[meteorite-list] Moon-forming impact leftscars in distant asteroids

From: Shawn Alan <shawnalan_at_meteoritecentral.com>
Date: Fri, 01 May 2015 08:38:21 -0700
Message-ID: <20150501083821.e8713c95af9984a493c5db01816d4c10.5060c0126f.wbe_at_email22.secureserver.net>

Hello Listers
 
I wonder what 32 meteorite samples they have used for this theory?

Enjoy!

Shawn Alan
IMCA 1633
ebay store http://www.ebay.com/sch/imca1633ny/m.html
Website http://meteoritefalls.com


Moon-forming impact left
scars in distant asteroids
Planetary collision dated through analysis of meteorites

by

Eric Hand

It was the biggest cataclysm the solar
system has ever seen. About 100 million
years after the planets began to take
shape, a Mars-sized body crashed into
the proto-Earth, creating a halo of hot
debris that coalesced into the moon.
There was collateral damage, it turns
out. Scientists now suspect that fragments
of the giant impact were flung all the way to
the fledgling asteroid belt. When this planetary
shrapnel crashed into bodies there, it
shock heated them, leaving an imprint that
can still be detected billions of years later
in meteorites. On page 321, planetary scientists
show that these shock-heating signatures
provide a new way to date the moon?s
formation, pegging it at 105 million years
after the beginning of the solar system
4.6 billion years ago.
The result could help settle debates
about the age of the moon and suggests that
meteorites, which are mostly fragments of
asteroids, could harbor other evidence of
tumult in the inner solar system. ?The asteroid
belt is almost primordial,? says Bill
Bottke, a planetary scientist at Southwest
Research Institute in Boulder, Colorado,
and lead author of the new study. ?A lot of
objects there have been witness to activity
in the inner solar system. We now have a

way to probe that.?
Scientists have long tried to pin down the
age of the moon by analyzing lunar samples
returned from the Apollo missions. But because
of disagreements about the isotope
systems used for dating, the calculated ages
vary from about 30 million years after the
start of the solar system
to 100 million or even
200 million years younger.
A more precise age would
help scientists work out
when the bumper-car process
of planet formation
began winding down. The
moon-forming impact is
thought to have come late
in the process, because
the composition of Earth?s
mantle reflects only a
short period of impacts
by smaller bodies after the
mammoth collision.
Researchers who study
the giant impact have
typically ignored the bits
that didn?t end up in the moon. But Bottke
realized that an event so large would have
created fragments moving fast enough to
escape the collective gravity of the Earthmoon
system. ?You create this huge swarm
of material,? he says. His models suggest

that 10 billion kilometer-sized bodies would
have been flung out into the solar system?
where many of them could strike asteroids.
Asteroids constantly collide with each
other, but at relatively slow speeds. Some
high-speed projectiles from the giant impact,
in contrast, would have struck at
speeds upward of 10 kilometers a second,
melting and transforming asteroid minerals
into darker, glassy materials. The shock
heating would also have altered a standard
radio active ?clock? used for dating, in which
a radioactive isotope of potassium decays
into argon that remains trapped in the crystal
structure of the rock. ?If you heat it up
enough, argon moves through the crystal
structures, and you can reset [the clock],?
says study co-author Tim Swindle, director
of the Lunar and Planetary Laboratory at
the University of Arizona in Tucson.
Searching through the literature for meteorites
that had already been dated, the
team found 34 samples that fit their profile:
those with shock-heating alteration and
ancient argon ages. A significant fraction
of these 34 samples have ages that cluster
around 105 million years after the solar system
began; that, the team believes, is the
age of the moon-forming impact.
Other scientists are excited about the
method but worried about the small sample
size. The authors used their own judgment
to identify meteorites with the right
type of shock heating, and their 34 meteorite
samples could hail from as few as five
or six parent asteroid bodies. ?Is that really
representative of everything the asteroid
belt saw?? asks Sarah Stewart, a planetary
scientist at the University of California,
Davis. ?It?s not a robust
conclusion, but it?s a
robust method.?
Swindle says the new
moon age estimate?a
signal ?strong enough
to look like more than a
curiosity??will improve
as his lab and others calculate
dates for more shockheated
meteorites. And
Bottke hopes the method
will be used for more than
just dates. He says meteoriticists
should return
to these 34 samples and
inspect them carefully.
Perhaps amid the veins
of glassy materials are
fragments of the giant impactor, or the
proto-Earth itself. ?There may still be
traces of the primordial Earth in the asteroid
belt, and they may be in our meteorite
collections today,? Bottke says. ?To me
that?s fun.?
Received on Fri 01 May 2015 11:38:21 AM PDT