CAMBRIDGE, Mass. — Acoustic techniques used by Massachusetts Institute of
Technology researchers to explore the Arctic Ocean may help determine
whether there is a vast liquid ocean under the ice blanketing Jupiter’s
moon, Europa.

MIT researchers report June 5 at the Chicago meeting of the Acoustical
Society of America that they may be able to use a technique similar to
ultrasound or the sonar navigation used by bats and dolphins to gather
information about Europa.

MIT ocean engineering professor Nicholas C. Makris said that implanting
soda-can-sized sensors in Europa’s icy exterior could provide researchers
with information on the temperature and structure of the planet. Current
sensor technology makes it possible to detect even tiny motions, and there
is evidence that massive ice fractures on Europa’s surface occur daily.

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.


Europa may be the only entity in our solar system besides Earth that
contains a great deal of water, researchers say, and this mission would
represent the first time ocean scientists have been involved in planetary

Gravity and magnetic data collected by the NASA Galileo Orbiter over the
past five years have provided increasing evidence that an ocean exists
underneath Europa’s uniform, 10- to 100-kilometer thick coat of ice. The
possible ocean on Europa may contain more liquid water than all the oceans
on Earth combined.

Magnetic studies have indicated that there must be a conducting layer in
Europa. A salty ocean would fit the bill. Researchers hope to discover
whether Europa is made up entirely of mushy ice or if it contains an ocean.
Where there is water, there may be life.


Pictures of the planet show odd, cusp-shaped cracks in the surface.
Europa’s numerous fractures and ridges 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 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. Meteors impact Europa about once a
month and these also could be used as sound sources.


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.

“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 kilometers through the water. Even if
they can’t be heard, instruments can pick up their vibrations from a

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. Makris is the
Secretary of the Navy/Chief of Naval Operations Scholar of Oceanographic