IMPACT MELTING ON VENUS - SOME CONSIDERATIONS FOR THE NATURE OF THE CRATERING RECORD

Modeling the volume of impact melt and its variation with the size of the impact event indicates that, for similar-sized final craters, venu sian impacts create about 25% more impact melt than terrestrial impact s. More significantly, venusian impacts result in approximately a fact or of three more impact melt than lunar events producing equivalent-si zed craters. This difference is due to the higher average impact veloc ity and higher ambient temperatures on Venus, which enhance impact-mel t production, combined with higher planetary gravity, which inhibits c rater growth for a given impact event. The initial, higher intrinsic t emperature of incorporated elastic debris also contributes to impact m elts with higher initial temperatures, lower viscosities, and longer c ooling times on Venus with respect to lunar impact melts. The enhanced production of relatively hot, low-viscosity impact melts under venusi an impact conditions may account for the long exterior runout flows an d also for the radar-smooth interior doors of some venusian craters. W e also argue that the anomalously deep character of Cleopatra may be a ttributed to drainage of its interior impact-melt pool to form the smo oth deposits in the adjacent Fortuna Tessera. Increasing depth of melt ing with increasing cavity size, resulting in the progressive weakenin g of transient-cavity floor material, is offered as a possible explana tion for the replacement of uplifted central peaks by rings with incre asing crater diameter. A consequence of this process is that interior rings will increase in diameter relative to the diameter of the final crater's rim crest with increasing crater size, a trend observed on Ve nus and other terrestrial planets. This weakening of the target due to relatively enhanced impact-melt production in the venusian environmen t makes it unlikely that Orientale-style impact basins ever formed on Venus. (C) 1995 Academic Press, Inc.