|Allan Hills 84001|
|Composition||Low-Ca Orthopyroxene, Chromite, Maskelynite, Fe-rich carbonate|
|Region||Allan Hills, Far Western Icefield|
|Related media on Wikimedia Commons|
Allan Hills 84001 (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 found features in the likeness of 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. Chemical analysis suggests that it originated on Mars when there was liquid water 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 2001 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, partly 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 a meteor about 17 million years ago, and fell on Earth about 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 when Mars may have had liquid surface 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.
In April 2020, researchers reported discovering nitrogen-bearing organics in Allan Hills 84001.
A later study in 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.
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." 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 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.
In popular culture
- "Meteoritical Bulletin Database: Allan Hills 84001".
- "President Clinton Statement Regarding Mars Meteorite Discovery". www2.jpl.nasa.gov. Retrieved 17 March 2018.
- Crenson, Matt (2006-08-06). "After 10 years, few believe life on Mars". Associated Press on USA Today. Retrieved 2009-12-06.
- Cassidy, William (2003). Meteorites, Ice, and Antarctica: A personal account. Cambridge: Cambridge University Press. pp. 122. ISBN 9780521258722.
- Lapen, T. J.; et al. (2010). "A Younger Age for ALH84001 and Its Geochemical Link to Shergottite Sources in Mars". Science. 328 (5976): 347–351. Bibcode:2010Sci...328..347L. doi:10.1126/science.1185395. PMID 20395507. S2CID 17601709.
- "Martian (OPX) Meteorites". The Meteoritical Society. Lunar And Planetary Institute. Retrieved 2014-05-07.
- "Information on the Allan Hills 84001". The Meteoritical Society. Lunar and Planetary Institute. Retrieved 2014-05-07.
- "The ALH84001 Meteorite". NASA. Jet Propulsion Laboratory. Retrieved 2014-05-07.
Orange carbonate grains, 100 to 200 microns across, indicate that the meteorite was once immersed in water.
- Eiler, John M.; Fischer, Woodward W.; Halevy, Itay (11 October 2011). "Carbonates in the Martian meteorite Allan Hills 84001 formed at 18 ± 4 °C in a near-surface aqueous environment". Proceedings of the National Academy of Sciences. PNAS. 108 (41): 16895–16899. doi:10.1073/pnas.1109444108. PMC 3193235. PMID 21969543.
- "Birthplace of famous Mars meteorite pinpointed". New Scientist. Retrieved March 18, 2006.
- "Evidence for ancient Martian life" (PDF).
- "How could ALH84001 get from Mars to Earth?". Lunar and Planetary Institute. LPI. 2014. Retrieved 2014-05-07.
- Nyquist, L. E.; Wiesmann, H.; Shih, C.-Y.; Dasch, J. (1999). "Lunar Meteorites and the Lunar Crustal SR and Nd Isotopic Compositions". Lunar and Planetary Science. 27: 971. Bibcode:1996LPI....27..971N.
- Borg, Lars; et al. (1999). "The Age of the Carbonates in Martian Meteorite ALH84001". Science. 286 (5437): 90–94. Bibcode:1999Sci...286...90B. doi:10.1126/science.286.5437.90. PMID 10506566.
- Koike, Mizuho; et al. (24 April 2020). "In-situ preservation of nitrogen-bearing organics in Noachian Martian carbonates". Nature Communications. 11 (1988): 1988. Bibcode:2020NatCo..11.1988K. doi:10.1038/s41467-020-15931-4. PMC 7181736. PMID 32332762.
- Dunn, Marcia (13 January 2022). "Study nixes Mars life in meteorite found in Antarctica". Associated Press. Retrieved 13 January 2022.
- Steele, A.; et al. (13 January 2022). "Organic synthesis associated with serpentinization and carbonation on early Mars". Science. 375 (6577): 172–177. Bibcode:2022Sci...375..172S. doi:10.1126/science.abg7905. PMID 35025630. S2CID 245933224. Retrieved 15 January 2022.
- McKay, David S.; Gibson, E. K. Jr.; et al. (1996). "Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001". Science. 273 (5277): 924–930. Bibcode:1996Sci...273..924M. doi:10.1126/science.273.5277.924. PMID 8688069. S2CID 40690489.
- McSween, H. Y. (1997). "Evidence for life in a martian meteorite?". GSA Today. 7 (7): 1–7. PMID 11541665.
- Clinton, Bill (1996-08-07). "President Clinton Statement Regarding Mars Meteorite Discovery". NASA. Retrieved 2006-08-07.
- Thomas-Keprta, K. L.; Clemett, S. J.; McKay, D. S.; Gibson, E. K.; Wentworth, S. J. (2009). "Origins of magnetite nanocrystals in Martian meteorite ALH84001" (PDF). Geochimica et Cosmochimica Acta. 73 (21): 6631–6677. Bibcode:2009GeCoA..73.6631T. doi:10.1016/j.gca.2009.05.064. Retrieved 2014-05-07.
- "NASA – Press Release #J04-025". Nasa.gov. Retrieved 2012-03-29.
- Covault, Craig (9 January 2010). "Three Martian meteorites triple evidence for Mars life". Spaceflight Now. Retrieved 2019-02-15.
- Garcia-Ruiz, Juan-Manuel Garcia-Ruiz (December 30, 1999). "Morphological behavior of inorganic precipitation systems – Instruments, Methods, and Missions for Astrobiology II". SPIE Proceedings. Instruments, Methods, and Missions for Astrobiology II. Proc. SPIE 3755: 74. doi:10.1117/12.375088. S2CID 84764520.
It is concluded that "morphology cannot be used unambiguously as a tool for primitive life detection."
- Agresti; House; Jögi; Kudryavstev; McKeegan; Runnegar; Schopf; Wdowiak (3 December 2008). "Detection and geochemical characterization of Earth's earliest life". NASA Astrobiology Institute. NASA. Archived from the original on 23 January 2013. Retrieved 2013-01-15.
- Schopf, J. William; Kudryavtsev, Anatoliy B.; Czaja, Andrew D.; Tripathi, Abhishek B. (28 April 2007). "Evidence of Archean life: Stromatolites and microfossils" (PDF). Precambrian Research. 158 (3–4): 141–155. Bibcode:2007PreR..158..141S. doi:10.1016/j.precamres.2007.04.009. Archived from the original (PDF) on 24 December 2012. Retrieved 15 January 2013.
- Vago, Jorge L.; et al. (2017). "Habitability on Early Mars and the Search for Biosignatures with the ExoMars Rover". Astrobiology. 17 (6–7): 471–510. Bibcode:2017AsBio..17..471V. doi:10.1089/ast.2016.1533. PMC 5685153. PMID 31067287.
- Brean, Joseph (January 14, 2022). "Not proof of life after all: 'Fossil' in famous Martian meteorite made by water, not aliens". National Post.
- Mittlefehldt, D. W. (1994). "ALH84001, a cumulate orthopyroxenite member of the SNC meteorite group". Meteoritics. lpi.usra.edu. pp. 214–221. Retrieved March 18, 2006.
- 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.