Amalthea: The Rubble Pile

Using results from the Galileo flyby of Amalthea in Nov. 2002, scientists have pieced together the density of this inner satellite of Jupiter. Rather than finding a rocky body that was expected from the density trend within the much larger Galilean satellites (innermost Io is densest while outermost Callisto is the least dense of the four), doppler shift data from the Galileo flyby revealed a very porous world with a density less than that of water. Even assuming a reasonable porosity that is in line with other worlds of its size, Amalthea must have a significant amount of ice. Such a result is inconsistent with the idea that a hot young Jupiter burned away water from the inner satellites like Io and Europa (though considered an icy body by some, its watery shell only constitutes the outer 100 km of Europa while the rest is silicates and iron). Either our understanding of the early period of Jupiter's history is incorrect (meaning Io and Europa lost their water through other means), or Amalthea formed elsewhere in the Jovian system or even outside the system itself.

The image above (a higher resolution version is linked), shows three views of Amalthea. The first is a shape model of Amalthea with escape velocity color coded (blue on the anti-Jovian end represents 1 m/sec while reds on the leading side represent 90 m/sec). The other two views include a mosaic of Voyager and Galileo images of the anti-Jovian hemisphere (middle) and a view of the leading hemisphere from Galileo (right). The bright spots on this satellite have intrigued scientists since the Voyager flybys. Given this result, it is quite possible that the spots maybe water ice exposed from beneath a mantling of sulfur from Io.