太陽系冰質小天體的協同研究 - 最後一哩路

Europa is an icy satellite orbiting Jupiter which is suggested to have a global subsurface ocean from the diversity of surface geological features and the magnetic field measurements made by the Galileo spacecraft. In addition, the HST observations also suggested water vapors outgassing from Europa, similar to Enceladus plume activity. In this proposed work, we have completed the ALMA (Atacama Large Millimeter/submillimeter Array) data analysis of Europa’s exosphere. The preliminary results showed the existence of some molecular gas. The local density enhancement in the exosphere and its high ejection velocity suggest that the detected gas species could be likely the outgassing activity associated with its localized liquid water reservoir and/or its subsurface ocean. In addition, we use a 3D Direct Simulation Monte Carlo (DSMC) model to study the plume structures of Europa. This model can describe the gas plumes with a broad flow regime ranging from close to continuum near the surface to free-molecular condition at higher altitudes. With a systematic study of a large parametric space of the total gas production rate and initial gas ejection velocity, the gas number density, temperature and velocity information of outgassing activity at various scales can be derived. Finally, the modeling results will be incorporated with a n-LTE radiative transfer model to simulate the water plume morphology which could be observed in the sub-millimeter and millimeter wavelengths. Understanding the plume of Europa and its effects (i.e., icy dust deposit on surface) plays a key role to probe its interior structure and outgassing mechanism (i.e., cryovolcanism). Our results can be well prepared for the space and ground telescope observations, and the in-situ measurements of the future spacecraft missions such as JUICE and Europa Clipper.