No signs of life
red planet

  15 JAN 1998 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.

U P D A T E
15 DEC 1998. Further evidence that the Mars meteorite has no signs of life came from a NASA meeting held Nov. 2 to 4. "The case has weakened dramatically," meteor scientist Horton Newsome told Science magazine. After two years spent examining the 2-kilogram rock by numerous researchers, he added, "a number of lines of evidence have gone away."

The presence of polycyclic aromatic hydrocarbons, for example, could easily represent a chemical found on early Mars, not life. The small, round objects that the NASA scientists originally thought were fossils of tiny bacteria could not represent life as we know it, because bacteria cannot get that small. And the oddly shaped molecules dubbed "worms" look like many structures built by entirely inorganic processes. Although the scientists who originally announced signs of life in the meteorite have not entirely retracted their claims, the notion is becoming more difficult to accept.

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 (see our coverage). Now the Jan. 16 Science carries a pincer attack on that speculation.

Brief background blurb

Maybe we are alone after all
  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 around 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.

  fossil

Magnification of "fossils" in the Martian meteorite and the "red planet" (left) courtesy NASA.


A sample of the Mars rock to scale against a paper clip. © UW-Madison/John Valley.

A mars meteorite sample clearly showing the orange-yellow carbonate globules. © UW-Madison/John Valley
  (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.

Oops!
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. okay, it's really smallThe 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.

I'm ready for my closeup 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.

--Dave Tenenbaum

  The Why Files
Further reading:
Requiem for Life on Mars? Support for Microbes Fades, Richard Kerr, Science, 20 November, 1998, pp. 1398-1400.

Isotopic Evidence for a Terrestrial Source... A.J.T. Jull et al, Science, 16 Jan., 1997.

Search for Endogenous Amino Acids in Martian Meteorite ALH84001, Jeffrey Bada et al, Science, 16 Jan., 1997.

The Case for Life on Mars, Everett Gibson, Jr., David McKay et al, Scientific American, Dec. 1997, p. 58 ff.

The Why Files covered weird "life" on Mars, and on Earth.

The Why Files covers the battle between evolution and creationism.

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