High and low velocity collisions between porous small bodies in our Solar System

Abstract

Recent space missions like New Horizons, observations and interplanetary dust particles found on Earth show that asteroids belonging to the taxonomic classes C and D, centaurs, as well as Kuiper-belt objects have low bulk densities (≤ 1300 kg/m3) compared to terrestrial planets. This indicates that the solar system in its early stages likely consisted of highly porous small bodies. For the purpose of studying the formation of asteroids and planetesimals alike, it is essential to understand the collisional process, the different behaviour, as well as the dissipative nature of porous materials. Impact studies suggest that due to their dissipative nature, porous bodies show a higher material strength than bodies without porosity (Stewart & Ahrens, 1999). Furthermore, cratering events could involve more compaction and less ejection (Housen & Holsapple, 1999). In this thesis, we mainly use the so-called P-α model (Herrmann, 1969) to account for the effect of porosity in order to investigate collisions and impacts between porous bodies. The model is described in detail including validation simulations and a display of the timestep resolution for the iterative relation between the pressure P and the distention α. The time resolution is of importance due to the existence of a specific pressure value for a specific distention value. If the relation of values does not agree, the timestep has to be reduced. We studied the formation of the largest crater on the Mars moon Phobos called Stickney, tightening the parameter space for the impactor size, velocity, as well as porosity. Furthermore, we investigated the comet-like activity of porous main-belt comets resulting from the sublimation of sub-surface water-ice getting exposed by impacts of meter-sized bodies. The crater of these porous bodies becomes larger and can be used as an upper limit for the depth at which ice needs to be present. In addition, we found a formation channel for the Kuiper-belt contact binary Arrokoth (2014 MU69), which has been visited by the New Horizons spacecraft, using semi-secular Lidov-Kozai oscillations. They are induced by the Sun and may lead to changes of the inner orbit’s mutual inclination and its eccentricity on timescales that are much longer compared to the orbital period. The collision velocity of Ultima and Thule to form Arrokoth approximately amounts to the escape velocity which is of the order of a few m/s. With Smooth Particle Hydrodynamics (SPH) simulations we scanned the parameter space of material strength and impact angle. Our studies conclude that porosity plays a major role in the outcome of porous body collisions. One can not simply reduce the bulk modulus, as well as the density, and use a model for non- porous material to account for porosity. Further investigations regarding porous materials will improve the understanding of formation processes of small bodies and planetesimals alike as well as the strength of these weak bodies.