Gas sublimation through cometary surface dust mantle

Recent observations of comet surfaces have prompted a closer examination of the discrepancy between the observed amount of exposed water ice and the measured water production rates within our solar system's inner regions. The visible surface ice seems insufficient to account for the measured rates of water production. Additionally, the levels of produced gas are notably lower than what would be anticipated from the sublimation of pure water ice bodies with a similar surface area. In light of the well-established understanding of comets' high porosity, a plausible explanation arises: the sublimation of water ice occurs beneath the visible surface. This sublimation generates gas that escapes through the pores, present within a matrix of hot dust that doesn't sublimate. This mechanism suggests that the produced gas may undergo additional heating, possibly explaining the temperatures required to match the measured terminal velocities. To delve into this phenomenon, we've conducted an investigation using numerical models based on the Direct Simulation Monte Carlo (DSMC) method for rarified gas dynamics. Our findings reveal intriguing insights. Once the thickness of the porous layer reaches a specific size, irregularities in the source aren't discernible in the flow field. Moreover, lateral flow through this porous layer, parallel to the surface, has the intriguing effect of enlarging the apparent source size, particularly for small and isolated sources. An important discovery is the significant increase in gas temperatures at the visible surface. These temperatures seem to mirror the distribution of temperatures within the non-volatile porous layer beneath. Surprisingly, the structural properties of this layer appear to exert only limited influence on the final gas parameters at the surface, emphasizing the complexity of this sublimation process. Thus, our investigation sheds light on the intricate dynamics occurring beneath comet surfaces, providing a more nuanced understanding of the mechanisms driving water production rates and gas behavior within these celestial bodies.