Allan Hills 84001
|Allan Hills 84001|
Meteorite fragment ALH84001
|Composition||Low-Ca Orthopyroxene, Chromite, Maskelynite, Fe-rich carbonate|
|Region||Allan Hills, Far Western Icefield|
|Related media on Wikimedia Commons|
Allan Hills 84001 (commonly abbreviated ALH84001) is a fragment of a Martian meteorite that was found in the Allan Hills in Antarctica on December 27, 1984, by a team of American meteorite hunters from the ANSMET project. Like other members of the shergottite–nakhlite–chassignite (SNC) group of meteorites, ALH84001 is thought to have originated on Mars. However, it does not fit into any of the previously discovered SNC groups. Its mass upon discovery was 1.93 kilograms (4.3 lb).
In 1996, a group of scientists claimed to have found evidence for microscopic fossils of bacteria in the meteorite, suggesting that these organisms also originated on Mars. The claims immediately made headlines worldwide, 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 the enormous scientific and public attention caused by it are considered turning points in the history of the developing science of astrobiology.
History and description
ALH84001 is thought 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 during a period when liquid water existed on the 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 away from the surface of Mars by the impact of another meteor 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 dating. 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 originating from a time period during which Mars is suspected to have supported liquid water.
In October 2011 it was reported that isotopic analysis indicated that the carbonates in ALH84001 were precipitated at a temperature of 18 °C (64 °F) 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, a team of researchers led by NASA scientists including lead author David S. McKay published an article in Science claiming that the meteorite may contain trace evidence of life from Mars. 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." The rest 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, 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.
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 hypothetical nanobacteria) and some are 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.
RNA world hypothesis
In 1999, a workshop on the size limitations imposed upon microbes by the requirements of life-sustaining processes found that though modern nanobacteria cannot be smaller in volume than the interior of a sphere of diameter 250 ± 50 nm, primitive microorganisms based on a single-polymer system could have been. They[who?] cite the example of RNA-based life, with ribozymes (catalytic RNA) taking the place of the much larger ribosomes as enzymes for cell replication. Such cells also need not 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 cells are one of the suggestions made by the RNA world hypothesis to bridge the gap between abiotic chemistry and modern cells.
- Cells could shrink after death;
- The fossils could be remains of fragments of larger organisms;
- They could be pathogens or symbiotes which depend on a host;
- They could live in consortia of smaller cells unable to survive independently on their own;
- They could be based on biological systems different from the ones we understand (such as the RNA world cells).
The RNA world hypothesis is still heavily debated by the scientific community, with chemist Steven A. Benner and physicist Paul Davies supporting it in the book Frontiers of Astrobiology, in the chapter "Towards a Theory of Life".
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Orange carbonate grains, 100 to 200 microns across, indicate that the meteorite was once immersed in water.
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It is concluded that "morphology cannot be used unambiguously as a tool for primitive life detection."
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- Steven a. Benner; Paul Davies (15 Nov 2012). Chris Impey; Jonathan Lunine; José Funes (eds.). Towards a Theory of Life, in Frontiers of Astrobiology (Page 37). Cambridge University Press. ISBN 9781139789226.
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- Stephan, T.; Jessberger, E. K.; Heiss, C. H.; Rost, D. (2003). "TOF-SIMS analysis of polycyclic aromatic hydrocarbons in Allan Hills 84001". Meteorit. Planet. Sci. uni-muenster.de. pp. 109–116. Archived from the original on 2007-08-12.
- Sawyer, Kathy (2006). The Rock from Mars: A Detective Story on Two Planets. Random House. ISBN 1-4000-6010-9.
|Wikimedia Commons has media related to Allan Hills 84001.|
- "NASA – The ALH84001 Meteorite". jpl.nasa.gov.
- "Allan Treiman's dissection of ALH84001 literature for the non-specialist". lpi.usra.edu.