Two Separate Styles of the Crater Outflow Emplacements on Venus

H. Miyamoto, S. Sasaki (University of Tokyo)

The numerical simulation code for laminar Bingham fluid was applied to Venusian crater outflows to estimate rheological properties and supply rates of outflow materials. This direct simulations on digital elevation models (DEMs) obtained by Magellan spacecraft provide constraints on materials of crater outflows through viscosity and yield strength.

From a number of calculation results, supply rates and durations of outflow materials are categorized into two types: the catastrophic style which has a high supply rate and a short duration of supply, and the gentle style which has a low supply rate and a long duration. Markham and Isabella crater outflows are typical of the former type, and they have high supply rate more than tex2html_wrap_inline11 m tex2html_wrap_inline13 /s. Therefore, durations of supply of outflow materials should be short, and estimated to be about tex2html_wrap_inline15 - tex2html_wrap_inline17 s. Willard and Xantippe crater outflows are, on the other hand, the latter type outflows. Their supply rates are estimated to be about tex2html_wrap_inline15 - tex2html_wrap_inline21 m tex2html_wrap_inline13 /s, and supply durations are more than tex2html_wrap_inline11 s. These results suggest there are two separate styles of outflow emplacements. A catastrophic emplacement may be caused by a debris flow or a vapor cloud triggered by impact itself, although a gentle emplacement was probably due to secondary processes: possibly segregation and drainage of impact melt materials from within previously emplaced ejecta materials, or post-impact volcanism.

Successful duplications of two of the largest outflows on Venus by a laminar Bingham fluid model show that outflows could emplace such a vast area only by impact melt flow whose total volume is estimated less than several tens percents of the total volume of impact melt inferred by the existing model. Therefore, the presence of outflows flowed vast area could not be a direct evidence of the presence of low-viscosity fluid, such as komatiite or carbonatite.