Jupiter moon 'full of holes'

Amalthea is a jumble of pieces

By Dr David Whitehouse BBC News Online science editor

Amalthea, the Jupiter moon recently visited by the Galileo spacecraft, is full of holes.

"The density is unexpectedly low," says Dr John Anderson of the US space agency's (Nasa) Jet Propulsion Laboratory (JPL) in California. "Amalthea is apparently a loosely packed pile of rubble."

The inner moons of Jupiter have undergone intense bombardment and break-up

Torrence Johnson

The spaces between chunks of rock and ice appear to take up more of the moon's total volume than the solid pieces.

Even the solid material 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," says JPL's Dr Torrence Johnson, project scientist for Galileo.

Small red moon

Tiny, red-tinted Amalthea is only about 270 kilometres (168 miles) in length and half that in width.

Its mass, estimated from its gravitational effect on Galileo when the spacecraft passed within about 160 kilometres (99 miles) of the moon on 5 November, is far smaller than had been predicted.

Amalthea's overall density is close to the density of water ice, Dr Anderson says. But the moon is almost certainly not a solid hunk of ice.

Amalthea: Blasted to bits by collisions

Amalthea's irregular shape and low density suggest the moon has broken into many pieces that now cling together due to their mutual gravity. In between there are empty spaces where the pieces do not fit tightly together.

"It's probably boulder-size or larger pieces just touching each other, not pressing hard together," Dr Anderson says.

Dr Johnson adds: "This finding supports the idea that the inner moons of Jupiter have undergone intense bombardment and break-up. Amalthea may have formed originally as one piece, but then was busted to bits by collisions."

It seems that Amalthea does not have quite enough mass to pull itself together into a consolidated, spherical body like the Earth's moon or Jupiter's four largest moons.

In addition, Amalthea's density estimate fits in with an emerging pattern of finding irregularly shaped moons and asteroids to be porous rubble piles.

Close encounter

Scientists are surprised Amalthea's 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 is puzzling because the favoured model for the formation of Jupiter's moons suggests moons closer to the planet would be made of denser material than those farther out.

Galileo's flyby of Amalthea brought the spacecraft closer to Jupiter than at any other time since it began orbiting the giant planet in 1995.

After more than 30 close encounters with Jupiter's four largest moons, the flyby was the last for Galileo, which has been put on course for an impact with Jupiter's atmosphere on 21 September 2003.