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Amalthea

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Scientists studying data from NASA's Galileo spacecraft have found that Jupiter's moon Amalthea is a pile of icy rubble less dense than water. Scientists expected moons closer to the planet to be rocky and not icy. The finding shakes up long-held theories of how moons form around giant planets.

"I was expecting a body made up mostly of rock. An icy component in a body orbiting so close to Jupiter was a surprise. This gives us important information on how Jupiter formed, and by implication, how the solar system formed," - Dr. John D. Anderson, an astronomer at NASA's Jet Propulsion Laboratory, Pasadena. Anderson is lead author of a paper on the findings that appears in the current issue of the journal Science.

Current models imply that temperatures were high at Amalthea's current position when Jupiter's moons formed, but this is inconsistent with Amalthea being icy. The findings suggest that Amalthea formed in a colder environment. One possibility is that it formed later than the major moons. Another is that the moon formed farther from Jupiter, either beyond the orbit of Jupiter's moon Europa or in the solar nebula at or beyond Jupiter's position. It would have then been transported or captured in its current orbit around Jupiter. Either of these explanations challenges models of moon formation around giant planets.

"Amalthea is throwing us a curve ball. Its density is well below that of water ice, and even with substantial porosity, Amalthea probably contains a lot of water ice, as well as rock." - Dr. Torrence Johnson, co-author and project scientist for the Galileo mission at JPL.
Analysis of density, volume, shape and internal gravitational stresses lead the scientists to conclude that Amalthea is not only porous with internal empty spaces but also contains substantial water ice.

One model for the formation of Jupiter's moons suggests that moons closer to the planet would be made of denser material than those farther out. That is 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.

Amalthea is a small red-tinted moon that measures about 168 miles in length and half that in width. It orbits about 181,000 kilometres from Jupiter, considerably closer than the Moon orbits Earth. Galileo passed within about 159 km of Amalthea on Nov. 5, 2002. Galileo's flyby of Amalthea brought the spacecraft closer to Jupiter than at any other time 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 moon flyby for Galileo.

The Galileo spacecraft's 14-year odyssey came to an end on Sept. 21, 2003. JPL, a division of the California Institute of Technology in Pasadena, managed the Galileo mission for NASA.

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Amalthea was discovered by the exceptionally keen eyesight of American astronomer Edward Barnard in 1892 using the 36 inch (91 cm) refractor telescope at Lick Observatory. This was the first discovery of a Jovian moon since Galileo's discovery in 1610.
Barnard's discovery of Amalthea was also the last moon of any planet to be discovered by direct visual observation.

It is one of Jupiter's four inner moons that orbits inside the orbit of Io.

Amalthea is a mass of icy rubble that could not have formed as close to the planet as its present orbit.
A new analysis does not pinpoint its true origin, but does indicate that the porous hunk of ice and rock is near its maximum possible size.

The new analysis is of data from the Galileo spacecraft, which sped past Amalthea at a distance of only 244 kilometres on 5 November 2002 on its way to a death-plunge into the Jovian atmosphere.

Amalthea is a dark object and reddish in colour.
In fact, Amalthea is one of the reddest object in the solar system. The red color is probably caused by sulfur from Io's volcanoes spiraling down to Jupiter and impacting on Amalthea. Bright patches observed on the major slopes of Amalthea are green in colour.

Astronomers had hoped to measure the moon's mass and density, but the ageing spacecraft lost its two-way radio link to Earth during the 200-second flyby.
Initial analysis of the little data retrieved indicated only that Amalthea appeared less dense than water.

Now astronomers have gone back through the data to estimate Amalthea's mass at just over two trillion tonnes (2.08 x 1015). Dividing that figure by the satellite's measurement of the moon's volume - 2.4 million cubic kilometres - gives a density of about 850 kilograms per cubic metre. This is lighter than solid ice.

But there is an uncertainty of 11% in this density calculation, which Is large enough that Amalthea might actually be as heavy as ice. But John Anderson of the Jet Propulsion Laboratory, in California, US, is convinced that Amalthea is a rubble pile with voids between chunks of ice and rock. Larger bodies that have melted tend to be spherical, but Amalthea is elongated, with three axes measuring 250, 146 and 128 kilometres.

Low pressure interior

Astronomers have spotted several other porous bodies recently. That had not been expected, but Anderson told New Scientist: "Now we understand that the pressures are so low in the interior that they can maintain their porosity."

The calculated pressure at the centre of Amalthea is just less than the strength of natural ice. But if a body was any larger, the internal pressure would be enough to flatten ice chunks at their cores, squeezing out any voids.

And while Amalthea's circular orbit of just 110,000 kilometres above Jupiter's surface makes it look old, ice could not have survived the heat at that distance as Jupiter's four largest moons formed, so Amalthea must have formed elsewhere.

It might have hailed from the asteroid belt, the Kuiper belt, or from a more distant orbit around Jupiter. Anderson suggests Amalthea may have collided with another object close to Jupiter, shattering into the fragments that later collected to form the rubble-pile moon.

Amalthea ("am al THEE uh") Quick-Look Statistics

Discovery: Sep 9, 1892 by Edward Barnard
Diameter (km): 270x170x150
Mass (kg): 7.17e+18
Mass (Earth = 1): 1.1988e-06
Surface Gravity (Earth = 1): 0.0055 - 0.0085
Mean Distance from Jupiter (km): 181,300
Mean Distance From Jupiter (Rj): 2.539
Mean Distance from Sun (AU): 5.203
Orbital period (days): 0.498179
Rotational period (days): 0.498179
Density (gm/cm?3) 1.8
Orbit Eccentricity: 0.003
Orbit Inclination (degrees): 0.40
Orbit Speed (km/sec): 26.47
Escape velocity (km/sec): 0.0842
Visual Albedo: 0.06
Surface Composition: ice and rock ?