Impact craters on Europa -- the jovian satellite that scientists say may hide a subsurface liquid ocean - show that the moon's brittle ice shell crust is more than 3 to 4 kilometers (1.8 to 2.4 miles) thick, two University of Arizona planetary scientists report in Science (Nov. 9 issue).

The Grainne Crater on Europa. Credit: Elizabeth Turtle Planetary Image Research Lab, Lunar and Planetary Lab, University of Arizona

The thickness of Europa's hard ice shell is a hot scientific debate. Some argue the crust must be only one or two kilometers (six-tenths mile to 1.2 miles) thick, given ridges, cycloid cracks and other geological features. Others contend the ice crust must be 10 times thicker, and that it includes a warm convecting ice layer that shapes observed surface features.

Beyond geology, the wider fascination with Europa is the possibility that it conceals a liquid water ocean, and, potentially, life. Instruments proposed for a future Europa orbiter mission include radar and other instruments to detect and explore the possible ocean. To explore an ocean - if it does indeed exist - scientists have to know the thickness of the overlying ice.

Elizabeth P. Turtle and Elisabetta Pierazzo of the UA Lunar and Planetary Laboratory numerically simulated impacts powerful enough to produce central peaks in impact craters imaged by the Galileo spacecraft.

At least six of 28 impact craters observed by Galileo and Voyager have well defined central peaks, Turtle said. They are found in craters larger than 5 kilometers (3 miles) in diameter.

The Brigid Crater on Europa. Credit: Elizabeth Turtle Planetary Image Research Lab, Lunar and Planetary Lab, University of Arizona

"There aren't many impact craters on Europa, but those that exist can tell us a lot because we understand the cratering process better than we understand many of the other processes that shape Europa's surface," Turtle said.

"The morphologies (structure) of some craters indicate that these impacts didn't completely vaporize or melt through a cold, brittle ice layer on Europa. So based on this observation, our impact simulations demonstrate that the ice crust must be more than 3 to 4 kilometers thick," Turtle said. "I should emphasize that what we've done is put a lower limit on the thickness of the ice. These simulations do not put an upper limit on ice thickness."

Central peak craters are observed on Earth, the moon, and Mars, Turtle said. "We have geologic evidence from Earth and the moon that shows that the material that collapses up into the central peak is material that was previously buried, but has been uplifted and broken up. Central peaks are deep bedrock that has been uplifted," much like a splash that results from dropping something into water, Turtle said.

"What we're seeing here on Europa appear to be standard central peaks. Since central peaks are deep material that's been uplifted, that means these impacts could not have penetrated through Europan ice to water. Water would not have been able to form and maintain a central peak."

Researchers also have hypothesized that Europa might have a thick ice shell composed of a thin brittle layer over warm convecting ice. But Turtle's and Pierazzo's research shows that the impacts couldn't have even penetrated to warm ice.

The Pwyll Crater on Europa. Credit: Elizabeth Turtle Planetary Image Research Lab, Lunar and Planetary Lab, University of Arizona

Europa's largest known central peak impact crater, the 24-kilometer (14-mile) diameter Pwyll, for example, contains a central peak roughly 5 kilometers (3 miles) in diameter and about 500 meters (three-tenths mile) high. Turtle calculated that if there were warm convecting ice beneath Pwyll's peak, the peak would have disappeared in less than a year.

This work is the first step in a multi-stage modeling project to determine ice thickness and better understand the geology and evolution of Europa, the UA scientists say.

The very sophisticated code that Pierazzo applied in this research to simulate the passage of the impact shock wave through water ice is very time consuming. It took two weeks to produce simulations of shock waves that occur in fractions of a second.

The next step is to use a less detailed and less time consuming code to simulate crater excavation and collapse to put further limits on the ice thickness, Turtle said.

In future research the team plans to simulate the temperature distribution during impacts for insight into structure of the solid ice, and to use information on temperatures and ice strength to model how long Europa's central impact peaks might exist.