Cometary activity: the case of 67P/Churyumov-Gerasimenko

Kramer_comet_graphical_abstract
Visualization of dust trajectories emerging from comet 67P/C-G from sun-lit (reddish) and shadowed areas (bluish lines). Taken from Advances in Physics: X, 3(1), 1404436, 2018

The Rosetta spacecraft has come to rest on comet 67P/Churyumov-Gerasimenko. The comet is retreating from the sun, but the analysis of the scientific data is an ongoing endeavour with many discoveries yet to be made. As described before, the coma structure of 67P/C-G followed a surprisingly predictable pattern: dust is emitted from the entire sunlit surface and later in space forms intricate dust bundles and rays, directly reflecting the surface topography. The rotation of the nucleus leads to a bending of the dust trajectories which allows us to read of the velocity of the particles: around 3 m/s at distances 2-3 km from the surface. Our detailed prediction of the dust coma from May 2015 recently appeared in Advances in Physics: X, 3(1), 1404436, 2018 (open access), where we compare the model with Rosetta images such as these ones: (1, 2, 3).

Figure5
Distribution of gas emitting regions on 67P/Churyumov-Gerasimenko (blue to red color scale: increasing gas emission). The black needles mark the reported locations of short-lived dust outbursts. Adapted from MNRAS 469, S20, 2017

The dust is propelled by the gas emitted from the surface by sublimation processes. It is a non-trivial task to back-out the surface ice distribution from the measured gas densities at Rosetta’s orbit via the COmetary Pressure Sensor (COPS), built by the ROSINA team (PI: Prof. Kathrin Altwegg, University of Bern, Switzerland). Using an analytical ansatz for the gas distribution, we managed to retrieve the “best-fit” distribution of the gas sources on the surface (T. Kramer, M. Läuter, M. Rubin, K. Altwegg: Seasonal changes of the volatile density in the coma and on the surface of comet 67P/Churyumov-Gerasimenko Monthly Notices of the Royal Astronomical Society, 469, S20, 2017).
Interestingly, the sources of higher gas emission are linked to reported short-lived outburst locations. A unified picture and modelling of gas and dust emission holds important clues about the composition of the cometary surface and we continue our investigation in that direction.

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