21 June 2016

Saturn moon Enceladus’ ice shell likely thinner than expected

Posted by edeatrick

By Elizabeth Deatrick

A vast ocean of water beneath the icy crust of Saturn’s moon Enceladus may be more accessible than previously thought, according to new research. A new study has revealed that near the moon’s poles, the ice covering Enceladus could be just two kilometers (one mile) thick—the thinnest known ice shell of any ocean-covered moon. The discovery not only changes scientists’ understanding of Enceladus’ structure, but also makes the moon a more appealing target for future exploration, according to the study’s authors.

Enceladus by Cassini

Cassini captured a great deal of data on Enceladus, but this study is the first to use them to determine the likely thickness of the moon’s ice.
Credit: Nasa/JPL-Caltech

Until recently, scientists saw Jupiter’s moon Europa as the moon most likely to yield new understanding into worlds with ice-covered oceans, according to Gabriel Tobie, a planetary scientist at the Laboratory of Planetology and Geodynamics of CNRS, the University of Nantes, and the University of Angers in Nantes, France and co-author of the new study.

Estimates of Europa’s ice shell thickness range from just a few kilometers to over 10 kilometers to over 20 kilometers (12 miles) thick. By comparison, Enceladus’ ice was previously thought to be 20 to 60 kilometers (12 to 37 miles) thick. But the new study suggests that at its south pole, Enceladus’ ice is less than five kilometers (three miles) thick, and possibly as little as two.

The thinness of the ice opens up future mission possibilities, according to authors of the new study published in Geophysical Research Letters, a journal of the American Geophysical Union. With ice this thin, an orbiting probe could use radar to see what lies beneath the moon’s shell. Though substantial engineering challenges would have to be solved first, scientists could even land a probe on the moon itself to drill down through the ice and sample the water below, Tobie said. Other scientists have proposed that ocean-covered moons like Europa could harbor life, and getting a look at Enceladus’ oceans could help us understand whether life could exist there, according to the authors.

The study yielded a second unexpected result: Enceladus’ core is likely much hotter than previously thought. Ice acts as an insulator, keeping the planet’s global oceans warm, but a thinner ice shell holds less heat. To maintain the same amount of water in the global oceans, with a thinner ice shell, Enceladus’ rocky core would have to generate much more heat than previously thought, according to the authors.

A new synthesis of data

In the new study, the research team used publicly available data collected by NASA’s Cassini spacecraft to measure Enceladus’ gravity and topography. The gravitational influence of Saturn causes liquid water on Enceladus to move differently than ice or rock, a movement called libration. The researchers used Enceladus’ gravity, shape, and libration data to build a computer model of the moon and determine how much of it is water, ice and rock.

The team concluded that the ice sheet over Enceladus was not only thinner than previously thought, but that its thickness varied over the surface of the planet. The ice sheet is noticeably thinner near the poles—especially the south pole, they found.

“I think it’s a very nice piece of work,” said Francis Nimmo, a planetary scientist at the University of California Santa Cruz who was not involved with the study. “On the one hand, you have the gravity and topography, and on the other hand you have the librations. They’re using both those pieces of data, which hasn’t been done before.”

—Elizabeth Deatrick is a science writing intern at AGU.