Bolides in the present and past martian atmosphere and effects on cratering processes

We investigate the action of the martian atmosphere on entering meteoroids for present and past atmospheres with various surface pressures to predict the smallest observable craters, and to understand the implications for the size distributions of craters on Mars and meteoroids in space. We assume different strengths appropriate to icy, stone, and iron bodies and test the results against available data on terrestrial bolides. Deceleration, ablation, and fragmentation effects are included. We find that the smallest icy, stone, and iron meteoroids to hit the martian ground at crater forming speeds of greater than or equal to500 m/s have diameters of about 2 in, 0.03-0.9 in (depending on strength), and 0.01 in, respectively, in the current atmosphere. For hypothetical denser past atmospheres, the cutoff diameters rise. At a surface pressure of 100 mb, the cutoff diameters are about 24 in, 5-12 in, and 0.14 in for the 3 classes. The weaker stony bodies in the size range of about 1-30 in may explode at altitudes of about 10-20 kin above the ground. These figures imply that under the present atmosphere, the smallest craters made by these objects would be as follows: by ice bodies, craters of diameter (D) similar to8 in, by stones about 0.5-6 in, and by irons, about 0.3 in. A strong depletion of craters should, thus, occur at diameters below about 0.3 in to 5 in. Predicted fragmentation and ablation effects on weak meteoroids in the present atmosphere may also produce a milder depletion below D similar to500 in, relative to the lunar population. But, this effect may be difficult to detect in present data because of additional losses of small craters due to sedimentation, dunes, and other obliteration effects. Craters in strewn fields, caused by meteoroid fragmentation, will be near or below present-day resolution limits, but examples have been found. These phenomena have significant consequences. Under the present atmosphere, the smallest (decimeter-scale) craters in sands and soils could be quickly obliterated but might still be preserved on rock surfaces, as noted by Horz; et al. (1999). Ancient crater populations, if preserved, could yield diagnostic signatures of earlier atmospheric conditions. Surfaces formed under past denser atmospheres (few hundred mbar), if preserved by burial and later exposed by exhumation, could show: a) striking depletions of small craters (few meter sizes up to as much as 200 in), relative to modem surfaces; b) more clustered craters due to atmospheric breakup; and c) different distributions of meteorite types, with 4 in to 200 in craters formed primarily by irons instead of by stones as on present-day Mars. Megaregolith gardening of the early crust would be significant but coarser than the gardening of the ancient lunar uplands.