24. Magnetite Grains in ALH 84001 Carbonate
The first kind of evidence for fossil life in ALH 84001 is some very small mineral grains inside the carbonate globules, as in this image, which is an extreme close-up of one of the dark bands in a carbonate globule (slide #22), made with a transmission electron microscope. The scale bar in the image is 20 nanometers long, or 20 millionths of a millimeter. The dark spots are grains of the iron oxide mineral magnetite (Fe3O4). Light does not go through magnetite, so the areas with these grains appear dark (slide #22) through a light microscope.
McKay and his co-workers showed that these magnetite grains (and similar iron sulfide grains) in ALH 84001 are similar to some produced by bacteria on Earth, so they suggest that these mineral grains in ALH 84001 may have been produced by martian bacteria. To show that the mineral grains might be products of martian bacteria, McKay and co- workers present evidence that the mineral grains did form on Mars, and that they have chemical compositions, crystal structures, sizes, and shapes like biologically produced grains on Earth. There is little doubt that the little magnetite and iron sulfide grains did not form on Earth, and so are martian. And there is also little doubt that grains of these minerals have been (and are being) made by bacteria on Earth. For instance, some bacteria make little magnetite grains, which are magnetic, as compasses to align themselves with the Earth's magnetic field. They use the magnetic field to aim themselves up or down.
However, similar mineral grains can grow without any action from living organisms. No one knows yet whether these iron oxide and sulfide minerals can grow in carbonate minerals without assistance from life.
Treiman (Lunar and Planetary Institute)