[meteorite-list] Latest New Scientist - Aliens Rock!

From: matt <metlist_at_meteoritecentral.com>
Date: Sat Aug 26 13:37:55 2006
Message-ID: <44F083D4.8030409_at_plu.to>

I don't think this has been posted to the list yet, an article from this
weeks issue (26th August 2006) of New Scientist (on a seperate note, has
anybody else's email address been demoted to a dwarf planet? ;):

Aliens rock!

It's 10 years since the world woke up to news of Martians. They turned
out to be a false alarm, but the story is far from over, says
astrobiologist Peter Ward

Mention the words "Mars" and "meteorite" and you'll probably think of
the 1.9-kilogram hunk of Mars rock discovered in the Allan Hills of
Antarctica in 1984. You might not recall its fiddly name, ALH 84001, but
exactly 10 years ago this meteorite shot to international fame when a
team of NASA scientists claimed it contained evidence of ancient Martian
life.

The team, led by David McKay of the Johnson Space Center in Houston,
Texas, announced in the journal Science that ALH 84001 contained
numerous "biosignatures", including chains of small rod-like structures
that they interpreted as fossil bacteria (vol 273, p 924). If they were
right, they were the first alien life forms to be encountered by humans.

A decade on, the vast majority of scientists who have been involved with
this most poked, prodded and analysed of all meteorites now believe that
ALH 84001 does not contain evidence of Martian life: all the supposed
biosignatures can be accounted for by inorganic processes.

The disappointment of ALH 84001 has not spelled the end of the Mars
meteorite story, however. Far from it. Just as one debate winds down,
another Martian meteorite is being quietly touted as possible evidence
that life once existed on the Red Planet. This time it is not small rods
but tiny holes that are raising hopes - and ire - once again.

The meteorite, called Nakhla, fell to Earth on 28 June 1911 at El-Nakhla
in Egypt, reportedly killing a dog. The material arrived in dozens of
pieces, together weighing around 10 kilograms and now housed in various
institutions around the world, including London's Natural History Museum
and the Smithsonian Institution in Washington DC.

Nakhla was one of the first meteorites to be identified as being from
Mars, based on an analysis of its chemical composition. This suggested
that it had crystallised out on a planet with a gravitational field
weaker than Earth's, but larger than the moon's. That meant Mars or
Mercury, but since it is highly unlikely that material blasted off
Mercury's surface could ever make its way to Earth, the consensus is
that it came from Mars.

Subsequent work on the meteorite's chemistry indicated that it
originated in thick lava flows that formed approximately 1.3 billion
years ago. Nakhla also shows clear signs of having been altered by water
1.1 billion years ago, some of the clearest evidence yet that liquid
water once flowed on Mars.

Like ALH 84001, Nakhla found its way to Earth after being blasted off
its home planet by an asteroid strike, in Nakhla's case around 11
million years ago. The collision was so violent that the material it
ejected into space escaped Mars's gravity and began a nomadic existence
wandering the solar system. Eventually, like the other 30 or so other
meteorites that have been identified as originating on Mars, Nakhla
encountered Earth's gravitational pull and fell onto its new home.
Signs of life

Interest in Nakhla was ignited in no small way by the furore over ALH
84001. In 1998, the Natural History Museum offered researchers samples
of the meteorite for analysis. The results are now starting to filter
in, and though astrobiologists are being cautious, there are intriguing
hints that Nakhla could become as famous - or maybe infamous - as ALH 84001.

The first of these results was announced in 2004 by geologist Martin
Fisk of Oregon State University in Corvallis, at the annual Lunar and
Planetary Science conference in Houston, Texas. Earlier this year,
Fisk's team published a full report of their findings in the journal
Astrobiology (vol 6, p 48).

Fisk's starting point was tiny tubular holes in the rock, of a sort
commonly seen in volcanic rocks on Earth. These tubes, when they are
found on Earth, average 1 to 3 micrometres in diameter and up to 100
micrometres long. They are believed to be the result of bacteria or
archaeans "eating" their way into recently formed basalts on the sea
floor to obtain essential nutrients. Nobody has ever seen this process
in action, but there is good evidence that the holes are of biological
origin: the tunnels are exactly the right size to have been bored by
bacteria, and often contain traces of biomolecules such as nucleic
acids. In 2004, geologists in Norway proposed that similar tunnels found
in ancient basalts around 3.5 billion years old are the earliest signs
of life on Earth.

When Fisk's team examined volcanic minerals inside Nakhla, to their
surprise they found small tunnels virtually identical in size and shape
to those in Earth rocks. They were unable to extract any nucleic acids
out of Nakhla's tunnels, but that's not so surprising: after enduring
blast-off and re-entry, not to mention floating in space for 11 million
years, the chance of this complex molecule surviving the journey in
recognisable form is pretty slim.

There are other interesting minerals inside Nakhla. At this year's Lunar
and Planetary Science conference in Houston in March, McKay and
colleagues reported that veins and cracks in the meteorite, including
the tunnels discovered by Fisk, contain carbon-rich matter that isotope
analysis suggests originated on Mars. Since the only kind of life we
know of is based on carbon, the abundance of this element in Nakhla -
plus the earlier evidence of contact with liquid water - only adds to
the intrigue.

Those involved have been careful not to get carried away by this new
evidence. Fisk and colleagues have stopped short of claiming that the
tunnels are proof of life on Mars, saying only that given the abundance
of volcanic minerals throughout the solar system, such features may be a
useful biosignature. McKay, meanwhile, told the meeting that there was a
"plausible basis for a hypothesis" that the carbon in Nakhla was of
biological origin.

Just as with ALH 84001, settling the debate will largely depend on
whether the supposed biosignatures could have been produced by
non-living processes. The search is now on for some abiotic way to make
the tunnels.

Already, some leading researchers are voicing expectations that Nakhla
will prove to be another red herring from the Red Planet. Joe Kirschvink
of the California Institute of Technology in Pasadena notes that the
tunnels are highly ordered, aligned roughly in parallel. While order is
often a sign of life, in this case it may be evidence of an abiotic
origin. All known tunnels made by Earth bacteria are random in
orientation. (Ironically, Kirschvink is one of the few scientists who
still maintains that ALH 84001 contains signs of life.)

Whatever the outcome of the Nakhla debate, the new evidence is likely to
have an important impact on the search for signs of life on Mars. In
2009, NASA is planning to launch a robotic mission, Mars Science
Laboratory, to search for biosignatures on the Red Planet. Thanks in no
small part to the work on Nakhla, one of the things it will look for is
volcanic rocks riddled with randomly oriented tubes, hopefully with
nucleic acids inside them. Finding them would probably spell the end of
the Mars meteorite story - but what an ending.
Received on Sat 26 Aug 2006 01:24:36 PM PDT