The icy surface of Jupiter’s moon Europa appears to continually supply its thin atmosphere with water vapor, according to Hubble observations from 1999 to 2015.
New research published in Geophysical Research Letters describes a stable aquatic atmosphere on Europa. Strangely, this apparent water vapor was only detected on the rear hemisphere of the moon, i.e. the side opposite to its orbital direction. Astronomer Lorenz Roth of the KTH Royal Institute of Technology in Sweden is the sole author of the article.
Europa presents a vast world ocean entirely covered with a layer of ice. Plumes of water are known to burst from cracks on the surface, sending steam more than 100 kilometers high. This results in scattered and transient pockets of water vapor in the extremely thin atmosphere of Europe.
But the water vapor described in the new study does not come from these geysers. On the contrary, it comes directly from the surface, because solid ice turns directly into gas. It seems to be a continual process leading to the perpetual renewal of water vapor in the atmosphere of Europe.
A paper earlier this year, co-authored by Roth, found similar traces of water vapor in the atmosphere of Jupiter’s moon Ganymede. Using the same observation technique, Roth has now shown that a similar process appears to be occurring over Europe, but on one side only. In both cases, the astronomer detected traces of oxygen as shown by ultraviolet observations made by the Hubble Space Telescope. In the case of Europa, Roth spotted these telltale spectral signatures in archived data sets, which were collected by the Hubble Imaging Spectrograph (STIS) in 1999, 2012, 2014 and 2015. Ultraviolet observations were do while Europa appeared in various places in its orbit. around Jupiter.
Hubble data showed the abundance of oxygen, a major component of water. Roth examined the strength of these emissions at different wavelengths to infer the presence of water vapor in Europe’s atmosphere. Roth considered other possibilities, such as isolated oxygen molecules, hydroxide, and even carbon dioxide, as all of them can produce oxygen signatures. But as Roth explained in an email, only water “is really consistent with the data and therefore we conclude that [water] must be present “, adding that” there remains however an indirect detection “.
This potential detection of stable water vapor on Europa came as a surprise, given the temperature differences between her and Ganymede; Europa, because it has a highly reflective surface, does not get hotter than -265 degrees Fahrenheit. That’s about 60 degrees Fahrenheit cooler than surface temperatures on Ganymede. But even with freezing temperatures in Europe, water ice on the surface is able to sublimate, passing directly from a solid to a gas.
Why water vapor appears over only one of the hemispheres of Europe is now an open question. All sides of Europe are exposed to the sun during its 42-hour day, and Hubble has always observed the sunny side. It’s a strange observation, but Roth has a few theories.
“The back hemisphere is darker and therefore probably warmer because ‘dark’ means more light is absorbed, resulting in heat. This means that the water molecules could be more easily released from the sublimation on the hotter side, ”explained Roth. “The rear hemisphere is also the side where charged particles flow to Europe. More charged particles could strike the surface from this side, but this is not clear because charged particles have complex trajectories.
Future work will be needed to confirm Roth’s detection of persistent water vapor in Europe’s atmosphere and to solve this latest celestial puzzle. A pair of upcoming missions, that of NASA Mower Europe and ESA Jupiter’s Frozen Moons Explorer, could greatly contribute to our understanding of Jupiter and its fascinating water-rich moons.