FIGURE F.1 Example of gullies present on the interior wall of a crater at 37.3°, 168°W. SOURCE: Malin and Edgett (2000).

as 60 K (to ~210 K) (Brass, 1980; Clark and Van Hart, 1981; Knauth and Burt, 2002). But the involvement of brines in the origin of the gullies is difficult to reconcile with the lack of any visible evidence of evaporite deposits associated with these features and with the inability of brines to explain either the latitudinal distribution or the poleward-facing orientation of the gullies.

Mellon and Phillips (2001) have advanced another hypothesis. They argue that the occurrence of shallow aquifers can be explained by the presence of thick mantles of extremely low thermal conductivity regolith, creating a sufficiently large geothermal gradient that the temperature of the local crust is raised above the melting point



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