ICARUS 49, 71-85 (1982) Deducing the Age of the Dense Venus Atmosphere RALPH KAHN Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14853 Received August 19, 1981; revised December 7, 1981 We show how crater size-density counts may be used to help constrain the history of the Venus atmosphere, based on the predictions of simple but reasonable models for crater production, surface erosion, and the effects of atmospheric drag and breakup on incident meteors in the Venus atmosphere. If the atmosphere is old, we may also be able to determine the importance of breakup as a mechanism for destroying incident meteors in a dense fluid. In particular, if the atmosphere is young, the old (uneroded) surfaces will have crater densities upward of 10O4 km- 2 and a ratio of small (4 km) craters to large (128 km) craters near 103. If the atmosphere is old and the breakup mechanism is dominant, absolute crater densities on Venus surfaces will be diminished by several orders of magnitude relative to the young atmosphere case. If atmospheric drag is dominant and the atmosphere is old, the absolute crater density will be lowered by perhaps an order of magnitude relative to the young atmosphere case, and the ratio of small to large craters will be reduced to a value near 10 1n according to the models. The comparison of crater populations on young, as well as old, surfaces on Venus can help in distinguishing the young and old atmosphere scenarios, especially since the situation may be complicated by currently undetermined erosional and tectonic processes. Once a large fraction of Venus surface has been imaged at kilometer resolution, as the VOIR project promises to do, it could be possible to make an early determination of the age of the Venus atmosphere. INTRODUCTION The formation history of planetary atmo- spheres is a subject of considerable impor- tance for understanding the geological evo- lution of the solid bodies of planets (Phillips et al., 1981), and as a constraint on models of the photochemical and climatological evolution of the atmospheres themselves. Recent improvements in ground-based ra- dar imaging of Venus (Campbell and Burns, 1980), the Pioneer Venus radar imaging results (Pettengill et al., 1980;Masursky et al., 1980), and the proposed Venus Orbiter Imaging Radar project indicate that we will soon be able to map large areas of the Venus surface with spatial resolution of a few kilometers or better. In this paper we discuss how impact crater size-density in- formation obtained from such maps can be used to help constrain the age of the Venus atmosphere. The modern atmosphere of Venus, which has a pressure at the ground nearly 100 times greater than that of Earth (Seiffet al., 1979), shields the surface of the planet from incident meteors by the deceleration of small-sized meteors, and possibly by the relatively size-independent process of me- teor breakup as well. An initial study of the behavior of meteors in a dense, Venus-like atmosphere was performed by Tauber and Kirk (1976). Age-dated lunar surfaces have been used to derive the time history of bombardment (e.g., Hartmann, 1972b; Soderblom et al., 1974)and the crater size distribution (e.g., Hartmann, 1970; McGill, 1977) for the moon. Comparisons of imaged surfaces of the Moon, Earth, Mars, and Mercury sug- gest that meteor fluxes in the inner solar system have been spatially uniform (Wetherill, 1975; Schaber and Boyce, 1977; Shoemaker, 1977). Hartmann (1977) has de- rived relations for the expected crater pop- ulations on the inner planets using gravita- tional scaling arguments and models of the incident meteor flux. From these results we 71 0019-1035/82/010071-15$02.00/0 Copyright j 1982 by Academic Press, Inc. All rights of reproductionin any form reserved.