Fear of Mimas by Dana Wilde Saturn's inmost-orbiting moon, not counting five or so smaller chunks of rock and ice, is Mimas. It might be the most genuinely dead place in the solar system. Voyager 1's 1980 photographs show so many craters, ruts, shadows and nooks over Mimas' trailing hemisphere (the one facing away from the direction of Mimas' orbit) that it resembles an ancient mass of coral rather than a planetary body. And a gigantic impact crater called Herschel covers more than a quarter of the surface of the leading hemisphere. Herschel's rim-walls loft 3 to 6 miles over the other craters, in a circle 80 miles wide, and a rocky peak rises some 4 miles out of the center. It looks like a still photograph of a liquid drop smashing into another liquid surface. Herschel is so huge in comparison with the rest of the moon that in the photos it blots out the sense of a sphere and transforms Mimas into an object, rather than a planetary body, the way a human cadaver is something other than a human being. Apparently Mimas barely survived the impact that created Herschel. The comet or rock that hit was six miles or so in diameter, and if that object had been slightly larger it would have blown Mimas to pieces. This would not have been the first time Mimas was destroyed. In fact, Mimas might have been annihilated and reborn as many as five different times since the planets and moons were originally formed 4.5 billion years ago. At some uncertain time in Saturn's history, a tremendous bombardment occurred, smashing the new moons in the system. Rocky and icy material left over from planet-formation struggled and surged around Saturn, and some of the larger satellites were outright annihilated by impacting comets or asteroids, or by other planetesimals whose orbits were disrupted in the mayhem. The Saturn system apparently was broken topieces as if there had been a cosmic war. The new debris settled into orbit around Saturn. Huge, flat rings formed from fine rock and ice material, and moons accreted again from the rest. In the midst of its own first death Mimas re-collected itself. Pieces of its original water ice and rock gravitated together in the original orbit, and formed a central mass which in turn attracted more rock and ice to those particular bends and folds in spacetime. It compacted itself tightly, in the normal process of planet formation, into essentially the same roughly spherical body, in the same orbit. The same moon. Then, after some uncertain eon, a second concentrated bombardment began. Mimas was repeatedly blasted, and Tethys, two orbits farther out, was hit by something which left a crater 250 miles wide. Two planetesimals, Telesto and Calypso, somehow ended up in Tethys' orbit, and another one is in Dione's. Beyond Dione, Rhea and Titan, a smaller, potato-shaped moon called Hyperion angles around Saturn bent over

45 degrees on its axis, suggesting it too was struck and half-killed by something enormous and violent. The rings themselves are mainly small chunks of water ice. Saturn is surrounded by rubble. After Mimas' most recent re-accretion into a small moon, it underwent another heavy bombardment which formed the craters on its trailing hemisphere. This suggests the impacts must have come from inside the system, because if they had come from outside in the form of, for example, comets or asteroids, then Mimas would have crashed into them headlong, and the craters would appear on the leading hemisphere. Instead, debris careening inside the Saturn system caught up to Mimas from behind. The bowl-shaped craters on the leading hemisphere are not as thick as the minefield on the trailing side, and the floor of Herschel, moreover, is relatively speaking not much pocked at all. This means the huge comet or asteroid or other debris that almost annihilated Mimas one last time came after the last serious bombardment, obliterating earlier craters. By that time Mimas must already have been geologically dead. All over its surface the craters are sharp and deep, suggesting Mimas does not shift or grind itself: rocks and dirt don't shake loose from hillsides because there are no tremors or erosions of any kind. Grooves up to 55 miles long rut parts of the trailing face, but these were probably caused by the giant impact. As far as anyone can tell, Mimas is not geologically active in any sense. It has neither volcanoes (which is not surprising, as only three places -- Earth, Jupiter's Io and Neptune's Triton -- are known to be active in this way) nor tectonic deformation nor even the internal heat source that most larger places have. It appears in its mass to be undifferentiated, which means it is a mixture of ice and rock that never shifted into natural places above and below each other. It seems dead. No one knows if this is true or not, of course. Some astronomers suggest that Mimas is not dead -- or has not always been dead -- explaining that its compressed football shape (called a "triaxial ellipsoid") and size (about 240 miles in diameter) combined with the tidal effects of its own rotation and Saturn's tremendous gravity imply it differentiated into a small rocky core and thin mantle of ice. But the craters are so deep and sharp-rimmed that Mimas clearly has not stirred for a long time. Nothing happens there, it is simply suspended in a normal orbit 115,000 miles from Saturn. It has no atmosphere. Its stillness is all, whirling round and round Saturn, like the stillness of a corpse. In scientific terms, the comparison of Mimas to a corpse implies the comparison of other planets to living bodies. The essential deadness of Mimas, for example, contrasts with the essential aliveness of Io, which is so tormented by the push and pull of Jupiter's gravity that it constantly Next page Fires of the Sun home The Mind Errant