Mary L. Urquhart

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 ≤ ζ ≤ 2.2 cm on Europa, where ζ 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 ζ 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.

Lunar thermal emission and remote determination of surface properties

Grain size, bulk density (or porosity), thermal conductivity, and rock abundance all play an important role in the thermal behavior of the lunar surface. Direct investigation of these properties of the lunar surface layer is not presently possible, with the exception of the samples returned from the Apollo landing sites. An indirect measurement of lunar surface properties may be possible using remote thermal infrared observations. In order to better understand the interplay between these properties, a diurnal thermal model for the lunar surface and near subsurface with temperature-dependent specific heat and thermal conductivity was developed. The inclusion of the temperature dependence of thermal conductivity and specific heat was found to be essential when attempting to derive regolith properties of the Moon due to the large difference in surface temperatures between day and night. Although particle size, bulk density, and thermal conductivity cannot be investigated completely independently, a clear relationship between these parameters, and their effects on lunar surface temperatures, is determined. An increase in the bulk density of the regolith is found to correspond to an increase in the nighttime temperature of the surface. Similarly an increase in the rock fraction also raises the predicted nighttime temperatures. Increasing grain sizes correspond to decreasing nighttime temperatures. No unique set of surface properties can be determined from thermal remote sensing measurements alone. Grain size is the most difficult regolith property to determine remotely, and rock abundance is by far the strongest contributor to the derived thermal inertia of the bulk surface.