A laboratory study of the photometric properties of Mars Global Soil Simulant MGS-1 and its variants

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Highlights

  • Measured the hyper-spectral bi-directional reflectance of Martian soil simulants MGS-1 and its three variants.

  • Performed photometric inversions using the Hapke model to obtain the single scattering albedo and phase functions.

  • Similar and different optical properties of the simulant and Mars in-situ data have been discussed.

Abstract

Laboratory photometric measurements of Martian soil simulants can provide useful information for analyzing in-situ optical measurements on the Martian surface. We measured the hyper-spectral bidirectional reflectance of Mars Global Soil Simulant MGS-1 and its three variants. By performing photometric inversions, we obtained the single scattering albedo, phase function, surface roughness, and the opposition effect parameters at eight discrete wavelengths, 432 ​nm, 450 ​nm, 530 ​nm, 601 ​nm, 750 ​nm, 900 ​nm, 1200 ​nm, and 1700 ​nm. By comparing these results with the in-situ measurements of Martian soils by the Mars Exploration Rovers Spirit and Opportunity and the Mars Science Laboratory Curiosity, we find the MGS-1 has very similar spectral single scattering albedo with the Curiosity soil, and all of its three variants have higher single scattering abledos than the Martian soils. However, the MGS-1S is spectrally similar to one of the sulfur-rich areas on Mars. The retrieved single scattering phase functions of all simulants exhibit stronger forward single scattering features than the average Martian soils measured by MER Spirit and Opportunity. The retrieved surface roughness parameters of the Martian soil simulants are between 17° and 26°, and this large roughness value is inconsistent with the macroscopic flatness of the sample surface, implying that the small-scale roughness plays an important role in the measurement. It was also found that the roughness parameters have clear anti-correlation with the single scattering albedo. In addition, the phase reddening effect was observed in all soil simulants and the maximum value of the color ratio is not sensitive to the incident zenith angle, but the phase angle at which the maximum value of the color ratio is located increases with incident zenith angle. Our results suggest that the similarities and differences of the physical parameters (i.e., the single scattering albedo, phase function, surface roughness, and opposition effect parameters) within the Martian soil simulants should be carefully considered in the interpretation on the hyper-spectral data, in particular the photometric data for the in-situ Martian observations.

Introduction

With the landings of Mars landers such as the Mars Exploration Rovers (MER) Spirit and Opportunity (Bresina et al., 2005), the Mars Science Laboratory (MSL) Curiosity (Grotzinger et al., 2012), the Tianwen-1 (Zou et al., 2021), and the Mars 2020 (Farley et al., 2020), more and more reflectance spectra with high spatial resolutions of the Martian surface have become available for scientific analysis. The spectral features such as spectral slope and absorption band depth are dependent on measurement geometries (i.e., the incident and viewing directions and the phase angle) and most in-situ measurements are carried out in various geometrical configurations (e.g., Shepard and Cloutis, 2011; Souchon et al., 2011; Hapke, 1993). The angular scattering behaviors of the Martian surface can provide important information on the physical properties of the regolith, such as single scattering albedo, particle transparency, size distribution, porosity, and surface roughness (Hapke, 1993). The accurate interpretation of the in-situ measurements would heavily rely on laboratory measurements on returned samples or analogs. Due to the lack of returned Martian samples, researchers have to rely on analog materials to simulate the mechanical, physical, chemical, and mineralogical properties of the Martian surface (e.g., Allen et al., 1998; Peters et al., 2008; Zeng et al., 2015; Stevens et al., 2018; Clark et al., 2020). The photometric properties of some Martian analogs have been studied by several groups (Georgiev and Butler, 2005; Johnson et al., 2013; Pommerol et al., 2013). For example, Johnson et al. (2013) carried out multispectral reflectance measurements of eight Martian soil simulants with different size distributions and added spherules on soil simulants to mimic the spherule-rich soils observed by MER Opportunity. The single scattering phase functions for Martian soil simulants measured by this group are similar to that of the Martian soils observed by MER Spirit. The added spherules greatly reduced the single scattering albedo, increased surface roughness, decreased the opposition angular width, and significantly increased backscattering. Pommerol et al. (2013) performed laboratory bidirectional reflectance measurements on dry, wet, and frozen surfaces of analogs including the JSC Mars-1 soil simulant and Hawaiian basaltic sand, and the results provided implications for identifying wet and icy soils on Mars.

So far, the published results of the photometric studies of Martian soil simulants mainly focused on JSC-1/1 ​A Mars soil simulants with compositions based on remote sensing data from bright regions of Mars. As more missions to Mars have been carried out, there has been an increasing need for more analogs to represent more types of soils on the Martian surface. In particular, Cannon et al. (2019) produced Mars soil simulants named Mars Global Soil Simulant (MGS-1) and its three variants. MGS-1 was designed to replicate Rocknest windblown deposit at Gale Crater (Cannon et al., 2019). Rocknest is a typical Martian basalt soil. It is the first sand deposit investigated by Curiosity, and its mineralogy measured by the CheMin (Chemistry and Mineralogy) X-ray instrument onboard Curiosity shows striking similarity with the estimated modal mineralogy information provided by the MER Spirit and Opportunity rovers (Achilles et al., 2017). Cannonet al. (2019) compared the reflectance spectra of the MGS-1 with that of the Rocknest soil and low albedo regions on Mars and found that MGS-1 has high similarity in spectral slope and band depth with the in-situ data but is brighter at all wavelengths. Variants of the MGS-1 include MGS-1C Clay ISRU (MGS-1C), MGS-1S Sulfate ISRU (MGS-1S), and JEZ-1 Jezero Delta Simulant (JEZ-1). Both the MGS-1C and MGS-1S can be used in the NASA Mars Water In-Situ Resource Utilization Study (Abbud-madrid et al., 2016. JEZ-1 was made to simulate the anticipated materials in the Jezero Crater delta which the NASA Mars 2020 rover is investigating. The photometric properties of these Martian soil simulants have not been reported yet.

In this work, we perform laboratory spectral bi-directional reflectance measurements on MGS-1 and its three variants, MGS-1S, MGS-1C, and JEZ-1. We first describe the samples, laboratory instruments, and data reduction procedures in Section 2. Then we present the measurement results and photometric model inversions. The photometric parameters of the samples have been obtained and compared with both other Martian soil analogs and the in-situ measurements. Finally, discussion and conclusions are made.

Section snippets

Samples descriptions

The Mars Global Simulant-1 (MGS-1) and its variant MGS-1S, MGS-1C, and JEZ-1, shown in Fig. 1, were produced by the CLASS Exolith Lab. The detailed information of the samples including the bulk chemistry can be found at https://sciences.ucf.edu/class/exolithlab and here we only summarize the mineralogical compositions of these samples in Table 1. The MGS-1S and MGS-1C are the modified versions of the MGS-1 and are enriched in polyhydrated sulfate gypsum and hydrated clay minerals (e.g.,

Reflectance spectra

Fig. 5a displays the reflectance spectra of MGS-1, typical Mars soil analog JSC-1A (data from Cannon et al. (2019) and our lab measurement), in-situ Rocknest soil and Mars remote sensing observations. Compared to JSC-1/1 ​A, which is similar in specta to the bright regions of Mars, MGS-1 is broadly similar in shape and magnitude to the spectra of Rocknest soil and Observatoire pour la Mineralogie, l’Eau, le Glace e l’Activitè (OMEGA) measured low albedo terrains. In particular, the absorptions

Model fitting results for laboratory measurements

In order to make easy comparisons with Mars in-situ measurements [Johnson et al., 2006a], model parameters at selected wavelengths, 432, 450, 530, 601, 633, 750, 900, 1200, and 1700 ​nm are summarized in Table 3, and the model-reconstructed BRDF at 601 ​nm for i ​= ​8° and i ​= ​45° are shown in Fig. 8. Due to the high noise level for wavelength larger than 1000 ​nm, the signal at 1200 and 1700 ​nm are obtained by averaging the data points from 1100 to 1300 ​nm and 1600–1800 ​nm, respectively.

Conclusions

We have measured the hyper-spectral bi-directional reflectance of Martian soil simulants MGS-1 and its three variants, the MGS-1S, MGS-1C, and JEZ-1, and obtained their Hapke model parameters by performing photometric inversions. The main results are.

  • (1)

    The spectral single scattering albedo of the MGS-1 is very close to that of the Curiosity soil over the latter one's measurement wavelength region. All of its three variants have higher single scattering albedos than the Martian soils, but the

Author-statement

All measurement data in this work can be downloaded from: https://doi.org/10.5281/zenodo.7426258.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank Frédéric Schmidt and an anonymous reviewer for helpful comments. This work was supported by the National Natural Science Foundation of China (12073024, 11941001), the State Key Laboratory of Lunar and Planetary Science at Macau University of Science and Technology (SKL-LPS(MUST)-2021-2023), the Civil Aerospace Pre-research Project (D020302; D020202), and the Science and Technology Development Fund, Macau (0020/2021/A1).

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