Constraints on the solid-state greenhouse effect on the icy Galilean satellites

Surface temperature data from the Voyager spacecraft provide a constraint on the magnitude of a possible solid-state greenhouse effect on the icy Galilean satellites. A solid-state greenhouse effect will occur if the regoliths of Europa, Ganymede, and Callisto are optically thin in the visible and opaque in the thermal infrared, which would be consistent with particulate water ice. We examine in detail the effects of using different values for light penetration depth and regolith thermal properties on the diurnal variation of surface temperature. We then compare model results with surface temperature measurements for all three satellites. We conclude that the solid-state greenhouse effect is limited to the approximate range of 0 cm less than or equal to zeta less than or equal to 2.2 cm on Europa, where zeta is the characteristic e-folding insolation absorption length; this magnitude of greenhouse effect produces an increase in subsurface temperatures of approximately 10 K or less. Similarly, Ganymede and Callisto both have allowable ranges for zeta of 0 to 0.5 cm based on Voyager surface temperature, with no significant heating of the subsurface layers. The amount of subsurface heating is strongly dependent on the assumed thermal properties of the regolith.