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Distant moons may have liquid oceans

Dec. 10, 2008
Courtesy Nature
and World Science staff

Mo­tions cre­at­ed by tides may gen­er­ate enough heat to main­tain liq­uid oceans with­in the icy moons of the so­lar sys­tem’s out­er plan­ets, a sci­ent­ist says in a new stu­dy.

If cor­rect, the claim could en­hance the pros­pects for find­ing life on dis­tant moons, since life as we know it de­pends on liq­uid wa­ter.

The re­search, pub­lished in the Dec. 11 is­sue of the re­search jour­nal Na­ture, draws on cal­cula­t­ions of a pre­vi­ously ne­glected tid­al force re­sult­ing from the mo­tions of moons with re­spect to their host plan­ets.

Thick ice blan­kets the icy moons, such as Jupiter’s Eu­ro­pa. But mount­ing ev­i­dence sug­gests the ma­te­ri­al be­neath the sur­face is liq­uid in at least some cases. 

The frig­id sur­face tem­per­a­tures and pal­try heat from with­in the lu­nar in­te­ri­ors in­di­cate the oceans should freeze, plan­e­tary sci­ent­ists say. But some re­search­ers have pro­posed that heat from tid­al forc­es with­in the sol­id in­te­ri­ors of the moon­s—caused by the moon or­bit­ing around the plan­et—may supply enough heat to keep the oceans liq­uid. 

In the new stu­dy, Rob­ert Tyl­er of the Uni­ver­s­ity of Wash­ing­ton found that a pre­vi­ously un­con­sid­ered tid­al force due to obliqu­ity—the tilt of the moon’s ax­is with re­spect to its or­bit—has the right prop­er­ties to cre­ate large waves with­in the oceans them­selves. 

These waves may pro­vide sig­nif­i­cantly more en­er­gy than the pre­vi­ously stud­ied tid­al mo­tions of their sol­id in­te­ri­ors, Tyl­er ar­gues. As the en­er­gy from the waves dis­si­pates, he wrote, that would gen­er­ate heat suf­fi­cient to be the “sig­nif­i­cant and po­ten­tially dom­i­nant” heat source for oceans on the out­er moons.


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Motions created by tides may generate enough heat to maintain liquid oceans within the icy moons of the solar system’s outer planets, a scientist said in a new study. If correct, the claim could enhance the prospect for finding life on distant moons, since life as we know it depends on liquid water. The research, published in the Dec. 11 issue of the research journal Nature, draws on calculations of a previously neglected tidal force resulting from the position of moons with respect to their host planets. Thick ice blankets the icy moons, such as Jupiter’s Europa, but mounting evidence suggests the material beneath the surface is liquid in at least some cases. The frigid surface temperatures and paltry heat from radiation within the lunar interiors indicate the oceans should freeze, planetary scientists say. But some researchers have proposed that heat from tidal forces within the solid interiors of the moons—caused by the moon orbiting around the planet—may supply enough heat to keep the oceans liquid. In the new study, Robert Tyler of the University of Washington found that a previously unconsidered tidal force due to obliquity—the tilt of the moon’s axis with respect to its orbit—has the right properties to create large waves within the oceans themselves. These waves may provide significantly more energy than the previously studied tidal motions of their solid interiors, Tyler argues. As the energy from the waves dissipates, he wrote, that would generate heat sufficient to be the “significant and potentially dominant” heat source for oceans on the outer moons.