Source: Scientific America

The sea sloshing beneath the icy surface of Jupiter’s moon Europa just might be the best incubator for extraterrestrial life in our solar system. And yet it is concealed by the moon’s frozen outer shell—presenting a challenge for astrobiologists who would love nothing more than to peer inside. Luckily they can catch a partial glimpse by analyzing the flavor of the surface. And the results are salty.  

A new study published this week in Science Advances suggests that sodium chloride—the stuff of table salt—exists on Europa’s surface. Because the exterior is essentially formed from frozen seawater, the finding suggests that Europa’s hidden sea is drenched in table salt—a crucial fact for constraining the possibilities for life on the alien world.

Not that scientists have tasted a slice of the distant moon. To analyze Europa’s composition, astronomers study the light emanating from its surface, splitting it into a rainbow-like spectrum to search for any telltale absorption or emission lines that reveal the world’s chemistry. There is just one problem: Ordinary table salt is white and thus gives off a featureless spectrum. But harsh radiation—which exists at Europa’s surface in abundance—just might add a dash of color. That much was realized in 2015 when two NASA planetary scientists Kevin Hand and Robert Carlson published a study suggesting the yellowish-brown gunk on Europa might be table salt baked by radiation. To reach that conclusion, Hand and Carlson re-created the conditions on Europa within vacuum chambers—or as Hand calls them, “stainless steel shiny objects that are humming and whizzing.” Next, they placed table salt into those chambers, lowered the pressures and temperatures to simulate Europa’s surface, and blasted the samples with an electron gun to simulate the intense radiation.

Many of the electrons were then captured in empty spaces within the salt’s crystalline structure, transforming the salt’s color into a yellowish brown that bore an uncanny resemblance to that of the mysterious material filling the fissures and fractures crisscrossing Europa’s surface. The parallel caused Hand and Carlson to speculate that the gunk just might be irradiated table salt. But a match in color is not proof. Instead, proof could arrive in the form of very distinct spectroscopic features—particularly an absorption line at 450 nanometers within the visible portion of the electromagnetic spectrum. “One of the upshots of this irradiation is that it can help make the previously invisible, visible,” Hand says.

So, Samantha Trumbo, a graduate student at the California Institute of Technology, her advisor Michael Brown and Hand (now also at Caltech) turned the Hubble Space Telescope toward the distant moon in search of that very feature. And after four observations that ran from May to August 2017, they spotted it. “The match between the lab data and the telescopic data was just beautiful,” Hand says. It proves that the surface of Europa is indeed covered with sodium chloride—at least within a region called Tara Regio that harbors young ice thought to have erupted from the ocean relatively recently. “You’d be able to taste it,” Trumbo says. “If you licked the surface of Europa, at least in this spot, it would taste salty.”

Reggie Hudson, an astrochemist at the Cosmic Ice Laboratory at NASA’s Goddard Space Flight Center who was not involved in the latest study, is impressed by the full picture. “It’s a very nice synthesis of lab work, Hubble data, radiation chemistry and planetary science,” he says.

For years, astronomers have argued that another type of salt, magnesium sulfate, was prevalent on Europa’s surface. Under that thinking, the moon’s subsurface ocean might have resembled Epsom salts mixed into a bath. But this new work suggests instead the ocean is more akin to table salt mixed with tap water—like the solution you might create to soothe a sore throat or what you might find in the Earth’s oceans. “It really is a shift from what we’ve thought for the past 20 years or so,” Trumbo says. “If this sodium chloride… is really representative of the ocean composition, then the ocean, at least salt-wise, would be more similar to what we see on Earth.”

And that could boost the chance for life to arise and evolve. Even so, scientists cannot yet say what quantities of salt exist beneath the ice. Europa’s seas could ultimately prove to be too salty for life as we know it to exist there at all.

Then again, a more equitable mix of salt and water could allow life there to thrive—especially if that ocean is as active as our own. On Earth, water gets pulled into the seafloor around the flanks of hydrothermal vents—seafloor springs that spew water at superhot temperatures—before being jettisoned back out from the vents themselves. In that process magnesium is captured within the rocks, whereas sodium and chlorine escape. In theory, Europa’s table-salt–laced surface could hint that the moon’s seawater is similarly cycled and even point toward hydrothermal vents—a tantalizing prospect given that these vents on Earth are usually overrun with life. But Hand and Hudson both caution against making such a bold leap. “I’m not 100 percent convinced that sodium chloride on Europa’s surface means hydrothermal activity down under, but the two things are not mutually exclusive,” Hudson says.

At the very least, the finding shows that Tara Regio is a good spot to study further with NASA’s Europa Clipper mission—a probe set to launch in the 2020s that will orbit Jupiter and fly past Europa 45 times. Trumbo, in particular, would like to search for any organics that might have spilled onto the surface along with the salt. And, just maybe, scientists could get close enough to Europa’s surface to (remotely) taste it, via a robotic lander. “As someone who has studied Europa’s surface chemistry for about 20 years, I’d have to say that nothing beats a lander for getting up close and personal,” Hudson says. And Hand similarly wants to be within reach.

“This is excellent work, but I doubt that it is the last word on the subject,” Hudson says. “As with a lot of good scientific research, it doesn’t close the door, it opens it to future studies.”