Binary asteroid in Jupiter's orbit may be icy comet from solarsystem's infancy (Forwarded)

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01 February 2006

Binary asteroid in Jupiter's orbit may be icy comet from solar system's
infancy

By Robert Sanders, Media Relations

BERKELEY -- A bound pair of icy comets similar to the dirty snowballs
circling outside the orbit of Neptune has been found lurking in the shadow
of Jupiter.

Astronomers at the University of California, Berkeley, working with
colleagues in France and at the Keck Telescope in Hawaii, have calculated
the density of a known binary asteroid system that shares Jupiter's orbit,
and concluded that Patroclus and its companion probably are composed
mostly of water ice covered by a patina of dirt.

Because dirty snowballs are thought to have formed in the outer reaches of
the solar system, from which they are occasionally dislodged and end up
looping closer to the sun as comets, the team suggests that the asteroid
probably formed far from the sun. It most likely was captured in one of
Jupiter's Trojan points -- two eddies where debris collects in Jupiter's
orbit -- during a period when the inner solar system was intensely
bombarded by comets, around 650 million years after the formation of the
solar system.

If confirmed, this could mean that many or most of the probably thousands
of Jupiter's Trojan asteroids are dirty snowballs that originated much
farther from the sun and at the same time as the objects now occupying the
Kuiper Belt.

"It's our suspicion that the Trojans are small Kuiper Belt objects," said
study leader Franck Marchis, a research astronomer at UC Berkeley.

Marchis and colleagues from the Institut de Mécanique Céleste et Calculs
d'Éphémérides (IMCCE) at the Observatoire de Paris and from the W. M. Keck
Observatory report their findings in the Feb. 2 issue of Nature.

The team's conclusion adds support to a recent hypothesis about the
evolution of the orbits of our solar system's largest planets, Jupiter,
Saturn, Uranus and Neptune, put forth by a group of researchers headed by
Alessandro Morbidelli, a theoretical astronomer with the Conseil National
de la Recherche Scientifique laboratory of the Observatoire de la Cote
d'Azur, Nice, France.

In a Nature paper last year, Morbidelli and colleagues proposed that icy
comets would have been captured in Jupiter's Trojan points during the
early history of the solar system. According to their scenario, during the
first few hundred million years after the birth of the solar system, the
large gas planets orbited closer to the sun, enveloped in a cloud of
billions of large asteroids called planetesimals, perhaps 100 kilometers
(62 miles) in diameter or less. Interactions with these planetesimals
caused the large gaseous planets to migrate outward until about 3.9
billion years ago, when Jupiter and Saturn entered resonant orbits and
began tossing the planetesimals around like confetti, some of them leaving
the solar system for good.

The bulk of the remaining planetesimals settled into orbits beyond Neptune
-- today's Kuiper Belt and the source of short-period comets -- but a
small number were captured in the Trojan eddies of the giant planets, in
particular Jupiter.

"This is the first time anyone has determined directly the density of a
Trojan asteroid, and it supports the new scenario proposed by Morbidelli,"
said coauthor Daniel Hestroffer, an astronomer at the IMCEE. "These
asteroids would have been captured in the Trojan points at a time when the
rocky planets were still forming, and this perturbation of the
planetesimals about 650 million years after the birth of the solar system
could have created the late bombardment of the Moon and Mars."

Though Marchis refers to the scenario as "a nice story," he admits that
more work needs to be done to provide support for it.

"We need to discover more binary Trojans and observe them to see if low
density is a characteristic of all Trojans," he said.

Trojan asteroids are those caught in the so-called Lagrange points of
Jupiter's orbit, located the same distance from Jupiter as Jupiter is from
the sun -- 5 astronomical units, or 465 million miles. These points, one
leading and the other trailing Jupiter, are places were the gravitational
attraction of the sun and Jupiter are balanced, allowing debris to collect
like dust bunnies in the corner of a room. Hundreds of asteroids have been
discovered in the leading (L4) and trailing (L5) points, each orbiting
around that point as if in an eddy.

The asteroid 617 Patroclus, originally discovered at L5 and named in 1906,
was found to have a companion in 2001, and so far is the only known Trojan
binary. The discoverers were not able to estimate the orbit of the
components because they had too few observations.

As experienced asteroid hunters, Marchis and his colleagues in August this
year discovered the first triple asteroid system, 87 Sylvia, much closer
to the Sun in the main asteroid belt between Mars and Jupiter, and used a
powerful 8-meter telescope of the European Southern Observatory's Very
Large Telescope in Chile to study the three objects. They were able to
chart the orbits of the asteroids to estimate the density of Sylvia, from
which they concluded it is a rubble-pile of loosely, packed rock.

The French and American team tried the same technique with the much more
distant Patroclus, employing imaging data from the Keck II Laser Guide
Star System at the W. M. Keck Observatory on Mauna Kea, which yields a
sharp resolution impossible with any other ground-based telescope.

"Before, we could only look at objects near a bright reference star,
limiting the use of adaptive optics to a small percentage of the heavens,"
Marchis said. "Now, we can use adaptive optics to view almost any point on
the sky."

The laser guide star system uses a laser beam to excite sodium atoms
within a small spot in the upper atmosphere. This artificial "star" is
used to measure atmospheric turbulence, which is then removed by the
movable mirrors of the Keck adaptive optics system.

With the system providing an unparalleled 58 milliarcsecond resolution,
the Keck team made five observations in the infrared between November 2004
and July 2005. Marchis and his colleagues determined that the density of
Patroclus and its companion, which are about the same size and circle
around their center of mass every 4.3 days at a distance of 680 kilometers
(423 miles), was very low: 0.8 grams per cubic centimeter, about one third
that of rock and light enough to float in water. Assuming a rocky
composition similar to that of Jupiter's moons Callisto and Ganymede, the
components of the system would have to be very loosely packed -- about
half empty space, an internal characteristic which is not expected for a
same-size binary system, the researchers concluded.

The team suggests a more reasonable composition of water ice with only 15
percent open space, which makes these objects similar to comets and small
Kuiper Belt objects, which have been determined to have densities less
than water.

Marchis suspects that the binary system formed when a single large
asteroid was torn asunder by the gravitational tug of Jupiter.

"The Patroclus system displays similar characteristics to the binary
Near-Earth Asteroids, which are believed to have formed during an
encounter with a terrestrial planet by tidal splitting," he said. "In the
case of a Trojan asteroid, it is only when the work of our collaborators
was published recently that we could suggest that this encounter was with
Jupiter."

Because in Homer's Iliad, Patroclus was Achilles' companion and a hero of
the Trojan War, Achilles would have been an appropriate name for one of
the two asteroids, which are about the same size. However, another
asteroid already has the name Achilles, so Marchis and his collaborators
proposed naming the smallest member of the binary system Menoetius, after
the father of Patroclus. The Committee on Small Body Names of the
International Astronomical Union has tentatively accepted the name. The
asteroid designated Menoetius is about 112 kilometers (70 miles) in
diameter, while Patroclus is about 122 kilometers (76 miles) wide.

In addition to Marchis, the team included astronomy professor Imke de
Pater and postdoctoral fellow Michael H. Wong of UC Berkeley; Daniel
Hestroffer, Pascal Descamps, Jérôme Berthier and Frédéric Vachier of the
Institut de Mécanique Céleste et de Calculs des Éphémérides (IMCCE); and
Antonin Bouchez, Randall Campbell, Jason Chin, Marcos van Dam, Scott
Hartman, Erik Johansson, Robert Lafon, David Le Mignant, Paul Stomski,
Doug Summers and Peter Wizinovich of the W. M. Keck Observatory.

The project was supported by grants from the National Science Foundation
through the Science and Technology Center for Adaptive Optics and by the
National Aeronautics and Space Administration. Most of the data were
obtained at the W. M. Keck Observatory, which is operated as a scientific
partnership between the California Institute of Technology, the University
of California and NASA, with additional observations obtained at the
Gemini Observatory operated by the Association of Universities for
Research in Astronomy, Inc., under a cooperative agreement with the NSF on
behalf of the Gemini partnership.

IMAGE CAPTIONS:

[Image 1:
http://www.berkeley.edu/news/media/d.../patroclus.jpg (127KB)]
Artist's rendering of the binary asteroids Patroclus (center) and
Menoetius. Jupiter and its four Galilean satellites are visible in the
distance, while the sun is out of sight to the left. The dirty snowballs
probably are fugitives from the Kuiper Belt now hanging out in Jupiter's
orbit.

Credit: W.M. Keck Observatory/Lynette Cook

[Image 2:
http://www.berkeley.edu/news/media/d...us_diagram.tif
(1.8MB)]
The asteroid 617 Patroclus and its companion are indicated by the green
triangle in Jupiter's trailing Lagrange point. The white dots indicate the
position, as of November 2005, of the 10,000 known asteroids in our solar
system with diameters greater than 10 kilometers. They cluster in the main
asteroid belt between Mars and Jupiter, plus the two Lagrange points of
Jupiter's orbit.

Credit: IMCCE-Observatoire de Paris

[Image 3:
http://www.berkeley.edu/news/media/r...oclus_keck.jpg
(3KB)]
Keck Telescope images of Patroclus and its somewhat smaller companion,
Menoetius. (Credit: F. Marchis et al., Nature)