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Naval Research: Listening to Ice

Jupiter's moon Europa
Jupiter's moon Europa
(courtesy of NASA)

Arctic ice is constantly moving, stretching, and compressing--and making a lot of noise as it does so. Dr. Nick Makris, an oceanographer who works at the Massachusetts Institute of Technology, has learned how to use that noise to learn about the ice and the ocean beneath it. Next he wants to use his methods to study a far away ice-covered ocean--one that is right now orbiting the planet Jupiter!

Sound is a great tool for studying ice and the oceans. Light fades very quickly in water, but sound can travel great distances underwater, especially if you use the right notes, or frequencies. Like light, sound bounces off of solid objects, so we can use it to èseeî in the dark water. To learn more about how people and animals use sound in the oceans check out our Acoustics page.

This enhanced color image of the crater Pwyll was taken by the Galileo spacecraft
(Courtesy of NASA)

By timing the speed of sound through water you can calculate the water's temperature. The amount of time it takes sound to travel through ice will tell you the ice's density (how tightly it's packed). Scientists sometimes make their own noise and record how it travels through sea and ice, but Dr. Makris has also learned to use the natural noises ice makes. From satellite images of Europa, one of the four largest moons of Jupiter, scientists have learned that its ice crust cracks regularly. What they don't know is how deep those cracks go or what is beneath the ice.

Dr. Makris is working with researchers at NASA on a plan to send several soda-can-sized microphones to Europa. The microphones would be placed a few feet into the ice, where they could listen for cracking. Scientists could then calculate the ice's density and thickness, and possibly discover what it covers. Some people believe that under the hard ice crust is a slightly warmer layer of slushy ice, but other scientists believe that the ice covers a saltwater ocean. Researchers at NASA had thought about sending a probe to Europa that would melt through the ice. When Dr. Makris heard this idea, he knew it would be too difficult because the ice could be tens of miles thick. He suggested that sound would be a more practical way to study Europa, so now NASA is considering the microphone probes for a future mission.

This false color image, created from 3 images taken by Galileo, shows different features in Europa's icy crust
(Courtesy of NASA)

Does it really matter if there's water or slush under Europa's ice? It does if you are looking for life in places other than the Earth. Water is necessary for life as we know it, so an ocean is the most likely place to support life. Europa could very well be home to the only other life in our solar system.