Arsenic and Old Life
From my presentation in Buenos Aires: Meteorite ALH84001, in which signs of fossil life were discovered in 1996.
Last Thursday astrobiology researchers working at Mono Lake in California announced discovery of microorganisms that can thrive on arsenic. This discovery could widen the definition of organic chemistry. Other scientists have already questioned these results. The science of astrobiology has long met with skepticism.
In 1996 NASA scientists found signs of fossil life on this meteorite, picked up from Antarctica in 1984. The rock forming ALH84001 is over 3.5 billion years old. Martian life could be very old, possibly predating life on Earth. Fossils from another world may be one of the great scientific finds of our time.
McKay et al proposed that a combination of features in ALH84001 could best be explained by a biogenic hypothesis:
1) Carbonates formed at relatively low temperatures involving water
2) Possible microfossils (biomorphs) simlar to Earthly bacterial forms
3) Polyaromatic hydrocarbons (PAH) associated with carbonates
4) Nanophase magnetite (Fe304) similar to that produced by bacteria
From time to time others try to attack these discoveries. Recently a paper claimed that the carbonates could have formed by other means, implying that all the findings are invalid. The most convincing evidence, and subject of the most recent work, is nanophase magnetite. As seen in a microscope, magnetites in ALH84001 form crystalline patterns that could not be formed by simple heating. Such patterns are formed by magnetitic bacteria on Earth.
Science thrives on skepticism, and great discoveries are almost always questioned. Several lines of data still point to life on a Martian meteorite. The question may not be completely settled until we have new samples from Mars. Humans may have discovered what they have long wondered about, life from another world.