Low-temperature reflectance spectra of meteorites: implications for space missions

Introduction:
The VNIR reflectance spectrum of a surface depends on its composition, physical properties, and the interactions between the surface and the environment. The temperature dependence of spectral properties may complicate the interpretation of planetary surfaces’ reflectance, but can also provide valuable information about their composition and state.

The VNIR reflectance spectra of common rock-forming minerals vary with temperatures [1][2][3][4], and these spectral changes can bias the interpretation of remote sensing data [5][6][4]. For example, Lucey et al. [7] showed that the A-type asteroids' spectra differ from those of olivines under terrestrial conditions but are consistent with those of olivines at the low surface temperatures expected on the main belt asteroids. Thereafter, Lucey et al. [8] detected temperature-dependent variations in the spectral properties of Eros, highlighting the importance of considering the effect of low temperatures on surface properties.

However, less has been done concerning carbonaceous chondrites. In this study, we report comparative low-temperature laboratory measurements of a panel of meteorites. This work may help us to understand the mineralogy of meteorites, but also to gain a better understanding of the solar system small bodies.

 Samples:
We studied carbonaceous chondrites from four different groups (CI, CM, CV, and CO), one ungrouped carbonaceous chondrite (UCC), an ordinary chondrite, and a HED meteorite (diogenite). More specifically, we used the CI Orgueil, the CMs Murchison and DOM 08003, the CVs QUE 94688 and Axtell, and the CO ALH 85003. The meteorite Tarda was used (UCC). Additionally, we studied the ordinary chondrite LL Saint-Severin and the diogenite Bilanga, whose possible parent body is the differentiated asteroid Vesta.

Measurements:
Low-temperature reflectance spectroscopy measurements were acquired at IPAG (France) using the bidirectional reflectance spectrogonio-radiometer SHINE (SpectropHotometer with variable INcidence and Emergence) [9]. Spectra were acquired from 400 to 4000 nm with an incidence angle of 0° and an emission angle of 20°. SHINE was coupled with the double environmental chamber CarboN-IR [10]. With this setup, we performed measurements from about 280 K to a minimum of 70 K with several intermediate temperature steps. The temperature error is estimated to be 1 K and the measurements were led in an Ar-filled inner cell, ensuring good thermalization to the granular samples. All of our samples were in the form of powder.

Results and implications for space missions:
Spectral changes induced by temperature were observed for all classes of meteorites, and these changes were generally linear as a function of temperature.

With decreasing temperatures, the band at 1000 nm associated with olivine and/or pyroxene generally becomes deeper (Bilanga, Axtell, QUE 94688) and narrower (Bilanga, Saint Severin, Axtell) (Figure 1 A, D). There is no trend in the change in minimum reflectance values associated with temperature variations, but the position of the minimum may shift towards longer (Axtell) or shorter (Bilanga) wavelengths.

Concerning the band at 2000 nm associated with pyroxene, although the minimum reflectance values do not seem to vary with temperature, the band shifts towards shorter wavelengths (Bilanga, Saint Severin, ALH 85003) (Figure 1 B, E). In addition, the band deepens (Bilanga, Saint Severin, ALH 85003) with decreasing temperatures. The only significant change in bandwidth is for Bilanga, for which the band becomes narrower.

With decreasing temperatures, the band at 3000 nm shifts towards longer wavelengths (Bilanga, DOM 08003, Tarda). In general, the band deepens (Bilanga and DOM 08003) and becomes narrower (Orgueil, Bilanga, Axtell, and DOM 08003). The slope between 2900 and 3900 nm increases at low temperatures (Orgueil, Bilanga, Saint-Severin, ALH 85003, QUE 94688, Murchison, DOM 08003) (Figure 1 C, F).

Other spectral features can be impacted by changes in temperature such as the maxima at 550 nm and 1500 nm, and the band at 700 nm.

Our results are important for the analysis of sample returns from the Hayabusa, Hayabusa2, and OSIRIS-REx missions, and the remote sensing measurement interpretations of the JAXA/MMX, ESA/Hera, ESA/Ramses, and JAXA Hayabusa2 extended missions. For example, the spectral properties of Phobos in the VNIR show that it could be analogous to a carbonaceous chondrite. Its surface temperatures vary significantly from 130 K to 350 K [11]. For a correct interpretation of the surface properties by the MMX Infrared Spectrometer (MIRS) [12], it is necessary to consider the spectral variations induced by temperature variations.

           

Figure 1: Reflectance spectra at different temperatures of (A) Bilanga, (B) Saint-Severin, and (C) DOM 08003. (D) shows the 1000 nm bandwidth changes for Bilanga. (E) shows the minimum reflectance of the 2000 nm band of Saint-Severin, and (F) shows the slope changes for DOM 08003.

 

Conclusion:
Our work shows the importance of surface temperature in the interpretation of VNIR reflectance spectra. Based on our present sample set, it seems that the spectral alterations generally show similar trends between ordinary chondrites, carbonaceous chondrites and achondrites. The data interpretation of several space missions may be affected by the present work, and future improvements of the quality and spectral resolution of the space mission measurements will increasingly necessitate taking into account the spectral alteration due to surface temperature.

 

References:
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[3] Schade, U., & Wäsch, R. (1999).
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[7] Lucey, P. G., Keil, K., & Whitely, R. (1998).
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