Other
Scientific paper
Feb 1968
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1968saosr.268.....c&link_type=abstract
SAO Special Report #268 (1968)
Other
5
Scientific paper
A catalog of nearly 300 craters and crater-like objects has been prepared from several sets of contrast-enhanced high-quality positive transparencies of the Mariner 4 photography. Craters were identified and counted by the same procedures used in the compilation of lunar crater catalogs; particular attention was given to crater class and quality. Counts of craters with diameters D < 20 km begin to show the effects of incompleteness. A direct inspection of the photographs as well as the constancy of crater class proportions with crater diameter interval above 20 km indicates that substantial erosion and obliteration of all but the largest craters have occurred during the history of Mars. The epochs of crater formation and crater erosion appear to be closely tied together in time. A statistical curve-fitting program for the observed crater diameter-frequency relations and a differential number-density distribution law of AD-B give B = 2.5 +/- 0.2 for D > 20 km or B = 3.0 +/- 0.2 D > 30 km. The population of impacting objects assumed responsible for these craters is taken as having a differential number density varying as X-β, where X is the diameter of the impacting object. The number of "live" comets and Apollo objects crossing the orbit of Mars is insufficient by more than 2 orders of magnitude to explain the observed number density of craters on Mars. For asteroidal objects with β = 2 or 3, the predicted and observed number densities cannot be brought into agreement unless we assume an early epoch of very high cratering rates on Mars. For β = 4 or 5, agreement between the predicted and observed number densities can be secured with a nearly uniform rate of asteroidal bombardment. The absence of saturation bombardment of Mars for very large craters points to a value of β significantly above 3. This is not inconsistent with expectations for asteroids with X > 1 km in the inner part of the asteroid b belt. In this case, crater diameter scales with kinetic energy, W, of the impacting object as W⅓. For all reasonable values of β, impact damage contributes to crater erosion and obliteration. The fraction of the surface covered by craters is of the order unity. For all reasonable values of β, asteroidal bombardment is capable of accounting quantitatively for the observed values of both A and B, particularly if the zone of obliteration around Martian craters is larger than that for lunar craters. For near saturation bombardment, the existing observations are of very little use in determining the value of β, or in distinguishing between saturation bombardment and such other erosion mechanisms as windblown dust of impact or of micrometeoric origin; liquid water on macro or microscales; mountain building; and flooding by lava. The dust produced by impact during the history of Mars is estimated to have depths between 0.1 and several kilometers. The diameter of the largest crater obliterated in 4.5 X 109 years is calculated to be between 60 and 180 km; for the lifetime against erosion, assumed to scale as Dα, we calculate α~0 for β = 2 or 3, and 1 <= α <= 2.5 for β = 4 or 5. For β < 3, the mean ages of Martian craters are found to be approximately equal to the age of the planet. However, for β significantly larger than 3, different craters will have different mean ages, ranging from about 2.25 X 109 years for the very largest craters, down to some tens of millions of years or less for craters smaller than 20 km. In this case, surface features of the order of 10 km in width or smaller may have been quite prominent in the early history of Mars and undetectable on the Mariner 4 photographs. Thus, the absence of such signs of running water as river valleys in the Mariner 4 photography is quite irrelevant to the question of the existence of bodies of water in early Martian history. These conclusions on ages are independent of estimates of the ages of lunar maria. Some weak evidence exists for a correlation between high crater density and dark areas on Mars.
Chapman Clark R.
Pollack James B.
Sagan Carl
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