Europa: Jupiter's Icy Moon

Europa is the smallest of planet Jupiter’s four largest moons and the second moon out from Jupiter. Until 1979, it was just another astronomy textbook statistic. Then came the close-up images obtained by the exploratory spacecraft Voyager 2, and within days, Europa was transformed—in our perception, at least—into one of the solar system’s most [#highlight1]intriguing[/highlight1] worlds. The biggest initial surprise was the almost total lack of detail, especially from far away. Even at close range, the only visible features are thin, kinked brown lines resembling [#highlight2]cracks in an egg shell[/highlight2]. And this analogy is not far off the mark.

The surface of Europa is almost pure water ice, but a nearly complete absence of craters indicates that Europa’s surface ice resembles Earth’s Antarctic ice cap. The eggshell analogy may be quite accurate since the ice could be as little as a few kilometers thick—a true shell around what is likely a subsurface liquid ocean that, in turn, encases a rocky core. The interior of Europa has been kept warm over the eons by tidal forces generated by the varying gravitational tugs of the other big moons as they wheel around Jupiter. The tides on Europa pull and relax in an endless cycle. The resulting internal heat keeps what would otherwise be ice melted almost to the surface. The cracklike marks on Europa’s icy face appear to be fractures where water or slush oozes from below.