Deciphering cometary outbursts: linking gas composition changes to trigger mechanisms

Dust and gas outbursts are recurrent cometary phenomena, playing a crucial role in shaping the coma. Proposed outburst trigger mechanisms include cliff collapse, pressure pockets, and amorphous-to-crystalline phase transition of water ice; however, the underlying processes remain inadequately understood. In this study, we analyse Rosetta/ROSINA data from multiple outbursts on comet 67P/Churyumov-Gerasimenko and present the evolution of the gas composition in the comet's coma during outburst events. We distinguish two distinct categories of cometary outbursts on the comet: water-driven events characterized by rapid (minutes to hours) changes in coma composition, and CO2-driven events displaying a slow, prolonged (hours to days) increase in highly volatile species. We tentatively associate these different gas composition patterns with different trigger mechanisms. Exposure of fresh ice due to cliff collapse leads to a notable water enhancement, while most perihelion outbursts coincide with substantial density increases of CO2. We propose that these CO2-driven events originate from subsurface gas-filled cavities, whose walls are suggested to have been sealed by earlier refreezing of CO2 migrating from warmer spots, hence increasing the cavity pressure required to burst.