Why is Europa spewing most of its water vapor out of one side?

Europa keeps releasing plumes of water vapor, something that Cassini already found out, and that's building up anticipation for the Europa Clipper mission even more. But there's something weirder going on.

The frozen Jovian moon is thought to have a subsurface ocean hiding beneath miles of ice, so it seems there should be no reason that only one of its hemispheres should persistently spew water vapor, but that is what Hubble observed. The space telescope revealed water vapor isn’t coming out of the leading hemisphere (which faces forward as it orbits) but only the trailing hemisphere (which faces the other way). But why? 

Finding water vapor in the atmosphere of Ganymede (which orbits on Europa’s trailing side) has given researcher Lorenz Roth of the KTH Royal Institute of Technology in Sweden -- who discovered it with his team this summer -- even more evidence of such a strange phenomenon. What is especially surprising about more water vapor being ejected by Europa is that Ganymede is the warmer moon.

“The trailing side is darker and thus warmer, likely increasing sublimation — gas molecules escaping the ice,” Roth, who led a study recently published in Geophysical Research Letters, told SYFY WIRE. “That favors the trailing hemisphere. So does the fact that this side is likely hit more by charged particles, which can also free water particles.”

It is possible that some of Europa’s water ice sublimated into vapor, going straight from solid ice to gaseous form. But whether or not this is actually happening remains to be proven. Ganymede’s water vapor has been found to arise from sublimation. This was also a discovery made by Roth and his team using Hubble data. Ganymede grows warmer around noon, and it is thought that temperatures rise enough around its equator for ice to sublimate.

The Space Telescope Imaging Spectrograph (STIS) aboard Hubble has observed both Europa and Ganymede in the far-ultraviolet, and looking into ultraviolet observations in its archive helped Roth find out how much oxygen was in Europa’s atmosphere. This was a breakthrough. Because water is made of oxygen, analyzing STIS observations at different wavelengths and finding out how much was there allowed him to get an idea of the amount of water vapor floating around Europa. The UV observations from STIS gave Roth some major insight.

“Europa has UV emissions at two wavelengths, and the relative brightness of these two emissions provides information about which species the light comes from,” he said. “Then we ran a special test, observing in and out of eclipse, and we looked into the images and where the emissions are on Europa.”

If water vapor on Europa sublimates, how much depends on temperature. Surface properties of its darker trailing hemisphere probably mean higher temperatures that result in more sublimation. Models have also shown thermal plasma running into Europa from behind, which might heat up the ice. Another way that water vapor can end up in Europa’s atmosphere is charged particles on the surface sending water molecules flying into space. Both that and sublimation can result in a difference between water vapor levels on its hemispheres.

Europa isn’t the only object that these observations could tell us about. They can contribute to future studies that reveal more information about how Europa, along with Jupiter and its other moons formed and evolved, which can also take us back billions of years. Jupiter is one of the planets which is believed to have had enough gravitational force to push planets and moons around from where they formed into their current orbits. Jupiter’s gravity keeps the Trojan asteroids, which NASA’s Lucy mission is now heading towards, in place in front of and behind it.

“Studies of the moons can allow us to learn about the age of the solar system when they formed,” Roth said. “Jupiter likely formed at the same time, and thus we can learn more about when giant gas planets form.”