Updating the Mercury Mean Spectra using 4.7 millions MASCS Spectra

Equipped with a suite of instruments looking for observing Mercury’s surface, the MESSENGER mission was able to retrieve resolved reflectance spectral parameters of most of the planet during its 5 years in orbit. Of particular interest are the ultraviolet-visible to near-infrared properties, which are diagnostic of Mercury’s diversity. Using the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument, the MESSENGER science team was able to define the average spectral response of Mercury’s surface from 300nm to 1400nm (Izenberg et al. 2014). This work makes use of more than 850 000 spectra acquired during the primary mission. Together with Mercury’s average spectra, the authors also characterized the spectral properties of various geological units (e.g. pyroclastic deposits, hollows, etc.). The spectral properties of Mercury’s units can also be compared to measurements derived from the Mercury Dual Imaging System (MDIS) imagers (Murchie 2018). While the absolute values may differ (for instance due to systematic differences in observing geometries and thus photometric corrections with a higher phase angle reprojection used for MASCS), the overall trends remain the same between the two instruments.

In this work, we present a revisited analysis of the Mercury Mean Spectra (MMS) together with spectral characteristics of Mercury geological and geochemical units. With respect to the previous analysis (Izenberg et al. 2014, Murchie et al. 2018), we make use of all the observations from the MASCS spectrograph from all mission phases (i.e. up to 4.7 million observations). Spectral properties are defined from 300nm to 1400nm using an improved inter-calibration of the two spectral channels (Besse et al. 2015). Analysis done after the end of the MESSENGER mission have allowed the scientific community to be more accurate in defining units on the surface of Mercury. Numerous units have been determined, for example, by geological/morphological and/or geochemical boundaries (i.e. High-Mg region). Making use of the Mercury Surface Spectroscopy (MeSS) architecture we can select more accurately and in greater number MASCS footprints that provide the representative spectral properties of a given unit. The MeSS PostGreSQL database project at ESAC was initially created to host MASCS MESSENGER science data and to provide a user friendly way to query MASCS data at once, for instance to characterise the spectral properties of surface features (Besse et al., 2020 ; Barraud et al. 2021). At this stage the MeSS database includes calibrated MESSENGER’s MASCS data together with the corresponding high-level products computed by the MeSS science team using those calibrated data. This methodology, by greatly increases the number of spectra used and improves significantly the signal-to-noise of the resulting average spectra.

The improved characterization of Mercury’s spectral units lets us better understand the properties of Mercury’s surface, and potential links to laboratory measurements. As shown in the case of hollows (Barraud et al 2022), the determination of an accurate average spectra for hollows (and other spectral units) is a necessary step in using laboratory measurements to decipher the chemical and physical properties of Mercury’s surface.