Geysers on Europa? Hubble Space Telescope finds more evidence for water vapour plumes

Composite image showing the possible water vapor plumes near the south pole of Europa, at about the 7 o’clock position. The image of Europa, from the Galileo and Voyager missions, is superimposed on the Hubble data. Image Credit: NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center
Composite image showing the possible water vapour plumes near the south pole of Europa, at about the 7 o’clock position. The image of Europa, from the Galileo and Voyager missions, is superimposed on the Hubble data. Image Credit: NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center

Intriguing new findings about Jupiter’s moon Europa were announced today by NASA, and while they don’t involve any direct evidence for life, they do provide more information on how scientists could better search for such evidence, without having to drill through the icy crust to the ocean below. The new observations, by the Hubble Space Telescope, have added to the evidence for active water vapour plumes on Europa – an exciting possibility, since they would possibly originate from the subsurface ocean, similar to the plumes on Saturn’s moon Enceladus. And just like the Cassini spacecraft has already done at Enceladus, those plumes – geysers really – could be sampled directly by a future spacecraft such as Europa Clipper.

The new findings announced today aren’t quite conclusive yet, but they are compelling. They will be published in the Sept. 29 issue of The Astrophysical Journal.

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

Annotated version of the illustration of Europa’s plumes. Image Credit: NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center/Z. Levay (STScI)
Annotated version of the illustration of Europa’s plumes. Image Credit: NASA/ESA/W. Sparks (STScI)/USGS Astrogeology Science Center/Z. Levay (STScI)

Possible plumes have been observed at Europa before, also using the Hubble Space Telescope, but confirmation is difficult given Europa’s immense distance from Earth. The new observations, led by William Sparks of the Space Telescope Science Institute (STScI) in Baltimore, Md., add to the evidence and strengthen the case that the plumes are real, albeit intermittent. In this case, the finger-like projections were seen emanating from Europa’s limb as the moon passed in front of Jupiter. The plumes are estimated to rise about 200 kilometres (125 miles) above the surface.

Sparks’ team was primarily trying to observe a possible thin, extended atmosphere, an exosphere, on Europa. It turned out that this would also be an ideal way to see Europa’s plumes, if they really were there.

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

While Enceladus’ plumes are ongoing, the ones at Europa appear to be more intermittent. The team observed Europa passing in front of Jupiter 10 times during 15 months, but the possible plumes were seen only during three of those occasions. This also correlates with the previous observations.

The earlier observations of the possible plumes were made in 2012, by a team led by Lorenz Roth of Southwest Research Institute (SwRI) in San Antonio, Texas. Using the Space Telescope Imaging Spectrograph (STIS) instrument on Hubble, they detected what appeared to be water vapour coming from Europa’s south polar region, and extending about 160 kilometers (100 miles) from the surface. In the new study released today, the plumes also appear to originate from near the south pole, although there was also another one closer to the equator. This is similar to Enceladus, where all the known plumes, over 100 in all, erupt from the south pole as well.

“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 and his team found,” 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.”

The fact that both teams have come up with similar results, using different methods, is exciting as that bolsters the case that these really are plumes of water vapour. It was also noted during the press briefing this morning that water vapor plumes are the only convincing natural explanation for the observations; the only other possibility would be some kind of instrument error on Hubble itself, but that is considered to be very unlikely and is pretty much ruled out now. At this point, the case for plumes on Europa has been almost but not quite yet proven. Only further observations will help to nail it down. Even these new observations from Hubble are near the limits of the telescope’s imaging capabilities.

Artist’s conception of the plumes on Europa. Image Credit: NASA/ESA/G. Bacon (STScI)
Artist’s conception of the plumes on Europa. Image Credit: NASA/ESA/G. Bacon (STScI)
Illustration depicting how the plumes were detected while Europa transited across the face of Jupiter. Image Credit: NASA/ESA/A. Feild (STScI)
Illustration depicting how the plumes were detected while Europa transited across the face of Jupiter. Image Credit: NASA/ESA/A. Feild (STScI)

Additional and better observations could be made using the James Webb Space Telescope, scheduled to launch in 2018. Unlike Hubble, it would use infrared cameras to look at Europa.

According to Paul Hertz, director of the Astrophysics Division at NASA Headquarters in Washington, D.C.: “Hubble’s unique capabilities enabled it to capture these plumes, once again demonstrating Hubble’s ability to make observations it was never designed to make. This observation opens up a world of possibilities, and we look forward to future missions – such as the James Webb Space Telescope – to follow-up on this exciting discovery.”

The big advantage at Enceladus was that the plumes were discovered by the Cassini spacecraft in 2005. This enabled, of course, much closer and more detailed studies of the plumes. Cassini has even flown through the plumes, “tasting” them as it were, and analyzing what is in them. They were found to contain water vapour, ice particles, salts, and organics of various complexity. Since they are thought to originate from the salty ocean below the surface (global, just as on Europa), this provides clues as to the conditions present in the ocean itself and how potentially habitable it might be. From this data, it is now thought that the subsurface ocean on Enceladus is likely quite habitable by earthly standards, at least for microscopic life. Cassini wasn’t designed to be able to detect life anywhere, but what it has found already is exciting. There is also now evidence for geothermal activity on Enceladus’ ocean floor, which could provide heat and nutrients, just as on Earth. The same could be true for Europa as well.

The geysers on Europa are thought to be similar to those on Saturn’s moon Enceladus, as seen by the Cassini spacecraft. Photo Credit: NASA/JPL-Caltech
The geysers on Europa are thought to be similar to those on Saturn’s moon Enceladus, as seen by the Cassini spacecraft. Photo Credit: NASA/JPL-Caltech

Confirmation of the plumes on Europa would also be a big factor in the planning of the upcoming Europa Clipper mission, sometime in the 2020s. It would provide an opportunity for the probe to be able to sample the spray and analyze it with its instruments, just like Cassini has done several times at Enceladus. This would provide clues as to the conditions in the ocean down below, and possibly whether any kind of life might be lurking below the ice crust. Otherwise, more ambitious future missions might have to drill through the ice, at least to one of the other lakes which are also thought to exist within the ice crust itself. Plus, while Europa is a long way from Earth, it is still a lot closer than Enceladus.

There is also, however, other evidence that material from the ocean below may make its way to the surface through some of the many large fissures in the surface ice, the ones which make Europa look like a giant cracked egg. The dark material seen in these fissures appears to be sea salt, which is then discolored by the harsh radiation surrounding Jupiter. A probe such as Europa Clipper could easily study these deposits.

Although the Juno spacecraft is now in orbit around Jupiter, its mission is to study Jupiter itself, and unfortunately can’t be used to search for Europa’s plumes.

Further in the future, a small submarine could one day explore the Europan ocean, similar to how robotic probes are used now to study the ocean below the ice at the Earth’s poles. No one knows what such an expedition might find, but it would undoubtedly be an exciting adventure. Last year, NASA selected 15 proposals for study under Phase I of the NASA Innovative Advanced Concepts (NIAC) program, and one of those is a “robotic squid” or aquatic rover which could explore the subsurface ocean of Europa or other similar moons like Enceladus. From the press release:

“One of the selected proposals calls for the use of a soft-robotic rover for missions that can’t be accomplished with conventional power systems. This rover would resemble an eel with a short antenna on its back that harvests power from locally changing magnetic fields. The goal is to enable amphibious exploration of gas-giant moons like Europa.”

Europa has long been considered one of the best places in the Solar System to search for evidence of alien life, and now the potential plumes discovery makes it more appealing than ever. Europa’s ocean has been calculated to contain twice as much water as all of Earth’s oceans combined. If some of that water is leaking into space, then it presents the perfect opportunity to learn more about what lies beneath the icy surface, in the dark waters below.

The new paper is available here.

This article was first published on AmericaSpace.

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