Radiation-induced alteration of apatite on the surface of Mars: first in situ observations with SuperCam Raman onboard Perseverance

Abstract

Planetary exploration relies considerably on mineral characterization to advance our understanding of the solar system, the planets and their evolution. Thus, we must understand past and present processes that can alter materials exposed on the surface, affecting space mission data. Here, we analyze the first dataset monitoring the evolution of a known mineral target in situ on the Martian surface, brought there as a SuperCam calibration target onboard the Perseverance rover. We used Raman spectroscopy to monitor the crystalline state of a synthetic apatite sample over the first 950 Martian days (sols) of the Mars2020 mission. We note significant variations in the Raman spectra acquired on this target, specifically a decrease in the relative contribution of the Raman signal to the total signal. These observations are consistent with the results of a UV-irradiation test performed in the laboratory under conditions mimicking ambient Martian conditions. We conclude that the observed evolution reflects an alteration of the material, specifically the creation of electronic defects, due to its exposure to the Martian environment and, in particular, UV irradiation. This ongoing process of alteration of the Martian surface needs to be taken into account for mineralogical space mission data analysis.

Figure 1

Mars Raman spectra acquired with SuperCam on apatite SCCT (TAPAG) on different sols throughout the mission, normalized to the mean signal. The plots are color coded with sol number. (A) Normalized spectra (x axis shows wavenumber in cm⁻¹); (B) Close-view in the 900–1000 cm⁻¹ spectral range; baseline-corrected experimental data ( +) and fitted pseudo-Voigt profile (full lines). (C) Intensity of the ${\nu }_1$ mode of apatite extracted from the normalized spectra (area of the fitted pseudo-Voigt profile) as a function of the sol of the observation. The error bars represent the uncertainty values of the parameters derived from the peak fitting procedure.

Figure 2

Laboratory Raman spectra acquired on an apatite target after different durations of UV irradiation (0 min, 5 min, 30 min, 270 min, 1270 min), normalized to the mean signal. The plots are color coded with irradiation time. (A) Normalized spectra (x-axis shows wavenumber in cm⁻¹); (B) Close-view in the 900–1000 cm⁻¹ spectral range; baseline-corrected experimental data ( +) and fitted pseudo-Voigt profile (full lines). (C) Intensity of the ${\nu }_1$ mode of apatite extracted from the normalized spectra (amplitude of the fitted pseudo-Voigt profile) as a function of the duration of target irradiation. The error bars represent the uncertainty values of the parameters derived from the peak fitting procedure (smaller than the markers, in this case).

Figure 3

Remote microimages (RMIs) of the apatite SCCT acquired on Mars with SuperCam, corresponding to the first and one of the latest Raman spectra considered in this study (sols 51 and 836). The yellow ellipses represent the fields of view of the Raman observation (dashed: 68%; full line: 95%).