An article published in the journal “Geophysical Research Letters” reports the detection of a stable presence of water vapor in the atmosphere of Jupiter’s moon Europa. Lorenz Roth of the Kth Royal Institute of Technology in Stockholm, Sweden, used ultraviolet observations conducted with the Hubble Space Telescope between 1999 and 2015 to infer the presence of water vapor. Its presence appears to be persistent only in one hemisphere of Europa, the one opposite the moon’s direction of motion in its orbit.

Lorenz Roth had led a team that had already used the same spectroscopic technique to detect the presence of water vapor in the atmosphere of Ganymede, another of Jupiter’s moons. Those are discoveries that help to better understand Europa and icy moons in general and offer information for upcoming space missions to explore Jupiter’s moons.

Europa has become one of the most studied moons in the solar system after the discovery of an underground ocean of liquid water. In recent years, geysers have been sighted that emitted water and molecules of various types, including complex ones, into space. However, that water quickly turns into vapor due to a lack of atmospheric pressure and tends to disperse into space.

Moons of that size can have an atmosphere, albeit a very thin one. It’s expected to be formed of oxygen and hydrogen along with water vapor. Even at that distance from the Sun, solar radiation can sublimate a small portion of the surface ice and cause some chemical reactions. The doubts concerned the amount of water vapor that could be present.

Studies of Ganymede’s atmosphere indicated that there was a different amount of molecular oxygen than expected. One explanation was the presence of atomic oxygen but a team led by Lorenz Roth detected a minimal presence of this element by analyzing observations conducted with the Hubble Space Telescope’s Space Telescope Imaging Spectrograph (STIS) and Cosmic Origins Spectrograph (COS) instruments. The apparent inconsistencies in the various observations were explained by the presence of water vapor, which also contains oxygen. These results were published in the journal “Nature Astronomy” in July 2021.

On Europa, the surface temperatures are lower than on Ganymede but Lorenz Roth still applied the examination technique already used. The result was the detection of water vapor in Europa’s atmosphere too. The unexpected result is that water vapor is persistently present only in this moon’s hemisphere opposite to the direction of motion while in the other there’s no indication of its presence.

It’s not possible to tell whether that water vapor comes from geysers, sublimation of surface ice, or a combination of both. Both NASA and ESA are planning space missions with probes to be sent in the coming years to study Europa as well, so each discovery is useful in defining these missions’ objectives.

In Europa’s case, the main objective is to understand if there are any life forms in its underground ocean. Other objectives are connected to the study of the formation and evolution of Jupiter and its moons. Those are useful studies to better understand the history of the solar system and in the growing studies of other planetary systems that include gas giant planets and large moons that could host life forms.