Magnetofossils from Ancient Mars: a Robust Biosignature in the Martian Meteorite ALH84001

Evidence of biogenic activity on Mars has profound scientific implications for our understanding of the origin of life on Earth and the presence and diversity of life within the Cosmos. Analysis of the Martian meteorite Allan Hills 84001 (ALH84001) revealed several lines of evidence that has led some investigators to suggest that microbial life existed on Mars approximately 4 billion years ago (45). One of the strongest lines of evidence is the presence of tens-of-nanometer-size magnetite (Fe3O4) crystals found within carbonate globules and their associated rims in the meteorite (57, 58). Approximately one-quarter of these magnetites have remarkable morphological and chemical similarities to magnetite particles produced by magnetotactic bacteria, which occur in aquatic habitats on Earth. Moreover, these types of magnetite particles are not known or expected to be produced by abiotic means either through geological processes or synthetically in the laboratory. We have therefore argued that these Martian magnetite crystals are in fact magnetofossils (57, 58). If this is true, such magnetofossils would constitute evidence of the oldest life forms known. In this respect, we note there is now considerable uncertainty concerning when the earliest terrestrial life forms existed. Until recently, results from the ~3.5-billion-year-old Apex cherts of the Warrawoona group in western Australia held this record (52), although this work is now in question (12).