Amino acid photostability on the Martian surface

Meteoritics amp Planetary Science 40 Nr 8 1185ndash1193 (2005)Abstract available online at httpmeteoriticsorg

1185 copy The Meteoritical Society 2005 Printed in USA

Amino acid photostability on the Martian surface

Inge Loes TEN KATE1 2 James R C GARRY2 Zan PEETERS2 Richard QUINN2 3 Bernard FOING4 and Pascale EHRENFREUND2

1Leiden Observatory Leiden University PO Box 9513 2300 RA Leiden The Netherlands2Astrobiology Laboratory Leiden Institute of Chemistry Leiden University 2300 RA Leiden The Netherlands

3SETI Institute NASA Ames Research Center Mail Stop 239-12 Moffett Field California 94035 USA4ESA Research and Scientific Support Department ESTECSCI-SR PO Box 299 2200 AG Noordwijk The Netherlands

Corresponding author E-mail tenkatestrwleidenunivnl

(Received 07 July 2005 revision accepted 18 July 2005)

AbstractndashIn the framework of international planetary exploration programs several space missionsare planned to search for organics and bio-signatures on Mars Previous attempts have not detectedany organic compounds in the Martian regolith It is therefore critical to investigate the processes thatmay affect organic molecules on and below the planetrsquos surface Laboratory simulations can provideuseful data about the reaction pathways of organic material at Marsrsquo surface We have studied thestability of amino acid thin films against ultraviolet (UV) irradiation and use those data to predict thesurvival time of these compounds on and in the Martian regolith We show that thin films of glycineand D-alanine are expected to have half-lives of 22 plusmn 5 hr and of 3 plusmn 1 hr respectively when irradiatedwith Mars-like UV flux levels Modelling shows that the half-lives of the amino acids are extended tothe order of 107 years when embedded in regolith These data suggest that subsurface sampling mustbe a key component of future missions to Mars dedicated to organic detection

INTRODUCTION

In the apparent absence of biogenic processes on theMartian surface exogenous delivery of organic material mustbe taken into account Organic compounds are present incertain classes of meteorites as well as in comets (Ehrenfreundet al 2002) Intact extraterrestrial delivery of such organicmaterial could have been a significant factor in determiningthe organic make-up of early Earth and Mars (Pierazzo andChyba 1999 Ehrenfreund et al 2002) Flynn and McKay(1990) have estimated that the mass of meteoritic materialreaching the surface of Mars is between 163 times 10minus6 and 736 times10minus8 kg mminus2 yrminus1 The major carbon component incarbonaceous meteorite samples is in the form of insolubleorganic macromolecules In the soluble fraction of suchsamples more than 70 extraterrestrial amino acids have beenidentified in addition to many other organic compoundsincluding N-heterocycles carboxylic acids sulphonic andphosphonic acids and aliphatic and aromatic hydrocarbons(Botta and Bada 2002 Sephton 2002 and references therein)The total abundance of all amino acids in the Murchisonmeteorite was determined to be 17ndash60 parts per million (ppm)by Botta and Bada (2002) Ehrenfreund et al (2001a)determined that the average of the amino acid abundance in the

CI carbonaceous chondrites Orgueil and Ivuna is 5 ppm Ifburial processes are ignored then unaltered carboncompounds should accumulate with an annual flux of sim15 ngmminus2 yrminus1 This flux level should yield detectable concentrationsafter a geologically brief period However the Viking gaschromatograph-mass spectrometer (GCMS) which had anaverage detection limit on the order of one part per billion(ppb) failed to find organic compounds that had not beenidentified as background contaminates (Biemann et al 1977)It is thus thought that some process may be at work on Marsthat destroys the infalling organic matter gaseous oxidizingagents have been postulated as encouraging the destruction ofmeteoritically delivered organic matter However impactingorganic matter may be displaced into deeper regolith layersand thus be protected from radiation and oxidation It istherefore possible that organic meteoritic material is destroyedon the exposed surface but may survive when protected atdepth The detection of organic material arising from eitherbiotic or abiotic sources as well as the characterization ofoxidants at the Martian surface (Zent and McKay 1994) areamong the main goals of future Mars missions

Laboratory simulations are a crucial step inunderstanding the chemical pathways on the Martian surface(ten Kate et al 2003) Organic compounds that are abundant

1186 I L ten Kate et al

in meteorites such as carbonaceous chondrites are a logicaltarget for those experiments since they may haveaccumulated to form significant deposits on the Martiansurface via exogenous delivery (Bland and Smith 2000) Wetherefore have examined the photostability of the simpleramino acids given their well-characterized properties andtheir ubiquitous presence in meteoritic samples This paperdiscusses photostability measurements on two amino acidsglycine and D-alanine that were exposed to Mars-like UVradiation D-alanine is used in this study to prevent biologicalcontamination of the samples the issue of chirality will not beaddressed in this paper The Equipment and ExperimentalProcedure section describes our simulation chamber as wellas the UV sources used for the irradiation studies Alsodescribed are the experimental procedure and thecharacterization of thin films In the Results section we reporton the IR spectroscopy of irradiated amino acid samplesFurthermore we tabulate the half-lives and UV destructioncross sections calculated from different experiments andpredict the values for Martian insolation intensity

EQUIPMENT AND EXPERIMENTAL PROCEDURE

The amino acids glycine (997 purity Merck) and D-alanine (98 purity Sigma) were ground using a pestle andmortar The powders were loaded into a vacuum sublimationsystem that holds a set of four silicon discs at a distance of15 cm above a resistively heated copper oven Each disc(24 mm diameter 08 mm thick) was polished on bothcircular faces and permanently fixed to a steel backing ring

with an inner diameter of 1 cm large enough for the beam ofan infrared spectrometer to pass through Four copper strapsone per disc have embedded magnets to hold the silicon discsabove the sublimation oven and permit the discs to be handledwithout touching their coated faces

The sublimation took place under vacuum at a typicalpressure of lt3 times 10minus4 mbar and at an oven temperaturebetween 150 degC and 175 degC During the deposition processthe layer thickness was monitored using standard laserinterferometry A schematic drawing of the arrangement isshown in Fig 1 Absolute thickness of the layers was obtainedfrom atomic-force microscopy (AFM Zeiss AxioPlan 2)scans which also showed that the films formed as solid pore-free layers of nanocrystals The sublimation rate of the solidamino acid and hence the layer thickness was controlled bymoderating the oven heater current A limited amount of H2Omolecules may have been trapped in the amino acid filmsduring deposition This will be discussed later

The charts in Fig 2 show the data for a sim330 nm thick D-alanine layer along with representative data that show thattwo areas on the disc experience near-identical depositionrates The thickness of the layer is calculated from the numberof interference fringes in the interferogram not from theabsolute voltage of the photodiode output

Although four discs can be coated simultaneously in thesublimation chamber only one can be accommodated in the

Fig 1 A schematic drawing of the sublimation chamber showing thelaser geometry The film thickness can be estimated from theinterference arising from the optical path difference taken by rays ACand AB

Fig 2 a) AFM image of a solid D-alanine layer the light grey parton the left side is the D-alanine layer the dark part on the right is thesilicon disc on which the layer is deposited b) A height profilemeasurement of the layer along the horizontal line plotted in 2a c)The corresponding interferogram from the layer deposition with twotraces showing deposition on the edge (upper trace) and on the centreof the disc (lower trace)

Amino acid photostability on the Martian surface 1187

simulation chamber which is shown schematically in Fig 3The body of this small stainless steel chamber is equippedwith four ports and a rotary feedthrough A mounting stage isfixed to the internal end of the feedthrough and the coppersupport tabs of the silicon discs can be clamped to this stage

One of the ports is equipped with a polished magnesiumfluoride (MgF2) window to admit the light from the UVlamps On the opposite face of the chamber a clear window ismounted with which the discrsquos alignment can be verified Theremaining two ports on the chamber hold potassium bromide(KBr) windows that allow the beam from a Fourier transforminfrared (FTIR) spectrometer to pass through the chamber Byturning the rotary feedthrough on the chamber the disc can beexposed first to UV radiation and then to the beam of theFTIR spectrometer Before during and after the irradiationphase absorption spectra were taken using an Excalibur FTS-4000 infrared spectrometer (BioRad) in the range of 4000 to500 cmminus1 at 4 cmminus1 resolution

UV Sources

Two UV sources have been used for the experiments Amicrowave-excited hydrogen flow lamp (Opthos) with anaverage flux of 46 times 1014 photons sminus1 cmminus2 is used to provideUV light between 120 and 180 nm The flux of this lamp wasmeasured by solid-state actinometry as described by Cottinet al (2003) and has an estimated uncertainty of 25 Thesecond UV source is a deuterium discharge lamp (Heraeus-Noblelight DX 202 range 190ndash400 nm) This lamprsquos outputwas measured before and after each experiment with a UVsensor (Solartech 80 model) that displayed the integratedintensity over the range from sim245 nm to sim265 nm (half-peaksensitivity full-width) From the known response curve theirradiance spectrum provided by Heraeus in arbitrary unitscould be transformed to give spectral irradiance in absoluteunits Similarly the UV sensor could be used to calculate the

absolute irradiance spectrum as a function of distance fromthe lamp In Fig 4 the irradiance of the deuterium lamp isshown for a distance of 145 mm from the lamprsquos dischargepoint At this distance and over the range of 190ndash325 nm thelamp generates 49 plusmn 1 times1013 photons sminus1 cmminus2 and delivers anintensity of 40 plusmn 5 microW cmminus2 If this intensity is compared tothe predicted noontime equatorial insolation at Marsrsquo surfacein a low atmospheric-dust scenario (Patel et al 2002) then thedeuterium lamp provides sim4 of the UV experienced onMars in the range of 190 nm to 325 nm The spectral profile ofthe lamp is deficient in the longer wavelengths compared tothe solar spectrum The spectrum of this lamp is shown inFig 4 along with a curve showing the lighting spectrum asexpected to exist on Mars

Experiments

Two sets of experiments have been performed using twoamino acids In the first set of experiments we used thehydrogen flow lamp as a light source From thoseexperiments photodestruction rates of solid glycine and D-alanine can be calculated The discs were irradiated with thehydrogen flow lamp for 25 hours and IR absorption spectraof the layers were recorded during this period In the secondset of experiments the deuterium discharge lamp was usedthis lamp has a lower total power output than the hydrogenlamp and longer exposures (gt40 hr) were needed to causemeasurable destruction During all of the experiments theirradiation chamber was pumped continuously and had anaverage internal pressure of 4 times 10minus6 mbar

An early concern was that the layer material may be lostby sublimation during irradiation of the disc Thus a separateexperiment was conducted to measure the in situ temperatureof the silicon disc during the irradiation process with both thehydrogen lamp and the deuterium lamp A Pt100 element was

Fig 3 A schematic drawing of the simulation chamber

Fig 4 a) A comparative chart of the irradiance of the surface noonlighting at the equator of Mars (from Patel et al 2002) b) The lightdelivered at the working distance of 145 mm from the dischargeregion of the deuterium lamp


glued to the illuminated face of the disc and showed thatwhile lit and under high vacuum the temperature of the discincreased by no more than 2 degC over the course of a three-hour hydrogen lamp exposure and by no more than 10 degCduring a 65-hour deuterium lamp exposure Thesetemperature increases are far below the sublimationtemperatures of 150ndash175 degC found for the glycine and D-alanine powders in our sublimation chamber at similarpressures A separate 28-hr experiment has also beenperformed to measure the effect of high vacuum (4 times 10minus6

mbar) alone on D-alanine no change in the layerrsquos absorptionspectrum could be seen

UV spectra of the glycine and D-alanine thin films havebeen recorded with a Varian Cary 3 Bio UV-Visiblespectrophotometer (Fig 7) The thin films were deposited onMgF2 windows because of its transparency in the shortwavelength range from 120 to 7000 nm Silicon is nottransparent for wavelengths shorter than 1000 nm

RESULTS

We have measured IR spectra of glycine and D-alaninesublimated as thin films onto silicon substrate discs Toconfirm the obtained absorption data and perform bandassignments the glycine spectra were compared with spectraof glycine in KBr pellets as measured by Ihs et al (1990)Uvdal et al (1990) and Rosado et al (1998) The D-alaninespectra were compared with spectra of L-alanine in KBrpellets determined by Cao and Fischer (1999 2000) andRozenberg et al (2003) IR spectra of D- and L-alanine are

similar because chirality does not effect IR spectroscopy(Yamaguchi et al 2005)

In Fig 5 glycine and D-alanine infrared absorptionspectra before irradiation are shown together with a traceshowing the changes in their spectra after sim48 hr of irradiationwith the deuterium lamp Table 1 describes the bandassignments of the main peaks that can be observed in theglycine and D-alanine spectra A comparison of the IR spectraof glycine in a KBr pellet and a solid layer of glycine on asilicon disc is given in Fig 6 to show their similarity

An assumption used in the data processing is that theamino acid films are optically thin We used the UV sensor tomeasure the amount of UV light from the deuterium lamp thatwas reflected from a silicon disc with and without a D-alaninecoating Silicon has an average reflectivity of sim70 (Edwards1985) for light in the wavelength range of 120 nm to 325 nmThis modest reflectivity was confirmed by measurementswith the UV sensor which additionally showed that theaddition of a 250 plusmn 10 nm thick layer lowered the reflectivityby sim10 in the spectral window to which the sensor wasresponsive (245ndash265 nm half-peak sensitivity full-width)Absorption spectra of glycine and D-alanine (Fig 7) at UVand visible wavelengths suggest that comparable amounts oflight would be reflected back through the amino acid films atwavelengths other than those measured by the UV sensor Thethickness of the layers and the reflectivity of the siliconsubstrate also meant that an additional illumination flux ofsim65 had to be considered from photons that pass throughthe layer and are then reflected by the disc back into theamino acid layer

Fig 5 The IR spectra of a) solid glycine and b) solid D-alanine in the range 4000ndash500 cmminus1 measured with a resolution of 4 cmminus1 Two spectraare shown (I) is recorded before irradiation with a deuterium discharge lamp and (II) is obtained by subtracting the spectrum of theunirradiated compound from the spectrum recorded after 483 hr (for glycine) and 502 hr (for D-alanine) of irradiation The vertical scale barsshow the infrared absorbance in arbitrary units The identification of the peaks can be found in Table 1


With the optically-thin assumption verified first orderreaction kinetics (Cottin et al 2003) were applied to calculatethe half-life and the UV destruction cross section of the solidglycine and D-alanine The destruction rate is the slope of alinear fit through the natural logarithm of the normalizedintegrated absorbance plotted against time (Fig 8) For thesecalculations the absorbance data of the peaks described inTable 1 are used From the destruction rate the UVdestruction cross section σ and the half-life are calculated(Cottin et al 2003)

In Tables 2 and 3 the UV destruction cross sections andthe half-lives are compiled for the hydrogen lamp irradiationof glycine and D-alanine UV photodestruction of matrixisolated glycine has been measured previously byEhrenfreund et al (2001b) and Peeters et al (2003) Thesedata however cannot be directly compared to UVphotodestruction of solid thin films Tables 4 and 5 containthese values for the deuterium lamp irradiation of glycine andD-alanine In our experiments all of the amino acid layershad thicknesses in the range of 160ndash400 nm From ourexperiments it appears as expected from an optically thinlayer that the layer thickness within this range does notinfluence the degradation cross section or the half-life

The uncertainty in the destruction cross section isdominated by two sources First the uncertainty of thehydrogen flow discharge lamp is estimated to be 25 byCottin et al (2003) and no appropriate independent sensorswere available to confirm this number Second the calibrationcertificate of the UV sensor used to calibrate the deuteriumUV lamp states an plusmn10 accuracy and in use the deuteriumlamp is stable to better than 1 of its output when measured

over multiple days The overall uncertainty in the measuredoutput of the deuterium lamp is estimated to be plusmn15 Asidefrom the uncertainties in the lamp flux smaller uncertaintiesare taken into account reflecting the error in the linear fit thatrepresents the destruction rate of the amino acid layers

The half-lives calculated from the laboratory data for thedeuterium lamp irradiation of both glycine and D-alaninewere extrapolated to Marsrsquo surface UV flux levels Using datafrom a model for the Martian atmosphere with a low dust load(Patel et al 2002) an intensity of 930 microW cmminus2 wascalculated for the wavelength range of 190 nm to 325 nmThis UV flux was used to calculate the half-lives of glycineand D-alanine shown in Table 6

DISCUSSION

Amino acids the building blocks of proteins are easilyformed and can be well-characterized by different analyticalmethods More than 70 different amino acids have beenobserved in meteorites (Botta and Bada 2002) Many of theseamino acids show isotopic excess confirming theirextraterrestrial origin (Sephton 2002 and references therein)We do not have any evidence for biogenic processes on MarsIf life as we know it developed in certain environments onMars amino acids may be present

We have studied the survival of amino acids exposed toMars-like UV conditions in the laboratory In theseexperiments we found half-lives of 18 plusmn 04 times 106 sec and 29plusmn 12 times 105 sec for glycine thin films and D-alanine thin filmsrespectively which correspond to half-lives of 22 plusmn 5 hr and 3plusmn 1 hr for glycine and D-alanine respectively at Mars UV

Table 1 Band assignments and wave numbers (in cmminus1) of some spectral features of the spectra of glycine and D-alanine The labels refer to the peaks in Fig 5

Label Glycine D-alaninea

Wave number Band assignmenta Wave number Band assignment

a 1599 νas (COO)b c d 1623 δ (NH3)e g νas (COO)g

b 1519 δs (NH3)b c d 1587 νas (COO)e f g δ (NH3)g

c 1446 δsc (CH2)b c d 1456 δas (CH3)e g

d 1414 νs (COO)b c d 1414 νs (COO)e

e 1336 ρw (CH2)b c d 1360 δa (CH3)g

f 916 ρr (CH2)d 1307 δ (CH)f g

g 896 δsc (CC)d 1239 β (NH3)g

h 703 ρr (COO)d 1115 ρ (NH3)g

i 1015 νs (CCNC)g

j 919 νa (CCNC) ρ (CH3)g

k 850 2ν (CCNC) ρ (CH3)g

aApproximate description of the band assignment υass asymmetricsymmetric stretching βass asymmetricsymmetric bending δass asymmetricsymmetricdeformation δsc scissoring mode ρwr waggingrocking mode

bIhs et al (1990)cUvdal et al (1990)dRosado et al (1998)eCao and Fischer (1999)fCao and Fischer (2000)gRozenberg et al (2003)


flux levels The difference in measured destruction ratebetween glycine and D-alanine can be explained in terms ofradical stabilization The destruction mechanism of glycinesuggested by Ehrenfreund et al (2001b) starts with theseparation of the carboxyl group

hνH2NCH2COOH rarr Η2ΝCH2sdot + COOHsdot

Rearrangement of the proton on the H2NCH2sdot radicalwould lead to methylamine (CH3NH2) and CO2 The stabilityof this intermediate radical determines the rate of this processSecondary and tertiary radicals appear to be more stable thanprimary radicals The ethylamine radical (H2NC2H4sdot) madefrom D-alanine is therefore more stable and so D-alaninebreaks down more rapidly than glycine

From these results we attempted to obtain knowledge onhow long amino acids could survive when embedded in theMartian regolith Our measured reaction rates can be used tocalculate estimated decomposition rates for amino acids onMars Using first-order decomposition kinetics theconcentration of amino acids at a given time can be calculatedfrom Equation 1

(1)

where J is the destruction rate a function of the destructioncross section and the flux Nt is the concentration (number ofmolecules) at time t N0 is the initial concentration and t is thetime (sec) From this relationship we can estimate the time ittakes for an initial regolith load of 1 ppb of amino acids in theMartian regolith to decrease to less than 1 part per trillion(ppt 1 per 1012) With an attenuation of the destruction rate by

a factor of 109 which is equivalent to the assumed regolithdilution ratio and caused by the shielding effect of theregolith the level of amino acids would decrease from 1 ppbto 1 ppt in approximately 4 times 107 yr for glycine and 5 times 106 yrfor D-alanine in the absence of regolith mixing andadditional amino acid inputs This calculation represents theexpected lower limit of the possible decrease in destructionrate due to dilution of amino acids in the regolith Animportant indication of this result is that the decompositionrate is not determined by complexity of the compound Therelationship between decomposition and decrease in reactionefficiency due to regolith dilution effects by erosion is shownin Fig 9

Continuous amino acid input into the regolith frommeteoritic infall and weathering processes is not consideredin Fig 9 The actual concentration of organics in the Martianregolith is expected to depend on the balance between thephotodecomposition rate and the input rate of amino acidsdue to physical weathering and mixing occurring on theplanetrsquos surface Physical weathering (processes that breakdown or fragment rocks and minerals) has many causesincluding particle collisions impact cratering glacialprocesses and volcanism Currently it appears that Mars isfar less geologically active than it has been in the past and itis likely that the primary mechanism of physical weatheringon Mars today is aeolian erosion caused by the collision ofparticles moved by wind Missions to Mars have returnedevidence of the extent to which aeolian activity is currentlyoccurring on Mars Bridges et al (1999) determined thatapproximately half of the rocks at the Mars Pathfinder sitehave been abraded by wind-borne particles Although thePathfinder site was formed by an ancient out-flow channelaeolian resurfacing of the site has clearly occurred and islikely to be the dominant geological process occurring on the

Fig 6 The IR spectra of a) glycine in a KBr pellet and b) solidglycine on a silicon disc in the range 4000ndash500 cmminus1 measured witha resolution of 4 cmminus1 The vertical scale bar shows the infraredabsorbance in arbitrary units

Nt N0e Jtndash=

Fig 7 a) The spectrum of the deuterium lamp between 190ndash400 nmrecorded at the same distance from the lamp as the amino acid filmscaled to the right ordinate The UV-visible absorption spectra of asim300 nm layer of (b) solid glycine and (c) solid D-alanine on a silicondisc in the range 190ndash400 nm scaled to the left ordinate


planet today The rate of physical weathering determines therate at which fresh surfaces are exposed to UV light Once asurface is exposed chemical weathering may proceedthrough UV-induced oxidation process

Carr (1992) has estimated the average rate of depositiondue to physical erosion on Mars to be 20 nm yrminus1 whichcorresponds to an approximate rate of 5 times 10minus5 kg mminus2 yrminus1

Assuming that surface rocks contain 1 ppb of amino acids theerosional contribution to the regolith would be 50 picogramsof amino acid mminus2 yrminus1 For accumulation of amino acids inthe regolith to occur input rates must exceedphotodecomposition rates Assuming a 2 microm penetrationdepth of UV photons into the regolith this condition is metwhen attenuation of the measured destruction cross section isgreater than 105 In cases where the attenuation is below 105amino acids are removed more rapidly than the input rate

It is reasonable to expect the decomposition rate of aminoacids in the regolith on Mars to exceed the input rate since theeffective attenuation of the destruction cross section willlikely be lower than the dilution ratio due to UV scattering bythe regolith There may be a further enhancement in thedegradation rate resulting from catalytic regolith surfaces(Quinn and Zent 1999)

There are two previously published laboratory studies

Fig 8 The natural logarithm of the normalized integrated absorbance (ln(SS0) plotted against time for the deuterium irradiation of (A)glycine (Table 4) and (B) D-alanine (Table 5)

Table 2 Glycine irradiated by the hydrogen flow lamp

Experimentσ(cm2molecule)

Half-life(sec)

1 21 plusmn 05 times 10minus19 43 plusmn 02 times 103





Average 24 plusmn 15 times 10minus19 44 plusmn 06 times 103

Table 3 D-alanine irradiated by the hydrogen flow lamp


Half-life(sec)






Table 4 Glycine irradiated by the deuterium discharge lamp


Half-life(sec)




Table 5 D-alanine irradiated by the deuterium discharge lamp

Experiment σ(cm2molecule)

Half-life(sec)





Table 6 Half-lives of glycine and D-alanine in the laboratory and extrapolated to Marsrsquo equatorial surface irradiation at local noon

CompoundHalf-life in the laboratory(sec)

Half-life on Mars(sec)

Glycine 18 plusmn 04 times 106 80 plusmn 18 times 104

D-alanine 29 plusmn 12 times 105 12 plusmn 05 times 104


that examined the photostability of amino acids in the Martiansurface material in both cases only glycine was studied OrUcircand Holzer (1979) demonstrated the UV decomposition ofpure glycine in the presence of small amounts of O2 gas in adry nitrogen atmosphere In their experiments 95 of theglycine was recovered after 109 hr of illumination at Mars fluxlevels No decomposition of glycine was observed in samplesirradiated in dry nitrogen Stoker and Bullock (1997)measured the photodecomposition of glycine mixed in apalagonitic regolith in the presence of a simulated Martianatmosphere and attributed their results to the effects of UValone Stoker and Bullock (1997) concluded that the presenceof oxidizing agents is not needed to explain the depleted levelof organics in Martian Viking regolith samples Thisconclusion is at odds with the results of OrUcirc and Holzer(1979) The decomposition rates reported in this paper areconsistent with the results of OrUcirc and Holzer (1979) forglycine decomposition in the presence of low levels ofoxygen indicating that radical induced decompositionmechanisms may have also played a role in our observed rates

In our amino acid films a limited amount of H2Omolecules may be incorporated in the amino acid thin filmsduring deposition andor irradiation The irradiation of H2Omolecules can lead to the generation of OHsdot and otherstrongly oxidizing radicals The photochemical production ofoxidizing radicals in both the near-surface atmosphere and onexposed surface material is thought to play a major role in thedecomposition of organics on Mars (Zent and McKay 1994)The background pressure in the simulation chamber duringsample exposure to UV light was lt10minus5 mbar In highvacuum systems the residual gas is predominately water andat 10minus5 mbar the partial pressure of water corresponds to theapproximately 1 ppm of water vapor in the Martianatmosphere Therefore the background gas phase waterabundance in the chamber is consistent with Mars number

densities and the uniformity of the IR spectra and therepeatability of the measured half-lives indicates that thewater abundance incorporated into the samples is smallconstant and not greater than expected Mars abundances

This work is part of a comprehensive multi-year effort todescribe the response of biologically important molecules toplanetary environments and in particular that of the Martiansurface Studying the survival of endogenous or exogenousbio-relevant compounds involves multi-parametersimulations of which the atmospheric interactions with theUV lighting found at Mars are of special interest and will beexamined in future work

CONCLUSIONS

We have constructed a system that allows in situmeasurements of the destruction rates of amino acid in solidthin films exposed to UV radiation These rates have beenused to predict the decomposition rates of two amino acidsglycine and D-alanine on the surface of Mars We estimatethat for a starting amino acid load of 1 ppb in the Martianregolith the amino acid concentration would decrease to lessthan 1 ppt in approximately 4 times 107 yr for glycine and 5 times106 yr for D-alanine Additionally for accumulation of aminoacids from weathering processes to occur in the regolith ourmeasured destruction rate for pure glycine would have to beattenuated by a factor of 105 by regolith mixing Given thisattenuation factor combined with the additional rate increasethat is expected due to radical generation in the regolith byUV decomposition of amino acids is expected to outstripaccumulation on the surface of Mars

Attempts to detect organic compounds by the Vikinglanders were unsuccessful and indicated that the surfacesamples contained no detectable amount of native organicmaterial These in situ results along with our laboratorymeasurements indicate that levels of amino acids in theMartian regolith are likely to be below the ppt levelSuccessful detection of organic compounds on Mars willrequire highly sensitive instrumentation such as the MarsOrganic Analyzer (Skelley et al 2005) Instrumentation withhigh sensitivity to organics coupled with measurementsdesigned to understand oxidative mechanisms occurring inthe regolith (Zent et al 2003) represent a promising strategyfor understanding Mars carbon chemistry

AcknowledgmentsndashThe authors wish to thank J Romstedt forproviding the AFM pictures ILtK is supported by theBioScience Initiative of Leiden University JG is supportedby the SRON National Institute for Space Research PE andZP are supported by grant NWO-VI 016023003 RQ issupported by NASA Ames cooperative agreement NCC2-1408

Editorial HandlingmdashDr Elisabetta Pierazzo

Fig 9 The concentration of glycine in the regolith as a function oftime The lines show the rates for different levels of destruction crosssection attenuation due to dilution of the amino acid in the regolithLevels of attenuation a) 1 b) 102 c) 105 and d) 109


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Bland P A and Smith T B 2000 Meteorite accumulations on MarsIcarus 14421ndash26

Botta O and Bada J L 2002 Extraterrestrial organic compounds inmeteorites Surveys in Geophysics 23411ndash467

Bridges N T Greeley R Haldemann A F C Herkenhoff K EKraft M Parker T J and Ward A W 1999 Ventifacts at thePathfinder landing site Journal of Geophysical Research 1048595ndash8615

Cao X and Fischer G 1999 New infrared spectra and the tautomericstudies of purine and L-alanine with an innovative samplingtechnique Spectrochimica Acta Part A 552329ndash2342

Cao X and Fischer G 2000 Infrared spectra of monomeric L-alanineand L-alanine-N-d3 zwitterions isolated in a KBr matrixChemical Physics 255195ndash204

Carr M H 1992 Post-Noachian erosion rates Implications for Marsclimate change Proceedings 23rd Lunar and Planetary ScienceConference pp 205ndash206

Cottin H Moore M H and BEgravenilan Y 2003 Photodestruction ofrelevant interstellar molecules in ice mixtures The AstrophysicalJournal 590874ndash881

Ehrenfreund P Bernstein M P Dworkin J P Sanford S A andAllamandola L J 2001b The photostability of amino acids inspace The Astrophysical Journal 550L95ndashL99

Ehrenfreund P Glavin D P Botta O Cooper G and Bada J L2001a Extraterrestrial amino acids in Orgueil and Ivuna Tracingthe parent body of CI type carbonaceous chondrites Proceedingsof the National Academy of Sciences of the United States ofAmerica 982138ndash2141

Ehrenfreund P Irvine W Becker L Blank J Brucato J RColangeli L Derenne S Despois D Dutrey A Fraaije HLazcano A Owen T and Robert F 2002 Astrophysical andastrochemical insights into the origin of life Reports on Progressin Physics 651427ndash1487

Edwards D F 1985 Handbook of optical constants of solids I editedby Palik E D OrlandoAcademic Press 547 p

Flynn G J and McKay D S 1990 An assessment of the meteoriticcontribution to the martian soil Journal of Geophysical Research9514497ndash14509

Ihs A Liedberg B Uvdal K Tˆrnkvist C Bodˆ P andLundstrˆm I 1990 Infrared and photoelectron spectroscopy ofamino acids on copper Glycine L-alanine and β-alanineJournal of Colloid and Interface Science 140192ndash206

OrUcirc J and Holzer G 1979 The photolytic degradation and oxidation

of organic compounds under simulated martian conditionsJournal of Molecular Evolution 14153ndash160

Patel M R Zarnecki J C and Catling D C 2002 Ultravioletradiation on the surface of Mars and the Beagle 2 UV sensorPlanetary and Space Sciences 50915ndash927

Peeters Z Botta O Charnley S B Ruiterkamp R andEhrenfreund P 2003 The astrobiology of nucleobases TheAstrophysical Journal 593L129ndashL132

Pierazzo E and Chyba C F 1999 Amino acid survival in largecometary impacts Meteoritics amp Planetary Science 34909ndash918

Quinn R C and Zent A P 1999 Peroxide-modified titaniumdioxide A chemical analog of putative martian soil oxidantsOrigins of Life and Evolution of the Biosphere 2959ndash72

Rosado M T Duarte M L T S and Fausto R 1998 Vibrationalspectra of acid and alkaline glycine salts VibrationalSpectroscopy 1635ndash54

Rozenberg M Shoham G Reva I and Fausto F 2003 Low-temperature Fourier transform infrared spectra and hydrogenbonding in polycrystalline L-alanine Spectrochimica Acta PartA 593253ndash3266

Sephton M A 2002 Organic compounds in carbonaceousmeteorites Natural Product Reports 19292ndash311

Skelley A M Scherer J R Aubrey A D Grover W H IvesterR H C Ehrenfreund P Grunthaner F J Bada J L andMathies R A 2005 Development and evaluation of amicrodevice for amino acid biomarker detection and analysis onMars Proceedings of the National Academy of Sciences of theUnited States of America 1021041ndash1046

Stoker C R and Bullock M A 1997 Organic degradation undersimulated Martian conditions Journal of Geophysical Research10210881ndash10888

ten Kate I L Ruiterkamp R Botta O Lehmann B GomezHernandez C Boudin N Foing B H and Ehrenfreund P 2003Investigating complex organic compounds in a simulated Marsenvironment International Journal of Astrobiology 1387ndash399

Uvdal K Bodˆ P Ihs A Liedberg B and Salaneck WR 1990 X-ray photoelectron and infrared spectroscopy of glycine adsorbedupon copper Journal of Colloid and Interface Science 140207ndash216

Yamaguchi M Miyamaru F Yamamoto K Tani M and Hangyo M2005 Terahertz absorption spectra of L- D- and DL-alanine andtheir application to determination of enantiometric compositionApplied Physics Letters 86053903-1-3

Zent A P and McKay C P 1994 The chemical reactivity of themartian soil and implications for future missions Icarus 108146ndash157

Zent A P Quinn R C Grunthaner F J Hecht M H Buehler M GMcKay C P and Ricco A J 2003 Mars atmospheric oxidantsensor (MAOS) An in situ heterogeneous chemistry analysisPlanetary and Space Science 51167ndash175


in meteorites such as carbonaceous chondrites are a logicaltarget for those experiments since they may haveaccumulated to form significant deposits on the Martiansurface via exogenous delivery (Bland and Smith 2000) Wetherefore have examined the photostability of the simpleramino acids given their well-characterized properties andtheir ubiquitous presence in meteoritic samples This paperdiscusses photostability measurements on two amino acidsglycine and D-alanine that were exposed to Mars-like UVradiation D-alanine is used in this study to prevent biologicalcontamination of the samples the issue of chirality will not beaddressed in this paper The Equipment and ExperimentalProcedure section describes our simulation chamber as wellas the UV sources used for the irradiation studies Alsodescribed are the experimental procedure and thecharacterization of thin films In the Results section we reporton the IR spectroscopy of irradiated amino acid samplesFurthermore we tabulate the half-lives and UV destructioncross sections calculated from different experiments andpredict the values for Martian insolation intensity

EQUIPMENT AND EXPERIMENTAL PROCEDURE

The amino acids glycine (997 purity Merck) and D-alanine (98 purity Sigma) were ground using a pestle andmortar The powders were loaded into a vacuum sublimationsystem that holds a set of four silicon discs at a distance of15 cm above a resistively heated copper oven Each disc(24 mm diameter 08 mm thick) was polished on bothcircular faces and permanently fixed to a steel backing ring

with an inner diameter of 1 cm large enough for the beam ofan infrared spectrometer to pass through Four copper strapsone per disc have embedded magnets to hold the silicon discsabove the sublimation oven and permit the discs to be handledwithout touching their coated faces

The sublimation took place under vacuum at a typicalpressure of lt3 times 10minus4 mbar and at an oven temperaturebetween 150 degC and 175 degC During the deposition processthe layer thickness was monitored using standard laserinterferometry A schematic drawing of the arrangement isshown in Fig 1 Absolute thickness of the layers was obtainedfrom atomic-force microscopy (AFM Zeiss AxioPlan 2)scans which also showed that the films formed as solid pore-free layers of nanocrystals The sublimation rate of the solidamino acid and hence the layer thickness was controlled bymoderating the oven heater current A limited amount of H2Omolecules may have been trapped in the amino acid filmsduring deposition This will be discussed later

The charts in Fig 2 show the data for a sim330 nm thick D-alanine layer along with representative data that show thattwo areas on the disc experience near-identical depositionrates The thickness of the layer is calculated from the numberof interference fringes in the interferogram not from theabsolute voltage of the photodiode output

Although four discs can be coated simultaneously in thesublimation chamber only one can be accommodated in the

Fig 1 A schematic drawing of the sublimation chamber showing thelaser geometry The film thickness can be estimated from theinterference arising from the optical path difference taken by rays ACand AB

Fig 2 a) AFM image of a solid D-alanine layer the light grey parton the left side is the D-alanine layer the dark part on the right is thesilicon disc on which the layer is deposited b) A height profilemeasurement of the layer along the horizontal line plotted in 2a c)The corresponding interferogram from the layer deposition with twotraces showing deposition on the edge (upper trace) and on the centreof the disc (lower trace)




UV Sources



Experiments









RESULTS












DISCUSSION










e 1336 ρw (CH2)b c d 1360 δa (CH3)g

f 916 ρr (CH2)d 1307 δ (CH)f g

g 896 δsc (CC)d 1239 β (NH3)g

h 703 ρr (COO)d 1115 ρ (NH3)g

i 1015 νs (CCNC)g










(1)





Nt N0e Jtndash=











Half-life(sec)









Half-life(sec)








Half-life(sec)






Half-life(sec)















CONCLUSIONS







REFERENCES


































UV Sources



Experiments









RESULTS












DISCUSSION










e 1336 ρw (CH2)b c d 1360 δa (CH3)g

f 916 ρr (CH2)d 1307 δ (CH)f g

g 896 δsc (CC)d 1239 β (NH3)g

h 703 ρr (COO)d 1115 ρ (NH3)g

i 1015 νs (CCNC)g










(1)





Nt N0e Jtndash=











Half-life(sec)









Half-life(sec)








Half-life(sec)






Half-life(sec)















CONCLUSIONS







REFERENCES



































RESULTS












DISCUSSION










e 1336 ρw (CH2)b c d 1360 δa (CH3)g

f 916 ρr (CH2)d 1307 δ (CH)f g

g 896 δsc (CC)d 1239 β (NH3)g

h 703 ρr (COO)d 1115 ρ (NH3)g

i 1015 νs (CCNC)g










(1)





Nt N0e Jtndash=











Half-life(sec)









Half-life(sec)








Half-life(sec)






Half-life(sec)















CONCLUSIONS







REFERENCES





































DISCUSSION










e 1336 ρw (CH2)b c d 1360 δa (CH3)g

f 916 ρr (CH2)d 1307 δ (CH)f g

g 896 δsc (CC)d 1239 β (NH3)g

h 703 ρr (COO)d 1115 ρ (NH3)g

i 1015 νs (CCNC)g










(1)





Nt N0e Jtndash=











Half-life(sec)









Half-life(sec)








Half-life(sec)






Half-life(sec)















CONCLUSIONS







REFERENCES




































(1)





Nt N0e Jtndash=











Half-life(sec)









Half-life(sec)








Half-life(sec)






Half-life(sec)















CONCLUSIONS







REFERENCES








































Half-life(sec)









Half-life(sec)








Half-life(sec)






Half-life(sec)















CONCLUSIONS







REFERENCES




































CONCLUSIONS







REFERENCES
































REFERENCES































Documents

Amino acid photostability on the Martian surface