Martian surface deposits, including polar deposits, represent a vast storehouse of data recording the evolution Of Mars' climate and surface environment. However, the greatest challenge to deciphering these martian geo-records is the need for absolute dating techniques, Particularly those techniques applicable to the timeframes and surface processes of the 'Martian Quaternary'. Lepper and McKeever [3,4] have proposed developing optical dating, an established terrestrial chronometric dating method based on principles eri'solid-state physics, for retnote in-situ dating oCrnartian surface seditnents

Lepper, K. E. (Kenneth E.)

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La-uR- 0 3^03 3 1Approved for public release;distribution is unlimited .Title:Author(s) :Submitted to:Los AlamosNATIONAL LABORATORYSINGLE -GRAIN OPTICAL DATING PROPERTIES OF JSCMARS - 1 : PRELIMINARY MEASUREMENTS OFRADIATION DOSE RESPONSE AND SENSITIVITYCHANGE .Kenneth E . Leppe r34th Lunar and Planetary Science ConferenceMarch 17-21, Houston T XLos Alam os N atio nal Labora tory, a n a ffi rm ative action/eq u a l o pportu n ity e mploye r, is ope rated by the•U n iversi t y of Califo rni a for the U . S .D epartment of Energy u nder contract W-740 5-ENG-36. By acceptance of t his a r ticle, th e publi she r recog nizes that the U .S . Governmentreta i n s a n onexclus ive, royalty-free l icense to p ubli s h o r re pro d uce the published form of this contri buti o n , or to a ll o w o th ers to d o so, fo r U .S .G overnment purpos es . Los A lamos N a tional Laboratory requ est s th a t t he p u b lisher iden tify th is a rtic le as wor k perfo rmed u nde r theauspices of the U .S . D epartme nt of En e rgy. Los Alamos National La boratory strongly su pports ac a dem ic freed om and a researcher's right topu bl is h ; as a n institu tion, however, t h e Laboratory does n ot en d orse th e vi ewpoint of a pu blication or g u arantee its tech nica l correctness .Form 836 (8/00)S1NGLE-GR41N OPTICAL DATING PROPERTIES OF JSC 11-tAR5 - I : PRELIMINARY 11tEASURE -M F.NT S OF RADIATION DOSE RESPONSE AND SENSITIVITY C HANGF, . K . Lepper, LuminescenceG eoch ro n o l ogy L a b , L os Alamos National L a bo ra t o ry, MS J495, Los Alarnos, NM 87545 ; l epper(r iIan l ,bov .Introduction : Martian surface deposits, includingpolar deposits, represent a vast storehouse of data re-cording the evolution Of Mars' climate and surfaceenvironment . 1[owever, the greatest challenge to deci-phering these martian geo-records is the need for ab-solute dating techniques [1], Particularly those tech-niques applicable to the timcf'ramcs and surface proc-esses of the "Martian Quatcrnary" [2] . Lepper andMcKeever [3,4] have proposed developing opticaldating, an established terrestrial chronometric datingmethod based on principles eri'solid-state physics, forretnote in-situ dating oCrnartian surface seditnents .We report here the results of ongoing experimentswith JSC Mars-I, a terrestrial analog of martian sur-face materials [5], to establish a broad fundamenta]knowledge base from which robust dating proceduresfor robotic missions may be e3evelopcd_ Such data willbe critical for determining the engineering requiR-ments of remote in-situ optical dating equipments in-tended for use on Mars .Gene r a l principles o f o p tica l da ting : Over gco-logic time, ionizing radiation from the decay of natu-rally occurring radioisotopes and from cosmic raysliberates charge carriers (electrons and holes) withinsilicate mineral grains . The charge carriers can subse-quently become localized at crystal defects leading toaccumulation of a "trapped" electron population . Ro-combination of the charge carriers results in photonemission, i_c . luminescence . The intensity of lumines-cence produced is proportional to the amount, Oftrapped charge, and thereby to the radiation dose ab-sorbed by the mineral grains since deposition at thesampled site . A determination of the ionizing radiationdose rate at the sample location allows the age of thedeposit to be determined (from Age = Ab.corhed Dose iUn.rc1 Rate) . Experimentally, optical stirnulatiun can beemployed to liberate trapped charge and initiate themeasurement process, which gives rise to the namc"opiical dating" .The event dated by optical techniques is actuallythe last exposure of the sediment grains to sunlight (i .e .a luminescence age is a depositional age) . This is thecase because solar radiation, particularly UV radiation,stimulates and removes trapped charge accumulatedprior to burial and resets the optical clock .Because an optical date is a depositional age, thetechnique is uniquely suited to address questions ofchronostratigraphy and Climate evolution recorded insedimentary deposits an Mars . Recent experimentaladvances have made it possible to perform opticaldatine measurements on single sand-sized sedimentgrains (Fig . 1 ) , paving the way for radical miniaturira-tion and optimization of inass and VO lumc that wouldireatlr• facilitate robotic optical dating experiments onh-ta rs .Fig . l . Single-grain optical dating samplc carousel (left) andenlargement of a sample substrate showing individuail grainswithin measurcmcnt pits (right) [lmagc courtesy of] . . 13cn Icr-lensen, Riso National Lab of Denrnark] .Ob ject ives : The goal of this set of experiments isto characterize the radiation dose response, the mostfundamental optical dating property, of sin~lc sand-sized grains extracted from the JSC Man-] simulant .As the measurement process itself can induce changesin signal response from one measurement cycle to thenext, these experiments will also, by necessity, char-acterize sensitivity change .Methods: A bulk sample of JSC Mars-1 [5] wassieved to obtain the I HO-2S0 µm size fraction and splitinto two subsets . One remained untreated, the otherwas treated with HF for 5 min . to remove the iron-oxide grain coatings . The etched sample was rinsedwith I I CI, dispersant, and methyl alcohol to removeprecipitates and particulates and then dried . Dry grainsof each subset were placed into the sample Substratcsshown above (Fig . 1) . The substrates were given in-creasing doses of irradiation from a 40 mCi ""Sr/"°Ybeta source (10, 25, 50, 75, 100, 150, 250 Gy) in he-twccn which they were preheated to 240`C for 10 s andthen measured . Stimulation was for I s by a€reenlaser (Nd:YVO4 - 532 nm) focused on each sample pitwith the resulting luminescence being measured in theUV emission range (340ORO) by a photomultiplier tube(PMT) .S ► N cLE-crt A« O i ' ricAL uA*rivc ;PR O PE? R T I[;S OF JSC 1 A Rti- 1 : K . Leprc rResults /Discussion : There were sever a l diffa--ences between t h e etched and untreate d s ing le -g rainsand samples . Ob scrv ation a lly , th e untr eate d sampl ere tained its red -brown field color, while the etc h e dsampl e wa s pred ominantl y white (pre s um ably pl ag i o-c lase g rains and glass partic l es ) w ith a ve ry sm a l l p r o-p o rti on o f black g rains (Fe-bearing mine r a l s su c h asmagn e tite and p y r oxene) . Addition a lly, the untreatedg r a ins we re s tatica lly charged a nd tend ed t o be re-pelled fro m e ach o ther and the subs tra te m a kin g it dif-li c ult to ensure the sub s trate wa s pro p e rl y l oad e d w ithsample g ra in s . Only I o f the 400 pits o f th e untrea t e dgr a in s meas ured y ielded s i gnal levels >2x in s trumentn o i se . Howev er , 61 o f 400 pit s of the etched gra in swere suitable fo r further a n a l ys i s ( >2x n o i se ) - - a d a t ay i e ld o f - 15 °% . Thi s data y i e ld diffe r e n ce may s uggcs tth a t Fe -oxid es can act a s lumines cence "poiso ns", al-l ow ing s timulat e d charge carrie r s t o be conductedaway and/ or by absorbing emitted luminescence .Sensitivity Changestudy of fine-grain sediment extracts (4-1]pun) iroitiJSC Mars-1 sensitivity correction procedures, com-monly used in terrestrial dating studies, were able toproduce dose recovery results within 5'! ,;, of the ex-pected dose for several sub-samples [6] .Reconstructed Dose Respons e12000~cU) 10000 ❑0.2 8000a~ ^0~j - 6000.> 4000 ❑.F0 0~ 2000 ❑ a o °o ~WR °A0 100 200 3001.0 XUntreated ; n= 10 .8 Etched; Ave . o fn=6 1c ( 7°' Z 0 . 6U) a) %c> Nio M 0 . 4 ~ .0)0 . 2X----~ -__ -~ x--X---X _-- ~ --- ~0 . 0 ~ .- - - ,2 3 4 5 6 7Cycle Numbe rF i g . 2 . Observed sensitivity change for the one usable tui-trcated single-grain (x) and the average behavior of the 61usable etched single-grains ( • ).Sc ns iti v itv Change . Sen s iti v ity c h a n g e was a c-cessed from the avail a bl e d a ta set by dividing the s i g -na l intensity o b se rved durin g eac h measurement cycl eb y th e dose appli ed durin g th a t cyc le (assuming a lin -ear dose re s p o n se model a s observed from a bulk s am -ple o f JSC M a r s -] ac ross a wide appli ed dos e ra n ge[ 4 ]) . The re s ult s of thi s analy s is are s hown in Fig . 2 .Both the unt rea ted a nd etche d grains exhibit s tr o n g l ydecreas ing s i gn al re spon se with increas ing numb e r o fmea s u rement s m a d e on each g r a in (cyc le # ) . fl o w-eve r , the occurrence of sen s iti v it y chan ge does n o t init self nega te a m a t erial 's utilit y for o pt ical dating. In aDose (Gy )Fig . 3 . Reconstructed dose response curves for 3 rrpresc nta-tivc etched single-grains .Radiation Dose Response . U s ing the aver age s en -s iti v i ty change values o bt a in e d for the etched samples(Fig . 2) as co rrecti on fact or s, dose re spon s e curves fore ach o f the 6 1 grains c an be reco n s tru c ted . Three ex -amples are s h own in Fi g . 3 . Linear reg re ss ions o f eachof the reconstructed curve s produced 5 5 of 61 R ' va l -ues >0 . 95, s u gg e s tin g that the g ra in s exhibit rel a t ivelyuniform s en s iti v it y change whi ch i s adequ a tel y r epre-sent e d by th e cur v e s h own in Fi g . 2 .As s tated thi s m e th o d o f sensitivity co rrectio n canonly y i e ld recon s tru c ted dose respo n se cur ve s th a tmimic t h e model u sed fo r sen s itivi ty a n a l ys i s, in thiscase linear . St a ndard terre strial s ing le- g r a in da tin gm e th o d s integ r ate a s en s iti v ity change corre c tion int othe data co llecti on pr ocedure s . Results of do se re-spo nse ex p erim ent s u s ing thi s integrated se n s iti v i tycorr ection w ill a l so be presented .References : [1] C liftb rd S . M . et al . ( 2 000 ) Icarus,14 4 , 2 1 0-242 . [ 2 ] E d gett K . (2001) LPSC LPI# 1 080, CD ROM [3] Lepper K . and McKeeve r S .W.S .(1 998) LPI Contibuti o n No . 953 . [ 4 ] Lepper K . andM cK eever S . W . S . (2000) Icurus . 144 , 295 - 3 0 I . [5]Allen, e t a l . (1999 ) EOS, 79(3 4 ) , 405,408 . [6] Banerj e eet a l . ( 2002 ) R ad. Prot . Dos . 101 , 32 1 - 3 26 .

Single-grain optical dating properties of JSC Mars-1: preliminary measurements of radiation dose response and sensitivity change.

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Single-grain optical dating properties of JSC Mars-1: preliminary measurements of radiation dose response and sensitivity change.

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