In 1979, the IRIS infrared spectrometers on the two Voyager spacecraft
obtained over 1000 disk-resolved thermal emission spectra of Europa, Ganymede,
and Callisto, Jupiter's three large icy satellites. This dissertation describes
the first detailed analysis of this data set.
Ganymede and Callisto subsolar temperatures are 10oK and 5oK respectively below equilibrium values. Equatorial nighttime temperatures are between 100oK and 75oK, Callisto and Europa being colder than Ganymede. The diurnal temperature profiles can be matched by 2-layer surfaces that are also consistent with the eclipse cooling observed from earth, though previous eclipse models underestimated thermal inertias by about 50%. Substrate thermal inertias in the 2-layer models are a factor of several lower than for solid ice. There are `cold spots' on Ganymede and Callisto that are not high albedo regions, which may indicate large thermal inertia anomalies.
All spectra show a slope of increasing brightness temperature with decreasing wavelength, indicating local temperature contrasts of 10--50oK. Callisto spectra steepen dramatically towards the terminator, a trend largely matched with a laterally-homogeneous model surface having lunar-like roughness, though some lateral variation in albedo and/or thermal inertia may also be required. Subsolar Ganymede spectra are steeper than those on Callisto, but there is no steepening towards the terminator, indicating a much smoother surface than Callisto's. The spectrum slopes on Ganymede may indicate large lateral variations in albedo and thermal inertia. A surface with similar areal coverage of dark, very low thermal inertia material, and bright material with thermal inertia a factor of 2--3 below solid ice, fits the diurnal and eclipse curves, and (less accurately) the IRIS spectrum slopes. Europa spectra have very small slopes, indicating a smooth and homogeneous surface.
Modelling of surface water ice migration gives a possible explanation for the inferred lateral inhomogeneities on Ganymede. Dirty ice surfaces at Jupiter are subject to segregation into high-albedo ice-rich cold spots and ice-free regions covered in lag deposits, on decade timescales. Ion sputtering and micrometeorite bombardment are generally insufficient to prevent the segregation. The reflectance spectra of Ganymede and Callisto may be consistent with this type of segregated surface.