[meteorite-list] Amalthea Had A Far-Flung Past

[Ron Baalke]

http://subarutelescope.org/Pressrelease/2004/12/23/index.html

Jupiter's Moon Had A Far-Flung Past
National Astronomical Observatory of Japan
December 23, 2004

The first ground based infrared spectrum of Jupiter's moon Amalthea
reveals that it must have formed far from its current location. This
new result, based on observations with the Subaru telescope and the
NASA Infrared Telescope Facility by a team of researchers from the
National Astronomical Observatory of Japan, the University of
Hawaii, and the University of Tokyo, sheds new light on our Solar
System's turbulent past.

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Planets like Earth and Jupiter formed from the disk of gas and dust
swirling around the Sun at the time of its birth. Rocky planets like
Earth formed in the high temperature environment close to the Sun,
while large gaseous planets like Jupiter formed in the cooler
regions farther away. Similarly, Jupiter, the largest planet in the
solar system, probably had its own disk of gas and dust. The four
moons of Jupiter discovered by Galileo (Io, Europa, Ganymede, and
Callisto) are likely to have been born from this disk.

In addition to the Galilean moons, Jupiter has two other types of
satellites: four small inner moons orbiting Jupiter within the orbit
of Io, the inner most Galilean satellite, and at least fifty five
small outer moons outside the orbit of Callisto, the outer most
Galilean satellite. All the outer satellites have tell-tale orbits
that reveal that they must have been captured by Jupiter during or
after the formation of the planet and its larger moons.

The origin of the four small inner moons remain a mystery, however.
They have orbits compatible with the hypothesis that they formed in
orbit around Jupiter like the Galilean moons. On the other hand,
their small irregular shapes and their comparatively low
reflectivity and low densities resemble asteroids and suggest that
they were captured by Jupiter's gravitational pull just like the
outer moons.

The mystery persists because of the challenge inherent in observing
Jupiter's small inner moons from Earth. The moons are small and
therefore faint, and they are obscured by the bright glare from
Jupiter. Although NASA's space probes Voyager and Galileo have
captured detailed images of Jupiter's small inner moons, these data
have been insufficient for resolving the question of their origin.

Naruhisa Takato from the National Astronomical Observatory of Japan
and his collaborators have now had success in obtaining the first
infrared spectrum of two of Jupiter's small inner moons, Amalthea
and Thebe. To obtain a spectrum over a wide range of infrared
wavelengths, the group combined the strengths of two instruments on
two telescopes on the summit of Mauna Kea, Hawaii. For high
resolution spectroscopy at wavelengths longer than 3 mirometers ,the group
used the Infrared Camera and Spectrograph on the Subaru telescope.
For shorter wavelengths, the group used SpeX on the NASA IRTF, which
has broad wavelength coverage.

The new spectrum of Amalthea shows the characteristic signatures of
water. The most likely location of this water is within water
containing hydrous minerals. Such minerals typically form in low
temperature environments, ruling out the possibility that Amalthea
could have formed in the high temperature environment of Jupiter's
immediate neighborhood while the planet was forming and where Amalthea
now is.

If Amalthea did not form near its present location, where did it
come from? The surface of Amalthea resembles regions of Callisto
that are not covered by ice. This suggests that Amalthea may have
been one of the many small "micro-satellites" orbiting Jupiter that
was sucked into an inner orbit when the Galilean moons formed.
However, the spectrum of Amalthea has similarities with asteroids
orbiting the Sun, suggesting that is was a "micro-planet" that was
pulled into Jupiter's orbit when Jupiter itself was forming.

Takato says "although we think Jupiter's moons formed as an assembly
of many smaller bodies, the same way we think planets formed from
'planetesimals', until now we have not found any example of the
original building blocks of a planet's moon. However, our results
strengthen the argument that Amalthea is one of the few remaining
pieces of the material that formed the Galilean moons. Amalthea may
have ended up in orbit close to Jupiter rather than get incorporated
into a larger moon or Jupiter itself. If this is the case, Amalthea
would be the first known example of a 'satellitesimal.'"

References:
These results were published in the December 24, 2004, edition
of the journal Science Vol. 306, 2224 - 2227.