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A spacecraft flying past Europa may be able to sample its colossal watery plumes – even if they stopped erupting weeks earlier. A new analysis suggests that jets spewing from Jupiter’s icy moon could produce complex, constantly shifting chemical patterns in its atmosphere, which we could use to figure out what is on, and even below, the surface.

Europa is thought to host a deep, salty ocean beneath an icy shell, and this ocean could be one of the best places to look for life in the solar system. In 2012, the Hubble Space Telescope spotted evidence that plumes of water vapour are vented from this subsurface sea into space.

That was good news for proposed missions to explore the moon and test its oceans for signs of life. Both NASA and the European Space Agency have missions in the works, targeted for launch in the early 2020s, that will fly past Europa.

”Those are free samples: we can just fly by and we can grab some of that material,” says at NASA’s Jet Propulsion Laboratory in California. “That’s our best way of understanding what’s going on ­­– not just at Europa’s surface but, in the case of plume sampling, the subsurface – from orbit.”

One potential problem with this plan is that the plumes appear intermittent: Hubble didn’t see another sign of them until 2016. But now and his colleagues at the Southwest Research Institute in San Antonio, Texas, have suggested a way to sample the plumes even if they aren’t active when a spacecraft gets there.

Luck not required

“You don’t actually have to be flying by Europa at the same location and same time as a plume is present,” he says. “You don’t have to get that lucky.”

Europa’s gravity may pull material from the plumes back to the surface, creating a layer of frost. Later, some of those particles would be propelled back off the surface, either by evaporating or by being thrown upwards when charged particles from Jupiter’s magnetosphere hit the surface, much like the splash when a pebble drops into a pond.

These particles feed Europa’s atmosphere, and could provide clues about the composition of the surface and the plumes – even if the plumes have already stopped. We could reconstruct a timeline of days or even weeks of previous activity, Teolis says. By calculating where the particles came from, we could begin to draw a map of fresh features on the moon’s surface.

“The potential to see the spatial distribution of the molecules and the change in the atmosphere over time is pretty spectacular,” Teolis says. “Understanding this problem is a major element in nailing down the composition of Europa’s subsurface ocean, and the potential for it to support life.”

“There’s a lot of variability on scales that we probably aren’t even close to understanding yet, so I think it’s going to be very difficult,” says Phillips.

But if researchers can overcome that difficulty, Europa holds a unique opportunity for a flyby mission to learn about the surface and interior of a world while soaring high above it.