CONSTRAINTS ON THE SOLID-STATE GREENHOUSE-EFFECT ON THE ICY GALILEAN SATELLITES

Surface temperature data from the Voyager spacecraft provide a constra int on the magnitude of a possible solid-state greenhouse effect on th e 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 cons istent with particulate water ice. We examine in detail the effects of using different values for light penetration depth and regolith therm al properties on the diurnal variation of surface temperature. We then compare model results with surface temperature measurements for all t hree satellites. We conclude that the solid-state greenhouse effect is limited to the approximate range of 0 cm less than or equal to zeta l ess than or equal to 2.2 cm on Europa, where zeta is the characteristi c e-folding insolation absorption length; this magnitude of greenhouse effect produces an increase in subsurface temperatures of approximate ly 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, w ith no significant heating of the subsurface layers. The amount of sub surface heating is strongly dependent on the assumed thermal propertie s of the regolith.