Effect of laser power on Raman analyses of lipids and amino acids: Implications for extraterrestrial life exploration

Abstract

One of the primary objectives of Mars exploration missions is to search for traces of past or present life. The search for bioorganic molecules is highly desirable because they have diagnostic features to indicate the presence of life. Lipids and amino acids are particularly favored because of their high preservation potential in the geological record. Raman spectroscopy has emerged as a promising tool for detecting and identifying organic molecules on extraterrestrial planets. However, cautions should be exercised in operating Raman instruments to detect organic samples, especially concerning laser power. Excessive laser power can potentially alter spectral characteristics and even damage samples, while low power levels suffer from attenuation of the measured Raman signal, making it challenging to interpret. Therefore, it is crucial to determine the optimal Raman laser power for organic detection and build a laser power-based spectral library to increase our capability to interpret the Martian spectroscopic data. As such, we performed a comprehensive Raman analysis of several lipids (decanol, hexadecane, and palmitic acid) and amino acids (alanine, aspartic acid, glycine, histidine, and tyrosine). The Raman laser power varies in two modes, i.e., increasing from 1% (0.63 ± 0.01 mW) to 100% (66.40 ± 1.85 mW) and vice versa, within the operational range of Raman payloads of Mars rovers. The results show that different spectra can be obtained for the organic samples when using different excitation powers. Using high-power lasers could cause damage to lipid samples, whereas amino acids exhibit stability even under the strong irradiation of 100% laser power. In addition, low-power lasers significantly reduce the Raman signal of lipids, while amino acids remain detectable at low laser powers even as low as 1%. Therefore, the most effective and secure operation is to gradually increase the laser power in the Raman analyses for detecting various organic compounds. Our study serves as a reference for operating Raman instruments and contributes to the accurate interpretation of spectral data returned from spacecraft. The supplementary material provides all the spectral data, facilitating further comparisons with future in situ and orbital measurements on Mars.