[meteorite-list] Sticks And Stones: The Martian Meteorite Debate Rages On

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:26 2004
Message-ID: <200204121553.IAA09851_at_zagami.jpl.nasa.gov>

http://www.csmonitor.com/2002/0412/p25s02-stss.html

Sticks and stones: the Martian Meteorite debate rages on

By Michelle Thaller
Christian Science Monitor
April 12, 2002

PASADENA, CA - Mars has always been a provocateur. The planet has a long
history of making us uneasy, from the portents of violence our ancestors
associated with its red glow, to our science-fiction nightmares of
malicious, technologically superior alien invaders.

And Mars is still stirring things up in the scientific community. For
several years now, there has been an on-going debate as to whether a
meteorite from Mars contains the fossilized remnants of microbial life. Some
scientists think we no longer have to wonder about whether there is other
life in the universe; we have the remains of tiny Martian cousins in our
laboratories at this very moment. Others remain skeptical, claiming that
every structure and chemical in the meteorite could have been formed by
natural processes that have nothing to do with life, like chemical
weathering and heating. Despite the controversy, the Martian Meteorite
debate has already taught us a lot about what kind of questions to ask the
next time we get our hands on a sample of Martian soil, as well as shown us
how little we understand about the threshold of life itself.

Backing up a little, how in the world did a piece of Mars find its way to
Earth? Would you recognize a Martian rock if it were sitting in your
backyard? The idea isn't as outlandish as it seems. We know of 24 meteorites
that were originally part of Mars.

The first was recovered after it boomed down into a field outside of
Chassigny, France, in 1815 (although the people of the time had no way of
knowing that it came from Mars). The famous Martian Meteorite (designated
ALH 84001) that has spurred all the debate was found more recently, lying on
the Antarctic ice in 1984. Antarctica turns out to be a rich ground for
meteorite hunters, as all of the indigenous rocks are buried beneath
thousands of feet of glacial ice. If you find a rock sitting on top of the
ice, there's a good chance it landed there from somewhere else. One of the
meteorite-hunting teams even has a mascot of a penguin standing with a
baseball glove aimed at the sky. Deserts like the Sahara and the Mojave are
also a good bet, as the meteorites stand out from the sandy, eroded rocks
around them.

So now that you've found a rock from space, how do you know it came from
Mars? We've never brought a rock back from Mars, and our robotic landers
have only been able to do crude chemical analysis of the rocks and soil
there. Interestingly, that issue is not part of the debate. Scientists are
almost certain that these meteorites are bits of the planet Mars due to
careful chemical analysis of bits of the Martian atmosphere trapped in the
rocks. We know the chemical composition of Mar's atmosphere very well (from
spectroscopic measurements), and the rocks match it exactly. Really, there's
no where else they could have come from.

Which leaves the next big question: How did they get here?

That might be the most amazing part of the whole story. The only way a rock
could get to here from there is to be blasted off the surface of Mars at
11,000 miles per hour (that's the speed needed to escape Mars' gravity).
There's no physical process on any planet that we know of that can achieve
such speeds. Even rocks hurled out of giant volcanic explosions don't go
anywhere near that fast. So, in fact, the only thing that can create a
Martian meteorite is another meteorite. Probably a very big one, as big as a
mile across.

That's right, scientists think these meteorites were chunks of Mars that got
blasted into space after a violent (think dinosaurs) impact from a meteor or
a comet. Judging from our maps of the Martian surface, this hasn't happened
for a while. We have no way of knowing exactly when this impact took place,
but we do have some idea how long ALH 84001 stayed drifting around between
Earth and Mars. High energy particles called cosmic rays irradiate anything
in space, leaving radioactive traces. ALH 84001 seems to have been exposed
to these particles for about sixteen million years, although if it was the
inner part of a larger meteor that broke up, it could have been in space
much longer. And we didn't find it the moment it fell to Earth either, not
by a long shot. Judging by other radioactive decay processes, the rock had
been cooling its heels in Antarctica for about 13,000 years.

Right away, planetary scientists knew they'd found something interesting, as
the rock showed signs of having been flooded with liquid water at least a
billion years ago, perhaps as much as three billion. This piqued everyone's
imagination, as the meteorite seemed to come from a lost age on Mars, when
life might have taken hold.

Billions of years ago, Mars was a very different place, with a thick
atmosphere and liquid water either on, or very near, the surface. Liquid
water seems to have changed the chemistry of the rock, dissolving parts away
and leaving globs of carbon-rich minerals. The globs were also rich in
organic compounds called PAH (polycyclic aromatic hydrocarbons). It's
actually not unheard of to find complex organic molecules in a meteorite.
Many meteorites contain them, and some scientists think that may be how
organic chemistry came to Earth in the first place. Still, the rock proved
to be intriguing.

When the scientists turned an electron microscope on these carbon globs,
they got the shock of their lives. Inside, clustered tightly together, were
hundreds of tiny, wormy shapes. Only about 100 billionths of a meter across,
the wormy things looked alarmingly similar to the fossilized remains of
ancient Earth bacteria. They certainly didn't look like anything that had
been seen inside a meteorite before.

And it wasn't only their shapes that were surprising; all around the "worm"
were pure strings of iron crystals, called magnetite. Similar magnetite
deposits are left behind when Earth bacteria die and decay. And at that
point, scientists knew of no natural process that could produce pure
magnetite crystals in the shapes and sizes observed in the meteorite. In
fact, up until then, similar magnetite deposits had been used as a tracer to
find bacteria in rocks. Did that still hold true? Had they, in fact, found
the first example of life outside Earth?

At this point there was a bit of a media circus, and a lot of facts got
distorted. In truth, no scientist had ever claimed that the meteorite
definitely contained life; there were just a lot of tantalizing loose ends,
and no good way of explaining them. Nature (and, it seems, the publicity
machine) abhor a vacuum, so in the absence of any conclusions, many people
got the idea that we had, in fact, discovered ancient Martian bacteria. But,
as is often the problem with front-page news, any subsequent detractions
seem to get buried somewhere on the back page.

In the last few months, scientists have done a bit of back-pedaling. What
happened, in the best of scientific tradition, is that people went back to
their labs and got to work. Was it possible, they wondered, to re-create
everything in the mysterious meteorite by natural geologic processes? The
wormy shapes were the first to go. There are plenty of ways to create
similar shapes from minerals embedded in the meteorite, no life needed. And
as stated before, PAH's, although highly-complex organic molecules, exist in
abundance in space. There is still some arguing back and forth as to exactly
what flavors of PAH's are commonly found in meteorites as opposed to those
in ALH 84001, but that particular debate has reached no closure.

In March of 2002, a team of scientists announced a discovery that may turn
out to be the last nail in the coffin for the Martian Meteorite. The team
had, in their laboratory, created very similar magnetite crystals to the
ones in ALH 84001, using nothing but repeated heating and shocking. From
what little we know of the meteorite's history, it seems to have undergone
plenty of both. Other scientists countered that the artificially created
magnetite didn't have the exact same structure as bacteria-produced
crystals, which may prove to be true. The crystals are so tiny that much of
the discussion has centered on inventing better ways to probe the chemical
structure of the crystals.

In the end, no one has proved beyond a shadow of a doubt that the shapes and
chemicals in ALH 84001 are due to the presence of fossilized bacteria. But
no one has disproved it either, and the whole debate brings up a fundamental
issue that NASA will have to face as it begins to search for life outside
the Earth: how do you recognize ancient life when you see it?

Billions of years ago, when ALH 84001 was forming inside some long-extinct
Martian volcano, life in our Solar System had just barely taken hold. We're
not just talking about Mars, either. Even on Earth, only the very first
bacteria were emerging. So much of the chemistry of primitive life is
indistinguishable from natural changes in rocks and minerals. That's no
coincidence; that's what early life had to work with. What was the subtle
change in chemistry, somewhere deep inside a rock or miles underneath the
oceans, that allowed life to begin? We just don't know. So, it seems that
before we can pass judgment on life elsewhere, we may need to get to know
ourselves, and our origins, a whole lot better.
Received on Fri 12 Apr 2002 11:53:48 AM PDT