Surface shapes, called double ridges, are found in every sector of Jupiter’s ice moon in Europe and can be hundreds of kilometers long. In a new study, planetary scientists at Stanford University and NASA’s Jet Propulsion Laboratory have identified a similar double ridge in the ice sheet of northwestern Greenland with the same geometry as Europe. To study ridge formation in Greenland, they used surface height and radar drilling data and said it was formed by a sequence of re-freezing, sealing and breaking shallow water thresholds in the ice sheet. They suggest that if this process is also responsible for the formation of double ridges in Europe, it could mean that shallow liquid water is present in the ice shell of the moon.
The artist’s concept shows how double ridges on the surface of Jupiter’s moon Europa can form on shallow, re-freezing water pockets in the ice shell. Image credit: Judge Blaine Wainwright.
Jupiter’s ice moon Europa, a major candidate for extraterrestrial habitability in our solar system, hides a global subterranean ocean beneath the outer ice sheet.
The thickness and thermophysical structure of this ice sheet is slightly limited, but models suggest that it may be 20-30 km thick with a layer of warm, convective ice underlying a cold, hard crust.
The detailed structure and dynamics of the ice sheet and the time scales through which they develop are crucial for understanding both the basic geophysical processes and the habitability of Europe.
Europe’s surface is young and geologically active, showing a wide variety of landforms, including ridges, troughs, bands, ribbons and chaotic terrain.
Of these, the most common are the double ridges, consisting of symmetrical pairs of ridges flanking the medial bed.
These ridges can stretch for hundreds of kilometers and include some of the oldest elements visible on the surface, with frequent crossings suggesting numerous cycles of formation throughout European history.
Planetary researchers have known about these characteristics since the surface of the moon was photographed by NASA’s Galileo spacecraft in the 1990s, but have been unable to come up with a definitive explanation for how they were formed.
Comparison of surface images of the double ridge of Europe (above) and the Earth (below). Image credit: Culberg et al., Doi: 10.1038 / s41467-022-29458-3.
Through analyzes of surface height and ice penetration radar data collected from 2015 to 2017 by NASA’s IceBridge operation, Ph.D., Stanford University. student Riley Culberg and colleagues revealed how the double ridge in northwest Greenland formed when ice broke around a pocket of pressurized liquid water that froze again inside the ice sheet, causing two peaks to rise in different shapes.
“In Greenland, this double ridge formed where water from surface lakes and streams often drains close to the surface and freezes again,” Culberg said.
“One of the ways in which such shallow pockets of water can form in Europe could be through groundwater that is pushed up into the ice shell through cracks – and that would suggest that there could be a reasonable amount of exchange,” which happens inside the ice shell.
Instead of behaving like a block of inert ice, Europe’s shell appears to be undergoing various geological and hydrological processes – an idea supported by this study and others, including evidence of water jets erupting on the surface.
The dynamic ice sheet maintains habitability as it facilitates the exchange between the subterranean ocean and nutrients from neighboring celestial bodies accumulated on the surface.
“Our explanation of how double ridges form is so complex that we could not imagine it without an Earth analogue,” said Dr. Gregor Steinbrugge, a planetary scientist at NASA’s Jet Propulsion Laboratory.
“The mechanism we set out in our document would be almost too bold and complicated to propose without seeing it happen in Greenland.
The paper was published in the journal Nature Communications.
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R. Culberg et al. 2022 Formation of a double ridge over shallow water thresholds of Jupiter’s moon Europa. Nat Commun 13, 2007; doi: 10.1038 / s41467-022-29458-3
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