[meteorite-list] Late Heavy Bombardment-- astroids, not comets

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu, 22 Oct 2009 00:01:04 -0500
Message-ID: <jlpvd5t99st22rqjadrtobtfit6nnku70m_at_4ax.com>

http://www.cosmosmagazine.com/node/3085/full


News
Jupiter shift pelted inner planets with asteroids
Wednesday, 21 October 2009

by Holly Hight
Cosmos Online

PORTLAND, OREGON: A shift in Jupiter's orbit early in the life of the Solar
System dislodged thousands of rocks from the Asteroid Belt, causing them to hit
the inner planets, including Earth.

Evidence for this cataclysmic bombardment comes from a reanalysis of lunar rocks
brought back by the Apollo astronauts and a careful study of lunar craters, said
David Kring, a planetary geologist at the Lunar and Planetary Institute in
Houston, Texas.

Kring presented his findings this week at the Geological Society of America's
annual meeting in Portland, USA.

Ancient cataclysm

Popularly known as the Late Heavy Bombardment, the cataclysm occurred during a
relatively short period of time - just 100 million years or so - representing a
spike in the number of large objects hitting the Earth, Moon, Mercury, Mars, and
Venus.

"Samples taken from the Apollo mission showed that a large number of impact
melts (lavas created by impacts) were generated at the same time," said Kring.

Moon rocks held in storage for more than four decades were restudied with
21st-century technologies to better date impact events during the cataclysm. "We
now understand impact rocks better," said Kring. "Forty years ago, when we went
to the Moon, we didn't know whether all the circular features were volcanic or
impact craters. Until we got those samples [from Apollo], we didn't have any
experience with impact craters."

Smoking gun

These techniques allow radiometric age dating, such as argon dating, with
smaller samples than before, effectively increasing the number of possible
tests. These tests (and others on asteroid fragments that have fallen to Earth
as meteors) revealed that the bombardment came from asteroids, not comets as
originally thought.

This is significant because an asteroid bombardment suggests an inner Solar
System cataclysm and not something that came from beyond the asteroid belt.
"Geochemical fingerprints point to asteroids," said Kring.

The scientists also calculated the size of the asteroids that formed lunar
craters known to be from the Late Heavy Bombardment, which occurred
approximately 3.9 billion years ago.

They then compared these to the size of the asteroids in the asteroid belt. The
conclusion: "The size distribution of craters on the Moon suggests the impacting
objects came from the Asteroid Belt," said Kring.

Models suggest a change in Jupiter's orbit would generate the resonances needed
to knock the asteroids off-course. "We now have increasing evidence in support
of the hypothesis that the giant planets' orbits have shifted over their
history," said Renu Malhotra, a planetary scientist at the University of Arizona
in Tucson.

Adds Kring: "The data tells us that asteroids were the dominant source. And for
the asteroids to move, Jupiter would have had to have moved."
Received on Thu 22 Oct 2009 01:01:04 AM PDT