It was a
science fiction fantasy come true: Ten years ago this summer, NASA announced
the discovery of life on Mars.
At a Washington, D.C., news conference, scientists showed magnified pictures of a
four-pound Martian meteorite riddled with wormy blobs that looked like
bacterial colonies. The researchers explained how they had pried numerous clues
from the rock, all strongly supporting their contention that microscopic
creatures once occupied its nooks and crannies.
It was
arguably the space agency's most imagination-gripping moment since Apollo.
Space buffs and NASA officials said that it just might be the scientific discovery
of the century.
"If
the results are verified," the late Carl Sagan pronounced, "it is a
turning point in human history."
Ten years
later, the results have not been verified. Skeptics have found non-biological
explanations for every piece of evidence that was presented on Aug. 6, 1996. And though they still vigorously
defend their claim, the NASA scientists who advanced it now stand alone in
their belief.
"We
certainly have not convinced the community, and that's been a little bit
disappointing," said David McKay, a NASA biochemist and leader of the team
that started the scientific episode.
But even
though the majority of his colleagues don't buy his "life on Mars"
theory--McKay's own brother, also a NASA scientist, is one of his most prominent
critics--many say they respect him and greatly appreciate his efforts.
The
announcement and the technical paper that followed it practically created
exobiology, the scientific field that investigates the potential for life on
other planets.
"Without
that paper I wouldn't be working in this field," said Martin Fisk, a
marine geologist who studies how bacteria survive under the sea floor, partly
because their harsh environment may resemble that of extraterrestrial life.
Debating
the claim has helped researchers develop standards that will eventually prove
useful for evaluating the presence of life in other Martian meteorites or a
sample from the red planet. It has given the scientific community ideas about
exactly where on the planet they would most like to scoop up a sample, should
they ever get to retrieve one.
And it is
undeniable that McKay and his colleagues have drawn attention to what is--whether
it contains evidence of life or not--a very interesting rock.
The rock in
question was discovered in Antarctica,
where rocks that fall from the heavens are easy to spot on the icy glacial
plains. Its name, ALH84001, indicates that it was the first meteorite found
during the 1984 research season in the Allan Hills, an especially meteorite-rich area
in the Trans-Antarctic Mountains.
At first
ALH84001 was misclassified, so it wasn't until 1993 that researchers even
realized the rock came from Mars. That was interesting enough, because at the
time fewer than a dozen Martian meteorites were known to science.
But ALH84001
also turned out to be much more ancient than the other known Martian
meteorites. At 4.5 billion years old, it dates from a period of Martian history
when liquid water--a requirement for the presence of life--probably existed at
the now barren planet's surface.
It made
sense to ask: Could there be fossils of ancient Martian microbes, or maybe
traces of them, preserved in the cracks and pore spaces of ALH84001?
The NASA
scientists proffered four reasons to support their view that the answer to that
question is "Yes."
First,
chemical analysis showed that the meteorite contained a variety of organic
molecules known as polycyclic aromatic hydrocarbons, or PAHs. PAHs can be
produced by biological processes, and that's what McKay and his colleagues
argued. But they are also commonly found in asteroids, comets and meteorites,
not to mention the Antarctic ice where ALH84001 is estimated to have lain for
13,000 years. For that reason, skeptics immediately dismissed the importance of
PAHs in the Martian meteorite.
A second
line of evidence--that the elongated blobs in the electron microscope images
could be fossils of ancient Martian bacteria--was also rejected pretty quickly
by most scientists.
The problem
was, those blobs were much smaller than any bacteria that have ever been
observed on Earth. A National Research Council panel concluded in 1998 that the
blobs were 100 to 1,000 times too small to be free-living organisms because
they couldn't have held all the proteins, DNA and other molecules necessary for
even the simplest metabolic processes.
You could
argue that perhaps Martian life evolved a more compact biochemistry, or that
the blobs shriveled as they fossilized. At one point McKay and the other NASA
scientists suggested the blobs might be pieces of larger organisms.
"That
was only mentioned once or twice and never brought up again," said Allan Treiman,
a geologist at the Lunar and Planetary Institute in Houston.
The two
other lines of evidence survived longer. Both revolved around minerals
sprinkled through the meteorite that could have been produced by microbes.
The first
mineral, carbonate, is typically formed on earth by the remains of living
organisms that make shells and other skeletal parts out of minerals they
extract from seawater. Some of those organisms can be quite tiny. So finding
carbonate in ALH84001 could indicate the presence of ancient microbes in the
rock.
The story
is similar for magnetite, the other mineral of interest in ALH84001. Some
bacteria produce extraordinarily small and pure magnetite crystals, then align
the magnetic grains to make a microscopic compass needle that helps them
navigate.
The
bacteria don't use their internal compasses to find north; they use them to
tell up from down. Earth's spherical shape means that a compass needle in
either hemisphere points at least somewhat downward, so the magnetite grains
help the microbes sense where they are with respect to the planet's surface.
Some of the
most evolutionarily ancient bacteria on Earth produce magnetite, McKay and his
colleagues pointed out. Perhaps ancient Martian microbes did as well; at least
some of the magnetite grains in ALH84001 share the shape, small size and remarkable
purity of those produced by bacteria on Earth.
Of all the
lines of evidence presented by the NASA scientists, it was the magnetite grains
that proved most provocative. They were embedded in the carbonate along with
other iron-containing minerals in such an unusual arrangement that something
out of the ordinary must have put them there--could it have been alive?
"The
shape of the magnetite grains is still rather distinctive," McKay said.
"If it were found on Earth it would be a very strong biosignature."
For years
McKay and his detractors argued about how distinctive the magnetite grains in
ALH84001 are, and whether a non-biological process could have produced them.
Certainly nobody had ever produced similar magnetite grains in the laboratory.
Then
somebody did. In 2001 a second team of NASA scientists, including McKay's
brother Gordon and a consultant to the space agency named D.C. Golden, managed
to cook up a batch of magnetite grains very similar to the ones in ALH84001.
Golden and Gordon McKay were also able to incorporate the magnetite grains into
balls of carbonate like the ones David McKay and his colleagues described in
1996.
"He
got a little testy about the results we were getting," said Gordon McKay,
whose office is down the hall from his brother's. ''What we have shown is that
it is possible to form these things inorganically."
What's
more, their laboratory method simulated conditions ALH84001 is known to have
experienced during its time on Mars.
Yet David
McKay insists his brother's team has not accurately described the synthetic
crystals' shape, and that they aren't sufficiently similar to the ones found in
ALH84001. He also suggests that the purity of the magnetite crystals stems not
from the lab process itself, but from using unrealistically pure raw materials
as a starting point.
Most of the
scientific community doesn't buy those arguments.
"Personally
I don't understand why (Gordon McKay's and) Golden's work hasn't just been the
final word on it," said Treiman, the Lunar and Planetary Institute
geologist.
Now David
McKay has added another meteorite to the mix. At a March scientific meeting he
presented microscopic images of the Nakhla meteorite, another Martian specimen.
The pictures resemble pits that terrestrial bacteria create as they literally
eat the volcanic rock of the sea floor.
"When
I first saw it I was really struck by the similarity," said marine
geologist Fisk, who is a professor at Oregon State University.
So far the
scientific community hasn't shown much interest in David McKay's analysis of
the Nakhla meteorite, partly because it dates from a more recent period of
Martian history when the planet was just as frigid and inhospitable to life as
it is today. In fact all of the 30-some Martian meteorites now known to
science, with the exception of ALH84001, are probably too young to have
contained living organisms.
But new
Martian meteorites turn up almost every year. Eventually, another 4.5
billion-year-old piece of the red planet is going to be discovered.
"Sooner
or later we're going to get another old rock," said Massachusetts
Institute of Technology geophysicist Benjamin Weiss.
And when
that happens, the talk about life on Mars will begin anew.
advertisement
