2. Crater Obliteration and Net Deposition Rates: Impact crater populations, especially those of small craters, can be used to characterize the environmental conditions on a given planetary surface. This technique was pioneered as long ago as 1965-66 in response to the Mariner 4 mission. Öpik (1965) remarked (without derivation) that on a surface of given age, below a critical size, erosion processes would lower the exponent of the power law size distributions by unity, relative to the production function observed on an airless body like the moon. Hartmann (1966) followed this reasoning and derived an average net deposition rate on crater floors of the order 10-4 cm/y. Chapman, Pollack and Sagan (1968) and Hartmann (1971) gave further quantitative derivations of what we might call the "Öpik effect" -- the flattening of the crater size distribution due to erosion/deposition processes that result in net infilling and obliteration of craters.
These general ideas led, as early as the Mariner 9 era, to the conclusion that Mars experienced a history of erosion and obliteration, possibly with stronger erosion rates in the past (Jones, 1994; Chapman, 1974; Hartmann, 1973). In the Viking era, an additional hypothesis was developed that the less-steep size distribution was due not to the "Öpik effect" but to a different population of ancient impactors (Strom, Croft, and Barlow, 1992).
More recent data, including MGS data, support the reality of the Öpik effect. Many MGS frames, such as Figure 1, reveal a range of degraded crater morphologies due to dust mobility and infill, including some craters virtually engulfed in dust. Crater diameter distributions appear to show the characteristic flattening of the slope by about unity, not only in the primary branch at 3 < D < 60 km in ancient regions, but also in the secondary branch at D2 or 3 km.
| Figure 1. MGS frame 02303 inside Schiaparelli shows range of crater morphologies due to infilling of craters by dust drifts. |
|
Figure 2 shows crater size distributions from two old, heavily cratered regions, combined with counts from lower resolution Viking frames of surrounding areas. The dashed reference line is for young lunar lava plains; the upper solid reference line is the empirical saturation equilibrium level (Hartmann and Gaskell, 1997). The heavy solid line is a curve predicted for a simplified model with dust infill at a net, average rate of the order 10-4 cm/y. This model assumes that craters act as potential wells trapping dust, with visible lifetimes proportional to their depth. It is modified from Hartmann (1971) by a more sophisticated treatment of crater depth/diameter ratios. The model appears to fit the data, indicating that Martian craters in old areas have populations in balance between crater production and environmental obliteration processes, which may crudely be characterized at modest latitudes by net infill or "leveling" rates of the order 10-4 cm/y. This agrees with earlier measures at much larger crater sizes; higher rates have been found for the polar regions (Plaut et al., 1987; Hartmann, unpublished).
|
|
The ubiquity of dust drifts, and their ability to cover topography, has consequences for future Mars exploration. To sample the maximum variety of rock sources and rock types, and, better yet, find bedrock outcrops, it will be important to avoid sites where topography and boulders are masked by relatively featureless drifts.
|
Figure 3 shows crater counts from a younger area, Solis Planum, where there has not been as much time for infilling, and the larger-crater population is more completely preserved, falling closer to the lunar mare reference line (dashed). |
References: Binder, A.B. et al. 1977. J. Geophys. Res., 82, 4439-4451; Chapman, C., J. Pollack, and C. Sagan 1968. Smithson. Astrophys. Obs. Spec. Report 268; Hartmann, W.K. 1971. Icarus 15, 410-428; Hartmann, W.K. 1966. Icarus 5, 565-576; Hartmann, W.K. and R.W. Gaskell 1997. Meteoritics & Plan. Sci. 32, 109-121; Öpik, E.J. 1965. Irish Astron. Journ. 7, 92; Öpik, E.J. 1966. Science 153, 255; Plaut, J., R. Kahn, E. Guinness, and R. Avidson 1988. Icarus 75, 357-377.
Back to MGS Mission at PSI