[meteorite-list] ALH84001 Not As Old And Still No Fossils

From: JoshuaTreeMuseum <joshuatreemuseum_at_meteoritecentral.com>
Date: Fri, 16 Apr 2010 13:27:15 -0400
Message-ID: <1C90B4960C5F421EB9E9AF2B4CA2491F_at_ET>

I completely agree with Alan Treiman on this one. I couldn't believe when
McKay and team made the same unfounded claims 10+ years later with no new
evidence to back them up. I think their 15 minutes of fame is finally over.


The Allan Hills meteorite, named for the site where it was found in
Antarctica, was once thought to contain fossil traces of life. That idea has
been mostly dismissed, and now the rock also appears to be not quite as old
as previously thought.

The oldest known Martian meteorite isn't so old after all. Though it's still
the oldest chunk of Mars scientists have ever found, new research suggests
the Allan Hills meteorite - officially known as ALH84001 - is about 400
million years younger than previously estimated.

A new analysis published in the April 15 Science pegs the meteorite's age at
a mere 4.091 billion years. Previously the meteorite was commonly accepted
to have formed 4.51 billion years ago, when the planet's surface was still
solidifying out of its primordial magma ocean. But the new age indicates the
rock would have formed during a later, chaotic period when Mars was being
pummeled by meteorites that fractured and shocked the planet's solid

The Allan Hills meteorite has been a lightning rod for controversy since
scientists announced in 1996 that it might hold fossils of Martian bacteria.
The scientific community has since mostly abandoned that idea, as one by one
every line of evidence for life has been given a non-biological explanation.

"People usually ask me about the life aspect, and I'm so sick to death of
that," says Allan Treiman of the Lunar and Planetary Institute in Houston,
who was not involved in the new work. Treiman and others now believe that
what once looked like fossils is actually rock that was shaped by ordinary
geological activity.

The previously accepted age of 4.51 billion years old was calculated in 1995
by measuring radioactive isotopes of samarium and neodymium. Radioactive
elements decay from a "parent" isotope (in this case, samarium) to a
"daughter" isotope (neodymium) at a set rate. By comparing the amount of the
parent element to the daughter element, scientists can infer how long a rock
has been around.

"To understand how the Martian mantle has evolved, it's critical to get
samples that are old, to see what the mantle sources were early in the
planet's history," says Thomas Lapen of the University of Houston, a
coauthor of the new study. "This is the only sample in that age range."

Lapen and his colleagues used radioactive isotope dating to calculate the
age of the meteorite, using different elements than the 1995 analysis did.
Lapen says that the elements used back then were mostly found in minerals
called phosphates, which succumb relatively quickly to weathering and
geological processes. Like hair dye or a fake ID, weathering could disguise
the rock's age in some ways, but not so thoroughly that more reliable
indicators are obscured.

"If it's subject to weathering, the phosphate would be the first to be
disturbed," Lapen says. "Then ages dependent on the phosphates are altered."

Instead of elements found in phosphates, Lapen's group used lutetium and
hafnium, elements that are mostly found in more change-resistant components
of the rock. This method showed that the meteorite is just 4.091 billion
years old.

Surprisingly, the researchers also found that several younger meteorites
have essentially the same composition as the Allan Hills meteorite, meaning
some of the same basic geologic processes have been at work on Mars for
almost its entire history.

"That connection is perhaps the most amazing outcome of this research,"
Lapen says. "Mars is a very steady state planet. Igneous processes were
happening the same way four billion years ago as they are happening right

The new age places the rock's birth date right at a period in the solar
system's history when all of the inner planets were being bombarded with
meteorites. That could clear up some confusion about the meteorite, Treiman
says. Parts of the rock show signs of having been melted and reformed a
second time since its birth, which would have been tough to explain if the
rock were all original Martian crust.

"That had been a bit of a problem," Treiman says. "You'd have to do whatever
mantle processing, whatever happened on the planet, before this rock came to
be formed. There's not a lot of time for that."

Read More

Phil Whitmer
Received on Fri 16 Apr 2010 01:27:15 PM PDT

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