to get a reliable depth estimate. This restriction means that shadow measurements from the Mariner 10 data were taken along the two narrow longitude bands with acceptable sun angles. Recently, Chappelow (LPSC 2008, Abs. #1441) developed a generalized shadow method that allows determination of crater shape and rim-floor depth for any crater with a conic section of revolution. The boundary of the interior shadow defines a portion of an ellipse. The shape of that ellipse and its offset relative to crater center can be used to solve for crater depth and interior shape (cone, ellipse, parabola, hyperbola). The method does not require that the shadow cross the crater center, but it does require that the viewing angle is close enough to nadir that the rim outline is circular. We are using this method with MESSENGER and Mariner data to expand the amount and areal coverage of depth data for small craters, and to determine the interior shapes of those craters. So far we have estimated depths and shapes for 133 craters (1.0 km < D < 8.0 km) in twelve of the frames from MESSENGER Flyby 1 NAC Mosaic #1 (images range from 117 to 150 m/pixel). Mean d/D is 0.17 (sd 0.04) with a range from 0.09 to 0.29. An exponential fit (in km) is d = (0.178±0.08) D0.89±0.04. Pike's previous results with Mariner data (Mercury, UA Press, 165- 273, 1988) show a unity exponent with a d/D ratio of 0.2, and are reasonably consistent with our work. The shapes of the crater interiors are slightly more conical than parabolic. We have observed some spatial clusters of craters with d/D ratios significantly different than the global mean, and we are investigating whether these areas correlate with a particular geomorphology.
Chappelow John E.
Herman M. W.
Herrick Robert R.
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