Low-temperature reflectance spectra of meteorites: implications for the MMX mission

Remote sensing visible to near-infrared (VNIR) reflectance spectroscopy measurements of a surface are influenced by its composition, physical properties, and interactions with the environment, including temperature variations. Laboratory measurements are used to isolate the effects of temperature variations from other phenomena. Temperature changes induce variations in the intrinsic absorption properties of common rock-forming minerals such as olivine and pyroxene (Singer and Roush, 1985; Roush and Singer, 1986; Schade and Wäsch, 1999). These changes result in modification of the position, depth, width, and shape of absorption bands. The surface temperature variations across small bodies and planetary surfaces must thus be considered for an optimal interpretation of spatially resolved observations. Several meteorites have been investigated at low temperature, however, due to their low reflectance, less has been done for carbonaceous chondrites (CCs). Laboratory studies focusing on spectral modifications induced by cryogenic temperatures in the VNIR are mostly limited to 2.5 µm, which prevents investigation of the hydroxyl and molecular water absorption band around 3 µm. In this work, we report a comparative analysis of temperature-dependent spectral modifications on a panel of nine meteorites over the 0.5-4 µm spectral range. We study CCs from four different class (CI, CM, CV, and CO) as well as an ungrouped one (UCC), an ordinary chondrite (OC), and a diogenite. We analyze a temperature range from ~280 K to ~70 K and detail the importance of spectral changes for the MMX mission.

Low-temperature reflectance spectroscopy measurements were acquired at IPAG (France) using the bidirectional reflectance spectro-gonio-radiometer SHINE (SpectropHotometer with variable INcidence and Emergence) (Brissaud et al., 2004). SHINE was coupled with the double environmental chamber CarboN-IR (Beck et al., 2015). Spectra were acquired from 0.5 to 4 µm with an incidence angle of 0° and an emission angle of 30°. We performed measurements from about 280 K to a minimum of 70 K with several intermediate levels. The temperature error is estimated to be 1 K. Measurements were performed under secondary vacuum (P~10^-6 mbar), preventing effects of oxidation by ambient air. All of our samples were in the form of unsieved powder. We use the position, depth, and width of the absorption bands (0.7, 1, 2, and 3 µm) to characterize spectral modifications induced by low temperatures. We also investigate interband peaks and average slopes.

There is a significant evolution of spectral properties of OCs, CCs, and diogenites with low temperatures in the visible to near-infrared. This includes changes in the position, depth (Figure 1), and width of the absorption bands at about 0.7, 1, 2, and 3 µm, as well as changes in the position of the interband peaks at around 0.55, 0.7, and 1.5 µm. The spectral slope is also modified. Spectral changes induced by low temperatures can influence the interpretation of the composition and degree of hydration of meteorites and planetary bodies.

The MMX InfraRed Spectrometer (MIRS) (Barucci et al., 2025) instrument onboard MMX will perform a spectral characterization of the Martian system between 0.9 and 3.6 mm. Although the specific mineralogy of the moons is currently difficult to determine, due to the lack of a strong absorption band in the VNIR, possible weak bands have been discussed and CCs have been highlighted as potential analogs (Rivkin et al., 2002b; Fraeman et al., 2012; 2014). If absorption bands are observed, the effects of low temperatures on their properties must be considered. In fact, surface temperatures on Phobos could vary between 100 K and 320 K (Giuranna et al., 2011; Sultana et al., 2025). With a spectral resolution of <32 nm, the instrument will be sensitive to the band’s position shifts as well as changes in the amplitude and width. MMX will also observe the Martian moons using the Optical RadiOmeter composed of Chromatic Imagers (OROCHI) instrument using seven spectral bands between 0.39 and 0.95 µm (Kameda et al., 2021). If the 0.7-µm absorption band is observed, low temperature effects must be considered for an optimal interpretation of the spectral band (Figure 2). MMX will return Phobos samples to Earth in 2031. We encourage to consider the effects of low temperature on spectral properties when comparing the future sample studied in the laboratory with remote sensing data. This work extends beyond the Martian moons and the MMX mission, and several other missions such as Hera (ESA), Lucy (NASA), Tianwen-2 (CNSA) and EMA (UAE) are concerned.

Figure 1: Spectral modifications induced by low temperatures on the band depth (BD) of the absorption band around 1 mm for three CCs.

Figure 2: Simulated MMX OROCHI spectra of Murchison (CM) meteorite, converted from laboratory reflectance measurements

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