WHEN astronomers found signs of oceans deep beneath the surfaces of two of
Jupiter鈥檚 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鈥檚 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. 鈥淵ou just need some sort of antifreeze,鈥� 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鈥檚 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. 鈥淚t
was a very big surprise on Callisto,鈥� which has an ancient surface and is the
most distant of Jupiter鈥檚 four large moons, says Louise Prockter of Johns
Hopkins University Applied Physics Laboratory.
Lorenz bases his predictions on a simple idea鈥攖hat 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鈥檚 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 鈥渨ould seem dubious鈥� 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鈥檚
predictions hold up.