Meteors impact Europa about once a month and these also could be used as sound sources. An Arctic exploration MIT researchers led by Makris, Doherty Professor of Ocean Utilization in MIT's Department of Ocean Engineering, 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. 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 kilometers through the water. Even if they can't be heard, instruments can pick up their vibrations from a distance. In addition to Makris, the research team includes ocean engineering postdoctorate associates Aaron M. Thode and Michele Zanolin and graduate students Sunwoong Lee, Purnima Ratilal and Joshua Wilson. This work is funded by the Office of Naval Research. The story of the historic flight of Apollo 11 is told in this 248-page book through offical NASA press kits and mission reports.