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.
Image of Himalia tkaen by the Cassini Space Probe on 19 December 2000. NASA/JPL.
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.
See also…
The 3 June 2010 Jovian Impactor. A small but growing number of observations of bolide impacts on other
worlds in the Solar System have been made. The most notable of these are
the impact of Comet Shoemaker-Levy 9 into the Jovian atmosphere in 1994
and the July 2009 impact of a similar but unnamed body with the same
planet. In addition a number of smaller events have been recorded in
recent years, on the Moon, Mars and Jupiter.
The nature and origin of the July 2009 Jovian Impactor. In 2009 the remains of the comet Shoemaker-Levy 9 were observed
impacting the Jovian atmosphere, the first time a body had been directly
observed colliding with a planet other than the Earth, and the first
time a comet had ever been seen impacting a planet. At the time this was
thought to be an extremely rare event, possibly happening as
infrequently as once every 500 years... 
Ripples in the rings of Jupiter. Although fainter and considerably less famous, the planet Jupiter has a
system of rings similar to that of Saturn, between the orbits of the
small moons Metis and Adriastea. In 1996 the Galileo spacecraft observed a series of ripples within these rings, with
material...
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