By Phil Plait
He killed Pluto, but he may have just breathed life into Europa exploration.
Astronomer Mike Brown — discoverer of the giant outer-solar-system iceball Eris, which is what started the machinery that kicked Pluto out of the planet club — has found some pretty strong evidence that Jupiter’s moon Europa has sprung a leak. Its undersurface ocean may be mixing with the icy surface, making it possible to understand its composition without having to dig down through dozens of kilometers of solid ice.
Mind you, we’ve been eyeballing Europa’s ocean as a potential habitat for life for decades. This news is not evidence of life, but it does add reason to look at Europa even more closely.
This evidence that the surface ice and undersurface ocean are in intimate contact comes in a series of steps. But first, you need to understand Europa. It’s an iceball 3,120 kilometers (1,940 miles) across, about a quarter the size of Earth, roughly the same size as our Moon. There’s a lot of evidence it has a global ocean about 100 kilometers deep under its icy surface: For one thing, pictures taken by probes show the surface is broken up like ice floes. There are also very few craters, meaning the surface is young, constantly resurfaced by some sort of erosion — probably shifting, grinding ice floes as they float on the ocean. There are more technical reasons to think that vast amounts of water exist under the surface as well, so most astronomers are pretty sure about this.
The water is kept liquid by heating via tidal forces, the effects of the gravity of its massive parent planet. But what kind of water is it? Salty, acidic, pure? One hint is that Europa’s density is more than that of rock, indicating it has a dense core (water is much less dense than rock, so to get such a high average density there must be a lot of rocks in its heart). When you put rocks in water they dissolve, giving up some chemicals. That makes it unlikely the ocean is fresh water.