A curious feature found on Greenland's ice sheet resembles those on the surface of Europa. If the same processes formed them, Europa may have pockets of water much closer to the surface than its internal ocean, greatly improving the chances of finding life on Jupiter's moon.
The Voyager spacecraft revealed that Europa has a liquid ocean beneath a shell of ice. The discovery instantly rocketed Europa to the top of the “possible locations for extraterrestrial life” charts. However, if the icy shell is too thick – current estimates are 20-30 kilometers (12-18 miles) – that life might as well be in a different star system for all our prospects of finding it. Saturn's Enceladus may be less suited to life, but if it's there it should be easier to access.
However, a new paper in Nature Communications presents evidence Europa may have water much closer to the surface.
The Galileo spacecraft revealed an abundance of double ridges on Europa, pairs of narrow ridgelines up to 300 meters (1,000 feet) high that can run for hundreds of kilometers separated by shallow troughs a little less than twice as wide as the peaks are high. As the icy moon's most common feature, explaining their origins could be key to understanding this world, but numerous efforts have proven unsatisfactory.
Stanford University PhD student Riley Culberg and co-authors noted the existence of a similar-looking double ridge in northwest Greenland, one of the closest proxies to Europa we can study directly. They seized the chance, examining its surface elevation and using radar to explore its interior.
The paper presents an explanation for how Greenland's double ridge formed after surface meltwater drained to a cavity within the ice sheet, and then refroze. The cavity occurred because porous ice was sitting above more solid material. The expansion on freezing meant the newly frozen water was under extreme pressure from the ice above, pushing up the ridges in response.
Culberg doesn't assume the same process is responsible for Europa's double ridges, and there are known differences. The Greenland ridge is only around 2 meters (6.5 feet) high, possibly because of Earth's much stronger gravity. Being so much colder, Europa doesn't have liquid surface water to drain. However, “[S]imilar shallow water pockets could form on Europa might be through water from the subsurface ocean being forced up into the ice shell through fractures,” Culberg noted in a statement.
If so, the implications are immense for future missions. It would mean, in the paper's words: “Shallow liquid water is spatially and temporally ubiquitous across Europa’s ice shell.” Or as senior author Dr Dustin Schroeder, also of Stanford put it, “It suggests there’s water everywhere.”
Not only would this bring Europa's water that much closer to our capacity to explore, but such near-surface water could also have a richer chemistry, and therefore an even better chance of life, than the ocean within. “It’s closer to the surface, where you get interesting chemicals from space, other moons and the volcanoes of Io,” Schroeder explained.
The authors weren't looking for Europa counterparts in Greenland. A subsurface of land and an internal ocean might be expected to produce different outcomes above. “We were working on something totally different related to climate change and its impact on the surface of Greenland when we saw these tiny double ridges,” Shroeder said. A presentation from members of the same lab on Europa alerted them to the similarity.
We observe Greenland frequently, rather than just when spacecraft pass by, so Schroeder and co-authors were able to track the development of Greenland's double ridge in three years of images.
“The mechanism we put forward in this paper would have been almost too audacious and complicated to propose without seeing it happen in Greenland,” Schroeder said. The fact planetary scientists have been seeking an explanation for the double ridges for 20 years without hitting on this one suggests he's right.
Europa landings, let alone drilling, may be a long way off, but the Europa clipper is set to launch in 2024 carrying ice-penetrating radar. Now its operators know exactly what signature to look out for to see if the Greenland mechanism explains Europa's ridges. If it does, the push to land on the Moon will heat up.