Why is Equatorial Water Ice Stable on the Galilean Satellites?

John Spencer, Lowell Observatory

The Galilean satellites are the closest bodies to the sun which have permanent surface exposures of water ice at low latitudes. Because of their relatively warm temperatures, total ice sublimation can be large on geological timescales (Figure 1). Ice with a peak noontime temperature of 146 K would loose the equivalent of 17 meters of ice in one million years, if there was no replenishment.

Figure 1 Upward sublimation rate for equatorial ice on the Galilean satellites, as a function of maximum daytime temperature, assuming instantaneous equilibrium with sunlight. Albedos corresponding to these maximum temperatures, on the same assumption, are also shown. Sublimation is very rapid on geological timescales.

Ice is replenished from re-impacting molecules sublimed from ice within a radius of tens of km (Purves and Pilcher 1980). However, because of the extreme dependence of sublimation on temperature, ice more than a degree or two warmer than its average suroundings will loose ice at a rate approaching the upward rates in Figure 1. If a surface brightens as it gains ice, and darkens as it looses ice, it is difficult to prevent rapid segregation of the surface into bright, cold, ice-rich regions, and warm, dark regions where the ice is covered in a nonvolatile lag deposit, on timescales of less than a million years (Spencer 1987a). This may explain many of the large albedo contrasts seen in Galileo's close-up images of Ganymede and Callisto (Belton et al. 1986), and the local temperature inhomogeneities suggested by Voyager thermal emission spectra (Spencer 1987b).

On longer, but still geologically important, timescales, there is a tendency for ice to migrate to higher latitudes, where vapor pressures are lower. Purves and Pilcher calculated that 5-10 meters of ice would be lost in a billion years from equatorial regions, for peak daytime temperatures of 137 K. Warmer ice would produce such rapid poleward migration that it would be difficult to resupply ice to the surface (by impact gardening, for instance) fast enough to maintain its presence on the equatorial surface. Local thermal segregation can help to prevent poleward migration by keeping the equatorial ice bright and cold.

The strong possibility of thermally segregated surfaces should be kept in mind when interpreting Galileo observations of the icy Galilean satellites. Galileo UVS observations of hydrogen escape from Ganymede may require ice as warm as 147 K, (Barth et al. 1997), which if confirmed would require re-evaluation of how equatorial ice can be stable on Ganymede.

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Contact Information:

John Spencer
Lowell Observatory
Flagstaff, AZ 86001
Telephone: (520) 774-3358 x229
email: spencer@lowell.edu