Macro and micro structures of pebble-made cometary nuclei reconciled by seasonal evolution

Mauro Ciarniello , Marco Fulle , Andrea Raponi , Gianrico Filacchione , Fabrizio Capaccioni , Alessandra Rotundi , Giovanna Rinaldi , Michelangelo Formisano , Gianfranco Magni , Federico Tosi , Maria Cristina de Sanctis , Maria Teresa Capria , Andrea Longobardo , Pierre Beck (1) , Sonia Fornasier (2, 3) , David Kappel , Vito Mennella , Stefano Mottola , Batiste Rousseau , Gabriele Arnold

1 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble
2 IUF - Institut Universitaire de France
3 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique = Laboratory of Space Studies and Instrumentation in Astrophysics

Comets evolve due to sublimation of ices embedded inside porous dust, triggering dust emission (that is, erosion) followed by mass loss, mass redistribution and surface modifications. Surface changes were revealed by the Deep Impact and Stardust NExT missions for comet 9P/Tempel 1 (ref. ¹), and a full inventory of the processes modifying cometary nuclei was provided by Rosetta while it escorted comet 67P/Churyumov-Gerasimenko for approximately two years^2-4. Such observations also showed puzzling water-ice-rich spots that stood out as patches optically brighter and spectrally bluer than the average cometary surface^5-9. These are up to tens of metres large and indicate macroscopic compositional dishomogeneities apparently in contrast with the structural homogeneity above centimetre scales of pebble-made nuclei¹⁰. Here we show that the occurrence of blue patches determines the seasonal variability of the nucleus colour^4,11,12 and gives insight into the internal structure of comets. We define a new model that links the centimetre-sized pebbles composing the nucleus¹⁰ and driving cometary activity^13,14 to metre-sized water-ice-enriched blocks embedded in a drier matrix. The emergence of blue patches is due to the matrix erosion driven by CO₂-ice sublimation that exposes the water-ice-enriched blocks, which in turn are eroded by water-ice sublimation when exposed to sunlight. Our model explains the observed seasonal evolution of the nucleus and reconciles the available data at micro (sub-centimetre) and macro (metre) scales.

Domains: Sciences of the Universe [physics]

Mauro Ciarniello, Marco Fulle, Andrea Raponi, Gianrico Filacchione, Fabrizio Capaccioni, et al.. Macro and micro structures of pebble-made cometary nuclei reconciled by seasonal evolution. Nature Astronomy, 2022, 6, pp.546-553. ⟨10.1038/s41550-022-01625-y⟩. ⟨insu-03705364⟩