REM%IMSSS@su-ai.arpa (Robert Elton Maas) (10/23/86)
From recent Nova episode on Voyager/Uranus, I believe that the prevaling theory of Miranda is that it was blown apart by some large collision and then the pieces recollected and partly melted. Where heavy pieces ended up in the center before the planet solidified, nothing subsequent happened because it was at equilibrium, but where heavy pieces remained near the surface during solidification those pieces later miagrated towards the center, displacing lighter material which flowed to the surface but cooled too quickly to damp out ripples (groves) and other evidence of the geologic activity. Thus we see radically different surface features depending on whether recent activity occurred or didn't occur. Based on that, I have a theory as to the surface terrain of Ganymede. Perhaps Ganymede suffered a similar fate, except Ganymede is larger and warmer so its more intense gravity pulled things toward equilibrium faster and its higher temperature kept it liquid longer relative to the resettling time of the chunks of rock and ice. When Ganymede finally froze, virtually all of the settling back to equilibrium was finished, leaving only some minor resettling to create the grooved terrain we observed. Callisto was lucky, not suffering such a disaster, although some of its large craters may have been caused by pieces from Ganymede that strayed too far out from Ganymede's orbit at the wrong time. Europa and Io of course have been so throughally melted recently to hide any evidence of their early fate. P.s. except for Titan and Miranda, the moons of Saturn and Uranus look remarkably similar. All (except those two) are super-cold very-frozen white-ice bodies which have frozen into them varying amounts of (1) craters, (2) geologic faults, and (3) colored/bright/dark material strewed across the surface. I predict the moons of Neptune will be similar except Triton and perhaps one of the mid-size moons that may be like Miranda. (Somebody, please archive this prediction and pull it out just before Voyager/Neptune in 1989?) By contrast, the moons of Jupiter are much warmer, ranging from green slightly-frozen ice, or ice over green water, to even warmer stuff.