Two Different Supply Styles of Crater Outflow Materials on Venus Inferred from Numerical Simulations over DEMs

Abstract

The three-dimensional numerical simulation code for nonisothermal Bingham fluid was applied to four typical crater outflows on Venus to estimate emplacement styles of outflow materials. We have succeeded in reproducing emplacement features qualitatively over digital elevation models (DEMs) obtained by the Magellan spacecraft, and we have shown that outflow materials can be formed by melts of basaltic crust. Therefore, the presence of crater outflows may not be evidence of the presence of previously suggested fluid materials, such as komatiite or carbonatite, although we failed to model the very low viscosity flow. Numerically estimated supply rates of flow materials are categorized into two types, a catastrophic style and a gentle style. Markham and Isabella crater outflows are examples of catastrophic styles, and the flow materials required to form them were probably supplied at higher rates than 10 10 m 3/s and the durations of material supply were less than 100 s. This very high supply rate is difficult to attribute to any volcanic process because typical eruption rates of terrestrial volcanic flows are less than 8 or 9 orders of magnitude lower than that. Such a style of emplacement is more plausibly considered as a result of the impact itself, for example, a catastrophic debris flow or a vapor cloud triggered by the impact. On the other hand, supply rates of Willard and Xantippe outflows, which are considered gentle-style flows, were estimated to be about 10 4 m 3/s and the durations were more than 10 5 s. Such a gentle style of outflow emplacement is probably not a direct result of the impact but is rather due to a secondary process, possibly segregation and drainage of impact melt materials from within previously emplaced ejecta materials, or post-impact volcanism.