MONTEREY, CALIFORNIA--Europa, one of Jupiter's strange moons, has welts on its icy face. The odd mounds could be a symptom of buoyant ice contaminated with salts or acids, according to a report 2 September at a meeting here of the American Astronomical Society's Division of Planetary Sciences. However, a separate analysis of cracks on Europa suggests that the ice--thought to cover a global ocean--is too thin to sustain large salty blobs.
Stunning images of Europa have come from NASA's Galileo spacecraft, which will plunge into Jupiter on 21 September after orbiting the planet since 1995. Cracks, domes, and chaotic jumbles point to active motions in the rind of ice that blankets Europa's ocean, planetary scientists believe. Galileo researchers proposed that Jupiter's intense tidal pull would churn the ice and ocean to form warmer blobs of ice. The blobs would rise toward the surface via convection within the crust. But under further scrutiny, it seemed that the convective motions weren't strong enough to lift the 100-meter-high mounds that Galileo sees.
The new study proposes that a pinch of salt or acid would do the trick. Planetary scientists Robert Pappalardo and Amy Barr of the University of Colorado, Boulder, calculated the effects of small proportions of sodium chloride and sulfuric acid within Europa's ice. Those substances exist in patches on the moon's surface, according to a spectrometer on Galileo. The contaminants would lower the melting temperature in blobs of ice deep in Europa's crust, the team found. Such blobs would flow more readily through regions of colder, harder ice, shedding impurities as they ascended. The resulting purer ice, more buoyant than the surrounding crust, would rise to the surface. The force could push up ragged domes 100 meters high, Pappalardo says.
The scenario doesn't sway planetary scientist Richard Greenberg of the University of Arizona, Tucson. Most of the raised regions aren't the size and shape Pappalardo's interpretation would suggest, he maintains. Further, convection requires the ice shell to be at least 20 kilometers thick. That's inconsistent with cracks in the ice--described in detail at the meeting by Greenberg's team--which split open, shift, and close in response to Jupiter's tides. "Everything we've done suggests that the cracks penetrate all the way to liquid water, which is hard to do if the ice is more than a few kilometers thick," he says.