Weathering grade A/B | Shock stage B | |
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Composition Low-Ca Orthopyroxene, Chromite, Maskelynite, Fe-rich carbonate | ||
Allan Hills 84001 (commonly abbreviated ALH84001) 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), ALH84001 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).
Contents
- History and description
- Hypothetical biogenic features
- List
- RNA World Hypothesis and other proposals that could be consistent with such small structures as cells
- References
The meteorite is best known for gaining intense media attention in 1996 when a group of scientists claimed to have found evidence for microscopic fossils of Martian bacteria in it, culminating in then U.S. president Bill Clinton giving a speech about the potential discovery. These claims were controversial from the beginning, and the wider scientific community ultimately rejected the hypothesis once all the unusual features in the meteorite had been explained without requiring life to be present. Despite there being no convincing evidence of Martian life, the initial paper and scientific and public attention caused by it are considered to be turning points in the history of the developing science of astrobiology.
History and description
This rock is considered to be one of the oldest Martian meteorites, 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 existed on the now arid planet's surface.
In September 2005, Vicky Hamilton of the University of Hawaii at Manoa presented an analysis of the origin of ALH84001 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 ALH84001 was blasted off from the surface of Mars by a meteor impact about 17 million years ago and fell on 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. Other meteorites that have potential biological markings have generated less interest because they do not contain rock from a "wet" Mars. ALH84001 is the only meteorite collected from such a time period.
In October 2011 it was reported that isotopic analysis indicated that the carbonates in ALH84001 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, ALH84001 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 S. 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 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 the President of the United States Bill Clinton to make a formal televised announcement to mark the event.
David S. McKay at NASA argued that likely microbial terrestrial contamination found in other Martian meteorites does not resemble the microscopic shapes in the ALH84001. In particular, the shapes within the 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. 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." The rest of the scientific community disagreed with McKay.
In November 2009, a team of scientists at Johnson Space Center, including 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.
List
These are some of the main features that have been interpreted as suggesting the presence of microfossils.
RNA World Hypothesis and other proposals that could be consistent with such small structures as cells
A workshop on the limitations of size of microbes in 1999 found that though modern nanobacteria can't be smaller in volume than the interior of a sphere of diameter 250 ± 50 nm, primitive microorganisms based on a single-polymer system could be. They cite the example of RNA based life, with ribozymes (catalytic RNA) taking the place of the much larger ribosomes as enzymes for cell replication. The cells also wouldn't have proteins and would have no need to translate DNA to mRNA. The result could be cells as small as a sphere 50 nm in diameter. These RNA World cells are one of the suggestions made to bridge the gap between abiotic chemistry and modern cells.
The main suggestions they gave that could explain the small size are that
The RNA World hypothesis is still under consideration, with Steven A. Benner (notable as the first person to synthesize a gene amongst many other accomplishments) and Paul Davies supporting it in a chapter in the book "Frontiers of Astrobiology" published in 2012 by the Cambridge University Press, in the chapter "Towards a Theory of Life"