By Lori Stiles

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

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

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. Images of the six
craters are online at

“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.

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

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.

Contact Information

Elizabeth Turtle


Elisabetta Pierazzo