FROM: Lori Stiles (520-626-4402; lstiles{at}u.arizona.edu)
February 25, 2009

UA Scientists Find Asteroids Are Missing, and Possibly Why

University of Arizona scientists have uncovered a curious case of
missing
asteroids.

The main asteroid belt is a zone containing millions of rocky objects
between
the orbits of Mars and Jupiter. The scientists find that there ought
to be more
asteroids there than researchers observe. The missing asteroids may be
evidence
of an event that took place about 4 billion years ago, when the solar
system's
giant planets migrated to their present locations.

UA planetary sciences graduate student David A. Minton and UA
planetary sciences
professor Renu Malhotra say missing asteroids is an important piece of
evidence
to support an idea that the early solar system underwent a violent
episode of
giant planet migration that might possibly be responsible for a heavy
asteroidal bombardment of the inner planets.

The scientists are reporting on their research in an article, "A
record of
planet migration in the Main Asteroid Belt," in the Feb. 26 issue of
Nature.

Minton and Malhotra began by looking at the distribution of asteroids
in the
main asteroid belt. Astronomers first discovered a series of gaps in
the
asteroid belt, now called the Kirkwood gaps, back in the 1860s when
only a
handful of asteroids were known. The gaps occur at distinct regions of
the
asteroid belt where Jupiter's and Saturn's gravity strongly perturbs
and ejects
asteroids. The present-day orbits of Jupiter and Saturn explain why
these
unstable regions are devoid of asteroids.

"What we wanted to know was, how much of the structure of the asteroid
belt
could be explained simply by the gravitational effects of the giant
planets, as
are the Kirkwood gaps," Minton said.

Minton and Malhotra looked at the distribution of all asteroids with
diameters
greater than 50 kilometers, or about 30 miles. All asteroids of this
size have
been found, giving the UA researchers an observationally complete set
for their
study. Also, almost all asteroids this large have remained intact
since the
asteroid belt formed more than 4 billion years ago, a time record
spanning all
but the very beginning of solar system history.

"We ran massive sets of simulations with computer planets where we
filled up the
asteroid belt region with a uniform distribution of computer
asteroids," Minton
said. The scientists then had the computers simulate the billions of
years of
solar system history.

Their simulations ultimately ended with far more asteroids remaining
than are
actually observed in the asteroid belt. When the simulated asteroid
belt was
compared with the actual asteroid belt, they discovered a peculiar
pattern in
the differences. The simulated asteroid belt matched the real asteroid
belt
quite well on the sunward-facing sides of the Kirkwood gaps, but the
real
asteroid belt seemed to be depleted in asteroids on the Jupiter-facing
sides.

"Then we simulated the migration of the giant planets," Minton said.
"The
perturbing effects of the migrating planets sculpted our simulated
asteroid
belt. After the migration was over, our simulated asteroid belt looked
much
more like the observed asteroid belt."

The UA scientists' research was funded by NASA and by the National
Science
Foundation.

"Our interpretation is that as Jupiter and Saturn migrated, their
orbital
resonances swept through the asteroid belt, ejecting many more
asteroids than
is possible with the planets in their current orbits," Malhotra said.
"And the
particular pattern of missing asteroids is characteristic of the
pattern of
Jupiter's and Saturn's migration."

"Our work explains why there are fewer asteroids on the Jupiter-facing
side of
the Kirkwood gaps compared to the sun-facing side," Minton said. "The
patterns
of depletion are like the footprints of wandering giant planets
preserved in
the asteroid belt."

Their results corroborate other lines of evidence indicating that the
giant
planets ? Jupiter, Saturn, Uranus and Neptune ? formed in a more
tightly
compacted configuration, and then Jupiter moved slightly closer to the
sun,
while the other giant planets moved farther apart from each other and
farther
away from the sun.

Minton and Malhotra say that their result has implications for how far
and how
fast the planets migrated early in solar system history, and the
possibility
that planet migration perturbed asteroids that may have contributed to
a heavy
bombardment of the inner solar system.

"Our result doesn't directly answer the question of whether the timing
of this
can be tied to inner solar system heavy bombardment ? that's open for
debate," Minton said. "But what it does say is that there was an event
that
destabilized asteroids over a relatively short period of time.

"All the asteroids being kicked out of the asteroid belt had to go
somewhere,"
he added. "The implication of this is that when all those asteroids
were
getting kicked out of the main belt, they could have become
projectiles
impacting the Earth and the moon, Mars, Venus and Mercury."

SCIENCE CONTACTS:
David A. Minton (520-621-7274; daminton{at}lpl.arizona.edu)
Renu Malhotra (520-626-5899; renu{at}lpl.arizona.edu)
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