Himalia is the largest of Jupiter’s irregular satellites; smaller moons, non-spherical in shape and with irregular, non-circular orbits, unlike the larger Galilean Moons, which are more planet-like bodies. It has a prograde orbit (i.e. it orbits in the expected direction, the same way as the planet rotates and the larger moons orbit), and may be part of a collisional family that includes other such moons (i.e. they may have originated from a common source-body, broken up in a collision event).
The Galilean Moons are thought to have formed at the same time as Jupiter, part of a circum-planetary disc that formed alongside the giant planet in the same way that the planets are thought to have formed in a circum-solar disk around the young Sun. However the irregular orbits of the smaller moons suggests that they originated elsewhere in the Solar System, and were later captured by Jupiter. The most likely origins for such objects would be the Outer Asteroid Belt, which is close to Jupiter, or the Kuiper Belt which is beyond the orbit of Neptune, but the source of the majority of comets, which fall into the Inner Solar System when their orbits are disturbed by close encounters with other bodies.
In a paper published on the arXiv database at Cornell University Library on 5 September 2014, Michael Brown of the Division of Geological andPlanetary Sciences at the California Institute of Technology and Alyssa Rhoden of the Johns Hopkins University Applied Physics Laboratory, describe the results of a spectrographic study of the surface Himalia carried out from the Keck Observatory on 26 and 28 November 2013, and the implications of the results of this.
The spectrographic analysis did not reveal any close similarity between Himalia and bodies of either the Outer Asteroid Belt or the Kuiper Belt, both of which tend to be rich in water ice and hydroxide compounds. Nor does Himalia appear similar to the rocky bodies of the Inner Asteroid Belt. Instead it has a spectrographic profile previously seen only in three bodies, 52 Europa, 31 Euphrosyne and 451 Patientia (collectively known as the ‘Europa-like Asteroids'), all of which are located in the Middle Asteroid Belt.
The precise nature of the surface of the Europa-like Asteroids is uncertain; it does not conform exactly to any known surface material, and is likely to be intermediate between those of the bodies of the Inner and Outer Asteroid Belts, probably consisting of a mixture of rock and ice particles (other bodies in the Middle Asteroid Belt show different intermediate spectrographic profiles, making the situation slightly more complex than it immediately seems).
This is a surprising finding, as there seems to be no obvious relationship between the orbits of Himalia and the Europa-like Asteroids. However the known sample size of these objects is very small, and the discovery of other bodies with similar spectrographic profiles could potentially resolve this riddle. Brown & Rhoden suggest that spectrographic studies of the Jupiter Trojan Asteroids, which are closer to Himalia and also currently thought to derive from the Kuiper Belt or possibly the Outer Asteroid Belt, might potentially produce similar objects.
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