Martian analogue samples, their spectroscopic biosignatures, and degradation by the cosmic radiation environment

Centre for Astrophysics and Planetary Science

Lewis R. Dartnell (lewis@lewisdartnell.com), Howell Edwards, Peter Muller, Laurent Desorgher

IntroductionThe success of an astrobiological search campaign onMars, or other planetary bodies in the solar system,relies upon the reliable detection of evidence of pastor present microbial life — biosignatures.Spectroscopic methods require little or no samplepreparation, can be repeated essentially endlessly,and may be performed in contact or even remotely.Such methods are therefore ideally suited to triagingfor targets containing biosignatures, which can beconfirmed with supporting instrumentation. Here wediscuss the use of Raman and FTIR (FourierTransform Infra Red) spectroscopy for the detectionand characterisation of biosignatures of microbial lifecolonising a diverse sample set. A further primeconsideration for the detection of biosignatures onMars is their long-term preservation in the face of thecosmic ionising radiation penetrating through themartian subsurface [2,3,4,5,6,7] (Fig.1).

Results & DiscussionRepresentative data from two of the samples in the completesample library are shown in Fig.3. Both samples werecollected on fieldwork to the Atacama desert in April 2015.The first is a quartz from the leeward side of the coastalmountains, colonised hypolithically by extremophilecyanobacteria. The second is a halite nodule from the Yungaysalar in the hyperarid core of the desert, exhibiting narrowdark bands of endolithic colonisation.

Cross-comparing between different Raman excitationwavelengths, blue and green excitation wavelengths elicit thestrongest response from carotenoids (due to the resonanceRaman effect) and phycobiliproteins of the cyanobacterialphotosystem, whereas red and IR are optimal for chlorophylland the UV-protectant scytonemin. The halite colonistsexhibit a strong signal for scytonemin, as their microhabitatdoes not provide UV shielding that the quartz hypolithsbenefit from. Overall, Raman provides identification ofspecific biomolecules (even down to the structure of thecarotenoids), whereas FTIR offers a more general descriptionof the repertoire of organic molecules present: components ofthe cell membrane, proteins and polysaccharides, as well ashydration.

Raman also provides a good description of the mineralogicalbackground, including quartz, carbonates and minor speciesin halite, as well as hematite deposition banding withinsandstones. Iron mobilisation is directed by endolithcolonisers within sandstones, such as those from theAntarctic Dry Valleys, to provide UV shielding.

Signs of past hematite mobilisation are also very conspicuoususing UV fluorescence imaging, with the region of iron-depleted quartz grains immediately beneath the colonisedlayer brightly fluorescent, as shown in Fig.4.

References[1] Preston, Louisa J, and Lewis R Dartnell. “Planetary Habitability: Lessons Learned From Terrestrial Analogues.” International Journal of Astrobiology 13.1 (2014): 81–98.[2] Dartnell, Lewis R, L Desorgher, J Ward, and A Coates. “Modelling the Surface and Subsurface Martian Radiation Environment: Implications for Astrobiology.” Geophysical Research Letters 34.2 (2007): L02207.[3] Dartnell, Lewis R, L Desorgher, John M Ward, and A J Coates. “Martian Sub-Surface Ionising Radiation: Biosignatures and Geology.” Biogeosciences 4 (2007): 545–558.[4] Dartnell, L R. “Ionizing Radiation and Life.” Astrobiology 11.6 (2011): 551–582.[5] Dartnell, L R, Stephanie Hunter, et al. “Low-Temperature Ionizing Radiation Resistance of Deinococcus Radiodurans and Antarctic Dry Valley Bacteria.” Astrobiology 10.7 (2010): 717–732.[6] Dartnell, Lewis R, Michael Storrie-Lombardi, et al. “Degradation of Cyanobacterial Biosignatures by Ionizing Radiation.” Astrobiology 11.10 (2011): 997–1016.[7] Dartnell, Lewis R, K Page, et al. “Destruction of Raman Biosignatures by Ionising Radiation and the Implications for Life-Detection on Mars.” Analytical and Bioanalytical Chemistry 403.1 (2012): 131–144.[8] Muller, J-P, M Storrie-Lombardi, and MR Fisk. “WALI - Wide Angle Laser Imaging Enhancement to ExoMars PanCam: a System for Organics and Life Detection.” EPSC Abstracts 4 (2009): EPSC2009–674–1

MethodThis on-going research programme combines threeaspects:1) Collecting a library of geological samples colonised by

extremophile microorganisms from a broad variety ofmartian analogue sites on Earth (Fig.2)

2) Characterising the detectable biosignatures from eachsample using a combined microscope system able toperform high-resolution spectroscopy with fourwavelengths of Raman laser excitation (blue, 473nm;green, 532nm, red, 633nm; IR, 784nm) and FTIRspectroscopy, all on the same target spot. This allowsnot only cross-comparison of the mineralogical signaland most prominent biosignature features detectable incolonised samples from different analogue sites, butalso determination of the optimum excitationwavelength for different sample types.

3) Exposing the samples to high doses of ionisingradiation and reanalysing to understand the rate ofbiosignature degradation

Mineralogical spatial features such as this potentiallyprovide a more robust biosignature after geologicaltimescales of exposure on the martian surface as they areimpervious to degradation by ionising radiation or UVirradiation, unlike remnant organic biomolecules.

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Fig.1: Previous radiation modelling (inset) [2,3] has found the top 2-3m ofthe martian subsurface to be dominated by penetration of cosmic ionisingradiation, the region we will be able to access in our search for biosignaturesin the foreseeable future with the ExoMars rover.

Fig.2: Our map reviewinganalogue sites on Earth [1].Samples used in this presentstudy include those fromBeacon Valley (9), the Atacamadesert (14), the Antarctic DryValleys (15), and the Mojavedesert (16).

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Fig.3: Field photography ofthe colonised quartz (left;highlighting the translucentmineral) and halite nodulefrom the hyperarid core(right) of the Atacamadesert, and 10x micro-scope images (inset) fromthe Raman-FTIR system.Representative spectrafrom four wavelengths ofRaman excitation aredisplayed, as well as theFTIR spectrum fromcolonies.

Fig.4: Endolithically colonised sandstones from Battleship Promontory inAntarctica (left) and the Mojave desert in the USA (right). Top row shows thesamples imaged under white light; the bottom row is imaging under 365 nmUV illumination using our WALI instrument [8], showing the conspicuousfluorescent region immediately beneath the colonised layer.