Case Study: Fossil Microbes on Mars?
Part of the Cosmic Horizons Curriculum Collection.
From the start, the evidence was both fascinating and controversial, and to this day it remains so.
The meteorite in question had escaped from Mars 16 million years ago when an asteroid or comet collided with the planet and blasted out a crater. The 2-kilogram fragment of Martian rock then moved in an elliptical orbit around the Sun until it was swept up by the Earth about 13,000 years ago. It landed in glacial Antarctica, where it remained until 1984, when a meteorite-hunting party picked it up it in the Allan Hills. The specimen was designated ALH84001. At first, no one suspected that it came from Mars.About ten years later, scientists examined ALH84001 more closely and found that it was not an ordinary meteorite, but one of the so-called SNC meteorites, which come from Mars. Meteorites of this class all contain traces of gas having a composition identical to the Martian atmosphere. Each of the dozen other SNC Martian meteorites then known had crystallized within the last 1.3 billion years, after Mars had become a frozen desert. But ALH84001 was over 4 billion years old, and had presumably existed at a time when liquid water was common on the surface of Mars. Liquid water is essential to life as we know it. For that reason ALH84001 attracted the attention of McKay and his team, who thought that the rock might preserve microscopic and chemical evidence of ancient life on Mars.
Three Types of Evidence
To avoid the possible terrestrial contaminants picked up by the meteorite in Antarctica, the team obtained their samples from the solid interior volume of the rock. They found that cracks within the meteorite contain orange-tinted carbonate globules, which resemble limestone cave deposits. This sort of material can form only in the presence of liquid water. McKay and his colleagues found three kinds of evidence that they interpreted in terms of ancient microbial life on Mars:
- The globules contained traces of complex organic compounds called polycyclic aromatic hydrocarbons (PAHs), which might be the decay products of microbes.
- The globules contained microscopic grains of magnetite (a magnetic iron oxide) and of iron sulfide, two compounds rarely found together in the presence of carbonates, unless produced by bacterial metabolism.
- The carbonate globules, when examined with an electron microscope, were found to be covered in places with large numbers of worm-like forms that resemble fossilized bacteria.
McKay and his colleagues conceded from the start that any one of these lines of evidence could be interpreted without recourse to biology. However they judged that the presence of all three in association with the carbonate globules made a persuasive case for ancient life on Mars. Other scientists at once began to subject the evidence to intense critical scrutiny, which is an expected and essential part of the scientific process.
Some geochemists found evidence that the carbonate globules were formed at temperatures up to 300º C, too hot for the survival of any known microbial life on Earth. But others concluded that the globules may have formed at temperatures below 100º C. This now seems to be the case.
What about the PAHs? Chemical theory and experiments show that organic (carbon-based) molecules are formed non-biologically within giant interstellar clouds of gas and dust. The solar system was born in such an environment. Organic molecules produce PAHs when heated, so these materials would have been present on the Earth and on Mars from the beginning. Ordinary soot contains PAHs. McKay and his colleagues soon conceded that the organic carbon evidence for fossil life on Mars was weak.
The evidence from the magnetite and iron sulfide grains is more substantial. In size and shape, most of the magnetite grains closely resemble those produced by terrestrial bacteria. No one has yet demonstrated a non-biological formation mechanism for such grains in association with iron sulfide. The evidence so far suggests that such materials require a biological origin.
What about the objects said to resemble fossil bacteria? Microscopists contend that shape alone is often misleading, because non-biological processes can produce objects that superficially resemble bacteria. The size of the supposed Martian fossils is also a contentious issue. Many of the worm-like objects in ALH84001 are just a few tens of nanometers across, or about a tenth the size of the smallest-known bacteria on Earth. But the minimum amount of molecular “equipment” needed to keep a bacterium alive (including the DNA and ribosomes to translate the genetic code into proteins) would require a volume equal to a 200-nanometer sphere. In other words, these so-called Martian fossils are just too small to have ever been alive.
But just a moment. How can we be sure that any hypothetical Martian life would use the same kind of biochemistry as terrestrial bacteria? Does all life have to be based on molecules as large and complex as DNA? Some scientists have reported finding so-called “nanobacteria” in a wide range of environments. These mysterious objects are as small as the alleged Martian microbes, and are conceivably living organisms, or fragments of organisms. Before the evolution of DNA, ribosomes, and complex proteins on Earth, simpler ancestral life forms must have existed. Those primitive organisms would have lost out in competition by the far more complex bacteria that later evolved on Earth, but their fossilized remains might still be found. And perhaps such things have been found in the Martian meteorite.
For several years, the general current of scientific opinion was running against the biological interpretation of the evidence from ALH84001. But recent studies of the magnetite grains provide increased support to a biological origin. Scientists continue to study ALH84001 and the other Martian meteorites. And they are planning a mission to bring samples back from Mars. While the evidence from ALH84001 remains controversial, it has without question stimulated a major new effort in the search for life, ancient or extant, on Mars.
The possibility that life exists today on Mars received a boost with the publication in June 2000 of orbiter images showing geologically fresh erosion channels on the slopes of Martian craters. The evidence suggests that there may be liquid water in geological formations not far below the surface of Mars. Perhaps this water breaks out at the surface intermittently and excavates channels before it rapidly boils into the thin Martian air. And where there is subsurface liquid water, there may be life. Stay tuned.
This is an excerpt from COSMIC HORIZONS: ASTRONOMY AT THE CUTTING EDGE, edited by Steven Soter and Neil deGrasse Tyson, a publication of the New Press. © 2000 American Museum of Natural History.