Double ridge ice formations seen on Europa are similar to formations detected on the Greenland Ice Sheet. This artist’s rendering shows how double ridges on the surface of Jupiter’s moon Europa may form over shallow, refreezing water pockets within the ice shell. This mechanism is based on the study of an analogous double ridge feature found on Earth’s Greenland Ice Sheet. (Justice Blaine Wainwright)

Europa’s ice crust is crossed by thousands of double ridges, pairs of long parallel raised lines with a small valleys in between, sometimes as much as hundreds of miles long and skyscraper-height tall rims. While these double ridges are ubiquitous on Europa’s surface, how they form remains something of a mystery to scientists.

Dustin Schroeder, an associate professor of geophysics at Stanford University’s School of Earth, Energy & Environmental Sciences, was working on an issue related to climate change when he saw double ridges similar to those seen on Europa here on Earth.  The ridges, in Northwest Greenland, were tiny when compared with those on Europa, but the found the same “M”-shaped crest as found everywhere on that Jovian moon.

“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 – and we were able to see the ridges go from ‘not formed’ to ‘formed,’ ” Schroeder said.

Could the double ridges be forming as a result of processes similar to those that form the double ridges on Europa?

If so, then Greenland would provide a possibly important new window into a central question about Europa:  Is that thick ice shell surrounding the subsurface ocean completely solid, or does it have what are called “water sills” within the shell?

This is important because, as the Nature Communications paper concludes, “If the same process is responsible for Europa’s double ridges, our results suggest that shallow liquid water is spatially and temporally ubiquitous across Europa’s ice shell.”

Or as Schroeder put it, “If the mechanism we see in Greenland is how these things happen on Europa, it suggests there’s water everywhere,” he said in a release.

They can make this inference because the double ridges formed in Greenland are the known, and detectable, result of the dynamics of subsurface water surrounded by the ice sheet.

Surface imagery comparison of a double ridge on Europa (a) and on Earth (b), on the Northwest Greenland Ice Sheet. The Europa image was a taken during the Galileo mission and the Greenland double ridge comes via the WorldView-3 satellite.  Signatures of flexing are visible along the ridge flanks, consistent with previous models for double ridges underlain by shallow sills. (Nature Communications)

If there are indeed many shallow water sills in the Europa ice shell — as the double ridges suggest — then the possibility of the moon being habitable increases significantly because the sills are wet and warm enough to possibly support life.

Since Europa is already seen (for other reasons) as among the best — if not the best — candidate for a second habitable environment in our solar system,  then the discovery could be quite promising.

Regarding those other suggestions that Europa could be habitable, the most crucial is that it supports a large salt-water ocean beneath the ice shell.

This has been confirmed in numerous ways over a long period of time, though there are differing views on what keeps the water liquid and how the craggy surface was formed and is maintained..

Europa is about a quarter of Earth’s size, or a bit smaller than Earth’s moon.  Evidence suggests Europa has a saltwater ocean possibly 40 to 100 miles deep, which means it could contain about twice as much water as Earth’s oceans. Some scientists hypothesize that Europa’s ocean could be reacting with super-heated rocks below its seafloor, possibly through hydrothermal vents. On Earth, such areas are hotbeds of chemical activity that nourishes innumerable creatures.

Because of Europa’s elliptical orbit, which sometimes brings it closer and sometimes farther away from Jupiter, plus Jupiter’s immense gravitational pull, more heat is generated in Europa from friction as it circles its host planet. Given that internal heat stimulates geological activity on rocky worlds, Europa is expected to have more extensive geology.

Some scientists predict that Europa has plate tectonics that shift and recycle the icy blocks making up the moon’s surface. If so, Europa could be circulating nutrients produced on the surface by radiation from Jupiter, such as oxygen, to pockets of liquid in the ice shell or perhaps to the ocean itself.

But all of this is hidden within the moon and in its ocean. NASA’s Europa Clipper mission, scheduled to launch in 2024 and arrive at Europa by 2030, will give scientists a chance to test some of their predictions by analyzing the chemical makeup of those infrequent, or hard to detect, plumes or the traces they may leave on the surface.

Until then, scientists will continue to study the Europan surface for clues about what is inside. And the double ridges — and the water sills they suggest — are such a clue.

Europa, one of Jupiter’s moons, has a surface crisscrossed by multiple sets of doubled ridged formations.(NASA/JPL/ASU)

Scientists are convinced that the Europan ice shell does not behave like a block of inert ice but rather undergoes a wide variety of geological and hydrological processes – an idea supported by this Stanford study and others.  A dynamic Europan ice shell could support habitability since it allows for an exchange between the subsurface ocean and nutrients accumulated on the surface from neighboring celestial bodies — just as a dynamic Earth, with tectonic plates, volcanos and constant interchange between the interior and the surface cyles essential compounds.

The discovery of what appears to be infrequent plumes of water vapor erupting to the surface adds to this view of Europa as an active body.   A different Stanford study including some of the same scientists suggests the plumes may originate in those shallow water sills.

Saturn’s moon Enceladus is known to have constant water vapor plumes shooting out of its subsurface ocean. But those on Europa would be quite different. They may be sporadic and they may be small and thin, and as a result impossible to detect from afar.  This is because Europa’s gravity is stronger than on much-smaller Enceladus, and would keep these water plumes close to the surface. In contrast,  Enceladus’ vapor column is always on and is bigger than the moon itself, spraying icy particles hundreds of miles above the surface.

This composite image shows suspected plumes of water vapor erupting at the 7 o’clock position off the limb of Jupiter’s moon Europa. The image of Europa, superimposed on the Hubble data, is assembled from data from the Galileo and Voyager missions. (W. Sparks / STScI / NASA / ESA / USGS Astrogeology Science Center)

Double ridges on Europa appear as dramatic gashes across the moon’s icy surface, with crests reaching nearly 1000 feet, separated by valleys about a half-mile wide. Scientists have known about the features since the moon’s surface was photographed by the Galileo spacecraft in the 1990s but have not been able to find a definitive explanation of how they were formed.

“People have been studying these double ridges for over 20 years now, but this is the first time we were actually able to watch something similar on Earth and see nature work out its magic,” said study co-author Gregor Steinbrügge, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL).

“We are making a much bigger step into the direction of understanding what processes actually dominate the physics and the dynamics of Europa’s ice shell,” he said in a release.

Through analyses of surface elevation data and ice-penetrating radar collected from 2015 to 2017 by NASA’s Operation IceBridge, the researchers discovered how the double ridge on northwest Greenland was produced.  When the ice fractured around a pocket of pressurized liquid water that was refreezing inside of the ice sheet, they found, two peaks would rise to rise into the distinct shape.

“In Greenland, this double ridge formed in a place where water from surface lakes and streams frequently drains into the near-surface and refreezes,” said lead study author Riley Culberg, a PhD student in electrical engineering at Stanford.

“One way that similar shallow water pockets could form on Europa might be through water from the subsurface ocean being forced up into the ice shell through fractures – and that would suggest there could be a reasonable amount of exchange happening inside of the ice shell.”

Water sills in Greenland likely occur via drainage of melt water, while on Europa they could come directly from the subsurface ocean via fractures or through heating from moving parts of the ice shell (a.)  The sill fractures due to internal pressure and water fills the central conduit (b.)  The sill is divided by refreezing of that central conduit or other fracture networks (c.) A double ridge develops as highly pressured water with the refreezing sill is forced out along the planes of weakness that form (d.) (Nature Communications)

The co-authors acknowledge that their explanation for how the double ridges form on Europa is quite complex,  so complex that they would never have proposed it without that analog on Earth.

“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 findings, the team reports, provides researchers with a signature that can quickly detect this process of double ridge formation using ice-penetrating radar.  An instrument that can accomplish that is currently planned for exploring Europa from space on the Clipper.

Regarding how the double ridges form, Culberg said that “We are another hypothesis on top of many – we just have the advantage that our hypothesis has some observations from the formation of a similar feature on Earth to back it up. It’s opening up all these new possibilities for a very exciting discovery.”

Scientists say there also could be large pockets of melted water in Europa’s ice shell, which are more likely than the ocean to be the source of plumes. These pockets could produce cozy habitats for organisms as well.