Numerical simulations of turbulent Rayleigh-Bénard convection in an ideal gas, using either the anelastic approximation or the fully compressible equations, are compared. Theoretically, the anelastic approximation is expected to hold in weakly superadiabatic systems with = ∆T /T r 1, where ∆T denotes the superadiabatic temperature drop over the convective layer and T r the bottom temperature. Using direct numerical simulations, a systematic comparison of anelastic and fully compressible convection is carried out. With decreasing superadiabaticity , the fully compressible results are found to converge linearly to the anelastic solution with larger density contrasts generally improving the match. We conclude that in many solar and planetary applications, where the superadiabaticity is expected to be vanishingly small, results obtained with the anelastic approximation are in fact more accurate than fully compressible computations, which typically fail to reach small for numerical reasons. On the other hand, if the astrophysical system studied contains ∼ O(1) regions, such as the solar photosphere, fully compressible simulations have the advantage of capturing the full physics. Interestingly, even in weakly superadiabatic regions, like the bulk of the solar convection zone, the errors introduced by using artificially large values for for efficiency reasons remain moderate. If quantitative errors of the order of 10% are acceptable in such low regions, our work suggests that fully compressible simulations can indeed be computationally more efficient than their anelastic counterparts.

The prospect of subsurface oceans in icy satellites presents an exciting area of research to understand their diverse processes and astrobiological potential. Induced magnetic fields were detected by Galileo on Europa, Ganymede, and Callisto (Khurana et al., 2009;Kivelson et al., 2000;Zimmer et al., 2000), which implies a subsurface ocean. Gravity measurements and surface features indicate that the icy shells are decoupled from the interior (cf. Hussmann et al., 2015). Direct imaging of erupting plumes on Enceladus from Cassini and potentially on Europa as well, from Hubble Space Telescope and magnetic field and plasma wave observations from Galileo (Jia et al., 2018) point to subsurface water sources. The surface observations offer clues about the composition of the subsurface ocean. Measurements of Saturn's E-ring indicate a sodium salt-rich source derive from Enceladus' interior (e.g., Postberg et al., 2009). Spectroscopic data from Galileo's NIMS suggests the presence of irradiated salts on the surface of Europa that may reflect the composition of the subsurface ocean (McCord et al., 1998;Trumbo et al., 2019). This non-water ice material is prominent in linear and chaos features on the surface. What are the sources of these salty materials? Can their presence on the surface reflect the composition in greater depths, for example, from the subsurface ocean or even from the silicate interior? How are they transported to the surface, and is the dynamics in the subsurface ocean expressed on the surface? Current understanding of icy subsurface oceans is drawn from studies of the Earth's ocean, fundamental knowledge of geophysical fluid dynamics, numerical simulations, and laboratory experiments. Topics of heat and material transport by hydrothermal systems and the circulation in the subsurface ocean have been explored with various assumptions about its conditions and physical properties (e.g., Amit et al., 2020;Goodman et al., 2004;Kvorka