Europa

Jupiter has 65 satellites, according to the Carnegie Institution for Science. They vary greatly (four of them make up almost all the mass, and Ganymede makes up a third of "almost all" by itself). It is these four, known as the Main Group or the Galilean Moons, that have captured the most interest, perhaps mostly because, as the largest, the are the easiest to see. They are known as the Galilean Moons because they were discovered in 1609 and 1610 by Galileo Galilei, who also famously recorded their orbits around Jupiter, bringing more support to the heliocentric view of the solar system.

As an aside, Galileo originally wanted to name the moons after the four brothers at the head of the powerful Medici family: Cosimo, Francesco, Carlo, and Lorenzo. This idea, though, proved to be less popular outside Florence, leading to the names we know today: Io, Ganymede, Europa, and Callisto, four lovers of the god Zeus in Greek mythology.

Europa, a Galilean moon and the sixth from Jupiter, is a bit smaller than Earth's moon, has a surface made up mostly of silicate rock, and probably has an iron core. It even has a thin atmosphere of oxygen. These and other geological features of the moon have led some to speculate that Europa holds large reserves of subterranean water, possibly entire oceans underground. Because of this, Europa is one of the leading candidates in the solar system for potential habitability and the presence of extraterrestrial life (well, at least since Mars let us down).

Which leads me into this new development. Geophysicists at UT Austin have possibly discovered a large (about the volume of the Great Lakes), surface lake of liquid water on Europa (and by surface, I of course mean several kilometers below the surface). The lake, like much of Europa's surface is mostly obscured by a thick ice shell, and it's not really known what is below it. A large subsurface ocean is one possibility, but another model claims that the top layer, composed of cold and brittle ice, covers a much thicker layer of warmer, convecting ice.

As far as I can tell, this new find doesn't necessarily support either model. This simulation of the formation of the lake seems to ignore that which lies far beneath the surface, instead showing how the lake can form just beneath the top ice sheet. It's possible, though, that I'm not interpreting this correctly.

This certainly isn't enough to get into the idea of actual life on Europa (though apparently some people have even been talking about colonizing it). Liquid water is, however, a step in that direction; it is also just interesting to learn more about the features of our solar system.