Detection of different biosignatures in the NWA 10628, NWA 1950 and DAG735 Martian meteorites by Raman spectroscopy

-    Introduction
In the absence of returned samples, the study of Martian meteorites allows us to classify the samples according to their composition and consequently to study the geochemical processes taking place in the Martian crust. Moreover, meteorites can be also good samples to find possible remains of organic matter that could shed some light on the existence of life on Mars.
In this sense, for this work several shergottites were analyzed with the aim of detecting remarkable biosignature signals by means of Raman spectroscopy to understand their origin, nature and structure. 
The selected meteorites were NWA 10628, 1950 and DAG735. Previous petrological studies on the specimens revealed that NWA 10628 consists on grains of pyroxene (~ 70 vol%) and interstitial plagioclase/maskelynite (~30 vol%) [1]; NWA 1950 is a cumulate peridotitic specimen consisting of olivine (~ 55 vol%), pyroxenes (~35 vol%) and plagioclase glass (~8% vol%) [2]; and DAG735 is composed of olivine megacryst and fine-grained mass of pyroxene, mesostasis and maskelynitized plagioclase [3]. 
-    Materials and method
In particular, the analyzed piece of NWA 10628 had a weight of 0.19 g, and “a brownish appearance with minor patches of fusion crust on the surface” [1]. At first sight, its composition seems quite uniform, but it is worth to highlight the difference in roughness between the two faces.
The sample of NWA 1950 weighed 0.081 g and, in contrast to the previous one, in this case it had a heterogeneous appearance with zones with a dark brown coloration and others with a more yellowish appearance.
Finally, the fragment of meteorite DAG735 tested weighed 0.66g and had a slightly rough matrix with a greenish-brown tone, Figure 1 shows, the comparison of the three samples.
 
Figure 1. Images of the studied meteorites a) NWA 10628, b) NWA 1950 and DAG735.

With the aim of avoiding alterations and modifications or consumption of such unique samples, non-destructive analytical techniques were employed for the study. On the one hand, an InVia confocal micro-Raman spectrometer provided with 785 nm and 532 nm excitation lasers and CCD detector and calibrated with the 520.5 cm^-1 silicon line, was used to obtain the molecular results. On the other hand, X-Ray fluorescence (XRF) allowed us to obtain the elemental composition of the meteorites, for which the M4 TORNADO micro-spectrometer was used.

-    Results and Discussion
The Raman analysis obtained for the meteorite NWA 10628 showed that some of the compounds were β-carotene (Figure 2) and a possible sterol known as Cholesteryl stearate (CSA). The spectrum of the β-carotene contains two strong bands at 1157 cm^-1 and 1515 cm^-1, typical of the stretching vibrations of the polyene chain v1(C=C) and v2(C-C) [4]. Furthermore, the presence of a complex phospholipid called sphingomyelin (SM) was determined by the existence of two bands at positions of 2852 cm^-1 and 2885 cm^-1 characteristic of CH2 stretching vibrations [5].

Figure 2. Raman spectra of β-carotene in the NWA 10628 meteorite.

In addition, in the specimen NWA 1950 Raman bands that corresponded to the presence of branched-chain saturated fatty acids were found. Obtained Raman spectra matched with those of compounds such as 14-methylpentadecanoid acid, 15-methylpalmitic acid and triacylglycerols, which have the v(C-C) stretching vibrations bands between 1050-1150 cm^-1, the δ(CH2) twist vibration at 1297 cm^-1 and δ(CH2) or δ(CH3) deformations between 1400-1500 cm^-1[6].
Finally, in the meteorite DAG735 a medium chain saturated fatty acid known as palmitic acid (CH₃(CH₂)₁₄COOH) was detected, whose characteristics stretching vibration bands v(C-C) are at 1063 cm^-1, 1099 cm^-1 and 1129 cm^-1. Moreover, δ(CH2) twisting vibrations appear at 1129 cm^-1 [7].

-    Conclusions
As a conclusion, it can be said that the analysis carried out in the three shergottites provided useful and valuable information about possible biomarkers present in them, and, therefore, on their probable implications with the potential origin of life on Mars. 
More specifically, a pigment belonging to the carotene family was found in the NWA 10628 meteorite. This may have been produced as a protection mechanism of microbial life against harmful exposure to UV radiation. Considering this, P. Vítek et al. [4] considered β-carotene as a biomarker of the organic fossil that could have colonized early Mars, 3-4 Ga ago. 
Additionally, in two of the meteorites, fatty acids that are part of the cell membranes of all life forms, including microbial organisms were found. In this sense, it should be noted that the samples were always treated taking into account biosafety and sample handling standards, following the MOMA protocol. Therefore, the presence of these compounds as a cross-contamination consequence can be rejected. This fact is supported by the detection of a sphingolipid in the meteorite NWA 10628, which is a compound also found in the structure of all cell membranes.
With these results, unfortunately, the origin of the detected compounds cannot be assured. These meteorites suffered a terrestrial weathering process in which the detected compounds could penetrate the meteorite matrix. However, regardless the Terrestrial or Martian origin of the detected compounds, it can be said that this work proves how an igneous rock, such as the analyzed shergottites, can be a good matrix for preserving biosignatures, compounds that can be detected by Raman spectroscopy. 

-    Acknowledgements
This work has been funded by the “Raman On Mars” project (Grant No. PID2019-107442RB-C31), funded by the Spanish Agency for Research (MICINN and the European Regional Development Fund), and the “Study of Alteration Processes in Terrestrial and Planetary materials” strategic project (ref. PES 21/88), funded by the University of the Basque Country (UPV/EHU.

-    References
(1)    https://www.lpi.usra.edu/meteor/metbull.php?code=63155
(2)    https://www2.jpl.nasa.gov/snc/nwa1950.html
(3)    García-Florentino C., Torre-Fdez I., Ruiz-Galende P., Aramendia J., Castro K., Arana G., & Madariaga J. M. (2021), Talanta, 224, 121863
(4)    Vítek P., Osterrothová K., & Jehlička J. (2009). 57(4), 454-459.
(5)    Czamara K., Majzner K., Pacia M. Z., Kochan K., Kaczor A., & Baranska M. (2015), 46(1), 4-20. Reference database of Raman spectra of biological molecules
(6)    Saggu M., Liu J., & Patel A. (2015).. Pharmaceutical research, 32(9), 2877-2888.