[meteorite-list] Asteroids Caused the Early Inner Solar System Cataclysm

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Sep 15 14:42:17 2005
Message-ID: <200509151841.j8FIf6l16405_at_zagami.jpl.nasa.gov>

ASTEROIDS CAUSED THE EARLY INNER SOLAR SYSTEM CATACLYSM
>From Lori Stiles, University Communications, UA, 520-621-1877
September 15, 2005

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Contact information listed at end of news release
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University of Arizona and Japanese scientists are convinced that evidence at
last settles decades-long arguments about what objects bombarded the early
inner solar system in a cataclysm 3.9 billion years ago.

Ancient main belt asteroids identical in size to present-day asteroids in
the Mars-Jupiter belt -- not comets -- hammered the inner rocky planets in a
unique catastrophe that lasted for a blink of geologic time, anywhere from
20 million to 150 million years, they report in the Sept. 16 issue of
Science.

However, the objects that have been battering our inner solar system after
the so-called Late Heavy Bombardment ended are a distinctly different
population, UA Professor Emeritus Robert Strom and colleagues report in the
article, "The Origin of Planetary Impactors in the Inner Solar System."

After the Late Heavy Bombardment or Lunar Cataclysm period ended, mostly
near-Earth asteroids (NEAs) have peppered the terrestrial region.

Strom has been studying the size and distribution of craters across solar
system surfaces for the past 35 years. He has long suspected that two
different projectile populations have been responsible for cratering inner
solar system surfaces. But there's been too little data to prove it.

Until now.

Now asteroid surveys conducted by UA's Spacewatch, the Sloan Digital Sky
Survey, Japan's Subaru telescope and the like have amassed fairly complete
data on asteroids down to those with diameters of less than a kilometer.
Suddenly it has become possible to compare the sizes of asteroids with the
sizes of projectiles that blasted craters into surfaces from Mars inward to
Mercury.

"When we derived the projectile sizes from the cratering record using
scaling laws, the ancient and more recent projectile sizes matched the
ancient and younger asteroid populations smack on," Strom said. "It's an
astonishing fit."

"One thing this says is that the present-day size-distribution of asteroids
in the asteroid belt was established at least as far back as 4 billion years
ago," UA planetary scientist Renu Malhotra, a co-author of the Science
paper, said. "Another thing it says is that the mechanism that caused the
Late Heavy Bombardment was a gravitational event that swept objects out of
the asteroid belt regardless of size."

Malhotra discovered in previous research what this mechanism must have
been. Near the end of their formation, Jupiter and the other outer gas giant
planets swept up planetary debris farther out in the solar system, the
Kuiper Belt region. In clearing up dust and pieces leftover from outer solar
system planet formation, Jupiter, especially, lost orbital energy and moved
inward closer to the sun. That migration greatly enhanced Jupiter's
gravitational influence on the asteroid belt, flinging asteroids
irrespective of size toward the inner solar system.

Evidence that main belt asteroids pummeled the early inner solar system
confirms a previously published cosmochemical analysis by UA planetary
scientist David A. Kring and colleagues.

"The size distribution of impact craters in the ancient highlands of the
moon and Mars is a completely independent test of the inner solar system
cataclysm and confirms our cosmochemical evidence of an asteroid source,"
Kring, a co-author of the Science paper, said.

Kring was part of a team that earlier used an argon-argon dating technique
in analyzing impact melt ages of lunar meteorites -- rocks ejected at random
from the moon's surface and that landed on Earth after a million or so years
in space. They found from the ages of the "clasts," or melted rock
fragments, in the breccia meteorites that all of the moon was bombarded 3.9
billion years ago, a true global lunar cataclysm. The Apollo lunar sample
analysis said that asteroids account for at least 80 percent of lunar
impacts.

Comets have played a relatively minor role in inner solar system impacts,
Strom, Malhotra and Kring also conclude from their work. Contrary to popular
belief, probably no more than 10 percent of Earth's water has come from
comets, Strom said.

After the Late Heavy Bombardment, terrestrial surfaces were so completely
altered that no surface older than 3.9 billion years can be dated using the
cratering record. Older rocks and minerals are found on the moon and Earth,
but they are fragments of older surfaces that were broken up by impacts, the
researchers said.

Strom said that if Earth had oceans between 4.4 billion and 4 billion years
ago, as other geological evidence suggests, those oceans must have been
vaporized by the asteroid impacts during the cataclysm.

Kring also has developed a hypothesis that suggests that the impact events
during Late Heavy Bombardment generated vast subsurface hydrothermal systems
that were critical to the early development of life. He estimated that the
inner solar system cataclysm produced more than 20,000 craters between 10
kilometers to 1,000 kilometers in diameter on Earth.

Inner solar system cratering dynamics changed dramatically after the Late
Heavy Bombardment. From then on, the impact cratering record reflects that
most objects hitting inner solar system surfaces have been near-Earth
asteroids, smaller asteroids from the main belt that are nudged into
terrestrial-crossing orbits by a size-selective phenomenon called the
Yarkovsky Effect.

The effect has to do with the way asteroids unevenly absorb and re-radiate
the sun's energy. Over tens of millions of years, the effect is large enough
to push asteroids smaller than 20 kilometers across into the jovian
resonances, or gaps, that deliver them to terrestrial-crossing orbits. The
smaller the asteroid, the more it is influenced by the Yarkovsky Effect.

Planetary geologists have tried counting craters and their size
distribution to get absolute ages for surfaces on the planets and moons.

"But until we knew the origin of the projectiles, there has been so much
uncertainty that I thought it could lead to enormous error," Strom said.
"And now I know I'm right. For example, people have based the geologic
history of Mars on the heavy bombardment cratering record, and it's wrong
because they're using only one cratering curve, not two."

Attempts to date outer solar system bodies using the inner solar system
cratering record is completely wrong, Strom said. But it should be possible
to more accurately date inner solar system surfaces once researchers
determine the cratering rate from the near-Earth asteroid bombardment, he
added.

The authors of the Science paper are Strom, Malhotra and Kring from the
University of Arizona Lunar and Planetary Laboratory, and Takashi Ito and
Fumi Yoshida of National Astronomical Observatory, Tokyo, Japan.

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Contact Information

Robert Strom rstrom_at_lpl.arizona.edu
520-621-2720 (office) 520-299-3742 (home)

Renu Malhotra renu_at_lpl.arizona.edu
520-626-5899 (office)
617-496-8380 or 520-241-8367

David Kring kring_at_lpl.arizona.edu
520-621-2024 (office)
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Received on Thu 15 Sep 2005 02:41:06 PM PDT