Allan Hills 84001

Hypothetical biogenic features

On August 6, 1996, a team of researchers led by NASA scientists including lead author David S. McKay announced that the meteorite may contain trace evidence of life from Mars. This was published as an article in Science a few days later. Under a scanning electron microscope, structures were visible that some scientists interpreted as fossils of bacteria-like lifeforms. The structures found on ALH84001 are 20–100 nanometres in diameter, similar in size to theoretical nanobacteria, but smaller than any cellular life known at the time of their discovery. If the structures had been fossilized lifeforms, as was proposed by the so-called biogenic hypothesis of their formation, they would have been 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 President of the United States Bill Clinton to make a formal televised announcement to mark the event.

McKay argued that likely microbial terrestrial contamination found in other Martian meteorites does not resemble the microscopic shapes in ALH84001. In particular, the shapes within ALH84001 look intergrown or embedded in the indigenous material, while likely contamination does 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. 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." Much of the scientific community disagreed with McKay.

In January 2010, a team of scientists at Johnson Space Center, including McKay, argued that since their original paper was published in November 2009, the biogenic hypothesis has been further supported by the discovery of three times the original amount of fossil-like data, including more "biomorphs" (suspected Martian fossils), inside two additional Martian meteorites, as well as more evidence in other parts of the Allan Hills meteorite itself. However, many in the scientific community have pointed out 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.

Features of ALH84001 that have been interpreted as suggesting the presence of microfossils include:

• The structures resemble some modern terrestrial bacteria and their appendages. Though some are much smaller than any known extant Earth microbes, others are of the order of 100–200 nm in size, within the size limits of Pelagibacter ubique, the most common bacterium on Earth, which ranges from 120 to 200 nm, as well as hypothetical nanobacteria. RNA organisms, which are expected to have lived on Earth during the time period when ALH84001 was ejected from Mars, may also have been as small or smaller than these structures, as modern RNA viruses and viroids are often as little as a few dozen nanometers. Some of the structures are even larger, 1–2 microns in diameter. The smallest structures are too small to contain all the systems required by modern life.
• Some of the structures resemble colonies and biofilms. However, there are many instances of morphologies that suggested life and were later shown to be due to inorganic processes.
• The meteorite contains magnetite crystals of the unusual rectangular prism type, and organized into domains all about the same size, indistinguishable from magnetite produced biologically on Earth and not matching any known non-biological magnetite that forms naturally on Earth. The magnetite is embedded in the carbonate. If found on Earth, it would be a very strong biosignature. However, in 2001, scientists were able to explain and produce carbonate globules containing similar magnetite grains through an inorganic process simulating conditions ALH84001 likely experienced on Mars.
• It contains polycyclic aromatic hydrocarbons (PAHs) concentrated in the regions containing the carbonate globules, and these have been shown to be indigenous. Other organics such as amino acids do not follow this pattern and are probably due to Antarctic contamination. However, PAHs are also regularly found in asteroids, comets and meteorites, and in deep space, all in the absence of life.

A later study in January 2022 concluded that ALH84001 did not contain Martian life; the discovered organic molecules were found to be associated with abiotic processes (i.e., "serpentinization and carbonation reactions that occurred during the aqueous alteration of basalt rock by hydrothermal fluids") produced on the very early Mars 4 billion years ago instead.

In popular culture

The 1996 The X-Files episode "Tunguska" was inspired by the Allan Hills 84001 announcement.

The 1997 film Contact used footage of Bill Clinton's television speech around the Allan Hills 84001, but edited to make it sound like he was discussing a search for extraterrestrial intelligence radio signal. The use of Clinton's Allan Hills 84001 speech led to a formal letter from White House Counsel Charles Ruff protesting against the use of Clinton's digitally composited appearance.

The 2001 mystery-thriller novel Deception Point by Dan Brown, about a discovered meteorite that seems to prove the existence of extraterrestrial life, was inspired by ALH84001.