The fourth member of the Galilean satellites had finally been seen, and Larry Soderblom happily introduced Europa. “Well, some few months ago, before the Voyager 1 encounter, we thought we had some idea of what planets were like—at least the planets in the inner solar system: Mars, Mercury, the Moon, the Earth. And we’ve discovered many times over in the last couple of months how narrow our vision really was. Included in the Jovian collection of satellites are the oldest (Callisto), the youngest (Io), the darkest (Amalthea), the brightest (Europa), the reddest (Amalthea and Io), the whitest (Europa), the most active (Io), and the least active (Callisto). Today we found the flattest (Europa).” In spite of the appearance of a cracked or broken surface, Europa showed no topography at all. Toward the sunset line, where the low angle of illumination should reveal even low relief, “the surface disappears—as if it were the surface of a billiard ball.” It seemed clear that Europa has much less relief than the other two icy satellites, Ganymede and Callisto. But why can’t Europa’s surface support relief? Perhaps Europa has a thick ice mantle—on the order of 100 kilometers. If Europa is affected by tidal heating, then such an ice mantle might be “sort of soft and slushy” rather than rigid as are the crusts of Ganymede and Callisto. “The fact that the surface of Europa cannot support relief of any substantial amount suggests that the surface must be soft.” But, Dr. Soderblom added, there does not appear to be much lateral motion or rotation causing the surface markings—they don’t seem to be offset—rather, “it’s as if Europa had been cracked, broken, by some process which crushed it like an eggshell and just left the pieces sitting there. Expansion and contraction of ice and water are a good way to crunch up the surface.” [Illustration: Regio Galileo is the largest remnant of the ancient, heavily cratered crust of Ganymede. This Voyager 2 color reconstruction was made from pictures taken at a range of 310 000 kilometers; the scene is about 1300 kilometers across. Numerous craters, many with central peaks, are visible. The large bright circular features have little relief and are probably the remnants of old large craters that have been annealed by the flow of icy near-surface material. The closely spaced, arcuate linear features are analogous to features on Callisto, such as the “ripple” marks surrounding the ancient impact feature Valhalla. [P-21761C]]