Biosignature detection is one of the most important goals in Mars missions. Since the Curiosity mission, the laser-induced breakdown spectrometer (LIBS) becomes an essential payload due to its convenience and versatility in profiling elemental chemistry. To test whether LIBS alone could filter potential biosignatures, a clastic quartz stone collected from a Mars analog setting, the western Qaidam Basin, was selected for LIBS analysis. Raman spectroscopy was used as an indicator of organic signals to support the presence of potential hypolithic communities and the dearth of epilithic biomass on the rock. A total of 344 LIBS spectra were determined and statistically analyzed using principal component analysis (PCA). Our results indicate that, with a sufficient sample size, PCA analysis can partially differentiate biotic and abiotic signals based on LIBS measures. This finding is significant since it indicates that multivariate analysis of LIBS data can be useful for biosignatures filtering on Mars exploration.

Located on “the roof of the world” Tibetan Plateau, the western Qaidam Basin is a cold, dry, and irradiative environment that shapes itself with landforms (e.g., dunes, yardangs, playas, wind streaks, polygonal terrains, and gullies) commonly found on Mars. A clastic quartz stone was sampled from a Cenozoic gravel deposit (38°35′44″ N, 90°59′6″ E, 3245.17 m altitude) from the hyperarid Dalangtan Playa, western Qaidam Basin, on 29 July 2021. The Cenozoic gravel deposit was likely derived from the weathering of Mesozoic (Pre-Jurassic and Jurassic) rocks, and quartz stones were common in the deposit. Visible light greenish color could be observed at the bottom of the quartz stone.

Multiple spots of four vertical lines (11 spots for line 1 Qz-l1, 6 spots for line 2 Qz-l2, 9 spots for line 3 Qz-l3, and 8 spots for line 4 Qz-l4) of the Qaidam quartz stone were selected to stereoscopically investigate the spatial distribution of Raman spectra-based mineralogical or organic/biotic signals. An alpha 300R confocal Raman imaging system (WITec, Germany) incorporated with a 50x objective lens of numerical aperture = 0.55 and an excitation laser source of 532 nm was used for Raman spectral measurements. The laser wavelength was corrected using the Raman peak of a Si wafer. All spectra were acquired in a spectral range of 0-4000 cm⁻¹ with a spectral resolution of 4.8 cm^-1. To retain the resolutions of both minerals and organic matter as much as possible, laser power was kept at 3.1 mW for an integration time of 3 s with the number of accumulations of 30.

To understand the elemental compositions and spectral features of chosen samples, the SciAps Z-300 Handheld LIBS Analyzer (SciAps Inc, Woburn, MA, USA) was employed for LIBS analysis (excitation source: 5-6 mJ·pulse^-1, 50 Hz repetition rate, 1064 nm laser source, argon purge). Z-300 LIBS Analyzer measured the signal intensity every 0.0333 nm from 200 to 900 nm. LIBS Analyzer was equilibrated with an internal standard prior to determining the peak patterns of respective target samples. LIBS was employed to construct a pseudo-three-dimensional geochemical profile of spots on the four vertical lines identical to Raman spectroscopic measurements. The LIBS spectrum of each spot was generated by the LIBS Analyzer in quadruplicate. In addition, 140 spots from 9 oval outlines of the top, side, and bottom faces of the Qaidam quartz were measured using LIBS: 8 spots of inner circle, 16 spots of middle circle, and 15 spots of outer circle on the top face; 16 spots of upper circle, 23 spots of middle circle, and 23 spots of lower circle on the side face; and 8 spots of inner circle, 12 spots of middle circle, and 19 spots of outer circle on the bottom face. Moreover, 68 random spots (20 from the top, 28 from the side, and 20 from the bottom) of the Qaidam quartz were chosen for singlicate LIBS measurements and the downstream statistical analysis.