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Allan Hills 84001 (commonly abbreviated ALH 84001) is a meteorite that was found in Allan Hills, Antarctica on December 27, 1984 by a team of U.S. meteorite hunters from the ANSMET project. Like other members of the group of SNCs (shergottite, nakhlite, chassignite), ALH 84001 is thought to be from Mars. However, it does not fit into any of the previously discovered SNC groups. On discovery, its mass was 1.93 kilograms (4.3 lb). It made its way into headlines worldwide in 1996 when scientists announced that it might contain evidence for microscopic fossils of Martian bacteria based on carbonate globules observed.


History

This rock is theorized to be one of the oldest pieces of the solar system, proposed to have crystallized from molten rock 4.091 billion years ago. Based on chemical analyses, it is thought to have originated on Mars from a period when liquid water — a requirement for the presence of life — probably existed at the now barren planet's surface.

In September 2005, Vicky Hamilton of the University of Hawaii at Manoa presented an analysis of the origin of ALH 84001 using data from the Mars Global Surveyor and Mars Odyssey spacecraft orbiting Mars. According to the analysis, Eos Chasma in the Valles Marineris canyon appears to be the source of the meteorite. The analysis was not conclusive, in part because it was limited to areas of Mars not obscured by dust.

The theory holds that ALH 84001 was shocked and broken by one or more meteorite impacts on the surface of Mars some 3.9 to 4.0 billion years ago, but remained on the planet. It was later blasted off from the surface in a separate impact about 15 million years ago and impacted Earth roughly 13,000 years ago. These dates were established by a variety of radiometric dating techniques, including samarium-neodymium (Sm-Nd), rubidium-strontium (Rb-Sr), potassium-argon (K-Ar), and carbon-14.

It is hypothesized that ALH 84001 originated from a time period during which liquid water may have existed on Mars. Other meteorites that have potential biological markings have generated less interest because they do not originate from a "wet" Mars. ALH 84001 is the only meteorite collected from such a time period.

In October 2011 it was reported that isotopic analysis indicated that the carbonates in ALH 84001 were precipitated at a temperature of 18°C with water and carbon dioxide from the Martian atmosphere. The carbonate carbon and oxygen isotope ratios imply deposition of the carbonates from a gradually evaporating subsurface water body, probably a shallow aquifer meters or tens of meters below the surface.


Hypothetical biogenic features

On August 6, 1996 ALH 84001 became newsworthy when it was claimed that the meteorite may contain evidence of traces of life from Mars, as published in an article in Science by David McKay of NASA.

Under the scanning electron microscope structures were revealed that some scientists interpreted as fossils of bacteria-like lifeforms. The structures found on ALH 84001 are 20-100 nanometres in diameter, similar in size to theoretical nanobacteria, but smaller than any known cellular life at the time of their discovery. If the structures are in fact fossilized lifeforms, as proposed by the so-called biogenic hypothesis of their formation, they would be the first solid evidence of the existence of extraterrestrial life, aside from the chance of their origin being terrestrial contamination.

The announcement of possible extraterrestrial life caused considerable controversy. When the discovery was announced many immediately conjectured that the fossils were the first true evidence of extraterrestrial life—making headlines around the world, and even prompting the President of the United States Bill Clinton to make a formal televised announcement to mark the event. David McKay at NASA argues that likely microbial terrestrial contamination found in other Martian meteorites do not resemble the microscopic shapes in the ALH 84001. In particular, the shapes within the ALH 84001 look intergrown or embedded in the indigenous material, while likely contamination do not. While it has not yet conclusively been shown how the features in the meteorite were formed, similar features have been recreated in the lab without biological inputs by a team led by D.C. Golden. David McKay says these results were obtained using unrealistically pure raw materials as a starting point, and "will not explain many of the features described by us in ALH84001." According to McKay, a plausible inorganic model "must explain simultaneously all of the properties that we and others have suggested as possible biogenic properties of this meteorite."

In November 2009, a team of scientists at Johnson Space Center, including David McKay, argued that since their original paper was published, the biogenic hypothesis has been "further strengthened by the presence of abundant fossil-like structures in other Martian meteorites." However, the scientific consensus is that "morphology alone cannot be used unambiguously as a tool for primitive life detection." Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.

Of the various tests used, three have been shown to be particularly useful:
- the mm-scale morphology of the objects in question;
- the carbon isotopic composition of organic matter associated with and/or comprising the fossil-like structures; and
- the chemical (molecular) makeup of the fossil-like objects. In principle, therefore, the question of biogenicity can be easily answered. But in practice, the answer has proven elusive, primarily because of a lack of analytical techniques having sufficient power to provide the high resolution three-dimensional morphological information needed to definitively address the question, and an absence of means by which to directly link, in individual microscopic specimens, morphological information to elemental-isotopic and structural-molecular compositions.