Evidence of plumes of water vapour or ice particles jetting 200 kilometres above multiple parts of the surface of Jupiter’s icy moon Europa has been detected.
The plumes were spotted as the Hubble Space Telescope watched the silhouette of Europa as it passed between us and the bright background Jupiter, something that happens each time Europa circles the giant planet.
The idea came from exoplanet studies, which have used the light of stars to probe extrasolar planets’ atmospheres as the planets pass between us and their suns.
In the case of Europa, the scientists were observing the plumes more directly, “but the principle is similar”, says William Sparks of the Space Telescope Institute in Baltimore, Maryland, lead author of a study to appear in the 29 September issue of The Astrophysical Journal.
The new finding supports an earlier finding made in 2012 and published in 2014, when a Hubble team led by Lorenz Roth of the Southwest Research Institute in San Antonio, Texas, used spectroscopic techniques to detect plumes of ionised water at similar elevations above Europa’s surface.
But that study only found the plumes on one occasion. This study found them on three separate occasions over the course of 15 months. That’s important, Sparks says, “because previously there’d just been one piece of evidence that these things exist. Now we have more”.
The new study also used a different method of detecting the plumes, looking for the degree to which they block solar ultraviolet light reflected from Jupiter.
That adds credence to the finding, says Jennifer Wiseman, senior Hubble project scientist at NASA’s Goddard Space Flight Centre in Greenbelt, Maryland, because the two approaches complement each other, providing independent evidence that plumes do occur.
And while Sparks’ observations can’t determine what exactly the plumes are made of, “we presume it to be water vapour or ice particles because that’s what Europa is made of”, he says.
Europa is believed to have a large, subsurface ocean beneath its icy surface – an ocean containing twice as much water as all of Earth’s oceans put together.
That makes it one of the most likely places in the solar system to harbour extraterrestrial life, and scientists have long dreamed of drilling through the ice to take samples.
But now, it appears, samples might be coming through the ice and fountaining into space, high enough that a future Europa mission should be able to analyse them. “We may be able to ‘explore’ that ocean for organic chemicals or even signs of life without having to drill through unknown miles of ice,” Sparks says.
Scientists are not sure how the plumes are produced, but one possibility, says Britney Schmidt, a space scientist at Georgia Institute of Technology, in Atlanta, Georgia, is that tidal flexion from Jupiter’s massive gravity field may create cracks in the icy shell.
But that’s not the only way it could happen, or possibly even the most likely. “There are a lot of ways we think water may reach the surface,” Schmidt says, but the important aspect of all of them is that that water should contain material from the subsurface ocean.
One potential problem is that the plumes appear to be intermittent. Roth’s 2014 study only detected them once, despite multiple attempts. The new study, which used observations taken over the course of 15 months, found plumes on three occasions, but not on seven other occasions.
Even if the plumes are intermittent and short-lived, that may not be a problem for NASA’s proposed Europa mission, tentatively scheduled for launch sometime in the 2020s.
In computer simulations, “we are constantly adjusting our trajectory for where and how we fly by Europa”, says Curt Niebur, NASA’s Europa mission program scientist.
“It’s amazing the degree of flexibility we have. We can, within a week or two, adjust trajectory so we can effectively fly right over it, if we find a plume or other [interesting] feature.”