Using the Meteorite Cratering Record to Study the Ancient Martian and Titan Atmosphere

Recent work opens new opportunities to apply the record of impact cratering on Mars and Titan to the study of the historical behavior of their atmospheres, in particular density fluctuations. Magellan showed that the Venus crater diameter distribution reveals the distinctive signature of a diameter cutoff, due the destruction of small bolides by the dense atmosphere. The work of Chyba et al (1993) extends earlier work by Melosh (1989) and others on the breakup of bolides in current-day planetary atmospheres. One of us (S.E.) has modified Chyba's program (kindly provided by Chyba); we use it to calculate the smallest bolides that would impact the surfaces of Mars and Titan, under different assumed ancient and modern atmospheric conditions. Mars. Our techniques include (1) calculation of the smallest bolides reaching the surface for various atmospheric densities, including the present density, assuming for various realistic asteroid and comet strengths; (2) prediction of the resulting crater diameter distribution; (3) search for atmosphere-induced cutoffs in the cratering size distributions on surfaces of various ages; and (4) recognition and cataloging of tight clusters of small craters that may be records of atmospheric breakup of weak bolides. Preliminary results to date in these four areas include: (1) Smallest bolides reaching the surface. Results depends strongly on the type of bolide. We consider four types of bolide of increasing strength: comet (very weak Weidenschilling, 1994) carbonaceous chondrite, chondritic stones, and irons. From the modified Chyba program for present Mars surface pressure, S.E. finds approximate cutoff bolide diameters of about 70m and 25m, for the first two categories, while stones and irons get through to the surface except at very small dust grain sizes. However, during episodes when the surface pressure exceeded roughly 200 mb, even stones are destroyed, leaving only irons to reach the surface. For example, at 500 mb, the cutoff sizes for bolides of the first three types are approximately 680m, 460m, and 180m. Smaller bodies explode in the atmosphere, like the Tunguska bolide did. (2) Predicted effects on cratering record. The loss of comets, and carbonaceous asteroids may reduce the total crater production rate on Mars by factors around two to three, since they may compose only around 30 to 70% of the total. A downturn in the cratering record by this amount might be hard to prove, in light of other erosive effects. However, the loss of stones as well, leaving only irons, produces a much larger effect, because only 2 to 4 percent of the flux is irons. We find that the cratering record for 500mb would be complete at D >~5 km, but would consist only of craters made by irons below D <~1.6 km. We conclude that any episode when an existing surface was exposed to an atmospheric pressure above roughly 200mb-500mb could leave a detectable signature. Current assertions of shorelines and other evidence of ancient oceans and glaciers would require that some surfaces are contemporaneous with ancient dense atmospheres, and would be likely to show the predicted size distributions. (3) Search for predicted effects. The program is only beginning, but we believe we have techniques at hand that could place upper limits on the atmosphere densities contemporaneous with any existing surfaces. (4) Crater clusters. We have noted a number of clusters of small craters that may be signatures of our predicted breakup of weak objects in the martian atmosphere. Titan. Although the atmospheric pressure of Titan is only somewhat above Earth's, the atmosphere of Titan is more like that of Venus in its very strong filtering effect on bolides; this results from the large scale height and total mass. Our preliminary predictions (Engel, Lunine, Hartmann, and Chyba, in preparation) indicate that the smallest craters produced under the current atmosphere would be tens of kilometers across, and there may good opportunities for Cassini radar studies of the crater diameter distributions to reveal characteristics of the ancient atmospheric density evolution. References: [1] Chyba C. et al. (1993) Nature, 361, 40. [2] Engel S. et al. (1994) Planet. and Space Sci. [3] Hartmann W. K. (1971) Icarus, 15, 410-428. [4] Melosh H. J. (1989) Impact Cratering, Oxford Univ.