page_title: Life on Mars
section_title: ALH84001
hierarchical_section_title: Life on Mars / Possible biosignatures / Meteorites / ALH84001
caption_attribution_description: Español: Vista microscópica de la estructura interna del ALH84001 English: Structures on ALH84001 meteorite Français&#160;: Un fragment de la météorite w:ALH84001. Il révèle par w:microscopie électronique à balayage des structures ressemblant à des w:bactéries, possibles formes de w:vie sur Mars. voir aussi&#160;: Category:Alien et Category:Astrobiology
context_page_description: The possibility of life on Mars is a subject of huge interest in astrobiology due to its proximity and similarities to Earth. To date, no proof has been found of past or present life on Mars. Cumulative evidence shows that during the ancient Noachian time period, the surface environment of Mars had liquid water and may have been habitable for microorganisms. The existence of habitable conditions does not necessarily indicate the presence of life.
  Scientific searches for evidence of life began in the 19th century, and continue today via telescopic investigations and deployed probes. While early work focused on phenomenology and bordered on fantasy, the modern scientific inquiry has emphasized the search for water, chemical biosignatures in the soil and rocks at the planet's surface, and biomarker gases in the atmosphere.
  Mars is of particular interest for the study of the origins of life because of its similarity to the early Earth. This is especially so since Mars has a cold climate and lacks plate tectonics or continental drift, so has remained almost unchanged since the end of the Hesperian period.
context_section_description: In 1996, the Martian meteorite ALH84001, a specimen that is much older than the majority of Martian meteorites that have been recovered so far, received considerable attention when a group of NASA scientists led by David S. McKay reported microscopic features and geochemical anomalies that they considered to be best explained by the rock having hosted Martian bacteria in the distant past. Some of these features resembled terrestrial bacteria, aside from their being much smaller than any known form of life. Much controversy arose over this claim, and ultimately all of the evidence McKay's team cited as evidence of life was found to be explainable by non-biological processes. Although the scientific community has largely rejected the claim ALH 84001 contains evidence of ancient Martian life, the controversy associated with it is now seen as a historically significant moment in the development of exobiology.
caption: An electron microscope reveals bacteria-like structures in meteorite fragment ALH84001
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