[meteorite-list] Earth Deemed Older, Calling Moon Formation Theory into Question

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:52:24 2004
Message-ID: <200208282243.PAA17759_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/older_earth_020828.html

Earth Deemed Older, Calling Moon Formation Theory into Question
By Robert Roy Britt
space.com
28 August 2002

A pair of new studies has helped pin down how long it took Earth to form,
breaking down the final barrier of disagreement over the precise timing but
creating a problem for the leading theory of the Moon's formation.

Earth reached mature size 30 million years after the Sun's birth, the two
independent results show. This is in line with the leading theoretical model
and most other indicators.

However, this is about 70 million years quicker than what was expected by
Moon formation theorists. These researchers' computer models have the
satellite being carved from a nearly mature Earth by a large impact about
100 million years after the origin of the solar

Earth is thought to have formed within a large, fairly flat and rotating
disk of gas and dust that circled the nascent Sun. Dust coalesced to form
rocks, which banged into each other. Some stuck and grew into asteroids and
larger objects called proto-planets, a few of which survived to become
planets.

Nearly all scientists agree on this scenario for building the four inner
planets.

Yet the method considered to be best suited to determine how long the
process took -- it looks at ratios of radioactive hafnium and tungsten in
ancient materials -- has in the past resulted in an answer of 60 million
years. The two new studies determined that previous tests were in error.

The results will be published in the Aug. 29 issue of the journal Nature.

One team was led by led by Qingzhu Yin at Harvard University, the other by
Thorsten Kleine at the University of Muenster in Germany.

"Earth is older than previously thought," Kleine told SPACE.com. "Our data
indicate that these collisions caused almost complete melting of Earth
resulting in a scenario called magma ocean, in which the Earth was covered
by a layer of magma."

Both groups reanalyzed the ratios of hafnium and tungsten in meteorites
thought to be about as old as the solar system itself. They studied
meteorites thought to have come from the asteroid Vesta, along with rock
from Earth.

Both studies concluded that Earth formed in the first 30 million years of
the solar system's existence. Mars, because it is smaller, took about 13
million years to develop, they say. Vesta, meanwhile, seems to have formed
within 4 million years, compared to previous estimates of 16 million years.

Kleine said the work suggests that the time it takes a planet's core to
develop is correlated to the planet's ultimate size. That might seem
intuitive, but it hadn't been shown before.

That doesn't mean, however, that planet formation is simple.

"The situation for the Earth and Mars is complicated because their growth
involves absorbing bodies similar to Vesta and then reprocessing the cores
within them," says Alastair Cameron of Harvard University and the Lunar and
Planetary Laboratory at the University of Arizona.

Cameron was not involved in the studies, but he wrote an analysis of them
for Nature.

Cameron said the new dates might call into question the leading theory of
the Moon's formation. That theory holds that about 100 million years after
the Sun's birth, the Earth was about 90 percent of its full size and was hit
by a single Mars-sized object. The impact kicked up material that went into
orbit around the planet and gathered together to become the Moon.

More computer simulations of Moon formation might now be required, given the
newly suggested time frames, Cameron says. He ought to know. He was on one
of two research teams back in the mid-1970s that developed the original idea
of the Moon being formed by an impact.
Received on Wed 28 Aug 2002 06:43:32 PM PDT