RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 105, NO. 5, 10 SEPTEMBER 2013 685 Endogenic water on the Moon associated with non-mare silicic volcanism: implications for hydrated lunar interior Satadru Bhattacharya*, Sriram Saran, Aditya Dagar, Prakash Chauhan, Mamta Chauhan, Ajai and A. S. Kiran Kumar Space Applications Centre, ISRO, Ahmedabad 380 015, India We have conducted spectral and spatial analysis of the Compton–Belkovich Thorium Anomaly (61.1°N, 99.5°E) region on the far side of the Moon based on high-resolution data from recent lunar missions. Chandrayaan-1 Moon Mineralogy Mapper data of Compton–Belkovich volcanic complex (CBVC) reveal the existence of a strong doublet feature near 2800 nm throughout the volcanic construct, which could be at- tributed to the presence of water and/or hydroxyl in the studied site. Very high resolution Lunar Recon- naissance Orbiter Camera–Narrow Angle Camera mosaic of the study area shows that the strongest of the hydration features within the CBVC is primarily related with either sunlit inner flanks of small-sized fresh craters or fresh escarpments associated with the central collapse structure. Moreover, Mini-RF Syn- thetic Aperture Radar data from Lunar Reconnais- sance Orbiter mission suggests the presence of a thick pyroclastic deposit in the volcanic complex. Our study indicates that the enhanced hydration at CBVC could possibly have originated from the episodic events of eruption and effusion involving silicic magma, which could probably be responsible for the tapping of a zoned magma body with a water-rich cap. Morphol- ogy of CBVC also confirms the presence of episodic effusive and eruptive events that probably had led to the formation of elevated topography, central col- lapsed feature and late eruptive domes in the study area. Keywords: Endogenic water, imaging spectrometer, Moon, pyroclastic deposits, silicic volcanism. RECENT detection of OH/H 2 O features on the Moon based on remote measurements revealed, contrary to prevailing ideas, the hydrous nature of the Moon 1–3 . Water and water-ice associated with the permanently shadowed cra- ters of the lunar poles have also been detected remotely 4 and measured directly by the LCROSS impact experi- ment 5 . Moreover, direct in situ measurements of lunar melt inclusions and lunar apatites indicated some parts of the deep lunar interior to be hydrated 6–8 . Here, we report the detection of strong OH/H 2 O features near 2800– 3000 nm based on Chandrayaan-1 Moon Mineralogy Mapper (M 3 ) observations over non-mare silicic Comp- ton–Belkovich Thorium Anomaly (CBTA) region, a vol- canic construct on the far side of the Moon 9,10 . Strength of the OH/H 2 O feature at CBTA ranges from ~ 6 to 17% (estimated based on convex hull continuum) relative to its surroundings having an average strength of ~ 3%. Based on radiative transfer models 11–13 , we have estimated a maximum OH/H 2 O concentration of ~ 0.55 weight per cent (wt%) from the strongest observed 2800-nm band strength of ~ 17% at 30% reflectance in the Compton–Belkovich volcanic complex (CBVC). Lunar Prospector Gamma Ray Spectrometer (LP-GRS) first identified an isolated small-scale feature with an unusually high Th abundance at Compton–Belkovich (CB) region (Figure 1) on the far side of the Moon 9 . Jollif et al. 10 interpreted CBTA as a compositionally evolved volcanic complex enriched in silica or alkali-feldspar indicative of rhyolitic volcanic materials based on Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer and LP-GRS FeO and Th measurements. CBVC is also characterized by higher albedo than its surroundings and is associated with a positive relief feature with several volcanic cones and domes 10 . High-resolution Digital Ter- rain Model (DTM) revealed the presence of irregular de- pressions within the topographic high that could probably represent collapse features associated with volcanism 10 . CBVC, being an isolated KREEP-rich (or similarly Figure 1. Lunar Reconnaissance Orbiter Camera–Narrow Angle Camera (LROC–NAC) mosaic of Compton–Belkovich Thorium Anom- aly (CBTA). Yellow dashed line demarcates the approximate boundary of Compton–Belkovich volcanic complex (CBVC). White arrows indi- cate volcanic domes within the Compton–Belkovich (CB) region. Red arrows mark irregular depression in the central part of the CBTA. White dotted curve represents an irregular depression along the western flank of the central collapse feature. Orange arrows near the southwest- ern boundary of CBTA mark a non-circular vent. Orange and white rec- tangular boxes represent areas of Figures 3 c and 4 c respectively. *For correspondence. (e-mail: satadru78@yahoo.co.in)