In 1979, images from Voyagers 1 and 2 showed Jupiter’s moon Europa to have a cracked but billiard-ball-smooth surface. Planetary geophysicists concluded that this icy world must have a deeply buried ocean, which must episodically flood the surface.
That’s when astrobiologists started talked earnestly about the possibility that Europa’s ocean might conceivably host life. But the putative ocean’s depth below the ice crust is unknown, though it must be at least several miles thick — a formidable barrier to exploring it with submersible, life-seeking probes.
It turns out that sampling Europa’s ocean might be easier than once thought. New results from the Hubble Space Telescope, announced this week, show what appear to be towering plumes of water jetting away from Europa’s surface. Hints that Europa might have water-powered geysers first came to light in 2012, when a team led by Lorenz Roth (Southwest Research Institute) used HST to spectroscopically detect localized clouds of hydrogen and oxygen atoms in Europa’s vicinity. It was tantalizing but not conclusive evidence.
The new results, though again not offering rock-solid proof, use an entirely different technique to suggest that Europa might be belching water into space. William Sparks (Space Telescope Science Institute) and others have used Hubble to record images of Europa as it crossed in front of Jupiter. They wanted to see if the moon has a thin atmosphere, which would show up as a dark aura around Europa when viewed in silhouette against Jupiter. (Other observers are trying to exploit this same transit technique to detect atmospheres around the planets of distant stars.)
Sparks’ team tracked 10 transits of Europa from December 2013 to March 2015, along with seven other sessions to model the appearance of Europa itself. They used a far-ultraviolet channel (centered at 150 nanometers) on HST’s Space Telescope Imaging Spectrograph, because that wavelength is scattered by Jupiter’s high-altitude hazes and its disk looks featureless. Moreover, Hubble’s resolution is best at ultraviolet wavelengths; with Jupiter 4 astronomical units away, this yields details as small as 38 km across.
Sparks and his team noticed what could be plumes on three occasions. Two of the presumed eruptions appear to come from the moon’s south polar region — the same general locale implicated by Roth’s team — and the third was nearer the equator. The vague puffs seem to extend at least 200 km (125 miles) above Europa’s limb. Details appear in September 29th’s Astrophysical Journal.
The researchers are hopeful but cautious about their results. In one image, for example, the base of one purported plume doesn’t coincide with the limb of Europa itself, as it should. And all three puffs cover roughly the same area as Europa itself, which on one hand might yield clues to the power of the eruptions — but also might imply that they’re unanticipated imaging artifacts. Forthcoming space observatories such as the James Webb Space Telescope should be able to reveal what’s really going on.
Still, the prospect of Europa episodically spouting samples of its hidden ocean directly into space has interplanetary planners salivating. Future spacecraft might be able to assess the ocean’s composition and life-hosting qualities merely by landing on the surface near an eruptive vent or flying through one of the plumes at close range.