Venus is the most Earth-like of the terrestrial planets, though very little is known about its surface composition. Thanks to recent advances in laboratory spectroscopy and spectral analysis techniques, this is about to change. Although the atmosphere prohibits observations of the surface with traditional imaging techniques over much of the EM spectral range, five transparent windows between ~0.86 µm and ~1.18 µm occur in the atmosphere’s CO2 spectrum. New high temperature laboratory spectra from the Planetary Spectroscopy Laboratory at DLR show that spectra in these windows are highly diagnostic for surface mineralogy [1]. The Venus Emissivity Mapper (VEM) [2] builds on these recent advances VEM is the first flight instrument specially designed to focus solely on mapping Venus’ surface using the windows around 1 µm. Operating in situ from Venus orbit, VEM will provide a global map of composition as well as redox state of the surface, enabling a comprehensive picture of surface-atmosphere interaction on Venus. VEM will return a complex data set containing surface, atmospheric, cloud, and scattering information. Total planned data volume for a typical mission scenario exceeds 1TB. Classical analysis techniques have been successfully used for VIRTIS on Venus Express [3-5] and could be employed with the VEM data. However, application of machine learning approaches to this rich dataset is vastly more efficient, as has already been confirmed with laboratory data. Binary classifiers [6] demonstrate that at current best estimate errors, basalt spectra are confidently discriminated from basaltic andesites, andesites, and rhyolite/granite. Applying the approach of self-organizing maps to the increasingly large set of laboratory measurements allows searching for additional mineralogical indicators, especially including their temperature dependence. [1] Dyar M. D. et al. 2017 LPS XLVIII, #1512. [2] Helbert, J. et al. 2016. San Diego, CA, SPIE. [3] Smrekar, S.E., et al. Science, 2010 328(5978), 605-8. [4] Helbert, J., et al., GRL, 2008 35(11). [5] Mueller, N., et al., JGR, 2008 113. [6] Dyar M. D. et al. 2017 LPS XLVIII, #3014.

To support the data analysis for the MErcury Radiometer and Thermal Infrared Imaging Spectrometer (MERTIS) instrument on the ESA-JAXA BepiColombo mission, we have measured the thermal infrared emissivity of finely grained silicates (<25 μm grain size) at different temperatures under vacuum to simulate the daytime conditions on the surface of Mercury. The silicates were selected to represent the mineralogy of Mercury as closely as possible (Helbert et al., 2007; Namur and Charlier, 2017; Vander Kaaden et al., 2017). The set includes one olivine (a Mg-rich forsterite), three pyroxenes (diopside, enstatite, and hypersthene), five feldspars (plagioclase group; anorthite, labradorite, andesine, oligoclase, and K-feldspar; microcline) and a feldspathoid (nepheline). The emissivity measurements for each mineral was carried out within the MERTIS spectral range of 7-14 μm with temperatures increasing from 100 C up to 500 C under vacuum (~0.1 mbar). The relationships between the spectral parameters such as the Christiansen Feature (CF) position, first Reststrahlen band (RB1) position, RB1 emissivity, and RB spectral contrast and temperature were investigated for all silicates. The study shows that the RB1 position shifts to longer wavelengths, RB1 emissivity decreases, and RB spectral contrast increases with increasing temperatures for all silicates studied. We apply the plot of CF vs RB1 as a tool to discriminate the major silicate groups such as feldspars, pyroxenes, and olivine, regardless of the temperatures at which they were measured. The CF vs RB1 plot can facilitate the first order discrimination of the mineralogy of Mercury’s surface with MERTIS. Moreover, this approach can be more widely used to map the igneous surface mineralogy of silicate targets such as the Moon, Mars, and S-type asteroids in the 7-14 μm spectral region with remote sensing from orbit and ground-based telescope observations.

Global mapping of the nature and distribution of volatiles such as sulfides on Mercury’s surface is essential for understanding the thermal evolution of the planet. The surface exposure of these sulfides over extreme day-night temperature cycles (176 days; 450 degC to -170 degC) on Mercury leads to thermal weathering of these sulfide compounds. It has been seen that among the proposed sulfides on Mercury (MgS, FeS, CaS, CrS, TiS, NaS, and MnS), CaS showed relatively stable and distinctive spectral features in the thermal infrared region (TIR; 7-14 μm) when studied under the simulated Mercury day conditions for temperatures ranging from 100 degC up to 500 degC under vacuum (0.1 mbar) (Varatharajan et al., 2019). In this study, we re-investigated the stability of CaS and its spectral emissivity spectral behavior. We exposed the sample for four consecutive Earth days simulating Mercury day cycles and measured the TIR spectra of CaS for temperatures up to 500 degC (with steps of 100 degC) every day. This time the spectral analysis is coupled and supported by XRD diffraction on the fresh and temperature-processed sample, showing the mineralogical evolution with temperature. We confirm that CaS is a stable compound and therefore it would remain stable on Mercury’s surface regardless of investigated peak surface temperatures. This study further implies that, for the hollows dominated by the sublimation of sulfides on Mercury (Blewett et al., 2013; Helbert et al., 2013a; Vilas et al., 2016), CaS could be the last of the sulfides that could be mapped on Mercury as other sulfides were lost by thermal decomposition, leaving behind hollows. This could make CaS an important tracer for other sulfides, which might be lost in the hollow-forming process and supports the detection of CaS within hollows by MESSENGER (Vilas et al., 2016). The emissivity spectra reported here are significant for the detection and mapping of CaS associated with hollows and pyroclastics using the Mercury Radiometer and Thermal Imaging Spectrometer (MERTIS) datasets.

The visible-infrared spectra of Mercury’s surface show little variation, displaying no distinct spectral features except for the possible spectral identification of sulfide within the hollows (Vilas et al. 2016). It is essential therefore to define and map any subtle spectral heterogeneity across Mercury’s surface and to correlate these differences where possible to geomorphological features, such as impact craters, volcanic vents, and tectonic features. The Mercury Atmospheric and Surface and Composition Spectrometer (MASCS) instrument onboard MESSENGER spacecraft is the only hyperspectral reflectance spectrometer to date that has mapped Mercury’s surface in the wavelength range 320 nm - 1450 nm. The limitation of MASCS is that it’s a point spectrometer that mapped Mercury’s surface at non-uniform spatial scale. In this study, we resampled the global MASCS hyperspectral dataset to a uniform spatial resolution of 1 pixel per degree. This enabled us to perform global multivariate analyses, including standard spectral parameter maps, k-means clustering, and principal component analysis (PCA) to spectrally characterize Mercury’s surface. Among these techniques, PCA significantly improved the identification of spectral heterogeneities across Mercury correlated to both chemical and physical properties of the surface, enabling us to identify units based on grain size, the presence of amorphous materials, and space-weathering associated alterations. The global MASCS PC color-composite map derived from principal components 1, 2, and 6 effectively distinguishes varying spectro-morphologies across Mercury’s surface, highlighting the spectral properties of various geochemical terrains. We further demonstrate that PCA spectrally differentiates between the two northern volcanic plains’ geochemical regions; the high-Mg and low-Mg terrains.

Specialized spectral library measured under controlled planetary surface conditions is important to accurately derive the chemical and physical properties from remote observations. It’s a general practice to powder the planetary analogues during spectroscopy studies as most surfaces are made up of fine-regolith materials. However, upon arrival at C-type asteroids Ryugu and Bennu, Hayabusa2 and OSIRIS-REx revealed these surfaces filled with rocks and boulders. In this study, we built a phase angle dependent ultraviolet (UV) to far-infrared (FIR) spectroscopy (0.2-100 µm) of a rocky piece of Mukundpura meteorite having five surfaces including fusion crust. Mukundpura meteorite is the freshest carbonaceous chondrite belonging to CM-chondrites in the entire collection which fell in the desert village of India on June 6, 2017. The two sets of varying viewing geometries having incident and reflectance angles includes ; a) asymmetric viewing geometry at 13°-13°, 13°-20°, 13°-30°, 13°-40°, and 13°-50°, and b) symmetric viewing geometry at 13°-13°, 20°-20°, 30°-30°, 40°-40°, and 50°-50°. This study found that overall spectral shape, reflectance values, and band depth of diagnostic absorption features are affected by viewing geometry and surface roughness; however, the fundamental band centers are not affected. The comparison of 2.72 µm absorption band of fusion crust and fresh interiors of Mukundpura with published Ryugu and Bennu spectra supports that Ryugu surface has experienced extensive heating in its geologic past compared to Bennu. Overall study shows that fusion crust and internal surfaces of the Mukundpura meteorite is a potential analogue of Ryugu and Bennu both spectrally and morphologically.