Data from the Galileo spacecraft have convinced scientists that Jupiter's largest moon probably has a global ocean.
As big as Mercury, Ganymede joins sister Jovian moons Europa and Callisto as the largest water-covered bodies, after Earth, known in the solar system.
Together, they now enable scientists to compare the geology of several planet-size bodies in which water plays a major role. Also, their apparent abundance of liquid water raises the question of whether or not any of them could support life.
The finding brings "a new revolution to efforts to understand the solar system," says Robert Pappalardo, a geologist at Brown University in Providence, R.I.
Galileo passed within 503 miles of Ganymede's surface May 20 and returned "absolutely spectacular data" that now lets scientists study the similarities and differences between the three Jovian moons in unprecedented detail, says Brown University colleague James Head.
Its close-up images - some with 3-D perspective - show a rough, ridged terrain broken by long lanes of smoother material.
Dr. Head said Ganymede appears to be covered by a brittle ice crust with more pliable material beneath.
Sometimes, the old crust breaks up, spreads apart, and material wells up to form a new smoother crust. At the same time, older crust moves apart along mid-ocean ridges, while new material wells up to replace it. It's a process similar to sea-floor spreading on Earth.
This finding does not, in itself, show that Ganymede's crust is underlain by liquid water. The upwelling material could be a warmer, more ductile form of ice.
The more-convincing argument for liquid water is written in the details of Ganymede's magnetic field. As the moon moves around Jupiter, its magnetic poles are "wiggling." One plausible reason: Jupiter's magnetic field is inducing electric currents in liquid salt water on Ganymede, jiggling Ganymede's own magnetic field.
It's "very strong evidence of a layer of [liquid] water," says Margaret Kivelson, a geophysicist at the University of California in Los Angeles. She estimates the layer to be a few miles thick and about 105 miles below the surface, although it could be higher up.
Others, however, aren't so sure. Some say scientists are settling on the liquid-water theory because "we can't think of any other explanation," says David Stevenson, a scientist at the California Institute of Technology in Pasadena.
Scientists are only beginning to learn the role of water oceans on bodies like the large Jovian moons. On Europa, as on Earth, the presumed liquid water beneath the surface ice extends down to a rocky crust. Dr. Stevenson notes this makes Europa the most likely of the three moons to support life. There is the possibility of life-sustaining energy sources such as warm hydrothermal vents similar to those on Earth's sea floor. The water itself probably is a few degrees above freezing, not much colder than the Arctic Ocean.
On Callisto and Ganymede, however, the liquid layer is sandwiched between layers of ice. "It's a new geological beast," says Torrence Johnson, a project scientist at Cal Tech's Jet Propulsion Laboratory, which runs the mission for NASA.
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