Land of gas and dust – exploring bursting cavities on comet 67P

Gas and dust outbursts are recurring phenomena on comets, offering critical insights into their subsurface activities. On comet 67P/Churyumov–Gerasimenko, two distinct outburst types have been identified: CO$_{2}$-dominated ‘summer fireworks’ near perihelion and water-driven events often linked to cliff collapses outside the perihelion period. While CO$_{2}$-dominated outbursts are thought to originate from subsurface gas cavities, the properties of these cavities remain poorly understood. In this study, we modelled the outgassing dynamics and dust velocities of outburst events using Rosetta/ROSINA data to estimate the characteristics of subsurface gas cavities and their impact on ejected particle dynamics. Our results indicate that CO$_{2}$-dominated events involve subsurface cavities with radii ranging from 15 to 62 m for an equivalent half-sphere geometry, depending on gas distribution assumptions. Conversely, water-driven outbursts would require subsurface temperatures far above equilibrium, supporting the hypothesis of mechanical processes like cliff collapses exposing ices to sublimation. Dust velocities in CO$_{2}$-dominated events – while aligning with results from other Rosetta instruments – were notably higher across all grain sizes compared to water-driven events, reflecting distinct dynamics in dust ejection. These findings highlight the critical role of subsurface gas reservoirs in driving explosive outbursts and suggest a strong connection between cometary activity, volatile distribution, and structural conditions. This study emphasizes the need for high-resolution data on subsurface volatiles from future missions and more refined modelling and experiments to further elucidate these mechanisms, with potential broader implications for our understanding of cometary activity.