Why Jupiter’s Moon Europa Is the Best Candidate For Finding Life Off Earth

Of all the worlds in our solar system, Jupiter's moon Europa is often raised as the best candidate for life, other than Earth. About the size of Earth's moon, it's got a briny ocean, containing twice as much water as what's on our entire planet, and a rocky core. Although it's covered in a thick crust of ice, the tidal forces produced as it orbits Jupiter—its monstrous host planet—could help create energy and chemistry that's right for life.

This target just got even more tantalizing.

On Monday, scientists announced that they've seen what look to be water vapour plumes spewing from Europa's surface, and here's why that's exciting: When we send a spacecraft there, in the next decade or so, it won't have to drill down through thick ice to access Europa's ocean. It could sample vapour plumes to learn about the ocean's composition.

"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," said team lead William Sparks, astronomer with the Space Telescope Science Institute in Baltimore, speaking to reporters on a call about the discovery. The plumes were observed by NASA's Hubble Space Telescope, which captured direct ultraviolet images of Europa as it passed in front of Jupiter—ten passages, spanning 15 months. They saw what looked to be plumes three of those times.

The plumes, if they really are there, are mighty. They were erupting about 125 miles up, spraying material that must land on the moon's surface, these scientists say.

Sparks and the others emphasized that their results, which will be published in The Astrophysical Journal on Sept. 29, are statistically significant—but they wouldn't confirm that these really are water vapour plumes, beyond a shadow of a doubt. "We are really working at the limits of Hubble's unique capabilities," Sparks explained.

Hubble Directly Images Possible Plumes on Europa. Video: NASA Goddard/YouTube

While no natural phenomenon would explain the sightings, maybe it was some bizarre quirk of the instrument they aren't yet aware of, he acknowledged.

It's not the first time that a science team has spotted what could be vapour plumes ejecting from Europa; in 2012, a similar announcement was made. I called up Kurt Retherford, staff scientist at the Southwest Research Institute in San Antonio, Texas, who was a collaborator on that research, which also used the same instrument aboard from the Hubble Space Telescope to spot what looked like vapour plumes on Europa.

"It's encouraging to see these results provide some supporting evidence," he said after NASA made its announcement.

Alien Ocean: NASA's Mission to Europa. Video: NASA Jet Propulsion Laboratory/YouTube

Sometime in the 2020s, NASA's Europa Clipper mission will launch to Jupiter's icy moon, and hopefully will have the chance to explore its deep ocean—maybe swooping through the vapour plumes, if they're confirmed by that point, to gather a sample. (NASA's Cassini mission managed to sample a geyser of Saturn's moon Enceladus.)

"The Clipper mission has a set of instruments that are all very capable of measuring plume composition," Retherford told me. It won't tell us if there are jellyfish or something else living in that sub-surface ocean, but rather, whether it might be habitable.

Europa is looking like the best place in our solar system to go looking for life. Not only does it seem to have the right ingredients—we have the capability to study them, soon.

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