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.
| 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.
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Amalthea and Jupiter's Ring
Image (375 KB)
Information |
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 μm ,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.'"
