The NASA/ESA Hubble Space Telescope has imaged what may be water vapor plumes erupting off the surface of Europa, the sixth-closest moon of Jupiter and the smallest of its four Galilean satellites.

Europa has a global ocean containing twice as much water as Earth’s oceans, but it is protected by a layer of extremely cold and hard ice of unknown thickness.

The plumes provide a tantalizing opportunity to gather samples originating from under the surface without having to land or drill through the ice.

“Europa’s ocean is considered to be one of the most promising places that could potentially harbor life in the Solar System. These plumes, if they do indeed exist, may provide another way to sample Europa’s subsurface,” said Dr. Geoff Yoder, of NASA’s Science Mission Directorate.

The plumes are estimated to rise about 125 miles (200 km) before, presumably, raining material back down onto Europa’s surface.

A research team headed by Space Telescope Science Institute scientist Dr. William Sparks observed these finger-like projections while viewing Europa’s limb as the moon passed in front of Jupiter.

The original goal of the team’s observing proposal was to determine whether Europa has a thin, extended atmosphere, or exosphere.

Using the same observing method that detects atmospheres around planets orbiting other stars, the researchers realized if there was water vapor venting from Europa’s surface, this observation would be an excellent way to see it.

“The atmosphere of an extrasolar planet blocks some of the starlight that is behind it,” Dr. Sparks said.

“If there is a thin atmosphere around Europa, it has the potential to block some of the light of Jupiter, and we could see it as a silhouette.”

“And so we were looking for absorption features around the limb of Europa as it transited the smooth face of Jupiter.”

The scientists observed ten transits of Europa across the face of Jupiter spanning over a year.

They saw what could be plumes erupting on three of these occasions.

About four years ago, Dr. Lorenz Roth from the Southwest Research Institute and the University of Cologne and co-authors detected evidence of water vapor erupting from the south polar region of Europa and reaching more than 100 miles (160 km) into space.

Although both teams used Hubble’s Space Telescope Imaging Spectrograph, each used a totally independent method to arrive at the same conclusion.

“When we calculate in a completely different way the amount of material that would be needed to create these absorption features, it’s pretty similar to what Roth et al found,” Dr. Sparks said.

“The estimates for the mass are similar, the estimates for the height of the plumes are similar. The latitude of two of the plume candidates we see corresponds to their earlier work.”

But as of yet, the two teams have not simultaneously detected the plumes using their independent techniques.

Observations thus far have suggested the plumes could be highly variable, meaning that they may sporadically erupt for some time and then die down.

If confirmed, Europa would be the second moon in the Solar System known to have water vapor plumes. In 2005, NASA’s Cassini spacecraft detected jets of water vapor and dust spewing off the surface of Saturn’s moon Enceladus.

The team’s results will be published in the September 29 issue of the Astrophysical Journal.

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William Sparks et al. 2016. Probing for Evidence of Plumes on Europa with HST/STIS. ApJ, in press

This article is based on a press-release issued by NASA.