Allan Hills 84001: Difference between revisions
→Hypothetical biogenic features
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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<ref>Allan H. Treiman, [https://www.lpi.usra.edu/lpi/meteorites/life.html Fossil Life in ALH 84001?]</ref>, 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/>
===List===
These are some of the main features that have been interpreted as suggesting the presence of life. Any hypothesis about ALH84001 has to explain them.
* Some of the structures resemble colonies and biofilms.<ref name="impact" /> However, there are many instances of morphologies that suggested life and were later shown to be due to inorganic processes.<ref name="impact" />▼
* 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 nonbiological magnetite that forms naturally on Earth.<ref name="impact" /> 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 experienced on Mars.<ref name="disbelief" />▼
* 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 found in asteroids, comets and meteorites, and in deep space, all in the absence of life.<ref name="disbelief"/><ref>[http://online.liebertpub.com/doi/pdfplus/10.1089/ast.2016.1533 Habitability on Early Mars and the Search for Biosignatures with the ExoMars Rover]. Astrobiology journal. Volume 17, Numbers 6 and 7, page 485. 2017 Mary Ann Liebert, Inc. DOI: 10.1089/ast.2016.1533</ref>▼
* The meteorite originates from a time on Mars 4.1 billion years ago<ref name="age3"/> when conditions for life on Mars were probably widespread, and carbonate globules in the meteorite suggest that it has been altered by water at low temperatures. <ref name="impact" />
* The structures resemble modern terrestrial bacteria and their appendages. Though some are smaller than any present day Earth microbes, others are of the order of 100 - 200 nm in size, within the size limits of [[Nanobacteria|nanobacteria]] and some are up to 1 to 2 microns in diameter.<ref name="impact" />.
* The smallest ones are too small to contain all the machinery of modern life <ref name="disbelief" />. However, 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, for instance, RNA based, with [[Ribozymes | ribozymes]] (catalytic RNA) taking the place of [[Ribosomes | ribosomes]] as enzymes for cell replication, could be as small as a sphere 50 nm in diameter. Cells could also shrink after death, or the fossils could be remains of fragments of larger organisms, or they could be pathogens or symbiotes which depend on a host, or they could live in consortia of smaller cells unable to survive independently on their own, or they could be based on biological systems different from the ones we understand. <ref>{{cite book|title=Size Limits of Very Small Microorganisms: Proceedings of a Workshop (1999)|date=1999|url=https://www.nap.edu/read/9638/chapter/2}}</ref>
▲* Some of the structures resemble colonies and biofilms.<ref name="impact" />. However
▲* The meteorite contains
▲* 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
* Evidence from isotopes of carbon originally favoured a biological origin.<ref name="impact" />. However on further analysis the findings were less clear, and may indicate an extra terrestrial origin but if so the organics could also be of meteoritic or cometary origin as for the PAHs.<ref>{{cite book|last1=By K. E. Peters, C. C. Walters, J. M. Moldowan|title=The Biomarker Guide: Volume 2, Biomarkers and Isotopes in Petroleum Systems|url=https://books.google.co.uk/books?id=RjcgAwAAQBAJ&pg=PA982#v=onepage&q&f=false}}</ref>
===RNA world hypothesis===
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