Is there life on Jupiter’s moon Europa? With its deep and potentially habitable global saltwater ocean, Europa is a tantalizing place to search for alien biology. And researchers at Stanford University said on April 19, 2022, that the moon’s ice shell – surrounding the ocean – might also be habitable. They suggest that pockets of water might be common within the ice shell. If true, it’s exciting because pockets of water in an ice shell might be easier to probe for life than probing below an ice shell.
The findings are based on a new study of unusual double ridges on the icy surface of northwest Greenland. Notably, those ridges, which form over subsurface liquid water, are remarkably similar to larger ones on Europa. The researchers published their intriguing peer-reviewed results in Nature Communications on April 19, 2022.
Double ridges in Greenland
The findings were actually fortuitous, made while the researchers were doing work related to climate change. More specifically, they noticed unusual small double ridges on ice sheets on Greenland’s icy and rocky terrain. These ice sheets grow and retreat, and are subjected to complex hydrology such as subglacial lakes, surface melt ponds and seasonal drainage conduits. Senior author Dustin Schroeder, an associate professor of geophysics at Stanford University, stated:
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.’
Significantly, Schroeder and his colleagues had seen similar double ridges before … on Europa. Indeed, the ones in Greenland appeared to be smaller versions of the ones that crisscross the surface of Europa. Could they, in fact, have formed in a similar manner? As described in the paper:
Double ridges are the most common surface feature on Europa and occur across every sector of the moon, but their formation is poorly understood, with current hypotheses providing competing and incomplete mechanisms for the development of their distinct morphology. Here we present the discovery and analysis of a double ridge in northwest Greenland with the same gravity-scaled geometry as those found on Europa. 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.
Europa’s odd ridges: an analog in Greenland?
While the double ridges on Europa form on the surface of the ice shell, the ones in Greenland form on the frozen land. However, they may have a similar formation mechanism. The researchers say that the ridge formations on Europa and in Greenland are extremely similar. As it turns out, there may be a common factor: subsurface pockets of water. In Greenland, the water comes from lakes and streams. On Europa, it could come from the subsurface ocean.
As the researchers noted, the ridges in Greenland formed when ice fractured around a pocket of pressurized liquid water. That water then refroze inside the ice sheet, causing two peaks to protrude from the surface in the distinct shape of the double ridge. Those findings are based on surface elevation data and ice-penetrating radar that NASA’s Operation IceBridge collected from 2015 to 2017. Lead author Riley Culberg at Stanford University explained:
In Greenland, this double ridge formed in a place where water from surface lakes and streams frequently drains into the near-surface and refreezes. 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.
Huge gashes and a geologically dynamic ice shell
Even though they look remarkably similar, the ridges on Europa are much larger. They appear as huge gashes on the moon’s surface, and the ridges can reach nearly 1,000 feet (305 meters) tall and are separated by 1/2-mile-wide (0.8 km-wide) valleys. NASA’s Galileo spacecraft first saw them in the 1990s. In fact, Europa’s surface is covered by so many ridges and cracks, it looks like a giant cracked egg.
The gashes, as well as their analogs in Greenland, also point to something rather unexpected: the ice shell is active. That is, rather than simply being a solid, unchanging outer casing of ice, the ice shell may actually be quite geologically dynamic. Why is that important? It increases the chances of habitability within the ice shell. Essential chemical nutrients on the surface could be transported down into the ocean. The nutrients originate from various places including the hyper-volcanically active moon Io, and accumulate on Europa’s surface. Schroeder said:
Because it’s closer to the surface, where you get interesting chemicals from space, other moons and the volcanoes of Io, there’s a possibility that life has a shot if there are pockets of water in the shell. If the mechanism we see in Greenland is how these things happen on Europa, it suggests there’s water everywhere.
Exchange of material between surface and ocean
The tentative – although elusive – water vapor plumes on Europa also support this possibility. If, indeed, they are similar to the water plumes on Saturn’s moon Enceladus, then water from deep below is able to make its way onto Europa’s surface. Between the plumes and ridges/water pockets, there could be an ongoing exchange of material between the surface and subsurface. This could create a habitable environment by earthly standards. Co-author Gregor Steinbrügge, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL) said:
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. 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.
Europa’s odd ridges point to suprisingly complex geological mechanisms
If their theory is correct, the geological mechanisms within the ice shell are quite complex. In fact, as Schroeder noted:
The mechanism we put forward in this paper would have been almost too audacious and complicated to propose without seeing it happen in Greenland.
Potential for exciting discoveries
As Culberg added:
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.
Indeed, we may start to make such new discoveries when NASA’s Europa Clipper mission reaches Europa. It’s scheduled for launch in October 2024.
Bottom line: Researchers at Stanford University say that unusual double ridges on Europa may form similar to ones in Greenland, over subsurface pockets of water in the ice shell. The existence of such water pockets could mean the ice shell above the subsurface ocean is geologically dynamic and potentially habitable.