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W. K. Hartmann, D. C. Berman, G. A. Esquerdo, E. B. Goldman, J. Grier (Planetary Science Institute), J. Anguita, M. de la Casa (Universidad Complutense)
Contrary to Viking-era expectations, impact crater populations extend to diameters 11 m or less. The size distribution of craters (and impacting planetesimals) derived from the moon and transferred to Mars by Neukum and Ivanov (1994, in Hazards Due to Comets & Asteroids, pp. 359-416) and by Hartmann (1999, Meteor. Planet. Sci. 34, 167-177) appear to apply to undisturbed Martian plains. Applying this fact, we can interpret ages (factor 4 uncertainty) and obliterative processes on Mars. Many broad lava plains on Mars, and the Arsia Mons, have model ages of a few hundred years (Hartmann et al. Nature 397, 586-589). Youngest flows on Elysium Planitia, 7-40 m thick, have model ages around 10 My or less (Hartmann and Berman, in preparation). These results are consistent with SNC basaltic meteorite ages, and establish fairly robustly that volcanism has continued on Mars into modern geologic history. At least two processes, subaerial deposition of dust and flooding by thin lava flows, tend to remove small craters from the size distribution on older surfaces (even as new ones are produced). This enables us to estimate "net deposition" or obliteration rates in different areas. Deposition is enhanced by a factor 10-100 in the polar areas, where 11 m-scale craters have survival times of less than 1 My. In many lower latitude areas, crater saturation equilibrium has set in at diameters of tens of m, producing regolith and dust, and inhibiting location of ancient outcrops. Permafrost layers, hundreds of m deep, may be a key to morphology in older areas (ages > 1-2 Gy). Episodes of early melting of such layers explain many features, such as the softening and infill of large, old craters; chaotic terrain; runoff channels; groundwater sapping; and a class of what appear to be isostatically adjusted, "melted-down" craters, of which we have discovered several examples (Hartmann and Esquerdo, 1999, Meteor. Planet. Sci. 34, 159-166).
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