Collisional environment in the early solar system from recent and future space mission

Kurzfassung

In the 5 years since the last Catastrophic Disruption Meeting, space missions have brought fascinating new data which challenges our concepts of collisions in the Early Solar System. For instance, Dawn has shown a lack of large craters on the dwarf planet Ceres (Marchi et al, 2016) while Rosetta has revealed a complex cometary world in which impacts, from small to catastrophic, may have played a significant role (Rickman et al 2015, Vincent et al 2017, and many others). This review talk will present the new observations and results from recent missions, with emphasis on Rosetta. I will discuss whether impact models developed before the rendez-vous with comet 67P (e.g. Holsapple et al 2007, Vincent et al 2013) still hold after Rosetta and how the comet has changed our perception of what an impact feature may look like on such a porous object. I will review some of the most intriguing topographic features observed on comets: pits and layers (Vincent et al 2015, Massironi et al 2016, Penasa et al 2017) and describe the challenges in relating those morphologies to what we think we know about the collisional environment in the Early Solar System. We will see how very recent work (Vincent et al 2017, Birch et al 2017) suggests that the large scale roughness of cometary surfaces (i.e. topography at 10-100m scale) is shaped by processes which are primarily ancients. Thus, reconstructing this early topography may eventually provide an estimate of the collisional rate shortly after the nucleus formation. That is, of course, if impacts are the dominating process. I will present how we explore this idea by building virtual comets with numerical experiments. Overall, the wealth of data provided by recent missions, and the significant advances in numerical modeling - now able to properly consider porous icy targets (deNiem et al 2017) - opens up a whole new world for our community, finally putting very strong constrains on what may have happened in our Early Solar System. This research is significant for other environments as well: 20% of Sun-like stars show a disk of planetesimals and dynamicists have demonstrated how important these are to keep gas giants far from their stars and let life emerge on the smaller planets (e.g. review in Fritz et al 2014). Yet, primitive Solar System conditions are an open problem. For instance, the absolute mass of planetesimals beyond Neptune is still debated, from a few to 30 Earth masses (see review in Davidsson et al 2016), yet this number would lead to different impact rate on the young comets and could mean their catastrophic disruption in the latter case. We will discuss whether Jupiter Family Comets like 67P may have experienced such collision. While models show that comets could be destroyed and reaccreted while keeping a high porosity and volatile content (Jutzi et al 2017, Schwartz et al 2018) it is not clear whether some of the deeper structural features like layers can be preserved (Belton et al 2007, Davidsson et al 2016). Finally, the talk will mention future missions which will bring more data to answer these questions: New Horizons, Lucy, and CAESAR. I will particularly discuss the last one, a comet sample return mission which will revisit comet 67P in 2029 after 2 orbits since Rosetta, providing new insights on cometary evolution and perhaps even a measure of the current impact rate.