Evolution of Cometary Surfaces

Kurzfassung

Comets are among the most primitive objects in our Solar System, yet they cannot have avoided some evolution over the 4.5 billion years since they formed. This is particularly true for Jupiter Family Comets (JFCs), who typically get close to 1 AU every 5-10 years. These objects have been the primary target of past cometary missions, as they are less challenging to reach and follow around the Sun than long period nuclei. However, their short period and heliocentric distance mean that their surfaces may have been significantly transformed by solar heating. Hence, understanding the relevant evolutionary processes is necessary if we want to use our recent observations to derive constraints on the primitive Solar System environment. In the last decades we have imaged 6 cometary nuclei, almost exclusively JFCs. Spacecraft data revealed complex worlds, with a large variety of morphological features (smooth plains, cliffs, pits, boulders, ...) and ongoing transformations (dust sedimentation, surface deflation, scarp retreat, avalanche, ...) on many timescales. Although these processes are visually impressive, they do not seem sufficient to build up the morphology we observe over the typical dynamical life time of a JFC in the Inner Solar System. We must therefore consider several phases in cometary evolution, with surface transformations taking place at different heliocentric distances (e.g. impacts in the Early Kuiper Belt, violent outbursts upon first entry in the Inner Solar System and the Centaur phase, slow sublimation in the JFC phase). We will review the current paradigms of cometary evolution and how recent data from Rosetta and New Horizons help us establish a link between surface changes in the Outer and Inner Solar System. We will also discuss how the next missions like ESA's Comet Interceptor will address these questions, and what models/experiments need to be developed in preparation of future data interpretation. The author thanks the International Space Science Institute (ISSI Bern), which supported the OCEOSS team discussions on this topic.