WHEN astronomers found signs of oceans deep beneath the surfaces of two of
Jupiter’s moons, Ganymede and Callisto, they were astonished. But Ralph
Lorenz of the University of Arizona says they should have been expecting it.
Lorenz told the Lunar and Planetary Science Conference in Houston this week
that his simple model of internal convection predicted liquid water below the
surfaces of those moons. And, he says, Saturn’s largest moon, Titan, should have
a hidden ocean too.
Subsurface oceans are possible because ordinary ice is less dense than water
and so forms at the surface. “You just need some sort of antifreeze,” such as
ammonia or salt, to keep the liquid layer from freezing, says Bill McKinnon of
Washington University in St. Louis. However, pinpointing exactly where these
oceans are has been difficult.
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Europa’s ocean, the first to be discovered, came as no surprise because its
surface looks recently frozen and it is heated by tidal forces from Jupiter. “It
was a very big surprise on Callisto,” which has an ancient surface and is the
most distant of Jupiter’s four large moons, says Louise Prockter of Johns
Hopkins University Applied Physics Laboratory.
Lorenz bases his predictions on a simple idea—that the way energy flows
from the warm rocky core of an icy moon tends to maximise the dissipation of
heat along the way to the cold surface. He says this promotes formation of a
liquid layer, because convection in a liquid can dissipate heat within an icy
moon more efficiently than a solid can. Lorenz has used the same principle to
describe heat transport from the equator to the poles of Titan, Mars and
Earth.
The big question now is whether Lorenz’s simple model will be able to explain
what actually happens inside real icy moons. Dave Stevenson of the California
Institute of Technology says the calculation “would seem dubious” because it
does not make a distinction between objects having the same heat flow and size,
but different composition and internal temperatures. As future space missions
get a closer look at these moons, researchers will be eager to see if Lorenz’s
predictions hold up.