MARS the warmonger is stirring up trouble again. When researchers announced
in 1996 that they had found the remains of Martian life inside a 3.9
billion-year-old meteorite, the reaction was passionate. Some people were
excitedly optimistic, many were sceptical, and the debate between the two sides
was fierce. But most of the evidence from the meteorite became discredited, and
within a few months most scientists felt the case was insupportable. Now the
debate has returned with a vengeance. In March, at the Lunar and Planetary
Science Conference (LPSC) at NASA鈥檚 Johnson Spaceflight Center in Houston, the
new champions of life on Mars set out to settle the matter鈥攁nd met
ferocious resistance. For most of us bystanders, the argument was both
fascinating and embarrassing. 鈥淚t was too emotional on both sides,鈥� says Frances
Westall, a geologist at the Lunar and Planetary Institute in Houston.
Perhaps the acrimony is understandable, because the significance of the
discovery鈥攊f it鈥檚 true鈥攊s awesome. For this reason, the meteorite,
named ALH84001, is probably the most heavily scrutinised rock on Earth. It is
thought to have been blasted off Mars鈥檚 surface by an asteroid or comet impact
about 17 million years ago. After drifting around the inner Solar System, it
landed in Antarctica, where it sat for a few thousand years before being picked
up by a prospector in 1984.
When David McKay of the Johnson Spaceflight Center and his colleagues
published the paper claiming that this meteorite held fossil life, they cited
several lines of evidence: subtle changes in mineral composition, traces of
complex organic molecules, and even microscopic elongated bumps that were
supposedly fossil bacteria. Since then, most of this has been shown to be
inconclusive. But one piece of evidence is proving hard to dismiss.
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The debate now hangs on the presence of tiny magnetic crystals in the
meteorite. Why should magnets be a sign of life? Because there is a class of
bacteria on Earth that manufacture their own magnetic crystals to orient
themselves in dark, muddy pools (see 鈥淐reatures from the black lagoon鈥�).
Could Mars have been home to similar bacteria 4 billion years ago?
Magnetite crystals can also be created by 鈥渁biogenic鈥� geological processes,
and many researchers believe the Martian crystals were made in this way. Hap
McSween of the University of Tennessee in Knoxville maintains that the crystals
from ALH84001 were made at very high temperatures, ruling out a biological
origin.
Then last year, Kathy Thomas-Keprta at the Johnson Spaceflight Center found
that the magnetite in ALH84001 has some remarkable properties. Magnetite
crystals formed by geological processes usually contain traces of titanium and
other metals. But those in the Mars meteorite are exceptionally pure鈥攋ust
like magnetite made by bacteria on Earth. They are also bullet shaped, whereas
most geological magnetite forms cubic or octahedral crystals. And the crystals鈥�
magnetic poles are perfectly aligned with their long axes. Bacterial magnetite
crystals also have this property, as it improves the strength of the magnet.
Westall is impressed by all this. 鈥淭homas-Keprta鈥檚 work is extremely thorough.
This is the strongest evidence for life on Mars.鈥�
So the Martian crystals are similar to those found in terrestrial bacteria.
But does that really mean they came from Martian life?
At the conference in March, D. C. Golden of Johnson Spaceflight Center put
forward a powerful counter-argument. He created very similar magnetite crystals
in the lab, simply by heating up a carbonate mineral called siderite. Golden鈥檚
discovery means the ALH84001 magnetites could have been made abiogenically, says
Allan Trieman of the Lunar and Planetary Institute. After all, they are found
among carbonate minerals, and scientists can show that the rock the meteorite
came from was once heated and stressed, probably by a separate asteroid impact
about 4 billion years ago.
Thomas-Keprta contends that Golden鈥檚 crystals have not been studied in three
dimensions. They were only revealed in a single electron microscope image, so it
is hard to say whether they have the same shapes as those in the meteorite.
But the real hullabaloo began in February, provoked by a paper in Proceedings
of the National Academy of Sciences. Its author, Imre Friedmann
of NASA鈥檚 Ames Research Center in California, is a comparative newcomer to the
debate. He was sceptical about the original claims, but has now been persuaded
by the curious arrangement of the magnetite grains. He claims they are arranged
in a chain, like a string of pearls.
A chain of magnets is perfect for orienting bacteria. But as you can see for
yourself with a set of small magnets, a chain is not stable鈥攐ne little
shake makes it collapse into a clump. So the bacteria use a membrane of elastic
material to keep the crystals apart. 鈥淲hen I saw the chains of magnetite, I was
convinced,鈥� says Friedmann.
Some opponents of the Martian-life theory think that these chains might have
been left behind by bacteria colonising the meteorite after it landed on Earth.
鈥淭he meteorite has been compromised by terrestrial contamination,鈥� says Westall.
Friedmann argues this is unlikely, because bacteria from Earth could not have
got into the rock. 鈥淭he magnetites are double sealed inside carbonates which are
inside glassy globules.鈥�
In the astrobiology session at the LPSC, chaired by Trieman, Friedmann鈥檚 talk
met with harsh criticism. One questioner accused Friedmann of 鈥渟eeing things鈥� on
his electron micrographs鈥攈e implied the chains are not really there, just
like the 鈥渃anals鈥� seen by early observers of Mars. Thomas-Keprta leapt to
Friedmann鈥檚 defence, but was soon cut off by Trieman, who insisted on keeping
the session on schedule and moved on to his own talk.
Trieman showed micrographs of Golden鈥檚 artificial crystals, which also lie in
a chain. Notably, though, they are not uniform in size like those in bacteria
and in ALH84001.
Most planetary scientists would like to think that there is life elsewhere.
Indeed, many believe that there probably was life on Mars early in the Solar
System鈥檚 history, when its atmosphere was thicker and heat from the newly forged
planet kept the surface warm enough for liquid water to exist in abundance on
the surface.
However, most still think that the evidence in the meteorite is not
conclusive. Faint traces of life on Earth at this time exist, but are hard to
find鈥攎ost are in ancient rocks in Australia and Greenland, dating back 3.5
to 3.9 billion years. Friedmann acknowledges that the meteorite fossils 鈥渁re by
far the best-documented microfossils of this age, on Earth or Mars鈥�. It would be
an incredible stroke of luck if one of the handful of rocks we have from Mars
just happens to be teeming with obvious fossils.
So if the evidence from ALH84001 isn鈥檛 powerful enough to persuade a cautious
community, perhaps the debate about life on Mars will only ever be resolved by
collecting samples from Mars, preferably from favourable sites such as dried-up
lake beds. NASA is considering plans to launch a mission in 2014 to bring back a
sample, and the European Space Agency has just hatched a plan to collect some
Martian soil by 2010
(快猫短视频, 31 March, p 10).
Meanwhile, the opposing factions have all but stopped listening to each
other. Along with most other planetary scientists, I don鈥檛 think this meteorite
has given us convincing evidence yet, but it is hard not to sympathise with
Friedmann and Thomas-Keprta. If no one ever stuck their neck out, scientific
revolutions would never happen. For those who do push controversial ideas, life
can be hard. Friedmann despairs of persuading his most ardent critics to abandon
their sceptical position. 鈥淭hey believe it like a religious belief. If they
close their eyes, I can鈥檛 do anything.鈥�
How did the Martians live? Imre Friedmann of Ames Research Center in
California believes that the magnetic crystals found in a meteorite from Mars
were produced by bacteria. If he鈥檚 right, it tells us not only that life has
existed outside planet Earth, but also something about what that life was
like.
It would mean that Martian bacteria were free-swimming. 鈥淢agnetotactic鈥�
bacteria on Earth thrive in the low-oxygen environment at the bottom of muddy
ponds. They can use their whip-like flagella to swim, but without directional
control they would go round in circles. This is where the magnets come in: they
align the bacteria with Earth鈥檚 magnetic field, like a compass needle. Except at
the magnetic equator, the field makes an angle with the ground, so these
bacteria can easily move downwards to the muddy depths.
But to have free-swimming bacteria, you need liquid water. The water would
have had to be stagnant enough to allow a tiny creature to swim against any
currents, which rules out the turbulent, deep-sea volcanic vents, where some
scientists believe life may have originated, or volcanically heated underground
streams. There鈥檚 only one alternative: Mars鈥檚 surface was covered in stagnant
ponds and puddles.
We also know that magnetic navigation is far too sophisticated a skill for
the very first life forms on Mars or Earth to possess. So for magnetotactic
bacteria to date back 3.9 billion years鈥攖he age of the
meteorite鈥攚ould be quite a surprise. Frances Westall of the Lunar and
Planetary Institute in Houston thinks this is the biggest problem for
Friedmann鈥檚 theory. But Friedmann points out that this ability has evolved
independently several times in the history of life on Earth. And perhaps the
deterioration of conditions on Mars, as the atmosphere evaporated and the
climate cooled, forced the bacteria to evolve quickly, and learn how to dive to
the bottom of their dark pools.