Allan Hills 84001

Allan Hills 84001

Meteorite fragment ALH84001
Type Achondrite
Clan Martian meteorite
Grouplet Orthopyroxenite
Composition Low-Ca Orthopyroxene, Chromite, Maskelynite, Fe-rich carbonate[1]
Shock stage B
Weathering grade A/B
Country Antarctica
Region Allan Hills, Far Western Icefield
Coordinates 76°55′13″S 156°46′25″E / 76.92028°S 156.77361°ECoordinates: 76°55′13″S 156°46′25″E / 76.92028°S 156.77361°E[1]
Observed fall No
Found date 1984
TKW 1930.9 g

Allan Hills 84001 (commonly abbreviated ALH84001[1]) 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). It made its way into headlines worldwide in 1996 when scientists announced that it might contain evidence for microscopic fossils of Martian bacteria based on carbonate globules observed.

History and description

ALH84001 on display at Smithsonian Museum of Natural History

This rock is considered to be one of the oldest pieces of the Solar System, proposed to have crystallized from molten rock 4.091 billion years ago.[2] Based on chemical analyses, it is thought to have originated on Mars[3][4] from a period when liquid water existed on the now barren planet's surface.[5][6]

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.[7] 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 meteorite impact about 17 million years ago[8] and fell on Earth roughly 13,000 years ago.[9] 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.[10][11] Other meteorites that have potential biological markings have generated less interest because they do not originate from a "wet" Mars. ALH84001 is the only meteorite collected from such a time period.[12]

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.[6]

Hypothetical biogenic features

On August 6, 1996,[12] 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.[13]

The electron microscope revealed chain structures in meteorite fragment ALH84001

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 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.[14]

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.[15]

David S. McKay at NASA argues that likely microbial terrestrial contamination found in other Martian meteorites do 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 do not.[16] 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.[17] David McKay says these results were obtained using unrealistically pure raw materials as a starting point,[12] 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."[17]

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."[16] However, the scientific consensus is that "morphology alone cannot be used unambiguously as a tool for primitive life detection."[18][19][20] Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.[18]

Of the various tests used, three have been shown to be particularly useful:[18] (1) the mm-scale morphology of the objects in question; (2) the carbon isotopic composition of organic matter associated with and/or comprising the fossil-like structures; and (3) the chemical (molecular) makeup of the fossil-like objects. In principle, therefore, the question of biogenicity can be easily answered. But in practice, the answer has proven elusive, primarily because of a lack of analytical techniques having sufficient power to provide the high resolution three-dimensional morphological information needed to definitively address the question, and an absence of means by which to directly link, in individual microscopic specimens, morphological information to elemental-isotopic and structural-molecular compositions.[18]

See also

Notes

  1. ↑ 1.0 1.1 1.2 "Meteoritical Bulletin Database: Allan Hills 84001".
  2. ↑ Lapen, T. J.; Brandon, A. D.; Debaille, V.; Beard, B. L.; Shafer, J. T.; Peslier, A. H. 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. Missing |last2= in Authors list (help)
  3. ↑ "Martian (OPX) Meteorites". The Meteoritical Society. Lunar And Planetary Institute. Retrieved 2014-05-07.
  4. ↑ "Information on the Allan Hills 84001". The Meteoritical Society. Lunar and Planetary Institute. Retrieved 2014-05-07.
  5. ↑ "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.
  6. ↑ 6.0 6.1 "Carbonates in the Martian meteorite Allan Hills 84001 formed at 18 ± 4 °C in a near-surface aqueous environment". PNAS.
  7. ↑ "Birthplace of famous Mars meteorite pinpointed". New Scientist. Retrieved March 18, 2006.
  8. ↑ "Evidence for ancient Martian life" (PDF).
  9. ↑ "How could ALH84001 get from Mars to Earth?". Lunar and Planetary Institute. LPI. 2014. Retrieved 2014-05-07.
  10. ↑ 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.
  11. ↑ 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.
  12. ↑ 12.0 12.1 12.2 Crenson, Matt (2006-08-06). "After 10 years, few believe life on Mars". Associated Press (on usatoday.com). Retrieved 2009-12-06.
  13. ↑ McKay, David S.; Gibson Jr, E. K. 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.
  14. ↑ McSween, H. Y. (1997). "Evidence for life in a martian meteorite?". GSA Today 7 (7): 1–7. PMID 11541665.
  15. ↑ Clinton, Bill (1996-08-07). "President Clinton Statement Regarding Mars Meteorite Discovery". NASA. Retrieved 2006-08-07.
  16. ↑ 16.0 16.1 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. doi:10.1016/j.gca.2009.05.064. Retrieved 2014-05-07.
  17. ↑ 17.0 17.1 "NASA - Press Release #J04-025". Nasa.gov. Retrieved 2012-03-29.
  18. ↑ 18.0 18.1 18.2 18.3 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. Retrieved 2013-01-15. It is concluded that "morphology cannot be used unambiguously as a tool for primitive life detection."
  19. ↑ Agresti; House; Jögi; Kudryavstev; McKeegan; Runnegar; Schopf; Wdowiak (3 December 2008). "Detection and geochemical characterization of Earth’s earliest life". NASA Astrobiology Institute (NASA). Retrieved 2013-01-15.
  20. ↑ 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. doi:10.1016/j.precamres.2007.04.009. Retrieved 2013-01-15.

References

Further reading

External links

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