Jupiter's moon just loose rubble and holes
Jupiter's potato-shaped inner moon Amalthea is so surprisingly light that scientists suspect it is made up of loose rubble and ice riddled with holes.
New measurements of the small moon, taken in a flyby conducted by NASA's Galileo spacecraft on November 5 this year, estimated its density by comparing the moon's mass with its gravitational effect on the probe as it passed within 160 km of the surface.
"The density is unexpectedly low," said Dr John Anderson, an astronomer at NASA's Jet Propulsion Laboratory in Pasadena, California, USA. "Amalthea is apparently a loosely packed pile of rubble."
The data suggests that empty gaps between solid chunks make up the majority of the moon's total volume. Even the solid matter that it does have might not be dense enough to fit some theories about the origin of Jupiter's moons, he told delegates at a meeting of the American Geophysical Union in San Francisco.
The red-tinted moon measures about 270 kilometres in length and half that in width. The calculations were a combination of the new estimate of the moon's mass with an earlier calculation of the volume from Galileo images of the moon.
Amalthea's overall density is close to the density of water ice, but the moon is almost certainly not a solid hunk of ice, Anderson told delegates: "Nothing in the Jupiter system would suggest a composition that's mainly ice."
Amalthea's irregular shape and low density suggest the moon has been broken into many pieces that now cling together from the pull of each other's gravity. Where the pieces don't fit together, there are simply empty spaces. "It's probably boulder-size or larger pieces just touching each other, not pressing hard together," he said.
One theory for why the moon has this composition is that the inner satellites of Jupiter have undergone intense bombardment and break-up. It is possible Amalthea originally formed as one piece, but was then broken up 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. However, the new density measurements dovetail into an emerging pattern in planetary science: more and more, scientists are finding irregularly shaped moons and asteroids resembling porous rubble piles.
What's more of a surprise, Anderson said, is the density estimate is so low that even the most solid parts of Amalthea are less dense than Io, a larger moon that orbits about twice as far from Jupiter. This is a problem for a leading theoretical model of the formation of Jupiter's moons, which requires that moons closer to the gas giant be made up of denser material than those farther out.
It had been assumed that the 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.
The structure and make-up of Jupiter's four largest moons support this model: Io, the densest and innermost moon, is made mainly of rock and iron. But the new finding suggests that even if Amalthea is mostly empty space, the solid chunks it does have are less dense than Io.
Galileo's recent flyby of Amalthea brought the spacecraft closer to Jupiter than at any other time since the probe began orbiting the giant planet on 7 December 1995.