Comparison of a pebbles-based model with the observed evolution of the water and carbon dioxide outgassing of comet 67P/Churyumov-Gerasimenko

Abstract

The Rosetta mission escorted comet 67P/Churyumov-Gerasimenko for approximately two years including the perihelion passage (1.24 au, August 2015), allowing us to monitor the seasonal evolution of the water and carbon dioxide loss rates. Here, we model 67P/Churyumov-Gerasimenko water and carbon dioxide production as measured by the Rosina experiment during the entire escort phase by applying theWater-ice-Enriched Block (WEB) model, namely a structural and activity model for a nucleus made of pebbles. Furthermore, we compare the surface temperature distribution inferred by VIRTIS-M observations in August 2014 (≈3.5 au inbound, northern summer) with the expected temperatures from our simulations in the nucleus’ northern hemisphere, investigating the relevance of self-illumination effects in the comet "neck" and assessing the active area extent during the northern summer. Our simulations imply that: 1) water production at perihelion is mostly from the dehydration of water-poor pebbles, continuously exposed by CO2-driven erosion; 2) at large heliocentric distances outbound the water loss rate is dominated by the self-cleaning of fallout deposits; 3) the outbound steep decrease of the water production curve with heliocentric distance results from the progressive reduction of the nucleus water-active area, as predicted by the proposed model; 4) in August 2014 the water production is dominated by distributed sources, originating in the active "neck"; 5) distributed sources originating in water-ice-rich exposures dominate the water production approximately up to the inbound equinox; 6) the time evolution of the CO2 loss rate during the Rosetta escort phase is consistent with the WEB model.