A fall of comet dust and a field of boulders

From the earliest clear views of comet 67P/Churyumov-Gerasimenko it was obvious that the surface is a collection of contrasts: smooth plains, imposing craggy cliffs and scatterings of boulders. Rosetta scientists are now digging into the detail to explain how some of these features may have arisen and what this means for our understanding of comets. The study of comet 67P/C-G reveals a dramatic surface environment where considerable amounts of material – up to 1000 kg per second – are ejected from the comet. Not all of this makes it into space, instead some falls back to coat the nucleus. These small, solid particles – typically with sizes ranging from micrometres to tens of centimetres – are ejected when icy material sublimes. As the ice turns from solid to gas, it escapes into space, propelling the solid particles with it. The smallest of these expelled dust grains – millimeter-sized or smaller – obtain sufficient velocity to escape the influence of the comet and become part of the comet’s tail, which can stretch for millions of kilometres through space. But some of the larger particles (centimetre-sized or greater) fall back, meaning that particles from one part of the comet can descend to the surface on another part of the comet’s double-lobed nucleus. This ‘airfall’ creates smooth plains that can be as much as a few metres thick. Nicolas Thomas from Universität Bern, Switzerland, and collaborators have used data from OSIRIS, the science camera on Rosetta, to study these dusty plains. They then used computer models to investigate the mechanisms at work. The neck, where the two lobes join together and which has been named the Hapi region, has been notably active and so provided an obvious site for investigation. Computer models show that particles ejected from Hapi with speeds below 0.8 m/s […]

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