What Can Europa’s Surface Tell Us About the Thickness of Its Ice?

You can tell a lot about a planetary body just by looking at its surface, especially if it has craters. Take Europa, for example. It has a fairly young surface—somewhere between 50 and 100 million years old. That’s practically “new” when you compare it to the age of the Solar System. And, Europa’s icy crust is pretty darned smooth, with only a few craters to change the topography.

Planetary scientists already know that Europa’s icy surface is a thin shell over a large interior ocean of salty water. How thin? To find out, a team of researchers led by Brandon Johnson and Shigeru Wakita at Purdue University studied images of large craters on Europa. They used what they saw, coupled with a variety of physical characteristics, to create computer models of that shell. “Previous estimates showed a very thin ice layer over a thick ocean,” said Wakita. “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.”

The thickness of that shell may well influence whether or not life exists at Europa. Its existence is a topic of intense interest since Europa could provide a reasonably habitable ecosystem for life. It has water, warmth, and organic materials for life to eat. That makes the search for life at Europa quite important. So, what do craters have to do with all this?

More About Craters

Impact cratering performs a lot of gardening in the Solar System, according to Johnson. He is the first author on a recently published paper discussing these features on Europa. “Craters are found on almost every solid body we’ve ever seen. They are a major driver of change in planetary bodies,” he said.

Four featured craters among many on the Moon: the triplet of Theophilus, Cyrillus and Catharina and Maurolycus. Many more craters can be seen across the lunar surface. Credit: Virtual Moon Atlas / Christian LeGrande, Patrick Chevalley

Just looking at images of different worlds in the Solar System, we can see some pretty heavily cratered surfaces. The Moon is a good example, as is Mars. And, we see it at many of the smaller bodies, such as the moons of the gas and ice giants. The more craters we see, the older the surface. In some places, multiple overlapping craters indicate a very old surface. In other places, such as at Europa, the craters are fewer and farther between. Something has “paved over” the craters such that any we CAN see were made after the repaving event. In addition, the craters reveal information about the surface as well as the “subsurface” of Europa.

“When an impact crater forms, it is essentially probing the subsurface structure of a planetary body,” said Johnson. “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.”

What Europa’s Craters Tell Us

This tiny moon is an enigma wrapped in shimmering ice. Its frozen surface hides a rocky inner core covered with a salt-water ocean. Like Earth, it experiences surface plate tectonics, driven by the core region’s heating. Inside, that heating drives currents of warmer water up from the core. That water gets forced to the surface, where it freezes and creates a new layer overlying any other features. This resurfacing happens every 50 to 100 million years.

Incoming impactors carve out new craters in that “freshened-up” surface, which gives scientists some pretty easy-to-study craters. They aren’t terribly deep, however, which tells scientists a lot about the structure of the icy shell. Johnson, Wakita, and their team studied images from the Galileo spacecraft to analyze Europa’s craters. In particular, they focused on two multi-ringed basins imaged on this moon. They show two or more concentric rings around the point of the impact that created them. Such basins are fairly rare and usually indicate some kind of large, energetic impact. On Europa, their appearance and formation give clues to the thickness of the icy shell and their thermal structure, which is a way to understand how the shell conducts heat.

Multi-ringed Crater Basins Tell a Tale

In their study, the Purdue team simulated a multi-ring basin with varying thicknesses of ice. Those thicknesses influence the degree of tidal heating in the shell itself. They also help scientists understand how heat exchange occurs between the bottom of the shell and the underlying ocean. The team found that icy shells thinner than about 15 kilometers don’t show the kinds of multi-ringed basins that exist on Europa. However, a thicker one does. In particular, the best-fit simulation used a 20+ kilometer-thick shell. It consists of two layers: a 6-8 kilometer-thick conductive “lid” that covers up a layer of warm, convecting ice.

One of Galileo’s images of the Tyre multi-ringed basin on Europa. There are at least 5-7 rings around the impact crater center. Courtesy: NASA/JPL/ASU.

In addition to studying the craters, the team also looked at the types of impactors needed to create those multi-ringed basins on Europa. From the structures seen in the Galileo images, they concluded that the impactors would need to be around 1.5 kilometers in radius to create the multi-ringed basins. Smaller ones wouldn’t create the structures they saw, and bigger impactors would result in very different-looking craters and rings.

What About Other Worlds?

Europa isn’t the only world at Jupiter with an icy crust. Both Ganymede and Callisto also show cratering, with multi-ring basins. This tells us that these worlds also have to have thick enough icy crusts where such basins can form. Planetary scientists have suggested their crusts are at least 80 to 105 kilometers thick. In their paper, the Purdue teams suggest that since Europa’s crust is likely to be at least 20 kilometers thick (if not more) it’s also likely that Ganymede and Callisto have much thicker crusts than current predictions suggest.

Callisto has many more craters than Europa and a thicker icy crust. Image credit: NASA/JPL

Finally, although the paper doesn’t specifically address this, the fact that the scientists can deduce impactor size from the characteristics of the resulting craters does provide insight into the sizes of impactors available in Jovian “airspace”. To sustain these kinds of multi-ringed basins, you need a good population of sizable impactors to do the job. Also, for Europa to be so recently “refreshed” really does give a clue to the impact environment in the Jupiter system. While Ganymede and Callisto both have very old surfaces, the existence of “fresh” ice at various cratering sites tells us that they’re still being bombarded in recent times, although they’re not actively resurfacing themselves. These are all additional data points to consider when understanding the habitability of environments, particularly at Europa (and possibly at places such as Enceladus at Saturn).

“Understanding the thickness of the ice is vital to theorizing about possible life on Europa,” Johnson said. “How thick the ice shell is controls what kind of processes are happening within it, and that is really important for understanding the exchange of material between the surface and the ocean. That is what will help us understand how all kinds of processes happen on Europa—and help us understand the possibility of life.”

For More Information

Planetary Scientists Use Physics and Images of Impact Craters to Gauge Thickness of Ice on Europa
Multiring Basin Formation Constrains Europa’s Ice Shell Thickness

Carolyn Collins Petersen

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