The SiO 2 abundance on the surfaces of the Moon and Mercury

The SiO2 abundance on rocky planetary surfaces is a key indicator for planetary crust composition and magmatic evolution. The Christiansen Feature (CF) in the mid-infrared provides a mineralogically grounded proxy for the abundance of SiO2. Here, we present a new experimental calibration based on synthetic glasses with SiO2 concentrations from 0.5 wt.% to 97.6 wt.% SiO2. The CF position shows a strong dependence on the SiO2 concentration, best described by a second-order polynomial (R2 = 0.957). To test the reliability of the new calibration we apply it to Diviner-derived CF data for the surface of the Moon to produce a global map of SiO2 on the lunar surface. The results reproduce the mare-highland bimodality and agree well with independent constraints from all lunar sample-return sites. We derive SiO2 abundances in high-silica regions, including Gruithuisen, Hansteen Alpha, and Lassell Massif with up to 76 wt.% SiO2. This is the first study that quantitatively derives the high SiO2 abundances expected in these high-silica regions with an independent compositional calibration. Finally, the new calibration allows the determination of SiO2 on the surface of Mercury. Using an existing Earth-based CF measurement, we obtain a low abundance of ~37 wt.% SiO2. High spectral and spatial resolution observations of Mercury by the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) on the BepiColombo mission will allow a global application of this new approach, which may be extended to other planetary bodies in the future.