If the moon Europa is tilted on its axis even slightly as it orbits the giant planet Jupiter, then Jupiter's gravitational pull could be creating powerful waves in Europa's ocean, according to Robert Tyler, an oceanographer with the University of Washington's Applied Physics Laboratory and author of a letter in the Dec. 11 Nature. As those waves dissipate, they would give off significant heat energy.

Depending on the amount of tilt, the heat generated by the ocean flow could be 100 to thousands of times greater than the heat generated by the flexing of Europa's rocky core in response to gravitational pull from Jupiter and the other moons circling that planet.

That's the current assumption - that oceans on moons are heated mainly by this flexing of their cores. In the case of Europa, it also has been thought that the thick ice covering its ocean probably generates some heat as two sides of cracked ice rub together in response to gravitational pull.

"If my work is correct then the heat source for Europa's ocean is the ocean itself rather than what's above or below it," Tyler says. "And we must form a new vision of the ocean habitat that involves strong ocean flow rather than the previously assumed sluggish flows."

Both are important considerations if exploratory missions are ever sent to Europa in search of life. Europa, which is slightly smaller than Earth's moon, is one of Jupiter's 63 moons. With surface temperatures as cold as minus 260 degrees Fahrenheit, Europa's surface is covered with a thick layer of ice.

There is evidence of a liquid ocean beneath the ice and, if there is volcanic activity on the sea floor, this could be a recipe for generating microorganisms that live without sunlight, perhaps like the microorganisms found at hydrothermal vents and other places on Earth.

Many planets and moons are known to be tilted within their orbital plane. The Earth, for example, has an axial tilt of about 23 degrees. It's why the northern and southern hemispheres have different seasons, depending on whether they are tilted more toward or away from the sun.

Previous theoretical calculations expected Europa to have an axial tilt of at least 0.1 degrees. It hasn't been measured and could be bigger than this. But even at this minimum value the tidal flow on Europa using Tyler's new calculation is quite strong - some 10 centimeters a second - and enough to cause significant heating.

The new calculation differs from previous ones in that it allows a more realistic dynamic response of the ocean to the tidal forces.

His assumptions and calculations led him to say that he thinks this kind of wave action could be the dominant heat source in the oceans of Europa and other moons.

"But this proposal is a relatively new contender - so let's see how it does," he says.

Tyler is the sole author of the letter in Nature. His work was supported by NASA's Outer Planets Research program.

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