Life on Mars: Difference between revisions
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Mars is of particular interest for the study of the origins of life because of its similarity to the early Earth. This is especially so since Mars has a cold climate and lacks [[plate tectonics]] or [[continental drift]], so it has remained almost unchanged since the end of the [[Hesperian]] period. At least two thirds of Mars's surface is more than 3.5 billion years old, and Mars may thus hold the best record of the prebiotic conditions leading to [[abiogenesis]], even if life does not or has never existed there.<ref>{{cite journal |doi=10.1029/RG027i002p00189 |title=The early environment and its evolution on Mars: Implication for life |date=1989 |last=McKay |first=Christopher P. |last2=Stoker |first2=Carol R. |journal=Reviews of Geophysics |volume=27 |issue=2 |pages=189–214|bibcode = 1989RvGeo..27..189M }}</ref><ref name="Fromproto">{{cite journal |bibcode=2007prpl.conf..929G |arxiv=astro-ph/0602008 |title=From Protoplanets to Protolife: The Emergence and Maintenance of Life |last=Gaidos |first=Eric |last2=Selsis |first2=Franck |date=2007 |pages=929–44 |journal=Protostars and Planets V}}</ref> In May 2017, evidence of the [[Earliest known life forms|earliest known life]] [[Evolutionary history of life#Colonization of land|on land]] on Earth may have been found in 3.48-billion-year-old [[geyserite]] and other related mineral deposits (often found around [[hot spring]]s and [[geyser]]s) uncovered in the [[Pilbara Craton]] of [[Western Australia]].<ref name="PO-20170509">{{cite news|author=Staff |title=Oldest evidence of life on land found in 3.48-billion-year-old Australian rocks |url=https://phys.org/news/2017-05-oldest-evidence-life-billion-year-old-australian.html |date=May 9, 2017 |work=[[Phys.org]] |accessdate=May 13, 2017 |deadurl=no |archiveurl=https://web.archive.org/web/20170510013721/https://phys.org/news/2017-05-oldest-evidence-life-billion-year-old-australian.html |archivedate=May 10, 2017 }}</ref><ref name="NC-20170509">{{cite journal|last1=Djokic |first1=Tara |last2=Van Kranendonk |first2=Martin J. |last3=Campbell |first3=Kathleen A. |last4=Walter |first4=Malcolm R. |last5=Ward |first5=Colin R. |title=Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits |url=https://www.nature.com/articles/ncomms15263 |date=May 9, 2017 |journal=[[Nature Communications]] |doi=10.1038/ncomms15263 |accessdate=May 13, 2017 |volume=8 |page=15263 |deadurl=no |archiveurl=https://web.archive.org/web/20170518082609/https://www.nature.com/articles/ncomms15263 |archivedate=May 18, 2017 |bibcode = 2017NatCo...815263D }}</ref> These findings may be helpful in deciding where best to search for [[Abiogenesis|early signs of life]] on the planet Mars.<ref name="PO-20170509" /><ref name="NC-20170509" />
On January 24, 2014, NASA reported that the [[Curiosity (rover)|''Curiosity'']] and [[Opportunity (rover)|''Opportunity'']] [[Mars rover|rovers]] started searching for evidence of past life, including a [[biosphere]] based on [[autotroph]]ic, [[chemotroph]]ic, or [[Lithotroph#Chemolithotrophs|chemolithoautotrophic]] [[microorganism]]s, as well as ancient water, including [[Lacustrine plain|fluvio-lacustrine environments]] ([[plain]]s related to ancient rivers or lakes) that may have been [[Planetary habitability|habitable]].<ref name="SCI-20140124a">{{cite journal|last=Grotzinger |first=John P. |title=Introduction to Special Issue - Habitability, Taphonomy, and the Search for Organic Carbon on Mars |url=http://www.sciencemag.org/content/343/6169/386 |journal=[[Science (journal)|Science]] |date=January 24, 2014 |volume=343 |issue=6169 |pages=386–387 |doi=10.1126/science.1249944 |bibcode=2014Sci...343..386G |pmid=24458635 |deadurl=no |archiveurl=https://web.archive.org/web/20140128113800/http://www.sciencemag.org/content/343/6169/386 |archivedate=January 28, 2014 }}</ref><ref name="SCI-20140124special">{{cite journal|authors=Various |title=Special Issue - Table of Contents - Exploring Martian Habitability |url=http://www.sciencemag.org/content/343/6169.toc#SpecialIssue |date=January 24, 2014 |journal=[[Science (journal)|Science]] |volume=343 |number=6169 |pages=345–452 |deadurl=no |archiveurl=https://web.archive.org/web/20140129042127/http://www.sciencemag.org/content/343/6169.toc |archivedate=January 29, 2014 }}</ref><ref name="SCI-20140124">{{cite journal|authors=Various |title=Special Collection - Curiosity - Exploring Martian Habitability |url=http://www.sciencemag.org/site/extra/curiosity/ |date=January 24, 2014 |journal=[[Science (journal)|Science]] |deadurl=no |archiveurl=https://web.archive.org/web/20140128102653/http://www.sciencemag.org/site/extra/curiosity/ |archivedate=January 28, 2014 }}</ref><ref name="SCI-20140124c">{{cite journal|title=A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars |url=http://www.sciencemag.org/content/343/6169/1242777 |date=January 24, 2014 |journal=[[Science (journal)|Science]] |volume=343 |issue=6169, number 6169 |pages=1242777 |doi=10.1126/science.1242777 |last=Grotzinger |first=J. P. |last2=Sumner |first2=D. Y. |last3=Kah |first3=L. C. |last4=Stack |first4=K. |last5=Gupta |first5=S. |last6=Edgar |first6=L. |last7=Rubin |first7=D. |last8=Lewis |first8=K. |last9=Schieber |first9=J. |last10=Mangold |first10=N. |last11=Milliken |first11=R. |last12=Conrad |first12=P. G. |last13=Desmarais |first13=D. |last14=Farmer |first14=J. |last15=Siebach |first15=K. |last16=Calef |first16=F. |last17=Hurowitz |first17=J. |last18=McLennan |first18=S. M. |last19=Ming |first19=D. |last20=Vaniman |first20=D. |last21=Crisp |first21=J. |last22=Vasavada |first22=A. |last23=Edgett |first23=K. S. |last24=Malin |first24=M. |last25=Blake |first25=D. |last26=Gellert |first26=R. |last27=Mahaffy |first27=P. |last28=Wiens |first28=R. C. |last29=Maurice |first29=S. |last30=Grant |first30=J. A. |display-authors=9 |bibcode=2014Sci...343G.386A |pmid=24324272 |deadurl=no |archiveurl=https://web.archive.org/web/20140214033931/http://www.sciencemag.org/content/343/6169/1242777 |archivedate=February 14, 2014 }}</ref>
The search for Life on Mars past and present is the first of [[NASA]]’S four science goals<ref>Hamilton, V.E., Rafkin, S., Withers, P., Ruff, S., Yingst, R.A., Whitley, R., Center, J.S., Beaty, D.W., Diniega, S., Hays, L. and Zurek, R., [https://mepag.jpl.nasa.gov/reports/MEPAG%20Goals_Document_2015_v18_FINAL.pdf Mars Science Goals, Objectives, Investigations, and Priorities: 2015 Version]</ref>:
{{quote|Goal I: determine if Mars ever supported life
* Objective A: determine if environments having high potential for prior habitability and preservation of biosignatures contain evidence of past life.
* Objective B: determine if environments with high potential for current habitability and expression of biosignatures contain evidence of extant life."
}}
This includes the search for evidence of [[Planetary habitability|habitability]], [[taphonomy]] (related to [[fossils]]), and [[organic carbon]] on the planet [[Mars]].<ref name="SCI-20140124a" />
In July 2017, researchers reported that the surface on the planet Mars may be more toxic to [[microorganism]]s, especially a common terrestrial type, ''[[Bacillus subtilis]]'', than thought earlier. This is based on studies with [[perchlorates]], common on Mars, in a simulated Martian [[ultraviolet]] atmosphere.<ref name="SM-20170706">{{cite news |last=Daley |first=Jason |title=Mars Surface May Be Too Toxic for Microbial Life - The combination of UV radiation and perchlorates common on Mars could be deadly for bacteria |url=http://www.smithsonianmag.com/smart-news/mars-surface-may-be-toxic-bacteria-180963966/ |date=6 July 2017 |work=[[Smithsonian (magazine)|Smithsonian]] |accessdate=8 July 2017 }}</ref><ref name="NAT-20170706">{{cite journal|last1=Wadsworth |first1=Jennifer |last2=Cockell |first2=Charles S. |title=Perchlorates on Mars enhance the bacteriocidal effects of UV light |url=https://www.nature.com/articles/s41598-017-04910-3 |date=6 July 2017 |work=[[Scientific Reports]] |volume=7 |number=4662 |doi=10.1038/s41598-017-04910-3 |accessdate=8 July 2017 |deadurl=no |archiveurl=https://web.archive.org/web/20170706185518/http://www.nature.com/articles/s41598-017-04910-3 |archivedate=July 6, 2017 |bibcode = 2017NatSR...7.4662W }}</ref>
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== Forward contamination ==
{{Details|Planetary protection}}
[[Planetary protection]] of Mars aims to prevent biological contamination of the planet.<ref name="strategy">{{cite book | author1=Committee on an Astrobiology Strategy for the Exploration of Mars | author2=National Research Council | date=2007 | chapter=Planetary Protection for Mars Missions | chapterurl=http://www.nap.edu/openbook.php?record_id=11937&page=95 | pages=95–98 | title=An Astrobiology Strategy for the Exploration of Mars | publisher=The National Academies Press | isbn=978-0-309-10851-5 }}</ref>
The prime concern of hardware contaminating Mars derives from incomplete spacecraft sterilization of some hardy terrestrial bacteria ([[extremophiles]]) despite best efforts.<ref name="Beaty" /><ref name="Debus">{{cite journal | bibcode=2005AdSpR..35.1648D | title=Estimation and assessment of Mars contamination | last=Debus | first=A. | volume=35 | date=2005 | pages=1648–53 | journal=Advances in Space Research | doi=10.1016/j.asr.2005.04.084 | pmid=16175730 | issue=9}}</ref> Hardware includes landers, crashed probes, end-of-mission disposal of hardware, and hard landing of entry, descent, and landing systems. This has prompted research on survival rates of [[Radioresistance|radiation-resistant microorganisms]] including the species ''[[Deinococcus radiodurans]]'' and genera ''[[Brevundimonas]]'', ''[[Rhodococcus]]'', and ''[[Pseudomonas]]'' under simulated Martian conditions.<ref name="Planetary protection – radiodurans">{{cite journal |bibcode=2010AsBio..10..717D |title=Low-Temperature Ionizing Radiation Resistance of Deinococcus radiodurans and Antarctic Dry Valley Bacteria |last=Dartnell |first=Lewis R. |last2=Hunter |first2=Stephanie J. |last3=Lovell |first3=Keith V. |last4=Coates |first4=Andrew J. |last5=Ward |first5=John M. |volume=10 |date=2010 |pages=717–32 |journal=Astrobiology |doi=10.1089/ast.2009.0439 |pmid=20950171 |issue=7}}</ref> Results from one of these experimental irradiation experiments, combined with previous radiation modeling, indicate that ''[[Brevundimonas]]'' sp. MV.7 emplaced only 30 cm deep in Martian dust could survive the cosmic radiation for up to 100,000 years before suffering 10⁶ population reduction.<ref name="Planetary protection – radiodurans" /> Surprisingly, the diurnal Mars-like cycles in temperature and relative humidity affected the viability of ''Deinococcus radiodurans'' cells quite severely.<ref name="simulation">{{cite journal |bibcode=2007AdSpR..40.1672D |title=Simulation of the environmental climate conditions on martian surface and its effect on ''Deinococcus radiodurans'' | last=de la Vega | first=U. Pogoda | last2=Rettberg | first2=P. | last3=Reitz | first3=G. |volume=40 |date=2007 |pages=1672–7 |journal=Advances in Space Research |doi=10.1016/j.asr.2007.05.022 |issue=11}}</ref> In other simulations, ''Deinococcus radiodurans'' also failed to grow under low atmospheric pressure, under 0 °C, or in the absence of oxygen.<ref name="serratia">{{cite journal | title=Growth of Serratia liquefaciens under 7 mbar, 0°C, and CO2-Enriched Anoxic Atmospheres | journal=Astrobiology | date=February 2013 | first=Andrew C. | last=Schuerger | first2=Richard | last2=Ulrich | first3=Bonnie J. | last3=Berry | first4=Wayne L. | last4=Nicholson. | volume=13 | issue=2 | pages=115–131 | doi=10.1089/ast.2011.0811 | url=http://online.liebertpub.com/doi/full/10.1089/ast.2011.0811 | bibcode=2013AsBio..13..115S | pmid=23289858 | pmc=3582281}}</ref>
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