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Planetary News: Jupiter (2005)

Amalthea Mystery: How Did a Snowball Get So Close to Jupiter?

By Emily Lakdawalla
10 June 2005

The Jupiter system is a very orderly place. The giant planet has four large moons and a few smaller ones orbiting it in nearly circular paths. Just like the planets in the Solar System, Jupiter's moons decrease in density systematically as you go out. Io is close to Jupiter and made almost entirely of rock. As you travel outward past Europa, Ganymede, and finally Callisto, the moons have higher and higher proportions of water ice, so become less and less dense. It is so orderly that scientists have always assumed that a little object called Amalthea, which is Jupiter's innermost moon and the next largest after Europa, must be a rocky body, as dense as or denser than rocky Io. But they were wrong.

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A team led by John Anderson of the Jet Propulsion laboratory has recently completed a two-year project to analyze Galileo data on Amalthea and made a surprising discovery. Not only is Amalthea not rocky, it is less dense even than water; according to the team's calculations, it is about 82% (plus or minus 9%) the density of the slightly dirty water ice typical of icy bodies in the Solar System. "We expected something perhaps asteroidal in density, or a solid rock object, which would have been much higher," Anderson says. "Amalthea just doesn't fit the pattern at all."

Why is the pattern so important? Because the orderly march of decreasing density out from Jupiter has been an important ingredient in models for how Jupiter formed. The outer moons of Jupiter (and also Saturn) have such low density. But these outer moons are all likely to have formed elsewhere in the Solar System, and been captured into their eccentric orbits around the giant planets. "You would think Amalthea was captured, because it doesn't fit the pattern with the other satellites," Anderson said. "But it has such a regular orbit. The orbit is almost circular and in the equatorial plane of Jupiter, so it looks like something that formed with Jupiter."

The measurement of Amalthea's density was derived from painstaking analysis of the radio signals sent by Galileo to the Earth as the spacecraft flew by Amalthea on November 5, 2002. The tiny gravity of Amalthea bent Galileo's course just slightly, changing its velocity by a mere few millimeters per second, and that change in velocity showed up as a minute Doppler shift in the radio signal that Galileo broadcast to Earth. Anderson and his team determined the mass of Amalthea from this Doppler shift in Galileo's signal, and found it to be unexpectedly small. Anderson and his team are the same people who discovered the anomalous acceleration of the Pioneer spacecraft through Doppler tracking.

If Amalthea is less dense than water, it pretty much has to have a composition of water ice with lots of open pores within it, like a snowball. And Amalthea's orbit is so close to Jupiter that the heat of the giant planet's formation would have vaporized any water so close to it, preventing it from condensing into a moon. That's why there's no water at Io, some at Europa, and more at Ganymede and Callisto -- because with increasing distance from Jupiter, the ambient temperature was lower when the system formed, so more water was available to condense at Callisto's orbit than Io's. There is no way an icy Amalthea could have formed where it is now.

"It has to be captured from somewhere," Anderson said. That means that it had to form outside the orbit of at least Io and probably farther out than that, and then somehow its orbit was perturbed to bring it in close to Jupiter, past the Galilean satellites in its way. The problem, Anderson says, is "there is no known mechanism for doing that. People haven’t been working that problem because they assumed it formed with Jupiter" as a rocky body.

Anderson's discovery means that scientists have to go back to the drawing board to figure out how the Jupiter system formed. Either Amalthea did not form at its current orbit, or it formed much later than all the other moons, after the system cooled; in either case, current theories for the formation of the Jupiter system don't stand up. It may well take another Jupiter mission to explain this mystery!