Published online 26 July 2001 | Nature | doi:10.1038/news010726-12

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Callisto's watery secret

One of Jupiter's moons may hold an underground ocean.

Over the moon: Callisto's icy surface may hide an underground ocean.Over the moon: Callisto's icy surface may hide an underground ocean.© SPL

One of Jupiter's largest moons, Callisto, may hold watery secrets beneath its surface, suggests a new analysis. The satellite's icy crust may be the planetary equivalent of a blanket, insulating an underground ocean.1

Radioactivity at Callisto's core provides ample heat to keep water from freezing. But scientists believed that the heat would escape through the satellite's crust of ice and rock.

"It was thought that a body of liquid water in a big icy satellite like Callisto would freeze within a few hundred million years," says Javier Ruiz, a geologist at the Complutensian University in Madrid, Spain. This seems like a long time but is short compared with the age of the Solar System.

Yet beneath Callisto's heavily cratered surface, a treasure may be hiding. Ruiz calculates that the ice holds in heat better than was previously thought. The temperature and pressure conditions on Callisto make the ice less conductive than normal, trapping in heat from the satellite's core that could keep water in a liquid form.

The finding offers "a new and remarkable insight into the icy rock world", says Kristin Bennett, a geophysicist at Los Alamos National Laboratory in New Mexico. Callisto's surface lacks volcanoes, mountains, rifts or any other sign of geologic activity, so the satellite has long been considered the 'ugly duckling' of Jupiter's large, icy moons, "Frankly, from a geologist's point of view, Callisto was boring," she says. Now "it might just be the 'swan' of the Solar System".

The first hints of Callisto's watery secret emerged in 1998. Data collected by sensors on the spacecraft Galileo revealed that the moon's magnetic field fluctuates as Jupiter turns. An underground ocean of salt water seemed the most plausible explanation, as salt water conducts electrical current, which could interact with Jupiter's magnetic field to produce the fluctuations.

It looked like indications of an ocean at depth, says Jeffrey Kargel of the US Geological Survey in Flagstaff, Arizona. "But people didn't quite believe it. How could Callisto have an ocean?"

Applying the analysis by Ruiz to other moons, such as Jupiter's Ganymede and Saturn's Titan, could also reveal underground seas. "This makes it a lot easier to have liquid water in icy satellites," Kargel says.

Whether such oceans could provide a home for hardy forms of life is now up for debate. Suggestions of life on icy Callisto have previously seemed incredible, says Kargel. But in recent years, researchers have found bacteria thriving in hostile environments on Earth, he points out, such as toxic thermal vents on the ocean floor. "Where we've found liquid water on Earth, we've found life," he says.

But the presence of life on Callisto should not be presumed, cautions Ruiz. The ocean's depths are probably packed with dense balls of ice and rock, which would block the circulation of heat and make it an inhospitable environment. 

  • References

    1. Ruiz, J. Stability against freezing of an internal liquid-water ocean in Callisto. Nature 412, 409 - 411 (2001).  | Article | PubMed | ISI | ChemPort |