Dusk/Dawn Atmospheric Asymmetries on Tidally-Locked Satellites: O₂ at Europa

Apurva Oza (1, 2) , Robert E. Johnson (3, 4) , François Leblanc (1)

1 HELIOS - LATMOS
2 Physikalisches Institut [Bern]
3 Engineering Physics Program [Charlottesville]
4 Department of Physics [New York]

We use a simple analytic model to examine the effect of the atmospheric source properties on the spatial distribution of a volatile in a surface-bounded atmosphere on a satellite that is tidally-locked to its planet. Spatial asymmetries in the O2 exosphere of Europa observed using the Hubble Space Telescope appear to reveal on average a dusk enhancement in the near-surface ultraviolet auroral emissions. Since the hop distances in these ballistic atmospheres are small, we use a 1-D mass conservation equation to estimate the latitudinally-averaged column densities produced by suggested O2 sources. Although spatial asymmetries in the plasma flow and in the surface properties certainly affect the spatial distribution of the near-surface aurora, the dusk enhancements at Europa can be understood using a relatively simple thermally-dependent source. Such a source is consistent with the fact that radiolytically produced O₂ permeates their porous regoliths. The size of the shift towards dusk is determined by the ratio of the rotation rate and atmospheric loss rate. A thermally-dependent source emanating from a large reservoir of O₂ permeating Europa’s icy regolith is consistent with the suggestion that its subsurface ocean might be oxidized by subduction of such radiolytic products.

Domains: Sciences of the Universe [physics]

Apurva Oza, Robert E. Johnson, François Leblanc. Dusk/Dawn Atmospheric Asymmetries on Tidally-Locked Satellites: O₂ at Europa. Icarus, 2018, 305, pp.50-55. ⟨10.1016/j.icarus.2017.12.032⟩. ⟨insu-01674281⟩