Interior responses to impacts by different impactor types

Kurzfassung

A number of numerical mantle convection studies from the past two decades have investigated the effects of very large meteorite impacts on mantle dynamics in terrestrial planets, especially Mars. On the grounds that most observed craters seem to have been produced by collisionally evolved bodies, probably main-belt asteroids, the impactors were generally assumed to be rocky and have material parameters similar to those of the target. However, the statistical analysis does not imply that all craters derive from an S-type asteroid, as there is a significant fraction of impactors whose properties differ substantially from those of the target, and the dynamical effects in the interior may be quite different even if the final crater is similar; candidate alternative impactors include C- and X-type asteroids, comets, and TNO-type bodies. It is generally not possible to deduce the nature of the impactor from the final crater, because the remnants of the impactor are rarely preserved. We show by analyzing scaling laws how different impactors may result in the same final crater on a given planet ("isocrater impacts") and investigate some dynamical effects for different impactor types for the bodies in the inner Solar System. The analysis shows how strongly the sizes of impactors of different types must differ in isocrater impacts for any combination of density and velocity. For the dynamics of the interior, it is the subsurface features of an impact rather than the crater that are of primary interest, but their geometry and properties are more difficult to study and less well described in terms of an analytical model. We extend the analysis of existing scaling laws to investigate the differences in penetration depth and size of shocked volume for different impactor types in isocrater impacts. Apart from the geometrical relations, there are also semi-empirical relations between impactor parameters and the amount of melt produced in an impact. The differences in melt production might allow to resolve the non-uniqueness of the impactor type. We also carry out numerical mantle convection simulations with a modified version of STAGYY in which the impact is represented as an instantaneous thermal anomaly. The models show that the effects on the interior of isocrater impacts by impactors of different types can vary considerably, especially between rocky impactors with low to intermediate velocities and fast, ice-rich impactors.