NASA’s Galileo spacecraft continues to deliver
surprises. Galileo’s seven-year run of discoveries continued
with the discovery of Jupiter’s potato-shaped inner moon
named Amalthea. It appears the moon has a very low density,
indicating it is full of holes.

“The density is unexpectedly low,” said Dr. John D. Anderson,
an astronomer at NASA’s Jet Propulsion Laboratory (JPL),
Pasadena, Calif. “Amalthea is apparently a loosely packed
pile of rubble, he said.”

The empty gaps between solid chunks likely take up more of
the moon’s total volume than the solid pieces, and even the
chunks are probably material that is not dense enough to fit
some theories about the origin of Jupiter’s moons. “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
Dr. Torrence Johnson, project scientist for Galileo.

This red-tinted moon measures about 270 kilometers (168
miles) in length and half that in width. Anderson and
colleagues estimated Amalthea’s mass from its gravitational
affect on Galileo, when the spacecraft passed within about
160 kilometers (99 miles) of the moon on Nov. 5. Dr. 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.

Johnson said, “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.”

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, Johnson and Anderson
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.

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. However, the new finding suggests, even if Amalthea
is mostly gaps, its solid chunks have less density than Io.

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 flyby was the last for Galileo. Galileo
has been put on course for a mission-ending impact with
Jupiter on Sept. 21, 2003. Galileo’s long and successful
career will come to an end on the Jovian surface. The
spacecraft, although still controllable from Earth, is
running out of propellant. Researchers are looking forward to
more surprises and new data, as Galileo approaches the
foreboding giant planet.

Galileo left Earth aboard NASA’s Space Shuttle Atlantis in
1989. JPL, a division of the California Institute of
Technology in Pasadena, manages the Galileo mission for
NASA’s Office of Space Science, Washington. Additional
information about the mission is available online at:
http://galileo.jpl.nasa.gov