Editing Miller–Urey experiment (section)

== Early Earth's prebiotic atmosphere ==
{{See also|Prebiotic atmosphere}}While there is a lack of geochemical observations to constrain the exact composition of the prebiotic atmosphere, recent models point to an early "weakly reducing" atmosphere; that is, early Earth's atmosphere was likely dominated by CO<sub>2</sub> and N<sub>2</sub> and not CH<sub>4</sub> and NH<sub>3</sub> as used in the original Miller–Urey experiment.<ref>{{Cite journal |last1=Zahnle |first1=K. |last2=Schaefer |first2=L. |last3=Fegley |first3=B. |date=2010-10-01 |title=Earth's Earliest Atmospheres |journal=Cold Spring Harbor Perspectives in Biology |language=en |volume=2 |issue=10 |pages=a004895 |doi=10.1101/cshperspect.a004895 |issn=1943-0264 |pmc=2944365 |pmid=20573713}}</ref><ref name="Catling-2017">{{Cite book |last1=Catling |first1=David C. |url=https://www.cambridge.org/core/books/atmospheric-evolution-on-inhabited-and-lifeless-worlds/CB3EE1D3F18A1DB234342E1FF410FC61 |title=Atmospheric Evolution on Inhabited and Lifeless Worlds |last2=Kasting |first2=James F. |date=2017 |publisher=Cambridge University Press |isbn=978-0-521-84412-3 |location=Cambridge |doi=10.1017/9781139020558}}</ref> This is explained, in part, by the chemical composition of volcanic outgassing. Geologist [[William Walden Rubey|William Rubey]] was one of the first to compile data on gases emitted from modern volcanoes and concluded that they are rich in CO<sub>2</sub>, H<sub>2</sub>O, and likely N<sub>2</sub>, with varying amounts of H<sub>2</sub>, [[sulfur dioxide]] (SO<sub>2</sub>), and H<sub>2</sub>S.<ref name="Catling-2017" /><ref>{{Citation |last=Rubey |first=W. W. |title=Development of the Hydrosphere and Atmosphere, with Special Reference to Probable Composition of the Early Atmosphere |date=1955 |url=https://doi.org/10.1130/SPE62-p631 |work=Geological Society of America Special Papers |volume=62 |pages=631–650 |access-date=2023-11-15 |doi=10.1130/spe62-p631}}</ref> Therefore, if the redox state of [[Earth's mantle]] — which dictates the composition of outgassing – has been constant since [[Formation of Earth|formation]], then the atmosphere of early Earth was likely weakly reducing, but there are some arguments for a more-reducing atmosphere for the first few hundred million years.<ref name="Catling-2017" />

While the prebiotic atmosphere could have had a different redox condition than that of the Miller–Urey atmosphere, the modified Miller–Urey experiments described in the above section demonstrated that amino acids can still be abiotically produced in less-reducing atmospheres under specific geochemical conditions.<ref name="bada20132" /><ref name="Miller-1983" /><ref name="Cleaves-2008" /> Furthermore, harkening back to Urey's original hypothesis of a "[[Impact event|post-impact]]" reducing atmosphere,<ref name="Urey-1952" /> a recent atmospheric modeling study has shown that an iron-rich impactor with a minimum mass around 4×10<sup>20</sup> – 5×10<sup>21</sup> kg would be enough to transiently reduce the entire prebiotic atmosphere, resulting in a Miller-Urey-esque H<sub>2</sub>-, CH<sub>4</sub>-, and NH<sub>3</sub>-dominated atmosphere that persists for millions of years.<ref name="Wogan-2023" /> Previous work has estimated from the [[Lunar craters|lunar cratering record]] and composition of Earth's mantle that between four and seven such impactors reached the Hadean Earth.<ref name="Zahnle-2020" /><ref name="Wogan-2023" /><ref>{{Cite journal |last1=Marchi |first1=S. |last2=Bottke |first2=W. F. |last3=Elkins-Tanton |first3=L. T. |last4=Bierhaus |first4=M. |last5=Wuennemann |first5=K. |last6=Morbidelli |first6=A. |last7=Kring |first7=D. A. |date=2014 |title=Widespread mixing and burial of Earth's Hadean crust by asteroid impacts |url=https://www.nature.com/articles/nature13539 |journal=Nature |language=en |volume=511 |issue=7511 |pages=578–582 |doi=10.1038/nature13539 |pmid=25079556 |bibcode=2014Natur.511..578M |s2cid=205239647 |issn=1476-4687}}</ref>
[[File:Three_phases_of_atmospheric_evolution_after_a_large_asteroid_impact_on_the_Hadean_Earth.jpg|center|thumb|712x712px|Conceptual figure from Wogan et al. (2023)<ref name="Wogan-2023" /> depicting three stages phases of atmospheric chemistry after a large [[Impact event|asteroid impact]] on the [[Hadean|Hadean Earth]]. In phase 1, the impactor vaporizes the ocean, and H<sub>2</sub> is generated after iron delivered by the impactor reacts with hot steam. In phase 2, H<sub>2</sub> reacts with CO<sub>2</sub> to produce CH<sub>4</sub> while the atmosphere cools for thousands of years and steam condenses to an ocean. Phase 3 represents the Miller-Urey atmosphere that persists for millions of years, where N<sub>2</sub> and CH<sub>4</sub> photochemistry generates HCN. The atmosphere returns to a CO<sub>2</sub> and N<sub>2</sub> dominated atmosphere after H<sub>2</sub> escapes from Earth to space. From: [[doi:10.3847/PSJ/aced83|Nicholas F. Wogan ''et al'' 2023]] ''Planet. Sci. J.'' 4 169. Licensed under [[CC-BY 4.0]].]]
A large factor controlling the redox budget of early Earth's atmosphere is the rate of [[atmospheric escape]] of H<sub>2</sub> after Earth's formation. Atmospheric escape – common to young, [[Terrestrial planet|rocky planets]] — occurs when gases in the atmosphere have sufficient [[kinetic energy]] to overcome [[gravitational energy]].<ref name="Catling">Catling, D., & Kasting, J. (2017). Escape of Atmospheres to Space. In ''Atmospheric Evolution on Inhabited and Lifeless Worlds'' (pp. 129–168). Cambridge: Cambridge University Press. {{doi|10.1017/9781139020558.006}}</ref> It is generally accepted that the timescale of hydrogen escape is short enough such that H<sub>2</sub> made up < 1% of the atmosphere of prebiotic Earth,<ref name="Catling-2017" /> but, in 2005, a [[Hydrodynamic escape|hydrodynamic model of hydrogen escape]] predicted escape rates two orders of magnitude lower than previously thought, maintaining a hydrogen [[mixing ratio]] of 30%.<ref>{{Cite journal |last1=Tian |first1=Feng |last2=Toon |first2=Owen B. |last3=Pavlov |first3=Alexander A. |last4=De Sterck |first4=H. |date=2005-05-13 |title=A Hydrogen-Rich Early Earth Atmosphere |url=https://www.science.org/doi/10.1126/science.1106983 |journal=Science |language=en |volume=308 |issue=5724 |pages=1014–1017 |doi=10.1126/science.1106983 |pmid=15817816 |bibcode=2005Sci...308.1014T |s2cid=262262244 |issn=0036-8075}}</ref> A hydrogen-rich prebiotic atmosphere would have large implications for Miller-Urey synthesis in the [[Hadean]] and [[Archean]], but later work suggests solutions in that model might have violated conservation of mass and energy.<ref name="Catling" /><ref>{{Cite journal |last1=Kuramoto |first1=Kiyoshi |last2=Umemoto |first2=Takafumi |last3=Ishiwatari |first3=Masaki |date=2013-08-01 |title=Effective hydrodynamic hydrogen escape from an early Earth atmosphere inferred from high-accuracy numerical simulation |url=https://www.sciencedirect.com/science/article/pii/S0012821X13003117 |journal=Earth and Planetary Science Letters |volume=375 |pages=312–318 |doi=10.1016/j.epsl.2013.05.050 |bibcode=2013E&PSL.375..312K |issn=0012-821X}}</ref> That said, during hydrodynamic escape, lighter molecules like hydrogen can "drag" heavier molecules with them through collisions, and recent modeling of [[xenon]] escape has pointed to a hydrogen atmospheric mixing ratio of at least 1% or higher at times during the Archean.<ref>{{Cite journal |last1=Zahnle |first1=Kevin J. |last2=Gacesa |first2=Marko |last3=Catling |first3=David C. |date=2019-01-01 |title=Strange messenger: A new history of hydrogen on Earth, as told by Xenon |url=https://www.sciencedirect.com/science/article/pii/S0016703718305349 |journal=Geochimica et Cosmochimica Acta |volume=244 |pages=56–85 |doi=10.1016/j.gca.2018.09.017 |arxiv=1809.06960 |bibcode=2019GeCoA.244...56Z |issn=0016-7037}}</ref>

Taken together, the view that early Earth's atmosphere was weakly reducing, with transient instances of highly-reducing compositions following large impacts is generally supported.<ref name="Wogan-2023" /><ref name="Urey-1952" /><ref name="Catling-2017" />