A First Look at Mars with PanCam: ExoMars 2022 Spectral Instrument Suite Emulator Observations of Martian Meteorites

Introduction:  The ExoMars 2022 Rosalind Franklin rover is scheduled to be launched in summer 2022 with a suite of instruments to investigate the Martian surface and near sub-surface [1]. The context instruments: the Panoramic Camera (PanCam), composed of the two Wide Angle Cameras (WACs), and High Resolution Camera (HRC), and the Infrared Spectrometer for ExoMars (ISEM) will be imperative in the selection of drill and analysis sites. The PanCam stereo imaging system will be the primary mode of scientific observation during the mission with two multispectral WACs in the Visible to Near Infrared (VNIR, 440-1000 nm) range mounted at the top of the 2 meter mast [2]. Within the 36° field of view of  the PanCam WACs, HRC will provide 5° field of view colour images at up to submillimetre resolutions [2]. Lastly, ISEM can them be utilised within the WAC/HRC field of views to provide 1° spot size, hyper-spectral coverage in the Near to Mid (1150-3300 nm) Infrared range [3]. In preparation for the mission, spectral analysis tools are being developed to automate as much of the analysis process as is feasible to reduce time and effort costs during mission tactical planning and analysis, as well as improving ability to discriminate between different minerals of interest. Here we report on our effort to constrain the spectral and spatial response of the context instruments for ExoMars using Martian meteorite targets

Martian Meteorite Imaging: This study used instruments emulators for PanCam WAC, ISEM (extended to 300-2500 nm range to provide coverage of PanCam filter wavelengths for comparison) and HRC to investigate the spectral response of a variety of SNC meteorites, to determine the instrument spectral and spatial capabilities and build reliable mission analysis tools. Preliminary analysis has been undertaken on the largest of the Martian meteorite samples. Meteorites were imaged at minimum mission configuration, in semi-directional lighting and operated under mission-similar protocols [2]. The Shergottite Tissint, BM2012, M1, was imaged to assess the instrument ability to distinguish visual and spectral features in the fresh face of the specimen. The PanCam emulator data was first flat-fielded, and environmentally colour corrected and radiometrically corrected using the ExoSpec Software developed by the PanCam science team [4].  

Preliminary Results. The fresh face of Tissint is comprised of olivine macrocrysts with black glass veins [5]. These features can be distinguished visually in both the WAC and HRC images (Figure 1). These features, however, are too small to target alone with the hyperspectral instrumentation. To probe the spectral response of these regions a 530-570-670 decorrelation stretch  was applied to highlight variation associated with a characteristic olivine spectral feature in this region, shown in Figure 2. This decorrelation stretch does show significant spectral variation between the dominant face material and the black glass veins.

Figure 1. Tissint BM.2012,M1, (Left) RGB imaged with RWAC PanCam emulator at 2m. (Right) Colour imaged with HRC emulator at 2 m.

The spatial resolution difference in the instrument, an order of five times higher for HRC at this distance, is clear. The multispectral data show a similar shape profile to the hyper-spectral data set over approximately the same region of interest (ROI) figure 3 with an offset in reflectance due in part to the instrument’s throughput of the different  narrow band filters [6], shown in Figure 3. A contribution could also be present from the viewing angle of the sample versus the target, hence surface corrections will also be investigated in this instance. To further aid in the detection of minerals of interest, the hyperspectral data will be utilised to develop a spectral parameter map to target olivine. This map will then be tested on the Tissint WAC images to evaluate the information yield.

Figure 2. Tissint BM.2012,M1, LWAC with decorrelation stretch applied at 530-570-670 (represented as red, green and blue respectively).

Figure 3. Tissint BM 2012,M1, VNIR reflectance spectra from PanCam and ISEM emulator, reflectance offset cause by the filter throughput and anomaly at 740nm where WAC transition occurs [4], (inset) Tissint BM2012, M1 with PanCam and Hyperspectral ROI.

Following the completion of the Tissint, the type specimens of the SNC group: Nahkla BM1913,26, Shergotty BM 41021 and Chassigny BM 1985M173, (Figure 4) will then be investigated to compare the spectral properties of the samples and the instrument ability to discriminate between them. This will inform mission processing while analysing a target against the Martian surface.

Figure 4. HRC colour images of other target Martian SNC’s (a) Nahkla BM 1913,26, (b) Chassigny BM1985M173 (c) Shergotty BM 41021 at 2m distance.

Further Meteorite Imaging: Iron, pallasite and chondritic meteorites have also been imaged to add variety to the spectral sampling pool. As well as in the interest of the instrument spectral and spatial response, meteorite finds on Mars have been a rising topic of interest [7], they represent an interesting avenue to study questions of habitability beyond earth [8, 9]. We also aim to develop methods of identifying meteorite targets on Mars from spectral and visual feature detection for the ExoMars instrument pipelines.

References: [1] Vago J. L. et al. (2017) Astrobiology, 17, 471-510 [2] Coates A. J. et al. (2017) Astrobiology, 17, 511-541. [3] Korablev O. I. et al. (2017) Astrobiology, 17, 542-564. [4] Allender E. J. et al. (2018) Image and Signal Processing for remote sensing XXIV, 10789, 1078901. [5] Chennaoui Aoudjehane H. et al. (2012) Science, 338, 6108, 785-788. [6] Cosuins. C. R. et al. (2012) Planetary and Space Science, 71, 80-100. [7] Ashley J. W. (2015) CosmoELEMENTS, 10-11 . [8] Schröder C. et al. (2016) Nature communications, 7, 13459. [9] Tait. A. W. (2019) LPSC L, #1387.