RAMAN SPECTRAL ANALYSIS FROM SHERLOC ONBOARD MARS 2020

The Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC) is one of the instruments mounted on the Perseverance rover’s robotic arm. SHERLOC focuses on searching for mineral species and organics that may contain spectral signatures of hydrous environments and past microbial life. It measures the Raman scattering in a narrow wavelength range (246-357 nm) with a significantly stronger fluorescence around 274-355 nm. Here the 40 measurements of Raman spectra from three detectors of SHERLOC, captured on Sol 4, were analyzed along with Matscam-Z images. Principal component analysis was applied to check the mineralogical diversity. The results include fluorescence signatures with maxima at 276 nm and Raman peaks similar to amorphous silicates.


INTRODUCTION
Raman spectroscopy is a powerful tool for identifying signatures of organic and mineral compositions. Spectral analysis shows that the peaks from the Raman spectra alone play a vital and additive role in the characterization and identification of minerals (Tripathi and Garg, 2021). Raman spectra play a vital role because we look for the possible unique spectral features at the molecular level for both the surfaces, primarily representing the characteristics of soil and rocks having silicates. Raman spectroscopy, named after Indian physicist Sir C. V. Raman, is a spectroscopic technique used to determine vibrational, rotational, and other frequency modes in a system (Gulari et al., 1984;Otto, 2012). Raman spectroscopy is also used to identify molecules by providing a structural fingerprint. When light passes through a medium without a change of frequency, it is called Rayleigh Scattering, which is also responsible for the blue color of the sky; it increases with the fourth power of the frequency, and it dominates at shorter wavelengths. The incident photons interact with the molecules and either loss or gain of energy. This gain and loss of energy are responsible for a frequency shift. Such inelastic scattering is called Raman scattering (Colthup et al., 1975). Like Rayleigh scattering, Raman scattering depends upon the polarizability of the molecules (Figure 1). This work utilizes the latest Raman spectral data from the Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC) instrument mounted on the Perseverance rover's robotic arm. It aims to search for mineral species and organics containing spectral signatures of hydrous environments and past microbial life.

MINERALOGY OF LANDING SITE -JEZERO CRATER
Jezero Crater is located in the Syrtis Major quadrangle with a diameter of around 45 km (18° 22′ 48″ N, 77° 34′ 48″ E). It was the landing site of the Mars 2020 rover mission ("Jezero Crater -Perseverance Landing Site -NASA Mars," 2021.; Mangold et al., 2021). The area holds a record of vast surface processes and significant aqueous alteration. This area was a high priority for robotic landed-mission exploration for a long time. The research * Corresponding author found that this site was once an ancient river delta with abundant olivine, carbonates, and organic molecules. It is a Noachian-aged crater and the only one on Mars where strong carbonate spectral features are found ( Figure 2) (Sun and Stack, 2020). Earlier, the high spatial and spectral resolution reflectance data recorded by the Mars Reconnaissance Orbiter's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) payload proved the presence of H2O-and SiOH-bearing minerals on the Martian surface. Spectroscopic observations show that amorphous silica is a weathering product of the Martian crust with basalts. A Western and Northern Delta have also been observed from the remote sensing observations. The Western Delta has Fe/Mg smectites, while the other is a less wellpreserved delta and is dominated by Mg-carbonates and associated olivine (Milliken et al., 2008). The Basin is also filled by olivine and Mg-carbonates, giving rise to three theories of primary detrital deposition, lacustrine carbonates, and the surrounding of regional Mg-carbonate/olivine in Nili Fossae. Volcanic processes trapped the eroded delta and surrounded the remnants of the delta, which had been separated from the parent delta bodies by the aeolian deflation process before the volcanism in the region. These features make the Jezero an ideal astrobiological site (Brown et al., 2020;Goudge et al., 2015;Horgan et al., 2020). CheMin proved the presence of amorphous silicates. There are a variety of amorphous silicates that can govern the earlier aqueous environments, including aqueous alteration at high water-to-rock ratios under a range of temperature and pH conditions, lowtemperature aqueous alteration at neutral to mildly acidic conditions (only under rapid weathering), and the presence of unaltered mafic glass which readily weathers in the presence of water ( (Gupta et al., 2022;Rampe et al., 2014).

DATASETS
The To conduct organic and mineral analysis SHERLOC finds and classifies astrobiological and organic minerals over the surface and near the subsurface of Mars. It works in three different (detector with different gains) regions. Another payload on Mars 2020 mission, Mastcam-Z, has zoom, focus, and 3D view functions mounted on the mast. This will allow a detailed view of distant objects with 150 microns/ pixel (0.15 millimeter or 0.0059 inches) to 7.4 millimeters (0.3 inches) /pixel of resolving power . Figure 1 shows many peaks recorded in the three regions by SHERLOC. These are the spectra taken at subsequent spacecraft clocks on Sol 4. To check the mineral diversity and the separation among major mineral peaks, a Principal Component Analysis (PCA) was performed for three regions of SHERLOC Raman spectra for Sol4. The PCA was carried out for all the Raman peaks to check the mineralogical diversity at the crater. Here, various comparisons between PC1, PC2, and PC3 are presented. The Dark-subtracted spectrum in calibration mode and in all three regions results in fluorescence signatures with maxima at 276 nm and Raman peaks similar to amorphous silicates. The major peaks in this wavelength region and their substitutes on the Earth are listed in Table 1 (Abbey et al., 2017;Beegle et al., 2020;Razzell Hollis et al., 2021; "The first 300 sols of the SHERLOC investigation on the Mars 2020 rover -NASA Technical Reports Server (NTRS)," 2022. Later Mastcam-Z images were also analyzed for Sol 4 and Sol 11. The Raman spectra on Sol 11 are also presented to compare how much the mineralogy differs from Sol 4.  There are many spectra recorded on Sol 4 ( Figure 3). The spectra were named and separately represented as per the regions. The three regions in which Raman spectra were acquired then subjected to PCA to check the diversity among the various spectra taken on Sol 4. Here 'A' is the wavelength, so it is ignored, and the numbers on components are the spacecraft clock with a prefix of the region (1,2 and 3).  Raman peaks observed in this region are also matched with the various amorphous and microcrystalline silica, hydrated sulfate, phosphate, and carbonate phases. Spectral features with sulfate phases were found at 300 and 325 nm. Also, aromatic ring organics with fluorescence are seen here (Beegle et al., 2020;Rampe et al., 2014). However, the comparison with PCs that contributed significantly less in representing the variability cannot be neglected. There may be few overlapping spectral features in fine spectral resolution SHERLOC responses.   Table 1, including carbonates and sulfates, were observed in the 240-360 nm region. PC1 X PC2 (Figure 9), separates the R2251, (R2191,2365) and R2203 majorly while PC1 X PC3 (Figure 10) distinguishes the R2365, R2305, R2251 and R2203 and PC2 X PC3 ( Figure 11) shows maximum variation among (R2365, R2305), R2203, R2251, R2251. PC2 X PC3 shows four major classes. On cross verification with spectra name and features, it is found that carbonates, phosphates, sulfates, and organics were the major reasons for these peaks and variations observed in PCA. For example, R2251, R2191, R2203, and R2365 show different separable features. However, PC3 X PC2 (Figure 11) shows the largest variance among all the comparisons. The SHERLOC Raman spectra for Region 3 recorded on Sol 4 are shown in Figure 12. This covers all the mineral features listed in Table 1 for the whole region of 240-360 nm. PC1 X PC2 (Figure 13), separates the (R3413, R3377), R3395, R3419 majorly while PC2 X PC3 (Figure 14) distinguishes the (R3305, The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLIII-B3-2022 XXIV ISPRS Congress (2022 edition), 6-11 June 2022, Nice, France R3341), R3209, (R3377, R3413) and ( Figure 15) shows maximum variation among (R3305, R3341), R3209, R3419, R3395. PC2 X PC3 shows four major classes. Similarly, these components show the presence of carbonates, phosphates, sulfates, and organics as the major source for these peaks and variations observed in PCA. Similarly, SHERLOC Raman spectra for Sol 11 ( Figure 16) were also analyzed, and similarly, fluorescence and spectral peaks features similar to amorphous silicates are observed. In Figure 1, along with significant peaks, the minute peaks also contribute to the mineralogical diversity in the Jezero crater. PCA proves that spectral diversity in 40 measurements for Sol 4 in three regions is not only due to one mineral species. The principal components (PCs) segregate the spectral features with major and minor spectral features contributing to the overall mineralogy of the Jezero crater (the area covered on Sol 4). SHERLOC provides a deep insight into the mineralogy of the crater, and PCA improves the understanding of the presence of these mineral species.

Figure 16. SHERLOC measurements for Sol 11 (All three regions)
Later the Mastcam-Z images from the Sol 4 and Sol 11 were also analyzed. Several boulders and aeolian activities were seen through Mastcam-z images, as seen in Figure 17, Figure 18, and Figure 19. The Mastcam-Z image from Sol 11 shows very few boulders and a considerable amount of luminance, which was not observed in Sol 4 ( Figure 20) (Mangold et al., 2021).

CONCLUSION
A near-range investigation by Mastcam-Z and chemical scanning from SHERLOC of these boulders, rocks, and other features by Perseverance will allow a detailed and quantitative study of Martian architecture, including soil and surface patterns. The mineral species extracted through PCA will help provide new insights on earlier lake evolution and continue searching for ancient biosignatures and organic matter. In the future, combined studies with SuperCam and Mastcam-Z will resolve several astrobiological mysteries.