One of the Solar System's most interesting worlds just got even more intriguing. Observations from the Hubble Space Telescope reveal the thin but persistent presence of water vapor on Jupiter's moon Europa, but for unknown reasons, it only exists above the hemisphere opposite its direction of orbit.
The Voyager probes' revelation that Europa's surface ice covers a deep ocean of water has made it a prime target for the search for life. The 2013 revelation that this water intermittently breaks through to create 100 kilometer (60 mile) high geysers added to the excitement. Some of this water subsequently renews the ice on Europa's surface, but some turns to gas, leading to detections of the presence of water vapor.
However, the team that made the detection only observed water vapor once in 17 observations, suggesting its presence was short-lived. Since the geysers only appear occasionally, it was thought the gas might only survive briefly after eruptions. However, reanalysis of Hubble observations published in Geophysical Research Letters reveals the vapor is always present on Europa's trailing hemisphere, at least to levels Hubble can detect.
Europa, like most moons, is “tidally locked” taking as long to turn as it does to orbit, so one face is always pointed towards Jupiter. This also means one side is always leading in its orbit, and the other permanently trailing. Repeated Hubble observations from 1999 to 2015 on the trailing side found water on the trailing side alone.
The discovery is not entirely surprising. Sole author Dr Lorenz Roth of the KTH Royal Institute of Technology recently used the same combination of archival far-ultraviolet observations and spectra to find a water vapor atmosphere around Ganymede. Observations were made under differing conditions, such as when part of Europa was in eclipse and the rest in sunlight.
"The observation of water vapor on Ganymede, and on the trailing side of Europa, advances our understanding of the atmospheres of icy moons," Roth said in a statement. "However, the detection of a stable water abundance on Europa is a bit more surprising than on Ganymede because Europa's surface temperatures are lower than Ganymede's."
The two moons are exposed to the same amount of sunlight, but Ganymede is much darker and absorbs enough radiation to be 35 ºC (60 ºF) warmer. At −162 ºC (−260 ºF) it's a remarkable feat for Europa to keep water gaseous, even if it's only a billionth of an Earth atmosphere worth over one hemisphere.
The study confirmed previous measurements that found 30-50 times more molecular (O2) than atomic (O) oxygen in what passes for Europa's atmosphere.
Europa's trailing hemisphere is darker, and therefore warmer, than the leading hemisphere, but Roth is skeptical the difference is large enough to explain their observations. In the end they conclude; “the source of the water vapor can not unambiguously [be] identified.”