Constraints on the ejecting-crust activity model on comet 67P/Churyumov-Gerasimenko

TL;DR

This study uses a pebble-based thermophysical model to simulate comet 67P's activity, revealing that low diffusivity and specific surface properties explain observed outgassing and dust ejection, especially in the southern hemisphere.

Contribution

It introduces a detailed thermophysical model incorporating pebble structure and surface properties to better understand cometary activity and dust ejection mechanisms.

Findings

Low diffusivity and high heat capacity are key to matching observed outgassing.

Dust and gas ejection predominantly occur in the southern hemisphere during perihelion.

Model struggles to simultaneously match dust, CO2, and CO emission rates.

Abstract

Reproducing the observed activity of comets with thermophysical models remains a primary challenge of cometary science. We use a pebble-based thermophysical model of gas-pressure build-up in the subsurface to reproduce the global emission rates of dust, water, CO$_{2}$, and CO observed by Rosetta at comet 67P/Churyumov-Gerasimenko (hereafter 67P). For sufficiently low diffusivities, the low tensile strength is overcome, leading to the ejection of $\sim$ millimetre- to decimetre-sized dust-particles as well as roughly the correct outgassing rates. All the ejections, and thus the bulk of the outgassing, come from the southern hemisphere during the time that it is strongly illuminated at perihelion. This leads to a 'blow-off' of the dust-crust that otherwise forms: volatiles are much closer to the surface in the south (within the top centimetre) than in the north (10-or-more cm deep), naturally explaining the strong southern water-outgassing expected from 67P's non-gravitational accelerations and torques. We find that low gas-diffusivity, as well as large heat-capacity and steeply decreasing tensile strength with depth or ice-content, are in best agreement with the outgassing data. However, even in these cases, we struggle not to exceed the observed emission rates of dust, CO$_{2}$, and CO. In the south, it is difficult for models to achieve a balance between triggering activity and generating too much of it (with CO$_{2}$ the critical driving-species here); while in the north, it remains challenging to generate activity at all. Strong constraints are placed on the nature of the activity mechanism by the location of dust-ejection and erosion.