KTH Royal Institute of Technology: KTH researcher finds persistent water vapour in Europa’s atmosphere

Nearly the size of Earth’s moon, Europa has been a subject of interest on account of its potential to harbour life. This speculation is based on the suspicion that beneath its ice crust—where the surface temperature is believed to hover around -170C—there lies an ocean containing more water than found on Earth.

To verify the water vapour, Roth used a technique that he applied to the recent revelations about water vapour on Ganymede, one of Jupiter’s other 78 moons. Water vapour had been suspected in plumes emanating from Europa’s surface, which had been documented over the years by the Hubble Space Telescope. But he says water vapour was not believed to be a part of the moon’s global atmosphere.

In the new findings, Roth reports that water vapour is present in Europa’s atmosphere and it’s constantly being replenished—although not by means of evaporation.

He says the vapour source could be sublimation—meaning water transforms from its solid ice to vapour phase without a liquid phase.

“It is unclear whether sublimation can be the source,” Roth says. “Actually it is a bit too cold, but we do not know temperatures and surface properties well enough to exclude sublimation.”

Additionally, Roth found that the vapour is only present on the moon’s trailing hemisphere, that is, the side that is opposite the direction of its orbit around Jupiter.

These revelations—from Europea and Ganymede—will help inform upcoming ESA and NASA missions to Europa to further understand the evolution of Jupiter, the solar system’s largest planet.

“The observation of water vapour on Ganymede and on the trailing side of Europa advances our understanding of the atmospheres of icy moons,” Roth says. He also expressed surprise about the detection of abundant H2O, given the moon’s low surface temperature.

Roth arrived at his findings after examining archival ultraviolet observations of Europa made by Hubble’s Space Telescope Imaging Spectrograph (STIS) over a 15-year period (1999 to 2015). From these he could determine the amount of oxygen—one of the constituents of water—in Europa’s atmosphere, and by interpreting the strength of emission at different wavelengths they inferred the presence of water vapour.