Impact features on Venus: Modeling craters and splotches

Our team has discovered first impact craters under the thick Venusian atmosphere with radar images during the Venera 15/16 mission. Later Magellan radar images of a better quality allowed us to count all impact craters and to find amazing features, splotches, resulted most probably from “airbursts” - total explosive disruption in flight of small celestial bodies. Splotches could have a central feature (possibly caused by terminal impacts of fragments), or could be diffusive patches of increased (bright) or decreased (dark) areas of changed radar reflectivity. The main explanation so far is that atmospheric shock waves, generated by airbursts, somehow change the surface radar reflectivity, e.g. creating smoother (radar dark) or more rough (radar bright) zones due to reflection of shocks. Size of splotches vary from ~10 km to ~200 km, being comparable with the characteristic atmosphere thickness. The exact mechanisms of air shock wave interaction with the surface is still under debates, but promises to help us better understand the presence of dust/sand/pebbles/boulders at the surface of Venus as well as to estimate mechanical properties of surface rocks. We start a small project to support the issue. The project includes the numerical modeling of atmospheric shock waves on Venus due to cratering impacts and due to airbursts. Our modeling is compared with results published in 1990s-2000s. Airbursts are modeled as a hot spheric volume gas explosion 10 to 40 km above the surface in the Venusian stratified atmosphere. In addition to trivial parameters like maximum pressure, dynamic pressure and the wind speed behind the shock front, necessary for the following analysis of a possible “aeolian” motion of surface’s fines, we try to formulate a general picture of shock wave propagation in the atmosphere after an airburst. We find that the large-scale hot gas bubble from the source zone creates a n x 10 km plume (a kind of a classical “mushroom”), which effectively expands laterally at high altitudes, pushing forward an enhanced shock wave. This wave is looking like a gradual conversion of the main shock wave from a hemispheric one to a conic front, returning back to surface. The other trivial (but not discussed quantitatively) phenomenon is the seismic wave, created by an air shock, but finally overrun the atmospheric shock front. It means that the surface air shock front at large distances arrive after the seismic wave shakes the surface. We plan to investigates all these phenomena and compare models with observations. An interesting possibility seems to be satellite observation of rare meteoroid entry to the Venusian atmosphere, as it now available for terrestrial bolides.