Galileo Relays Surprising News About Jupiter’s Moons

Halfway through its two-year reconnaissance mission to Jupiter, the Galileo spacecraft has found a mysterious dusting on the supposedly dead moon Callisto that one NASA scientist said “makes you want to go up and sweep it off.”

The discovery that these “dust slides” are erasing craters on Callisto’s surface was completely unexpected, said planetary scientist Kelly Bender of Arizona State University during a press briefing at the Jet Propulsion Lab in Pasadena on Thursday. “It wasn’t a puzzle people were expecting from old Callisto,” said Bender of the moon generally perceived as “boring.”

Galileo also sent back audible radio signals from Callisto’s twin, Ganymede, which squawked and squealed as the spacecraft swept through its strong magnetic field. The signals painted a picture of the charged halo of particles around Jupiter’s largest companion like an ultrasound creates an image of a baby inside a mother’s womb.

This “Ganymede symphony,” as UCLA astrophysicist Margaret Kivelson called it, showed that the moon’s magnetic field is strong enough to punch a hole in Jupiter’s own magnetic field. Researchers also showed off reconstructions of Ganymede’s inner structure--the first time the insides of a distant moon have been examined.

Callisto and Ganymede are the twins of the Jupiter system, sharing the same size (comparable to the planet Mercury), the same density and the same age. Yet they seem to have evolved into very different bodies.

“That’s what makes it a good mystery,” said Galileo project scientist Torrence Johnson. “That’s going to be a lot of fun for the theorists.”

Callisto shows no traces of a magnetic field, while Ganymede “acts like it has a little bar magnet inside that’s tilted by 10 degrees--like Earth,” Kivelson said.

The question is: Why?

Indeed, the presence of a magnetic field on Ganymede is part of a much larger planetary puzzle. Of all the solid bodies in the solar system, only Earth, Mercury and now Ganymede are known to have magnetic fields. No one understands why, especially because Earth’s closest neighbors, Venus and Mars, don’t have magnetic fields.

Magnetic fields are generated by the fluid motion of molten metal inside a body. Yet Ganymede should have cooled solid billions of years ago. “It gives you an idea of how puzzling the whole thing is,” Kivelson said.

UCLA geophysicist Gerald Schubert speculated that Ganymede somehow got reheated recently--about half a billion years ago--presumably by getting pulled and flexed by Jupiter’s enormous gravity. However, such strong tugging would require that the moon once swept much closer to Jupiter, and somehow has since altered its orbit.

A reconstruction of Ganymede’s inner structure confirmed that it probably does have a molten metal core, surrounded by a rocky mantle and icy crust. This first-of-a-kind look inside was derived from radio signals traveling from Galileo to Earth as it swept past Ganymede. As the spacecraft got pulled to and fro under the influence of the moon’s changing gravity, the radio signal got pulled and pushed as well. By putting together the data from two Ganymede passes, scientists were able to reconstruct the moon’s inner structure. “It’s a bit like having an X-ray or MRI,” Kivelson said.

As for Callisto, its surface turned out to be “nothing like what we expected,” said Johnson. For one thing, Galileo found a surprising dearth of craters, an indication that something is smoothing out the surface. The craters “seem to be disappearing,” said Bender. “We don’t know what’s obliterating them.”

Part of the answer may lie in the mountain-like ridges that surround the craters. Bright at the tops where new ice has been exposed, and dark at the bottom, they appear to be shedding dust and rocks that are sliding off the high ridges and into the plains.

As a finale to their show-and-tell, the researchers produced new images of Jupiter’s most exotic moon, Europa, which they speculate may hide an ocean beneath its icy surface. “We were blown away by this image,” said Bender. The image revealed a complex overlapping of lines and faults and plates that appear to be spreading, as they do on Earth’s ocean floor.

The researchers haven’t had time to study the new pictures. “We’ve only had 27 hours with this image,” Bender said. “And we’re back to square one” trying to understand what they saw.

The best of Europa is yet to come, with a close fly-by planned for Dec. 19 and another in February. Those images will have 20 times higher resolution than those viewed Thursday.