LAUSR

Abstract Quadrangle Ac-H-10 ‘Rongo’ (Lat 22°S to 22°N, Lon 288°–360°E) shows a fairly homogeneous topography, with the presence of notable elevations such as Ahuna Mons, Liberalia Mons, and part of Samhain and Uhola Catenae. The deepest areas correspond to the Rongo crater region, the areas between Samhain and Uhola catenae, and the region of the quadrangle south of Ahuna Mons. A substantial variability in the 2.7-µm band depth distribution is observed across the Rongo quadrangle, indicating an east-west gradient in the abundance of Mg-phyllosilicates. The NH4-phyllosilicates distribution appears quite homogeneous, except some localized regions, such as crater Haulani's ejecta, the flanks of Ahuna Mons, and crater Begbalel. The two band depths at 2.7 and 3.1 µm display an overall low correlation, suggesting a variable degree of mixing between Mg-phyllosilicates and NH4-phyllosilicates. At the local scale, mineralogical phases other than phyllosilicates are observed. Quadrangle Rongo includes sodium carbonate-rich regions, such as the flanks of Ahuna Mons, the ejecta of Xevioso crater located in the southern edge of Liberalia Mons, and crater Begbalel, which often display a reduction in both the 2.7- and 3.1-µm band depths, associated with an increased band depth at ∼4 µm, related to the presence of Na-rich carbonate phases. This suggests recent hydrothermal activity in this area, due to several episodes of cryovolcanism, or impacts that unveiled a peculiar composition in the shallow subsurface. Alternatively, the crust in this region might show a variable degree of compactness, such that the formation of Na-carbonates is favored only in specific locations (De Sanctis et al., 2016; Ruesch et al., 2016; Zambon et al., 2017). From a geological standpoint, quadrangle Ac-H-10 Rongo shows a correlation between its two main geologic units (Platz et al., 2017) and the distribution of Mg-phyllosilicates, suggesting a link between geology and mineralogy in this area.