Jupiter's second largest moon, Callisto, has been upgraded from dead and boring to possibly wet and electrifying.
In a new study, Javier Ruiz of the Universidad Computense de Madrid found that heat generated in the interior of Callisto may be trapped there, warming a subsurface ocean but not detectable on the surface.
The research, published in the July 26 issue of the journal Nature, shows that the heat would be generated radioactively, in the moon's core, and that it warms a subterranean ocean but does not radiate through Callisto's dense, thick crust of ice. The work supports previous suspicions that Callisto might harbor subsurface water.
"I find it a very interesting work," said Krishan Khurana, a Jet Propulsion Laboratory researcher who works on the Galileo magnetometer team but was not directly involved in the new study. "It may be the groundbreaking research we've all been looking for."
What Khurana and others have been looking for, since the Galileo spacecraft first observed Callisto's supposed magnetic field in 1998, is an explanation for why the magnetometer noted variations in Jupiter's magnetic field whenever Callisto was observed. It turns out the moon does not have its own magnetic field, but instead it acted like an enormous electromagnet: a magnet made from a conductive material, and interacted with Jupiter's colossal magnetic field.
The only explanation that scientists could come up with was that Callisto was filled with some sort of conductor.
"Any conductor would do," explained Khurana, "However it's hard to imagine very large amounts of conductors inside the moon beside salt water."
The misunderstood moon
Callisto's fate was decided, that it be considered the most boring of the four Galilean moons of Jupiter.
Several strikes were against Callisto. For one, unlike Europa, whose underground ocean is kept in a liquid state from Jupiter's gravity tugging on it, an effect called tidal heating, Callisto is only heated from its small radioactive core. Because it was more obvious Europa had an ocean, this difference made Callisto a poor candidate for water.
Also, scientists relied on evaluations of Callisto that used only average temperatures, surface thickness and ice viscosity. The results suggested water in Callisto had to be frozen, despite its ability to behave electromagnetically.
But Ruiz modeled Callisto with a wide range of these properties and found that the moon could hold water if the ice was thick enough and dense enough. Ice viscosity, or how easily the ice grains slide past each other, was one major source of variation in the modeling.
"The viscosity (of the ice) in the past was modeled as a fixed number for the whole crust. Where as in this work, it's found that the top-most layer would be stable against convection of heat, because it is so very cold and rigid. When that is taken into account, the rest of the ice would have a higher viscosity than one imagined before," said Khurana.
A higher viscosity would hold in the small amount of heat from the core and also explains Callisto's unique appearance.
David Stevenson, a professor of planetary science at the California Institute of Technology, cautions that settling on one value of ice viscosity is still just guesswork.
"The parameters of Callisto are still poorly known, and not well enough known to settle on a definitive characterization," Stevenson said.
Unlike Europa, whose ocean continually cracks and smoothes its patina, Callisto's icy terrain appears to have gone unchanged since its creation roughly four billion years ago, except for the countless dimples of meteorite impacts. This undynamic appearance enforced Callisto's reputation as an unlikely candidate for having a liquid middle.
But Ruiz's evaluation states that the surface on Callisto must be so thick, as much as 93 miles (150 km), that the minimal heat from within would never smooth out the surface.
Still, cautioned Stevenson, "This is not a problem that you can calculate and find an answer of 'yes' or 'no' to."
Stevenson said some properties that are being estimated, and which therefore leave the results lacking a definitive answer, are the grain sizes of the ice and the heat flow of the planet. Nonetheless, Stevenson believes the results are promising.
Of course, where there is water there is a possibility for life as we know it to flourish. But Khurana and Stevenson agreed that it's very unlikely that a fertile incubator lies underneath Callisto's ancient surface.
Khurana believes that because Callisto's insides are in a form of equilibrium, there will be no environmental niches like temperature gradients or reactions between water and minerals for life to colonize. Nonetheless, Ruiz's calculations allow science to expand its strict parameters to include Callisto as a fascinating satellite of Jupiter.
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