Allan Hills 84001: Difference between revisions
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[[File:ALH84001 structures.jpg|thumb|300px|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 {{Nowrap|20–100 [[nanometre]]s}} in diameter, similar in size to theoretical [[nanobacteria]], but smaller than any [[cellular life]] known at the time of their discovery. If the structures
The announcement of possible [[extraterrestrial life]] caused considerable controversy. When the discovery was announced, many
David S. McKay at NASA presented three main lines of evidence
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.<ref name="jsc2009">{{cite journal |title=Origins of magnetite nanocrystals in Martian meteorite ALH84001 |journal=Geochimica et Cosmochimica Acta |date=2009 |last= Thomas-Keprta |first=K. L. |last2=Clemett |first2=S. J. |last3=McKay |first3=D. S. |last4=Gibson |first4=E. K. |last5=Wentworth |first5=S. J. |volume=73 |issue=21 |pages=6631–6677 |url=http://www.nasa.gov/centers/johnson/pdf/403099main_GCA_2009_final_corrected.pdf |format=PDF |accessdate=2014-05-07 |doi=10.1016/j.gca.2009.05.064|bibcode = 2009GeCoA..73.6631T }}</ref> 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.<ref name="jsc2004">{{cite web|url=http://www.nasa.gov/centers/johnson/news/releases/2004/J04-025.html |title=NASA – Press Release #J04-025 |publisher=Nasa.gov |accessdate=2012-03-29}}</ref> David McKay says these results were obtained using unrealistically pure raw materials as a starting point,<ref name=disbelief/> 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."<ref name="jsc2004"/> The rest of the scientific community disagreed with McKay.<ref name=disbelief/>▼
* Detection of polycyclic aromatic hydrocarbons, or PAHs. These however are common in comets, asteroids and the Antarctic ice
* The shape of the blobs resembling bacteria. The problem here is that they are so small, down to 20 nm in diameter. The smallest microbes known today are less than 100 nm in diameter but not as small as this.
* Presence of carbonate, often used for microbes to make shells
* Extraordinarily small and pure crystals of magnetite.
[work in progress]
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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."{{Citation needed|date=March 2017}} <!-- <ref name="jsc2009"/> -->However, the scientific consensus is that "morphology alone cannot be used unambiguously as a tool for primitive life detection."<ref name=morphology>{{cite journal | title = Morphological behavior of inorganic precipitation systems – Instruments, Methods, and Missions for Astrobiology II | journal = SPIE Proceedings | date = December 30, 1999 | first = Juan-Manuel Garcia-Ruiz | volume = Proc. SPIE 3755 | pages = 74 | doi = 10.1117/12.375088 | url = http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=995013 | accessdate = 2013-01-15 | quote = It is concluded that "morphology cannot be used unambiguously as a tool for primitive life detection."| series = Instruments, Methods, and Missions for Astrobiology II | last1 = Garcia-Ruiz }}</ref><ref>{{cite news|author=Agresti|author2=House|author3=Jögi|author4=Kudryavstev|author5=McKeegan|author6=Runnegar|author7=Schopf|author8=Wdowiak|title=Detection and geochemical characterization of Earth’s earliest life|date=3 December 2008|publisher=NASA|url=http://astrobiology.ucla.edu/pages/res3e.html|work=NASA Astrobiology Institute|accessdate=2013-01-15|deadurl=yes|archiveurl=https://web.archive.org/web/20130123132429/http://astrobiology.ucla.edu/pages/res3e.html|archivedate=23 January 2013|df=}}</ref><ref>{{cite journal | title = Evidence of Archean life: Stromatolites and microfossils | journal = Precambrian Research | date = 28 April 2007 | first = J. William|last= Schopf |first2=Anatoliy B.|last2=Kudryavtsev|first3=Andrew D.|last3=Czaja|first4=Abhishek B.|last4=Tripathi| volume = 158 | issue = 3–4 | pages = 141–155 | url = http://www.cornellcollege.edu/geology/courses/greenstein/paleo/schopf_07.pdf | format = PDF | accessdate = 2013-01-15 | doi=10.1016/j.precamres.2007.04.009|bibcode = 2007PreR..158..141S }}</ref> Interpretation of morphology is notoriously subjective, and its use alone has led to numerous errors of interpretation.<ref name=morphology/>
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