[meteorite-list] Planet Migration May Be Why Asteroids Are Missing

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 25 Feb 2009 13:26:24 -0800 (PST)
Message-ID: <200902252126.NAA12428_at_zagami.jpl.nasa.gov>

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)
Received on Wed 25 Feb 2009 04:26:24 PM PST