Scientific Method / Science & Exploration

Europa’s ocean could help explain its jigsaw surface

Models try to puzzle through the moon’s icy shell to the ocean beneath it.

A snapshot from the model on the left, showing warmer and cooler water. Image of Europa's surface on the right.
J. Wicht and NASA/JPL/University of Arizona

Part of the beauty of Jupiter’s icy moon Europa is its incredible smoothness. But like most things, if you look closely, cracks appear in this facade. In Europa’s case, the cracks come in the form of jumbled pieces of ice that make up what are called the moon's “chaos terrains.” Just what caused the chaos is an open question.

There is, however, an obvious candidate. Europa’s most exciting characteristic is probably the ocean of liquid water that is thought to exist beneath that icy crust. It seems likely that the ocean has something to do with the chaos terrain, especially given the presence of salt there. To figure that out, however, we’d have to know something about how water circulates in that ocean. And, unlike our own oceans, you can’t just chuck a buoy in and see where it goes.

Circulation in the ocean would be driven by the heat from Europa’s interior. It’s been thought that the big-picture pattern might look something like the atmosphere of Jupiter, with alternating bands of eastward or westward flow. Ultimately, this pattern carries the greatest amount of internal heat to Europa’s polar regions. A new study, led by University of Texas at Austin researcher Krista Soderlund, suggests the circulation pattern could actually look quite different.

The researchers created a mathematical model of Europa’s ocean that could simulate the Jupiter-like circulation pattern. Then, they played with a key parameter in the equations—a term known as the Rossby number. It has to do with how strongly the Coriolis effect—the force that causes air to spiral in toward the low pressure at the center of a hurricane—influences the flow of the ocean water.

Estimates for what value of the Rossby number best represents the conditions on Europa have varied, but only the smaller values produce a Jupiter-like circulation pattern. By using a larger value in their model, the researchers push the circulation pattern into a very different configuration.

The flow is a little more complex, and the neatly separated bands of eastward and westward flow break down. It’s a little less Jupiter-like and a little more like Earth’s atmosphere. On Earth, a convection cell in the tropics carries air upwards at the equator and back down around 30 degrees north and south latitude. That same kind of behavior shows up in the ocean in this model, with a Coriolis-induced westward component to the flow, just like the trade winds on Earth. At high latitudes, the simulated water flows in the opposite direction.

Most importantly, heat is distributed much differently. Rather than warming the poles most, warm water generally rises at the equator, with cool water sinking at the poles. That’s intriguing, because the chaos terrains mostly exist between 40 degrees south and north of Europa’s equator. If this picture of Europa’s circulation is accurate, it would seemingly link the delivery of heat to the underside of Europa’s icy shell near the equator, which could then lead to the unknown process that forms the chaos terrain.

In an accompanying article, Wheaton College astronomer Jason Goodman laments the difficulty of studying Europa’s intriguing subsurface. “If we hope to directly sample Europa’s ocean, traversing the last few kilometers of ice may prove a greater challenge than crossing a billion kilometers of interplanetary space. In the meantime, indirect observations and computer simulations continue to provide new insights into this mysterious alien ocean.”

Nature Geoscience, 2013. DOI: 10.1038/NGEO2021, 10.1038/NGEO2034  (About DOIs).

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