“All these worlds are yours, except Europa.
Attempt no landing there.”
— 2010: The Year We Make Contact
In the fictional universe of ‘2010’, people on Earth receive a message being described as the most important one ever transmitted, concerning the presence of life on Europa. In the real world, scientists have possibly discovered the most important evidence to date: the presence of a liquid water ocean underground this fascinating Jovian moon.
This year has turned out to be a really good one for planetary science, with important and fascinating discoveries concerning the possible habitability of various places in the Solar System. Just days after we have reported here on AmericaSpace new evidence from the Mars Reconnaissance Orbiter about current flowing water on the Red Planet, it’s time for the fascinating Jovian moon Europa to come to the spotlight. Scientists working with the Hubble Space Telescope announced the results of their observations of water plumes on Europa’s surface.
The new Hubble observations were reported at a press conference during the recent 2013 Fall meeting of the American Geophysical Union, held in San Francisco, Calif., between 9-13 December. A scientific team, led by Lorenz Roth of the Southwest Research Institute in San Antonio, Texas, used Hubble’s Space Telescope Imaging Spectrograph, or STIS, to conduct observations of Europa in ultra-violet wavelengths during November and December 2012. The team was particularly interested in confirming the presence of water vapors coming from Europa, which were hinted at by previous observations from Hubble in 2009. During their 2012 observing campaign, the scientists discovered a vast amount of hydrogen and oxygen ions coming from the moon’s south polar region.
“With these images, we were able to detect water vapor in Europa’s south pole, for the first time,” says Kurt D. Retherford, co-author of the study, also of the Southwest Research Institute in San Antonio. “These images point to the existence of water vapor, about 200-km high, at or near the south pole of Europa.”
The presence of ions around Europa might not be seen as evidence of water vapors at first glance, but the moon lies inside Jupiter’s radiation-laden massive magnetosphere. This magnetosphere traps and accelerates charged particles along its magnetic field lines to very high speeds, creating radiation belts of very high intensity while producing auroras on Jupiter and its Galilean moons, including Europa. As Europa completes an orbit around Jupiter every 3.5 days, it sweeps across this intense magnetic field, dragging its magnetic field lines along, which pass right through the interior of the moon. Since Jupiter completes a rotation around its axis approximately every 10 hours, the magnetic field and radiation environment around Europa display a periodic 10-hour variability. By winnowing out these observed variabilities from the Hubble data, the scientists saw that some patterns of ionised hydrogen and oxygen were fixed above the limb of the moon near the south pole.
This observed pattern was consistent with the hypothesis of water coming out of Europa and breaking apart into hydrogen and oxygen ions by the intense radiation around Jupiter. Furthermore, these emissions varied as Europa orbited Jupiter. During its pericentre, when the moon was at its closest to Jupiter, the team observed no emissions at all. These were only seen at or near Europa’s apocentre, when the moon was at its farthest distance from the planet. This bode well with the suggestion of an underground water ocean on Europa and the presence of plumes on the surface that erupt this water into space, driven by Jupiter’s tidal forces which vary according to Europa’s distance from the massive planet.
Jupiter’s massive gravitational pull exerts tidal forces on Europa the same way the Moon exerts tidal forces on the Earth, creating the tides. This stretching and squeezing of the moon’s interior is at its strongest when Europa is at its pericentre, and at its lowest when at its apocentre. Consequently, any fissures on Europa’s surface would close while the moon was closest to Jupiter and would relax and open when Europa was at its farthest, allowing for subsurface water to erupt through fissures at the surface, accounting for the Hubble observations. “The plume variability supports a key prediction, that we should see this kind of tidal effect if there is a subsurface ocean on Europa,” says Retherford.
A similar phenomenon has been observed on Saturn’s moon Enceladus from the Cassini spacecraft, albeit in a much higher resolution. Cassini discovered the now-infamous fissures, better known as “tiger stripes,” near Enceladus’s south pole in 2005. These stripes consist of a series of four parallel fissures on the moon’s surface that spew out water ice, dust, and organic compounds high into space. Enceladus is another Solar System object believed to hold a subsurface liquid water ocean just like Europa. Although similar water plumes on Europa have long been hypothesised, they were never directly observed until now.
With all these different, exciting lines of evidence coming together, the question about the possibility of life comes naturally. “The discovery that water vapour is ejected near the south pole strengthens Europa’s position as the top candidate for potential habitability,” says Roth. “However, we do not know yet if these plumes are connected to subsurface liquid water or not.”
“We pushed Hubble to its limits to see this very faint emission,” says Joachim Saur of the University of Cologne, Germany, co-lead author and principal investigator of the Hubble observing campaign. “Only after a particular camera on the Hubble Space Telescope had been repaired on the last servicing mission by the Space Shuttle, did we gain the sensitivity to really search for these plumes.”
As powerful as it is observing from afar, the Hubble Space Telescope can’t provide the high-resolution views that a Europa-bound spacecraft would. And although Europa had been studied by the Galileo spacecraft that was in orbit around Jupiter from 1995 to 2003, its cameras and instruments lacked the necessary sensitivity and resolution to detect the plumes. “It’s really unfortunate that Galileo had only 11 passes of Europa,” said Dr. Jim Green, director of NASA’s Planetary Science Division during the press conference. “And all the knowledge we have of Europa is based on observations from those flybys. So the south pole was not covered very well, and my understanding is, that the highest resolution we have for some of those passes is about 100 km per pixel, so looking for these details, would not be found on the Galileo data.”
Indeed, even with Galileo’s several flybys of Europa, approximately 40 percent of the moon was left uncharted, including the south polar regions. And the recent Hubble observations have raised a whole series of new questions. “Do the vents extend down to a subsurface ocean or are the ejecta simply from warmed ice caused by friction stresses near the surface?” asks Roth. At the heart of it all, a key question remains: Is there life in that underground ocean?
In order to gain a better understanding, a dedicated mission to Europa is required. “We really do need a dedicated Europa mission,” Roth points out.
Still, the chances for such a NASA mission in the foreseeable future don’t look good. Although Europa as a destination was ranked as the second-highest priority in the 2013-2022 Planetary Science Decadal Survey and a Europa mission concept called Europa Clipper has been proposed to NASA, lack of funding has forced the space agency to postpone any plans for such a mission indefinitely. Although the European Space Agency has approved the JUpiter ICy moons Explorer, or JUICE mission, which would make some Europa flybys, it is not slated for launch until 2022, arriving at the Jovian system around the 2030 timeframe. With current budget realities, however, even such a mission occurring more than 15 years into the future may be the best we can hope for.