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UV Production of Methane from Surface and Sedimenting Idps On Moores et al., 2017, P&SS_2017_232 R1 Martian Methane in the light of REMS UV production of methane from surface and sedimenting IDPs on Mars in light of REMS data and with insights for TGO Corresponding Author: John E. Moores Center for Research in Earth and Space Science (CRESS), York Uni ersit!, "#$$ %eele Street, 'oronto, () M*J +,*, Canada -.oores/!orku.ca Contri0uting Authors: Christina L. S.ith: CRESS, York Uni ersit!, chrs.ith/!orku.ca Andre2 C. Schuerger: 3ept. of ,&ant ,atholog!, Uni ersit! of 4&orida, USA, schuerg/u5.edu 1 Moores et al., 2017, P&SS_2017_232 R1 Martian Methane in the light of REMS !stract This paper refines .odel predictions for the production of .ethane from U7-irradiated interplanetar! dust partic&es (IDPs) no2 that the Ro er En iron.enta& Monitoring Station (REMS) instru.ent on0oard the Mars Science 1a0orator! (MSL) Ro er has .ade the 6rst .easurements of the U7 en iron.ent on the surface of Mars, at :a&e Crater. (nce these .easurements are inc&uded in a U7 radiati e transfer .ode&, 2e 6nd that .odel&ed U7 sol-integrated energies across the planet are &o2er than pre-measurement esti.ates 0! *;< on a erage, considering a&& &atitudes and seasons. This reduction, in turn, reduces the predicted production of .ethane from indi idua& accreting 93,s, e=tending their &ifeti.es and increasing the surface concentration of organics that .ust accu.u&ate in order to emit su>cient .ethane to 0a&ance the accretion of organic compounds to Mars. E.ission from reasona0&e accu.u&ations of 93Ps cou&d range up to ?#.@A+$8" ppb sol8+. Richer deposits of organic car0on at the surface .a! e.it .ethane at no .ore than *.@ ppb sol8+. An e=a.ination of 93,-deri ed .ethane production during at.ospheric sett&ing indicates that no .ore than $.*B< of organic car0on from .eteor strea.s .a! 0e deposited in the at.osphere. Thus, such a process cannot e=plain either the spikes obser ed in .ethane nor the &o2 equi&i0riu. a&ues o0ser ed 0! MSL. Instead, this discrepanc! .a! 0e e=plained if D E$ tons per !ear of organic car0on sur i es to the surface, the at.ospheric &ifeti.e of .ethane is D ++$ !ears or the efficienc! of the U7-CF" process is D #%. Under the assu.ption of reduced car0on input c!c&ing in the Martian s!stem from these processes, 0oth soi& concentrations of organic car0on and at.ospheric .easurements of .ethane obser ed 0! MS1 are consistent 2ith the U7-CF" process. This refinement of .ethane production from 93,s and its geographica& and ertica& distri0ution 2i&& 0e an i.portant input for .odels attempting to understand the resu&ts to 0e deri ed 2 Moores et al., 2017, P&SS_2017_232 R1 Martian Methane in the light of REMS from the 'race :as (r0iter (T:() .ission that 2i&& .ap .ethane concentrations in the .artian at.osphere in B$+E at $.$+ ppb . Index Terms: Mars, methane, atmospheric chemistry, UV ". Introduction La0orator! studies (Stoker and Gu&&ock, +@@EH Schuerger et a&., B$+BH %eppler et a&., B$+B) demonstrate that .ethane is readi&! e ol ed from interplanetar! sources of organic car0on 2hen irradiated 0! u&tra iolet (U7) photons 0et2een B$$ and "$$ n.. 4urther.ore, Mars shou&d continuous&! accrete such partic&es, as dyna.ica& .odel&ing of interplanetar! dust partic&es (IDPs) in the plane of the solar s!stem suggests that se era& hundred tons of this organic car0on shou&d 0e col&ected 0! the upper at.osphere of Mars each !ear (Fl!nn, +@@I). It 2as this com0ination 2hich &ed to the de elopment of a U7-CF" .odel that &inked together .odel&ed U7 irradiation of Mars (e.g. Moores et a&., B$$#) 2ith the input of organic car0on to ca&cu&ate the surface &oading of organics (Moores and Schuerger, B$+B) and the equi&i0riu. .ethane concentration in the at.osphere (Schuerger et a&. B$+B). This .odel&ing found that if the entire input of organic car0on 2as con erted to .ethane 2ith an at.ospheric &ifeti.e of *B@ !ears (Atre!a et a&., B$$#), ++ ppb 2ou&d accu.u&ate as a steady state concentration o er geologica& ti.e. Ho2e er, a .ore reasona0&e upper 0ound of B.B ppb 0ased upon the 93, FJC ratio 2as considered .ore &ikel! (Schuerger et a&. B$+B). These a&ues 2ere rough&! consistent 2ith .an! of the telescopic and or0ita& .easurements that 2ere .ade prior to the &anding of the Mars Science La0orator! (MSL) ro er in &ate B$+B. 3 Moores et al., 2017, P&SS_2017_232 R1 Martian Methane in the light of REMS The MS1 ro er carried t2o instru.ents capa0&e of pro iding infor.ation to test the U78 CF" .odel: the Sa.ple Ana&!sis at Mars 'una0&e Laser Spectrometer (SAM8'1S) 2hich 2ou&d direct&! .easure the at.ospheric concentration of .ethane to a precision of K$.+ ppb (We0ster and MahaM!, B$++H MahaM! et a&., B$+B) and the Ro er En iron.enta& Monitoring Station (REMS) 2hich 2ou&d Cuantify, for the 6rst ti.e, the a.ount of U7 radiation recei ed at the surface of Mars (GN.eO-E& ira et a&., B$+B). 4ol&o2ing initia& nu&& resu&ts (Webster et a&., B$+*) that &i.ited .ethane a&ues 0e&o2 + ppb , SAM8'1S found e idence for &arge spikes of .ethane up to ?# ppb 2hich persisted o er relati el! short ti.esca&es of tens of so&s and a 0ackground concentration of .ethane of D $.# ppb (Webster et a&., B$+;). This a&ue 2as .ore than * ti.es &o2er than predicted 0! the U78 CF" .odel and is &ess than can 0e e=plained 0! the discrepanc! 0et2een pre-flight .odel&ing of U7 5u= (Moores et a&., B$$#) and the obser ed U7 5u= (S.ith et a&., B$+I). This disagreement is particu&ar&! interesting in &ight of the organic content of soi&s at :a&e crater (Freissinet et a&., B$+;) 2hich are &o2er than anticipated, 0ut &ie 2ithin pre-flight predictions .ade using the U7-CF" .odel (Moores and Schuerger, B$+B), consistent 2ith a&& other &anding sites !et isited on Mars. Lhat cou&d 0e causing the disagreement for at.ospheric .ethane 0et2een the U7-CF" .odel and the SAM8TLS resu&tsP (ne possi0i&it! is that the effecti e &ifeti.e of .ethane in the Martian at.osphere is signi6cant&! &ess than *B@ !ears. A &o2 at.ospheric &ifeti.e for .ethane, perhaps due to an unkno2n destruction .echanis., 2as cited 0! LefQ re and 4orget (2$$@) as a necessar! condition to e=plain the rapid disappearance of the .ethane plu.es reported 0! Mu..a et a&. (B$$@). Another possi0i&it! is that relati el! &itt&e organic car0on reaches the surface in the 6rst place (Fries et a&., B$+I), due to photol!sis of 4 Moores et al., 2017, P&SS_2017_232 R1 Martian Methane in the light of REMS fa&&ing 93Ps high in the at.osphere !ielding .ethane that is i..ediatel! destro!ed 0! the high 5u=es of &!.an8R radiation a0o e #$ k. in a&titude (Wong et a&., B$$*). Thirdl!, the efficac! of the U7-CF" process cou&d 0e &ess than B$< (Schuerger et a&., B$+B). 4ina&&!, there are .echanis.s that cause .ethane-c!c&e compounds to 0e redistri0uted across the surface, inc&uding the aeolian transport of s.a&& partic&es that 2ou&d inc&ude 93,s (Moores and Schuerger, B$+B), at.ospheric circu&ation (Mischna et a&., B$++), and adsorption and desorption of .ethane onto regolith (Gough et a&., B$+$H Mes&in et a&., B$++). Each of these processes .a! aMect the obser ed &oca& concentration of .ethane. Ho2e er, neither of these possi0i&ities funda.enta&&! cha&&enges the U7-CF" production process. :i en the i..inent obser ation of .ethane 2ith a precision of K +$ ppt (0.$+ ppb ) 0! the Mars 'race :as (r0iter (T:() in B$+E (Robert et a&., B$+I), it is necessar! to refine the U7-CF" .odel using the REMS resu&ts to pro ide infor.ation on U78.ediated production of .ethane. Ho2e er, note that this paper focuses solel! on production and 2i&& not discuss the redistri0ution of .ethane in the at.osphere ia circu&ation. The production of .ethane 2i&& 0e a critica& input into .odels 2hich contain destruction .echanis.s and 2hich can then infor. the interpretation of the ':( .ethane resu&ts. 4urther.ore, as noted 0! Moores and Schuerger (2$+B), .easurements of .ethane emissions from the surface can 0e used to constrain the Cuantit! and distri0ution of organic car0on at the surface of Mars. As such, this paper pro ides the needed update to the .ode&, the speci6cs of 2hich 2i&& 0e discussed in section B. Section * 2i&& then descri0e the refined U7-CF" production as applied to Mars. Fina&&!, this refinement 2i&& 0e used as a fra.e2ork to critica&&! e a&uate the organic car0on 0udget of Mars inc&uding the proposed car0on inputs, .ethane destruction .echanis.s and their &ikel! geographic and 5 Moores et al., 2017, P&SS_2017_232 R1 Martian Methane in the light of REMS tempora& ariation (sections ".+ and ".B) 0efore pro iding speci6c i.p&ications for ':( obser ations (4."). The Cuestion of aeria& deposition of .ethane from sedi.enting 93Ps 2i&& a&so 0e considered in section ".*.
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