Surface landforms called double ridges occur across every sector of Jupiter’s icy moon Europa and can be hundreds of kilometers in length. In new research, planetary scientists from Stanford University and NASA’s Jet Propulsion Laboratory identified a similar double ridge in the ice sheet in northwest Greenland with the same geometry as those on Europa. To explore the formation of the ridge in Greenland, they used surface elevation and radar sounding data and indicate it was formed through a succession of refreezing, pressurization and fracture of a shallow water sill within the ice sheet. They suggest that if this process is also responsible for the formation of double ridges on Europa it may indicate that shallow liquid water is present in the moon’s ice shell.
This artist’s conception shows how double ridges on the surface of Jupiter’s moon Europa may form over shallow, refreezing water pockets within the ice shell. Image credit: Justice Blaine Wainwright.
Jupiter’s icy moon Europa, a prime candidate for extraterrestrial habitability in our Solar System, harbors a global subsurface ocean beneath an outer ice shell.
The thickness and thermophysical structure of this ice shell are poorly constrained, but models suggest it may be 20-30 km thick with a layer of warm, convecting ice underlying a cold, rigid crust.
The detailed structure and dynamics of the ice shell and the timescales over which they evolve are critical for understanding both the fundamental geophysical processes and habitability of Europa.
Europa’s surface is young and geologically active, displaying a wide variety of landforms including ridges, troughs, bands, lenticulae, and chaos terrain.
Of these, double ridges are the most common, consisting of symmetric ridge pairs flanking a medial trough.
These ridges may extend for hundreds of kilometers and include some of the oldest features visible on the surface, with frequent cross-cutting implying numerous formation cycles over Europa’s history.
Planetary researchers have known about these features since the moon’s surface was photographed by NASA’s Galileo spacecraft in the 1990s but have not been able to conceive a definitive explanation of how they were formed.
Surface imagery comparison of a double ridge on Europa (top) and Earth (bottom). Image credit: Culberg et al., doi: 10.1038/s41467-022-29458-3.
Through analyses of surface elevation data and ice-penetrating radar collected from 2015 to 2017 by NASA’s Operation IceBridge, Stanford University Ph.D. student Riley Culberg and colleagues revealed 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, causing two peaks 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,” Culberg said.
“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.”
Rather than behaving like a block of inert ice, the shell of Europa seems to undergo a variety of geological and hydrological processes — an idea supported by this study and others, including evidence of water plumes that erupt to the surface.
A dynamic ice shell supports habitability since it facilitates the exchange between the subsurface ocean and nutrients from neighboring celestial bodies accumulated on the surface.
“Our explanation for how the double ridges form is so complex, we couldn’t have conceived it without the analog on Earth,” said Dr. Gregor Steinbrügge, a planetary scientist at NASA’s Jet Propulsion Laboratory.
“The mechanism we put forward in our paper would have been almost too audacious and complicated to propose without seeing it happen in Greenland.”
The paper was published in the journal Nature Communications.
_____
R. Culberg et al. 2022. Double ridge formation over shallow water sills on Jupiter’s moon Europa. Nat Commun 13, 2007; doi: 10.1038/s41467-022-29458-3
