Skeptics say the potato-sized meteorite shows no key evidence of biological activity.
January '98 of The Why Files Wrote:
In a double whammy powerful enough to vaporize the nastiest
extraterrestrial bacteria, scientists report that the signs of life in a Martian meteorite
probably came from Earth.
Since the life-on-Mars theory was announced by scientists from NASA and
Stanford University in August, 1996, it's ranked as one of the most tantalizing
propositions of the decade. Now the Jan. 16 Science carries a pincer attack on that
speculation.
Brief Background
Recall that the meteorite in question originated on Mars, then was
blasted into space by an asteroid collision. It was found in Antarctica in 1984 after
laying about there for some 12,000 years.
After inspecting part of the meteorite, NASA's David McKay and
colleagues located tiny carbonate structures and organic molecules on the meteorite that
seemed to have been formed by bacteria on
Mars more than 1.3 billion years ago.
(The Red
Planet is now frozen and dry, but it
was much warmer and wetter back then.)
Although the assertion was controversial from the first, only now have
scientists analyzed the meteorite's main organic compounds. The results are fueling the
doubters. "This is bad news with respect to using these meteorites to assess whether
there ever was or is life on Mars," says Jeffrey Bada. Bada, director of the NASA
Specialized Center of Research and Training in Exobiology at the Scripps Institution of
Oceanography in California, was the main author of one of the new reports.
While not eliminating the possibility that life once existed on Mars,
the new findings do seem to nullify the evidence for it.
Time Poorly Spent
Bada's team spent years looking at amino acids in fragments from
various meteorites. In August, 1997, they received fragments of the potato-sized meteorite
that caused the sensation in 1996. The researchers studied amino acids because they are
the building blocks for proteins and enzymes and thus are essential to all known forms of
life.
Amino acids and some other molecules can form in either of two mirror
images. Like your own hands, both versions of these so-called "handed" chemicals
look alike. But just as your left hand won't fit a right-hand glove, the opposite versions
of handed chemicals are not identical. This distinction matters: biological systems only
work if the handed chemicals are all in the same form. (On Earth, all amino acids used to
make proteins are left-handed.)
Normally, you'd expect that the discovery of amino acids -- a
foundation of life -- in a rock from Mars would be exciting news. But Bada's analysis
showed that almost all of those amino acids were lefties -- just like life on Earth and
other amino acids in the ice where the meteorite lay for 12,000 years.
More telling was the particular amino acids found in the meteorite.
While earthly life uses more than 20 of these building blocks, only four were found in the
meteorite -- exactly the same cast of characters as found in the ice. To Bada, the
conclusion was inescapable. "They are clearly terrestrial and they look similar to
amino acids we see in the surrounding Antarctic ice."
Believe it or not, scientists usually choose simple explanations over
complex ones. And Bada says the simplest explanation for his findings was that the organic
chemical had originated on Earth. And the second new study only reinforced that line of
thought.
Another Shoe Drops
To
understand the second study, we must return to McKay's 1996 report, which speculated that
microbes had formed peculiar mineral grains found inside carbonate structures in the
meteorite. As further evidence of life, McKay pointed to organic molecules in the
meteorite.
But the study by A.J. Timothy Jull of the University of Arizona, also
reported Jan. 16, undercuts that notion. Rather than inspect amino acids, Jull looked at
the concentrations of two isotopes of carbon, the most important element in living things.
(Isotopes are types of elements containing different numbers of neutrons; an isotope's
number denotes its atomic weight.)
Since the carbonates and the organic material were supposedly formed by
microbes that were taking in the same chemicals from the Martian environment, Jull
reasoned that both should have contained the same proportions of carbon-13 and carbon-14.
But he found "the organic material contains carbon-14 and the carbonate
doesn't."
Carbon-14 often forms in Earth's upper atmosphere when an energetic
particle like a cosmic ray blasts into a nitrogen nucleus, ejecting a proton. Thus Jull
concluded that the carbonate minerals, which lack carbon 14, must have come from
"somewhere in space, presumably Mars, and the organic material is a recent addition
which took place while the meteorite was sitting on the ice." The disparity between
the isotopes proved "there is no connection between the two things."
Ouch!
Some of
us Why Filers were hoping there was life in this life-in-space biz...
Bad enough that the organic carbon was made in a different place than
the carbonate minerals that supposedly formed it. It also seems the organic carbon formed
after the meteorite smashed into the Antarctic about 12,000 years ago. Jull says the rock
went through "several episodes of contamination" after the collision, when
liquids containing amino acids seeped inside.
Where do these findings leave the hypothesis that Mars once had life?
Although Bada, the amino acid expert, concedes diplomatically that the meteorite is not
"going to give us a definitive answer," the new findings leave intact precious
little evidence for life. With luck, he adds, Martian samples scheduled to be returned to
Earth in 2008 will reveal whether life ever graced the Red Planet.