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.
These new observations seem to indicate that there’s an undersurface ocean on Europa, with salty water loaded with sodium-, potassium-, and magnesium chloride. It gets to the surface somehow, and on the trailing hemisphere gets whacked with high-speed sulfur from Io. These form sulfates. The sodium and potassium sulfates go away, leaving behind the magnesium sulfates. That’s what Brown and his colleague Kevin Hand found.
Also, that “gets to the surface somehow” part is intriguing. It means that the water under the surface, and the ice above it, can get mixed. Maybe it’s from cracks in between the ice floes, or maybe it’s from weak geysers, lazier versions of what we see from Saturn’s moon Enceladus. The exact mechanism isn’t known, and that will be the target for future work, no doubt. But the clear conclusion is that the liquid interior and solid exterior get mixed.
And why is this so very cool? Well, it means that if we want to find out what the ocean under Europa is made of, we don’t have to punch a hole through kilometers of ice (some missions have been proposed to do just that). All we have to do is either aim better telescopes at the surface, or send more sophisticated probes to do the same. Digging would be better, of course, but may not be so urgent.
And another thing. On Europa we have warm salty water, interesting chemicals carbon-based molecules — called organic molecules — have been found on two of Jupiter’s other moons, and energy supplied both from tides and from Jupiter’s crazy strong magnetic field. We have no direct evidence for life on or in Europa, but wow, the ingredients really do look like they’re there, and this new observation indicates the surface and interior can mix. Between Europa and Enceladus (and maybe even Titan), the prospects for finding some sort of biological presence in our solar system outside of Earth are still very, very interesting.