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Planetary News: Galileo (2004)

A Mine Dug in Galileo Data Yields Discovery of Mass Anomalies within Ganymede

By Emily Lakdawalla
August 18, 2004

Radio scientists examining data from Galileo's second flyby of Ganymede have determined that Ganymede's relatively smooth surface is hiding something. Beneath the icy outermost layer, there must be anomalous concentrations of rocky material -- or their lack -- in order to explain tiny gravitational tugs felt by the Galileo spacecraft as she flew by Ganymede on September 6, 1996.

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The discovery was made by a team led by John Anderson, of the Jet Propulsion Laboratory, and Jerry Schubert, of the University of California, Los Angeles. "It took us this long to straighten out the anomaly question, mostly because we were analyzing all 31 close flybys for all four of Jupiter's large moons," said Anderson. "Once we were convinced that anomalies exist on Ganymede, we looked for similar evidence on other satellites, especially Europa, which has a similar structure to Ganymede. Oddly enough, none of the other 30 close flybys showed any evidence of mass anomalies. There is only one flyby, the second flyby of Ganymede, where mass anomalies are evident. The second Ganymede flyby is unique."

Examining the influence of gravity on a spacecraft is the second most powerful method that scientists have to study the interior structure of terrestrial planets. (The most powerful method -- seismology -- is only possible for planets with seismographic stations, which have so far only been placed on the Earth and Moon.) The gravity field of a planet or moon depends not only on how much mass it has, but also upon how that mass is distributed within the body's interior. Under the influence of gravity, all planetary bodies are denser near their centers than they are near their surfaces. Large bodies, like planets and moons, tend to be "differentiated," meaning that they are divided into concentric layers, with the innermost layer being the densest.

A spacecraft flying by the body experiences a gravitational pull from all of the mass within it. This pull causes the spacecraft to accelerate, which in turn causes a Doppler shift in the spacecraft's radio signal back to Earth. If the spacecraft passes sufficiently close to the planet, local differences in mass distribution within the planet will cause tiny fluctuations in the Doppler shift of the radio signal. The challenge that scientists then face is to reconstruct the distribution of mass within the three-dimensional structure of the planet from these tiny Doppler shifts.

For all but one of the Galileo flybys of Jupiter's moons, Anderson and coworkers were able to explain the Doppler shifts in Galileo's radio signal with relatively simple models of mass distribution. "Jupiter's four Galilean satellites can be approximated by fluid bodies that are distorted by rotational flattening and by a static tide raised by Jupiter," they wrote in their article, published on August 13 in Science magazine. But "it is impossible to obtain a satisfactory fit to the Doppler data from the second Ganymede flyby."

In order to explain the accelerations they observed, the researchers were forced to assume that there must be at least two locations where the mass distribution does not fit the simple model. One is a place where there is more mass than expected at a high latitude, and another is a place where there is less mass than expected, at a low latitude. A model with a third extra mass fits the observed Doppler data slightly better than the two-mass model. The amount of "extra" and "missing" mass is small compared to the mass of Ganymede as a whole, but they must represent huge features, equivalent in size to mountain ranges. Puzzlingly, there are no obvious surface features in positions corresponding to the locations of the theorized mass.

What does this all mean about the interior of Ganymede? Positive mass concentrations might mean that there is extra rock at a location where one would otherwise expect to see ice or water; for example, it could be a subsurface "mountain range" at the rock-ice interface, or a large pile of rocky material within Ganymede's outer ice shell. Negative mass implies extra ice where one would otherwise expect rock. But Ganymede, like Europa, is theorized to have a liquid ocean underneath its icy outer shell. Such a liquid ocean would be expected to "seek its own level" over time, flowing away from positive mass anomalies and toward negative mass anomalies in order to even out the gravitational field.

"If there is a liquid ocean under the surface, any anomalies near the surface are most likely located at the bottom of the ocean," Anderson says. "This is a possibility, but the single Galileo flyby cannot discriminate between anomalies at the bottom of an ocean and anomalies near the surface in a completely frozen ice shell. With additional work, we can probably distinguish between anomalies at the rock-ice interface about 800 kilometers (500 miles) beneath the surface and anomalies near the surface, but any interpretation beyond that would be stretching the limited data from the Galileo mission."

This discovery appears to raise more questions than it answers and suggests avenues for further research. "A Ganymede orbiter is needed before the entire satellite can be mapped in detail for mass anomalies. If we are to resolve this puzzle raised by the Galileo mission, both gravity mapping and altimeter [topographic] data are important. This should be an objective of the JIMO [Jupiter Icy Moons Orbiter] mission, in my opinion," says Anderson. This view is shared by JPL's Torrence Johnson, former Galileo project scientist. "The fact that these mass anomalies can be detected with just flybys is significant for future missions. Knowing more about the interior of Ganymede raises the level of importance of looking for gravity anomalies around Jupiter's moons and gives us something to look for."