Galileo's close flyby of Jupiter's small moon Amalthea earlier this month shows that the rocky satellite is riddled with holes and probably contains more empty space than solid rock, scientists said today.
"The density is unexpectedly low," said John D. Anderson, an astronomer at NASA's Jet Propulsion Laboratory (JPL). "Amalthea is apparently a loosely packed pile of rubble."
Astronomers have long suspected that some asteroids might be rubble piles, boulders that congregated loosely together over the eons. This is the first look at a planetary satellite that is constructed in this haphazard fashion.
Further, even the solid chunks of Amalthea are not very dense, raising questions about its origin.
"Amalthea now seems more likely to be mostly rock with maybe a little ice, rather than a denser mix of rock and iron," said JPL's Torrence Johnson, project scientist for Galileo.
The red-tinted moon is about 168 miles long (270 kilometers) and about half as wide. Anderson and colleagues estimated Amalthea's mass from its gravitational affect on Galileo, when the spacecraft passed within about 99 miles (160 kilometers) of the moon on Nov. 5. Peter Thomas at Cornell University, Ithaca, N.Y., had calculated Amalthea's volume from earlier Galileo images of the moon.
Amalthea's overall density is close to the density of water ice, Anderson reports today at the fall meeting of the American Geophysical Union in San Francisco. However, the moon is almost certainly not a solid hunk of ice.
"Nothing in the Jupiter system would suggest a composition that's mainly ice," Anderson said.
Amalthea's irregular shape and low density suggests the moon has been broken into many pieces that cling together from the pull of each other's gravity, mixed with empty spaces, where the pieces don't fit tightly together.
"It's probably boulder-size or larger pieces just touching each other, not pressing hard together," Anderson said.
Amalthea does not have quite enough mass to pull itself together into a consolidated, spherical body like Earth's moon or Jupiter's four largest moons. The density estimate, obtained from Galileo's flyby, extends an emerging pattern of finding irregularly shaped moons and asteroids to be porous rubble piles. What's more of a surprise, the scientists said, is the density estimate is so low that even the solid parts of Amalthea are apparently less dense than Io, a larger moon that orbits about twice as far from Jupiter.
"This finding supports the idea that the inner moons of Jupiter have undergone intense bombardment and breakup. Amalthea may have formed originally as one piece, but then was busted to bits by collisions," Johnson said.
One model for the formation of Jupiter's moons suggests moons closer to the planet would be made of denser material than those farther out. That's based on a theory that early Jupiter, like a weaker version of the early Sun, would have emitted enough heat to prevent volatile, low-density material from condensing and being incorporated into the closer moons. Jupiter's four largest moons fit this model, with the innermost of them, Io, also the densest, made mainly of rock and iron.
Galileo's flyby of Amalthea brought the spacecraft closest to Jupiter since it began orbiting the giant planet on Dec. 7, 1995. After more than 30 close encounters with Jupiter's four largest moons, the Amalthea flyby was the last for the craft.
An encounter with strong Jovian magnetic fields has given the spacecraft fits, and other data collected from the flyby has not been recovered.
Galileo has been put on course for a mission-ending impact with Jupiter on Sept. 21, 2003. The spacecraft, although still controllable from Earth, is running out of propellant.
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