Didn't Malin and Edgett conclude that the gullies were not due to geothermal heating because there were no obvious volcanic sources nearby?

Mellon and Phillips may not have been aware of the recent TES measurements of temperatures in the southern hemisphere. The argument about the water sublimating before melting was also applied to the small amounts that could supply microbes with water on Mars. However, recent experiments suggest this is not the case, as Gil Levin has argued. And I found an article cowritten by Carl Sagan where he discusses performing experiments that support Levin's contention of liquid water forming close to the surface:

"Experiments under Martian pressures and temperature cycling, and with a variety
of wind velocities, performed in the laboratory of one of us (C.S.), tend to support the conclusion that thin layers of liquid water are generally formed each morning at tropical latitudes on Mars when diffusion is limited. A variety of experiments (23) indicates that many terrestrial microorgansms can grow, even if liquid water is available for only 15 minutes each day, in otherwise subzero conditions; or in soil microenviroments where liquid water is available as a consequence of freeze-thaw cycling."
"Contamination of Mars"
Carl Sagan, Elliott C. Levinthal, Joshua Lederberg
Science, 15 March 1968, vol. 159, no. 3820, p. 1191-1196

Then the question of whether this would happen for the large amounts of water needed to form the gullies should also be subjected to experiment to decide the issue definitively. My guess is that there would be plenty of time for the water to remain liquid to carve the gullies before it evaported or boiled off.

Bob Clark