28. ALH 84001 Possible Fossil Object
This image, taken with a scanning electron microscope, is one of the most striking examples of a possible fossil micro-organism that McKay and co-workers have found so far. The rod in the center of the image, only about two ten-thousandths of a millimeter long (0.2 micrometers), appears to be segmented as if it were a filament composed of many separate cells. This rod looks like some very small bacteria found on Earth, and Dr. McKay and his co-workers interpreted it as a bacterium. It might also be a fragment of a bacterium, but some researchers have suggested that it is an inorganic mineral structure. Its position in the rock and close association with the martian organic chemicals suggests that the rod, too, formed on Mars. Slide #27 shows comparable bacteria from Earth.
Even this image, which looks very much like a micro-organism, has aroused controversy. Some mineral grains, including clays and asbestos, can have the same sizes and shapes as these possible martian bacteria. And there are questions about whether there really are bacteria on Earth as small as the segments in this filament (see slide #29).