Short Communication The spatial distribution of molecular Hydrogen in the lunar atmosphere—New results Smitha V. Thampi a,n , R. Sridharan b,1 , Tirtha Pratim Das a , S.M. Ahmed c , J.A. Kamalakar d , Anil Bhardwaj a a Space Physics Laboratory, VSSC, Indian Space Research Organisation, Trivandrum, India b Physical Research Laboratory, Navarangpura, Ahmedabad, India c Central Instruments Laboratory, University of Hyderabad, India d Laboratory for Electro Optical Systems, Indian Space Research Organisation, Bangalore, India article info Article history: Received 26 June 2014 Received in revised form 12 December 2014 Accepted 13 December 2014 Available online 23 December 2014 Keywords: Lunar atmosphere Chandrayaan-I Surface boundary exosphere abstract The measurements carried out by Chandra's Altitudinal Composition Explorer (CHACE) onboard the Moon Impact Probe (MIP) of Chandrayaan I mission is used to obtain information on the 2-D distribution of the lunar atmospheric H 2 by a novel approach that makes use of the basic fact that the Moon has a Surface Boundary Exosphere (SBE).These are the ‘first’ daytime in situ measurements of lunar H 2 covering the 201S to 881S latitude region centered 141E longitude. A critical examination of the observed spatial features of the surface number density of H 2 vis-à-vis the surface topography delineated from the Lunar Laser Ranging Instrument (LLRI) in the main orbiter Chandrayaan-I, indicates that that lunar surface process may be important in introducing small scale variations in the H 2 number density. Another constituent which exhibited spatial variation in the observed partial pressure is 40 Ar and it was hypothesized that it is indicative of the spatial heterogeneity in the radiogenic activity of the Lunar interior (Sridharan et al., 2013a). The absolute number density at the surface and also the latitude/ altitude variation of the densities that are reported for the first time, highlight the complexities of the sunlit lunar atmosphere. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction The lunar atmosphere is known to be extremely tenuous and falls in the category of Surface Boundary Exosphere (SBE), imply- ing that the scale height is much smaller than the mean free path, which means there are no inter-molecular collisions and the region is nearly isothermal. The first in situ measurements of the compo- sition of the lunar atmosphere were made during the Apollo-17 mission using a mass spectrometer—the Lunar Atmosphere Com- position Experiment (LACE), which was physically deployed by the astronauts on the lunar surface (  201N latitude). The LACE ran into saturation during sunlit times mainly for want of a larger dynamic range and, due to this limitation, it had provided data only during night time conditions. The LACE night time data confirmed the existence of Helium, Neon and Argon in the lunar atmosphere, and the several other detected species were doubted to be of non-lunar origin, essentially from the out-gassing during the astronauts 0 extra-vehicular activity and also from the materials left behind, including the Lander (Hoffman et al., 1973). It was suggested that most probably molecular Hydrogen [H 2 ] existed in the lunar atmosphere, and it must have had its origin in the lunar surface (Hoffman et al., 1973). In fact, the LACE had measured a H 2 surface density of 6.5  10 4 cm 3 at night, which agreed with the predictions by Hodges (1973). On the other hand, the 40 Ar number density reported by LACE was  10 4 cm 3 during pre-dawn hours. Apart from LACE, there was a UV spectrometer on Apollo 17, which also detected similar levels of H 2 (  10 4 cm 3 ). On the other hand, the estimated atomic Hydrogen abundance was too low, i.e.,  10 cm 3 (Fastie et al., 1973). This was interpreted as an indication for the conversion of the majority of the solar wind protons as H 2 at the lunar surface and its re-emission to the atmosphere in accordance with the lunar surface temperature. The upper limits of H 2 at the Apollo-17 site (Hodges et al., 1974) also qualitatively agreed with this hypothesis. Later, using the Apollo-17 UV spectrometer data, Feldman and Morrison (1991), provided more stringent upper limits to the number densities of H (17 cm 3 ), and H 2 (9000 cm 3 ). On the other hand, Wurz et al. (2012), based on the previously reported upper limits, arrived at H 2 number density of  5  10 4 cm 3 and 40 Ar number density of  4  10 4 cm 3 near 10 km above the lunar surface on the day- side, at a temperature of 400 K. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/pss Planetary and Space Science http://dx.doi.org/10.1016/j.pss.2014.12.018 0032-0633/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: smitha.v.thampi@gmail.com (S.V. Thampi). 1 NASI-Sr. Scientist. Planetary and Space Science 106 (2015) 142–147