Solar radiation is the primary energy source for surface planetary life, so that pigments are fundamental components of any surface-dwelling organism. They may therefore have evolved in some form on Mars as they did on Earth. Photosynthetic microbes are major primary producers on Earth, but are concurrently vulnerable to ultraviolet (UV) damage. Using non-intrusive laser Raman spectroscopy to recognize the component parts of biomolecules, we have shown not only the abundance of microbial photosynthetic and photoprotective pigments in situ, but also their spatial distribution within their microhabitat. This essential aspect of their screening or avoidance survival strategies is lost on extraction with solvents. This precise approach is eminently suited to analysis of epilithic (surface) and endolithic (within rocks) communities in Antarctic desert habitats, which are putative analogues of early Mars. Raman spectra for key biomolecules (e.g. the UV screen parietin and the antioxidant β-carotene in epilithic lichens) enable not only the detection of organics in light-stratified habitats, but also the characterization of unknown pigments. Typical biomarkers of astrobiological relevance in our Raman spectral database include scytonemin (a UV screen), chlorophyll (primary photosynthetic pigment), phycocyanin (accessory pigment for shade adaptation) and a hopanoid extracted from 2n5 Gya microbial stromatolite from Australia. This compound dates from the same time period when a wetter Mars could have had a potentially flourishing surface microbial community of its own. Analyses with a laboratory Raman instrument have been extended to a novel miniature Raman spectrometer, operating at the same optimal excitation wavelength (1064 nm) via an In-Ga-As detector. After evaluation in Antarctica, this instrument will be space-qualified for a proposed Mars rover mission to detect biomolecules in the nearsurface sediment profile of palaeolakes, using experience with Antarctic biomarkers to interpret alien spectra of fundamental components, without the need for prior knowledge of the identity of the target compounds.

Fourier Transform laser Raman spectroscopy was used to generate diagnostic spectra for pigments and biodegradative calcium oxalate in situ in two yellow-pigmented species of the lichen genus Acarospora from contrasting sites in the Antarctic and the Mediterranean. This non-intrusive technique was used to identify the photoprotective pigments rhizocarpic acid and β-carotene by their unique Raman spectral fingerprints. The use of low energy near-IR excitation at 1064 nm eliminated interference from autofluorescence of photosynthetic pigments. The insensitivity of the technique to water permitted the use of field-fresh material. The dominant yellow pigment, rhizocarpic acid, gave a diagnostic pattern of corroborative bands at wavenumbers (ν) 1596, 1665, 1620 and 1000 cm −" . It was possible to discriminate between hydration states of calcium oxalate ; the monohydrate (whewellite) featured a ν(CO) stretching band at 1493 cm −" whereas the dihydrate (weddellite) had a contrasting ν(CO) stretching band at 1476 cm −" . Fourier Transform deconvolution and intensity measurements were used to obtain relative quantitative data for rhizocarpic acid by using its ν(CO) and ν(CONH) amide modes, for carotenoid pigment by its ν(C l C) band at 1520 cm −" and for calcium oxalates by their ν(CO) bands. ν(CO), ν(CONH) and ν(C l C) are the vibrational stretching modes of the carbonyl C l O, protein amide 1 and alkenyl C l C moieties, respectively, in the pigments and metabolic products of the Acarospora lichens. The ability to determine the precise (20 µm spot diameter) spatial distribution of these key functional molecules in field-fresh thallus profiles and variegations has great potential for understanding the survival strategies of lichens, which receive high insolation, including elevated levels of UV-B, under extremes of desiccation and temperature in hot and cold desert habitats.

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