Europa's surface exchange with underground ocean
Sen—Astronomers have found the strongest evidence yet that salty water, from the ocean beneath the frozen exterior of Jupiter's moon Europa, actually makes its way to the surface.
Using the Keck II Telescope in Hawaii, and its OSIRIS spectrometer, Mike Brown, from the California Institute of Technology and Kevin Hand from NASA's Jet Propulsion Laboratory, have identified a spectroscopic feature on Europa's surface indicating the presence of a magnesium sulfate salt. The mineral, called epsomite, could have formed by oxidation of a mineral likely originating from the ocean below.
Europa's ocean is believed to cover the whole moon under a thin ice shell, and is thought to be about 60 miles (100 kilometres) deep. The moon remains locked in relation to Jupiter, with the same hemisphere always leading and the other trailing in its orbit. The leading hemisphere has a yellowish appearance, while the trailing hemisphere seems to be splattered and streaked with a red material. It is thought that Jupiter's moon, Io, spews volcanic sulfur from its atmosphere, and Jupiter's strong magnetic field sends some of that sulfur toward the trailing hemisphere of Europa.
Chloride salts bubble up from Europa's liquid ocean and reach the frozen surface where they are bombarded with sulfur from volcanoes on Jupiter's innermost large moon Io. This illustration of Europa (foreground), Jupiter (right) and Io (middle) is an artist's concept. Credit: NASA/JPL-Caltech
"Magnesium should not be on the surface of Europa unless it's coming from the ocean," Brown says. "So that means ocean water gets onto the surface, and stuff on the surface presumably gets into the ocean water."
The finding suggests there is a chemical exchange between the ocean and surface, making the ocean a richer chemical environment. This exchange, Brown said, "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."
Brown and Hand mapped the distribution of pure water ice versus anything else. The spectra showed that even Europa's leading hemisphere contains significant amounts of non-water ice. Then, at low latitudes on the trailing hemisphere, the area with the greatest concentration of the non-water ice material, they found a tiny dip in the spectrum. They tested everything but the signature of magnesium sulfate persisted.
The magnesium sulfate appears to be generated by the irradiation of sulfur ejected from Jupiter's moon Io and, the authors deduce, magnesium chloride salt originating from Europa's ocean. Chlorides like sodium and potassium chlorides, which are expected to be on the Europa surface, are not usually detectable because they have no clear infrared spectral features. But magnesium sulfate is detectable. The authors believe the composition of Europa's ocean may closely resemble 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," Hand said. "And of course our ocean is a nice, salty ocean. Perhaps Europa's salty ocean is also a wonderful place for life."