26. ALH 84001: Bacteria Shapes
As a third line of evidence, Dr. McKay and co-workers identified objects and structures in ALH 84001 that they think are fossil martian bacteria. It may not be easy to recognize life on an alien world, so scientists look for proofs in many directions: objects with shapes and sizes that seem reasonable for life, evidence that the organisms grew, and evidence that they reproduced. Commonly, at least for Earth life, micro-organisms form colonies — clusters of cells close together. Sometimes, colonies are just the descendants of a single cell that landed in a spot and multiplied. Other times, colonies are organized groups of cells, helping each other grow and multiply.
This electron microscope image shows many rod-shaped objects on the surface of a carbonate grain in an ALH 84001 globule. The objects are mostly 0.01 micrometer long but are as large as 0.1 micrometer. The objects look like some kinds of micro-organisms on Earth, and could possibly be a fossil colony of micro-organisms that grew (and died) long ago on Mars. Most micro-organisms on Earth are no smaller than 0.5 micrometer in diameter, so these objects in 84001 are very small indeed. But some tentatively identified bacteria on Earth, the nanobacteria, are this same size. While new equipment and techniques are aiding scientists in their investigation of microscopic life forms, study of minute microscopic bacteria has, until recently, been constrained by technological limitations.