On Monday, NASA announced what most of us thought they would. Evidence of water vapor plumes erupting from Jupiter’s moon Europa. Using the Hubble Space Telescope, a team of astronomers discovered what they believe to be plumes of water vapor rising an estimated 125 miles before raining back down onto Europa’s surface.
The team, led by William Sparks Telescope Science Institute in Baltimore, were not even hunting for water plumes. Their time with the Hubble was originally to determine if Europa had a thin exosphere (extended atmosphere).
“The atmosphere of an extrasolar planet blocks some of the starlight that is behind it,” says Sparks. “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.”
But that’s not all they could see. This same method would show water vapor plumes erupting on Europa’s surface, if they existed.
And that’s what they saw. Sparks’ team watched Europa pass in front of Jupiter 10 times over the course of 15 months. They saw plumes three times. Here’s a composite image showing the water vapor plumes. Note the whiter areas extending from the surface at the 7 o’clock position.
These plumes are thought to come from a subsurface ocean on Europa.
Yesterday’s announcement isn’t the first time Hubble saw evidence of water vapor plumes on Europa. Researchers saw the same plumes in the same area (Europa’s south polar region) in 2012. “By far the simplest explanation for this water vapor is that it erupted from plumes on the surface of Europa,” said lead author Lorenz Roth at the time.
But in 2014, data from Cassini’s 2001 Jupiter flyby showed no evidence of water vapor plumes around Europa. “We found no evidence for water near Europa, even though we have readily detected it as it erupts in the plumes of Enceladus,” Larry Esposito, UVIS (one of Cassini’s instruments) lead at the University of Colorado at Boulder, said in 2014.
Which brings us back to yesterday’s announcement. What’s different here is Sparks’ team used a different method than Roth’s team to find the same plumes at the same spot.
“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.”
What yesterday’s announcement does is places another piece of supporting evidence in the water vapor column.
There are still some issues, though. Both teams have yet to detect the plumes at the same time using their different methods. Roth’s team tried to spot plumes within a week of one of the detections by Sparks’ team and couldn’t find them.
Right now, the evidence points to plumes sporadically erupting on the surface of Europa. It would explain why Cassini’s data doesn’t show evidence of plumes and why the two teams have yet to detect them at around the same time. It appears Europa isn’t like Saturn’s moon Enceladus. Plumes on Enceladus erupt on a constant basis.
Now, we wait for confirmation. NASA’s next generation telescope, the James Webb Space Telescope, should be able to do it by observing the moon in infrared. But that telescope isn’t scheduled to launch until 2018.
If the James Webb Space Telescope can’t give firm confirmation, a planned NASA orbiter will. The unnamed spacecraft will be equipped with a thermal instrument to survey Europa’s icy surface at altitudes as low as 16 miles above the surface. Europa’s potential water vapor plumes won’t stay secret for much longer.