Image: Courtesy of ELIZABETH TURTLE/University of Arizona

Just how thick the layer of ice covering the surface of Jupiter's moon Europa is lies at the center of an ongoing scientific debate. Current theory holds that the icy layer covers a huge liquid ocean, so its thickness holds import for possible Europan oceanic explorations in the future. But estimates of the ice breadth have ranged from merely one or two kilometers to upwards of 30. Now new research, published in the current issue of Science, provides support for an intermediate thickness of at least three to four kilometers.

To investigate the thickness of the ice layer, Elizabeth Turtle and Elisabetta Peirazzo of the University of Arizona's Lunar and Planetary Laboratory exploited impact craters on Europa's surface. The scientists modeled the impact of comets into a layer of ice overlying liquid water and compared the results to six actual Europan craters imaged by Galileo and Voyager that exhibit central peaks (see image). As Turtle explains, because "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." The authors thus conclude that the ice shell must have been greater than three to four kilometers deep. They stress, however, that their observation can only be interpreted as a lower limit.