Still nailing down that ocean
Although most planetary scientists are comfortable with the idea of an ocean at Europa, its existence still remains unproven. So Massachusetts Institute of Technology researchers plan to use acoustics to explore the Arctic Ocean and apply that work to the question of a vast liquid ocean under the ice blanketing Europa.
MIT researchers will report this week at a Chicago meeting of the Acoustical Society of America that they may be able to use a technique similar to ultrasound, the sonar navigation used by bats and dolphins, to gather information about Europa.
While such an experiment may be a decade or more away, this unconventional approach to planetary exploration would have to begin to be developed now, Makris said. An array of geophones on the icy surface could simultaneously localize discrete events such as fractures and determine the moon's ice-layer thickness as well as the thickness of a potential ocean layer.
Using sound to 'see'
Europa's numerous fractures and ridges, shown in Galileo images taken in the past five years, are believed to have formed in response to tidal deformations generated by the moon's slightly eccentric 85-hour orbit around Jupiter.
Inspired by evidence for these regularly occurring ice fractures, the MIT researchers propose probing Europa's interior by deploying an array of surface microphones that listen to naturally occurring sound. Knowledge of ice mechanics suggests that these propagating fractures would generate significant acoustic energy in the frequency range 0.1-100 hertz (Hz).
Studying the ice sounds would allow researchers to see if there was a connection between the moon's orbital period and the ice fractures, which occur on Europa once every 30 seconds.
Starting in the Arctic
MIT researchers led by Makris have used sound-based techniques to explore the Arctic Ocean. By inserting vibration-sensitive hydrophones in the water, researchers used ambient sound to listen for changes in noise levels. They found that noise levels increased when winds and currents put stresses on the ice.
"Noise levels are like a thermometer for stress on the ice," Makris said. "The ice is very sensitive and conducive to sound." Sound waves made by large fractures go through the ice and penetrate into the ocean.
These low-frequency sound waves, akin to those created by whales, get trapped and can propagate hundreds of miles (kilometers) through the water. Even if they can't be heard, instruments can pick up their vibrations from a distance.
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