Astronomers have found the strongest evidence yet that the ocean on Jupiter's moon Europa may consist of salty water, like our own. And, they say, that salty water appears to be making its way to the surface.
"We now have evidence that Europa's ocean is not isolated - that the ocean and the surface talk to each other and exchange chemicals," says Mike Brown of Caltech.
"That means that energy might be going into the ocean, which is important in terms of the possibilities for life there. It also means that if you'd like to know what's in the ocean, you can just go to the surface and scrape some off."
For the first time, it's been possible to establish some of the materials present on Europa's surface: including the presence of a magnesium sulfate salt, a mineral called epsomite, that could only originate from the ocean below.
"Magnesium should not be on the surface of Europa unless it's coming from the ocean," says Brown. "So that means ocean water gets onto the surface, and stuff on the surface presumably gets into the ocean water."
Europa's ocean is thought to cover the entire moon, to a depth of 100 kilometers. Europa's locked in relation to Jupiter, with the leading hemisphere having a yellowish appearance.
The trailing hemisphere appears splattered and streaked with a red material - which has caused debate for 15 years. The prevailing theory is that one of Jupiter's largest moons, Io, spews volcanic sulfur from its atmosphere, which is channeled by Jupiter's strong magnetic field towards Europa.
It's also clear that there's something other than pure water ice on the trailing hemisphere's surface.
"From Galileo's spectra, people knew something was there besides water. They argued for years over what it might be - sodium sulfate, hydrogen sulfate, sodium hydrogen carbonate, all these things that look more or less similar in this range of the spectrum," says Brown. "But the really difficult thing was that the spectrometer on the Galileo spacecraft was just too coarse."
Tests have indicated the presence of magnesium sulfate only on the trailing surface of Europa, indicating that the ocean can't be sulfate-rich, but instead must be rich in chlorine, with sodium and potassium present as chlorides.
Therefore, says Brown, the composition of Europa's sea is probably very like the salty oceans of Earth.
"If we've learned anything about life on Earth, it's that where there's liquid water, there's generally life," says JPL's Kevin Hand. "And of course our ocean is a nice salty ocean. Perhaps Europa's salty ocean is also a wonderful place for life."