Allan Hills 84001 - Possible Biogenic Features

Possible 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 were interpreted as remains—in the form of 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.

Several tests for organic material have been performed on the meteorite and amino acids and polycyclic aromatic hydrocarbons (PAH) have been found. The debate over whether the organic molecules in the meteorite are in fact of exobiologic origin or are due to abiotic processes on Mars or contamination from the contact with Antarctic ice on Earth is still ongoing.

Early on, Ralph Harvey of Case Western Reserve University and Harry McSween of University of Tennessee reported evidence that the carbonate globules found in the meteorite were formed at high temperature (above 650°C) by volcanic or impact processes on Mars. At such high temperatures, it would be very unlikely that the morphology of the globules could have had any kind of biological origin. Later, however, the same authors published papers supporting a hypothesis in which the globules formed at low temperature from an aqueous solution. Most scientific papers published in the past 10 years now accept that carbonates on Mars formed this way.

Other initial skepticism towards the biogenic hypothesis focused on the idea that the nanometer-sized filaments were too small to contain RNA, but evidence continues to grow that nanobacteria do exist in nature. Furthermore, microbiologists have successfully cultured nanobacteria in the lab, with sizes within the range of at least some of the purported microfossils in ALH 84001.

Some experts argue that the biomorphs found in the meteorite are not indicative of life on Mars, but instead are caused by contamination by terrestrial biofilms. However, scientists at NASA argue that likely microbial terrestrial contamination found in other Martian meteorites do not resemble the texture of the biomorphs in ALH 84001. In particular, the biomorphs in 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 of Hernandez Engineering Inc. in Houston. 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, reasserted that there is "strong evidence that life may have existed on ancient Mars", after having reexamined the meteorite using more advanced analytical instruments now available, in light of the objections that had been made since the biogenic hypothesis for the biomorphs first had been put forward. Overall, the team concluded that:

None of the original features supporting our hypothesis for ALH84001 has either been discredited or has been positively ascribed to non-biologic explanations.

In addition, they argued that since the original paper was published, the biogenic hypothesis has been "further strengthened by the presence of abundant biomorphs in other Martian meteorites."