Ganymede, the largest moon of Jupiter, is a water world.
Researchers using the Hubble Space Telescope have confirmed an ocean beneath Ganymede’s icy crust that are likely ten times deeper than Earth’s oceans.
The moon may have more liquid water beneath its surface than there is on Earth.
To reach this conclusion, scientists measured fluctuations in the moon’s aurorae. Aurorae, which are electrified ribbons of heated gas, are generated when a planetary body’s liquid metal core produces a magnetic field.
On Ganymede, the aurorae around its north and south poles move, influenced by fluctuations in Jupiter’s huge magnetic field.
Scientists used computer modeling to verify that the only explanation for the extent of the “rocking” of Ganymede’s aurorae is the presence of a saline ocean, which would cause Ganymede’s magnetic field to offset to some extent Jupiter’s magnetic impact.
Several measurements indicated that the aurorae moved by about two degrees, but without the presence of an ocean Jupiter’s influence would have caused movement of about six degrees.
“When there is an electrically conductive ocean present, this counteracts Jupiter’s influence,” Joachim Saur, a professor geophysics at Germany’s University of Cologne and the lead researcher, said.
Saur explained that four independent measurements with HST verified the two-degree movement of Ganymede’s aurorae.
Scientists had speculated for several decades that Ganymede could be an oceanic moon. The Galileo probe, which flew by the moon several times during its 1995-2003 mission, provided further grist for that supposition when it measured the moon’s magnetic field. But those flybys did not last long enough for scientists to detect the movement of Ganymede’s aurorae.
“The flybys lasted only 20 minutes each,” Saur said. “In the new Hubble observations we have seven hours of data, so we do not have the ambiguity anymore.”
Ganymede’s sea may also be impacting the surface of the moon. Mapping of the satellite by the U.S. Geological Survey reveals areas of the surface that are smoother and less cratered than other areas, which could indicate the presence of tectonic forces that permit movement of sea water from beneath the icy crust.
“These lighter shaded regions are believed to be formed by flooding on the surface by water coming to the surface by faults or even cryovolcanoes,” Jim Green, NASA’s director of planetary science, said.
Ganymede is Jupiter’s largest moon, with more mass than Mercury. It joins fellow Galilean moon Europa and Saturn’s moon Enceladus on the list of moons known to have water oceans.
Another of Jupiter’s four largest moons, Callisto, is also thought likely to have liquid water.
“Every observation that we make, every mission that we send to various places in the solar system, is taking us one step closer to finding that truly habitable environment, a water-rich environment in our solar system,” Heidi Hammel, executive vice president of the Association of Universities for Research in Astronomy, said. “Everywhere we look there’s water.”
Image courtesy Wheaton College, Johns Hopkins University Applied Physics Laboratory, Brown University, Jet Propulsion Laboratory, Unites States Geological Survey.
Ganymede, one of the four satellites of Jupiter discovered by Galileo in 1610, has been mapped.
The geologic map of Ganymede’s entire surface is the first completed of any moon of the solar system’s outer planets.
Researchers led by Wes Patterson of the Johns Hopkins University Applied Physics Laboratory and Geoffrey Collins of Wheaton College used images obtained by the NASA probes Voyager and Galileo to produce the map.
“This is our best compilation of what we know so far,” Collins said.
He explained that the map was drawn on the basis of data obtained by the Voyager and Galileo probes, which visited the Jovian system in the 1970s and 1990s.
“We were working off a base mosaic, a mosaic put together from the Galileo camera and the Voyager camera that was done by the U.S. Geological Survey,” he said. “They have a lot of cartographic experts there to make sure that the data is located in the right place. The cartographic image data is all based on the cameras from Voyager and Galileo.”
Jupiter’s largest satellite and the largest moon in the solar system, Ganymede is larger than both Mercury and the dwarf planet Pluto. It has an icy outer shell, about 500 miles thick, that was probably primarily the result of tectonic activity.
Scientists suspect this because orbiting spacecraft have observed two different landforms on the moon’s surface. As explained by NASA’s Solar System Exploration site:
Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain, which forms intricate patterns across Ganymede. . . . This grooved terrain is probably formed by tensional faulting or the release of water from beneath the surface. Groove ridges as high as 700 m (2,000 feet) have been observed and the grooves run for thousands of kilometers across Ganymede’s surface. The grooves have relatively few craters and probably developed at the expense of the darker crust. The dark regions on Ganymede are old and rough, and the dark cratered terrain is believed to be the original crust of the satellite. Lighter regions are young and smooth (unlike Earth’s Moon).
Collins said that the map may help scientists figure out the source of Ganymede’s tectonic forces.
“This map is helping us concentrate on separating which part happens at the end and which part at the beginning,” he explained. “By doing that, we can pull back and look at this pattern of motion that was happening on Ganymede’s surface while it was being tectonically ripped apart and compare it to ideas of how those tectonics may be driven by internal processes.”
Ganymede’s tectonic activity is likely the result of either tidal forces or a heat source inside the moon. One clue as to its genesis lies in the fact that changes in Ganymede’s surface do not appear to be constantly occurring.
“The fact that it turned on and turned off again may have something to do with some short-lived internal activity, maybe some heat pulse that was happening inside Ganymede,” Collins said. “One of the theories is that Ganymede may have undergone a tidal heating episode that was short-lived and due to interactions between Ganymede and Europa. Right now they are in orbital resonance. Perhaps when they fell into that resonance Ganymede became very hot on the interior. That would have implications for what’s happening on the surface.”
While the map may help researchers gain a more complete understanding of the moon’s history, it is not likely to answer all the questions about Ganymede.
For example, while scientists are now reasonably sure that volcanism has not been the dominant shaper of Ganymede’s surface, there is a possibility that some craters are actually locations of volcanoes.
“There are these very odd, strange pits that we found in a few different locations on Ganymede,” Collins explained. “On the map they are labeled as depressions. There are about four clusters of them.
“If there were cryo-volcanoes on Ganymede, that’s our best guess about where they might be. They almost look like volcanic calderas.”
The map also provides no information about elevation on Ganymede. Scientists lack the detailed topographic data that could provide an answer to the question whether the more recent, light-colored areas on Ganymede’s surface have a different elevation than the older, dark-colored areas.
“Unlike Mars we’ve never orbited Ganymede,” Collins said. “So we’ve never collected the topographic data that Mars scientists enjoy. We don’t know the answer to that question.”
Despite the uncertainties about Ganymede that remain, Collins thinks the new map is a big step forward for the scientific community.
“It’s gone from being a point of light to being a world in its own right, with its own geology, its own history, its own geography,” he said.
While the map is the first of an outer solar system moon, it is the fourth made of any moon. Earth’s moon, Luna, and two other Jovian moons, Io and Callisto, have been mapped.
A map of Europa should be finished soon.
“There have been preliminary copies floating around,” Collins said.
The European Space Agency is scheduled to send the next probe to the Jovian moons. Known as the Jupiter Icy Moons Explorer, it will launch in 2022 and, upon arriving, spend three years studying Ganymede, Callisto, and Europa.
The map was published by the U.S. Geological Survey. You can download it here.
This image of Ganymede was obtained by NASA’s Galileo probe. Image courtesy Wikimedia.
NOTE: This story was updated on March 3, 2014.