OBSERVATIONS OF A POTENTIALLY HYDRATED PYROCLASTIC MANTLE OVER THE MENELAUS DOMES/TACQUET FORMATION OF SOUTHERN MARE SERENITATIS

The Tacquet Formation was noted as a distinct map unit in the geologic map of Mare Serenitatis by Carr (1966). The unit was also called out by Carter et al. (2009) as having anomalously low circular polarization ratios in S band radar data, similar to those of known lunar pyroclastic deposits. Two domes within the Tacquet Formation (TF) were described by Lena et al. (2013) as the Menelaus domes. The TF and Menelaus domes are characterized by several distinctive compositional properties with respect to the surrounding mare units. They are lower in albedo, higher in TiO₂, higher in FeO, and have pyroxene 1 and 2 μm band minima positions and band depths distinct from the surrounding mare.

Application of a “Wavelength Mapper” approach (van Ruitenbeek et al., 2014) that determines the band minimum position and band depth of spectral absorption features within a desired wavelength interval was applied to M³ data over the area. Areas on or near the domes have marginally longer “1 μm” and shorter “2 μm” band minima positions. This is consistent with higher Fe²⁺ content pyroxenes (Klima et al., 2011). Examination of higher spatial resolution Lunar Reconnaissance Orbiter Camera Wide Angle Camera (LROC WAC) and of SELENE Kaguya Multiband Imager data provided more detail than global maps, indicating that the western Menelaus dome is higher in TiO₂ and that it and the area to the east has higher FeO than the surroundings.

Using a thermal correction approach for longer wavelengths of M³ data, near the 3 μm water and OH absorption feature, based on the anisothermal, roughness-incorporating correction of Bandfield et al. (2018), recently updated by Tai Udovicic et al (this meeting), it was found that the TF has deeper absorption than the units to its south and north. This finding is consistent with that of Milliken and Li (2017) finding increased hydration in association with some lunar pyroclastic deposits. The association of increased OH or H₂O in association with the TF bolsters the case that it is a mantling deposit as suggested by the radar polarization evidence of Carter et al. (2009).