The study of impact craters on Europa, a moon of Jupiter thought to hide a liquid ocean beneath an icy crust, has shed new light on the thickness of the satellites surface.
Researchers set a minimum surface thickness of between 2 and 2.5 miles (3-4 kilometers) for the jovian moon. Past estimates of the crusts depth have ranged from under six tenths of a mile to 10 times that thick based on what's known about the surface geology and composition.
The study, by Elizabeth P. Turtle and Elisabetta Pierazzo, based at the Lunar and Planetary Laboratory of the University of Arizona, appeared in the Nov. 9 issue of the journal Science.
The new estimate stems from examination of the moons craters, caused by asteroid or comet impacts. Some of the craters contain central peaks, formations of uplifted material at a craters center. Crater research of impacts on Earth, the Moon and Mars has shown that central peaks form from deep bedrock material thrown up when after object hits the surface in a sort of rebound effect.
The researchers numerically simulated the Europan impacts using computer-modeling programs.
The central peaks on Europa appear to be fairly standard, Turtle said. She added that since peaks are formed from material found deep underground, the crater impacts that formed them could not have penetrate through to the moons ice and water; water would not have been able to form and maintain a central peak.
Of 28 craters on Europa imaged by the Galileo and Voyager spacecraft, at least six are large enough have central peaks, which are typically found in those with diameters of at least three miles (five kilometers).
Turtle said the structure of some Europa craters studied showed that the impacts that caused them did not completely melt through an initial ice layer or vaporize the moon's surface.
A crater called Pwyll, Europas largest crater with a peak structure, sports a central peak three miles across and three-tenths of a mile high in a depression that spans 14 miles. If there were a layer of warm convecting ice immediately beneath Pwylls peak, the formation would have disappeared in less than a year, the recent study said.
Though Turtle and Pierazzos study set a minimum thickness for Europas crust, they stressed that their work did not set a maximum limit.
The extent of Europas shell of ice has been one of much debate, with some scientists estimating a thin crust up to just over a mile thick because of the ridges, racks and other geological features seen on the surface. Others, however, maintain the crust should be at least 12 miles thick and includes a layer of warm, convecting ice that shapes observed surface features.
The importance of an accurate measure of Europas crust applies to more than just geology. A Europa orbiter mission is tentatively planned by NASA for launch in 2008 to determine the possibility of a subsurface ocean. If one were found, knowledge of the surface thickness would be imperative before the ocean could be explored.
More About the Solar System:
advertisement
