Icy impacts: Planetary scientists use physics and images of impact craters to gauge the thickness of ice on Europa

Sometimes planetary physics is like being in a snowball fight. Most people, if handed an already-formed snowball, can use their experience and the feel of the ball to guess what kind of snow it is comprised of: packable and fluffy, or wet and icy.

Using nearly the same principles, planetary scientists have been able to study the structure of Europa, Jupiter's icy moon.

Europa is a rocky moon, home to saltwater oceans twice the volume of Earth's, encased in a shell of ice.

Scientists have long thought that Europa may be one of the best places in our solar system to look for nonterrestrial life.

The likelihood and nature of that life, though, heavily depend on the thickness of its icy shell, something astronomers have not yet been able to ascertain.

A team of planetary science experts including Brandon Johnson, an associate professor, and Shigeru Wakita, a research scientist, in the Department of Earth, Atmospheric, and Planetary Sciences in Purdue University's College of Science, announced in a new paper published in Science Advances [ES1] that Europa's ice shell is at least 20 kilometers thick.

To reach their conclusion, the scientists studied large craters on Europa, running a variety of models to determine what combination of physical characteristics could have created such a surface structure.

"This is the first work that has been done on this large crater on Europa," Wakita said.

"Previous estimates showed a very thin ice layer over a thick ocean. But our research showed that there needs to be a thick layer -- so thick that convection in the ice, which has previously been debated, is likely."

Using data and images from the spacecraft Galileo, which studied Europa in 1998, Johnson analyzed the impact craters to decode truths about Europa's structure.

An expert in planetary physics and colossal collisions, Johnson has studied almost every major planetary body in the solar system.

Scientists have long debated the thickness of Europa's ice shell; no one has visited to measure it directly, so scientists are creatively using the evidence at hand: the craters on Europa's icy surface.

"Impact cratering is the most ubiquitous surface process shaping planetary bodies," Johnson said.

"Craters are found on almost every solid body we've ever seen. They are a major driver of change in planetary bodies. When an impact crater forms, it is essentially probing the subsurface structure of a planetary body. By understanding the sizes and shapes of craters on Europa and reproducing their formation with numerical simulations, we're able to infer information about how thick its ice shell is."

Europa is a frozen world, but the ice shelters a rocky core.

The icy surface, though, is not stagnant. Plate tectonics and convection currents in the oceans and the ice itself refresh the surface fairly frequently.

This means the surface itself is only 50 million to 100 million years old -- which sounds old to short-lived organisms like humans, but is young as far as geological periods go.

That smooth, young surface means that craters are clearly defined, easier to analyze and not very deep.

Their impacts tell scientists more about the icy shell of the moon and the water ocean below, rather than conveying much information about its rocky heart.

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Explanation for formation of abundant features on Europa bodes well for search for extraterrestrial life

Europa is a prime candidate for life in our solar system, and its deep saltwater ocean has captivated scientists for decades. But it's enclosed by an icy shell that could be miles to tens of miles thick, making sampling it a daunting prospect. Now, increasing evidence reveals the ice shell may be less of a barrier and more of a dynamic system -- and site of potential habitability in its own right.

Ice-penetrating radar observations that captured the formation of a "double ridge" feature in Greenland suggest the ice shell of Europa may have an abundance of water pockets beneath similar features that are common on the surface. The findings, which appear in Nature Communications April 19, may be compelling for detecting potentially habitable environments within the exterior of the Jovian moon.

"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," said study senior author Dustin Schroeder, an associate professor of geophysics at Stanford University's School of Earth, Energy & Environmental Sciences (Stanford Earth). "If the mechanism we see in Greenland is how these things happen on Europa, it suggests there's water everywhere."

A terrestrial analog

On Earth, researchers analyze polar regions using airborne geophysical instruments to understand how the growth and retreat of ice sheets might impact sea-level rise. Much of that study area occurs on land, where the flow of ice sheets is subject to complex hydrology -- such as dynamic subglacial lakes, surface melt ponds and seasonal drainage conduits -- that contributes to uncertainty in sea-level predictions.

Because a land-based subsurface is so different from Europa's subsurface ocean of liquid water, the study co-authors were surprised when, during a lab group presentation about Europa, they noticed that formations that streak the icy moon looked extremely similar to a minor feature on the surface of the Greenland ice sheet -- an ice sheet that the group has studied in detail.

"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.

Upon further examination, they found that the "M"-shaped crest in Greenland known as a double ridge could be a miniature version of the most prominent feature on Europa.

Prominent and prevalent

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 conceive a definitive explanation of how they were formed.

Through analyses of surface elevation data and ice-penetrating radar collected from 2015 to 2017 by NASA's Operation IceBridge, the researchers 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," 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."

Snowballing complexity

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.

"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) who started working on the project as a postdoctoral researcher at Stanford. "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."

The co-authors said their explanation for how the double ridges form is so complex, they couldn't have conceived it without the 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 equip researchers with a radar signature for quickly detecting this process of double ridge formation using ice-penetrating radar, which is among the instruments currently planned for exploring Europa from space.

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