ENHANCEMENT OF OH/H2O ASSOCIATED WITH THE COMPTON-BELCOVICH VOLCANIC COMPLEX ON THE FARSIDE OF THE MOON: ENDOGENIC OR EXOGENIC LUNAR WATER?

The presence of OH/H₂O has been identified in three distinct regions of the Moon. First, the positive identification of inherent water in samples revealed the presence of water in its deep interior. Second, the distribution of OH/H₂O was remotely measured and mapped across the surface in the likely form of a surface layer on the regolith, with stronger abundances in polar regions. Third, the presence of polar water buried in the upper few meters of regolith was first measured remotely and in situ by LCROSS. Subsequent to these discoveries, hydroxyls have been identified in association with the central peak of Bullialdus crater, the formation of which likely exposed a deep-seated KREEP-bearing Mg-Suite lithology. Here we describe the enhancement in OH/H₂O associated with the lunar farside Compton-Belcovich volcanic complex as measured by the Moon Mineralogy Mapper (M³) experiment flown on Chandrayaan-1.

Spectral data from M³ reveal the presence of a strong 3.0µm absorption (suggesting the presence of OH/H₂O) band directly associated with the regolith in Compton-Belcovich, significantly stronger than the background regolith outside of the volcanic feature. Data from the Diviner instrument on the Lunar Reconnaissance Orbiter (LRO) show that the Compton-Belcovich is silicic in composition. Investigations of other silicic volcanic sites by other authors has shown that those other sites are also enhanced in OH/H₂O implying that perhaps the source of the water may be internal. However, we must also consider that the hydration is due to preferential retention of OH/H₂O by the silicic mineralogy at these volcanic sites. Data from Deep Impact flybys of the Moon suggested that there was preferential trapping of OH/H₂O in the highlands relative to the mare; perhaps we are detecting a variation of that at Compton-Belcovich. The Compton-Belcovich volcanic complex is also observed to be mantled by a block-poor deposit, illustrated by data from the Mini-RF instrument on LRO.