Europa's Elusive Water Plume Paints Grim Picture For Life

A meteorite may have been responsible for a water plume briefly spotted above Europa two years ago, implying it takes a very rare event to breach the ice on the Jovian moon.

Astrobiologists worldwide received news in December 2013 that water vapor was detected in Hubble Space Telescope observations of Jupiter's moon Europa, which is considered one of the top potential locations in our solar system for life. Those results were published in the journal Science and led by Lorenz Roth, a planetary scientist at the Southwest Research Institute in Texas.

However, follow-up observations of Europa have found no plume emanating from the moon. A new paper based on observations made by NASA's Saturn-orbiting Cassini spacecraft reveals that Europa's atmosphere is 100 times less dense than claims in previous publications, and composed mainly of atomic rather than molecular oxygen.
Furthermore, the magnetosphere plasma, or superheated gas, at Europa’s orbit is very hot, with properties indicating that the plasma is mainly composed of ions, or charged particles, from the nearby moon Io.

Based on the plasma properties, the rate of injection of neutral gas from the surface of Europa is probably very low. This means the plasma in Europa's atmosphere has a low density, and a low escape rate into its magnetic field or magnetosphere.

No one knew what the plasma was made of before Cassini's measurements, which led some researchers previously to believe that the plasma came from Europa. Plasma can be comprised of different molecules, including hydrogen and oxygen, but Cassini's spectroscope was not designed to measure which species of molecules were present in the gas.

Because molecular hydrogen and atomic hydrogen were not found in Cassini's measurement of the plasma, this means that the plasma did not come from Europa as was previously suspected. The plasma that as detected was made of sulfur dioxide — a product of volcanoes on Io.

“Our conclusion was that almost everything we were seeing was plasma from Io,” said Don Shemansky, chief scientist of the planetary and space science division of space weather company Space Environment Technologies (PSSD/SET).

However, a meteorite that briefly threw water aloft from Europa's surface cannot be ruled out, he said