Chandrayaan-1, India's first planetary exploration mission to Moon carries a suite of payloads including a High Energy X-ray spectrometer (HEX) designed to study low-energy (30-270 keV) natural gamma rays emitted from the lunar surface due to decay of uranium and thorium. The primary science objective of HEX is to study transport of volatiles on the lunar surface through the detection of the 46.5 keV line from Pb-210 decay, which is a decay product of volatile Rn-222, both belonging to the U-238 decay series. HEX is designed to have a spatial resolution of similar to 33 km at energies below 120 keV. The low signal strength of these emissions requires a large area detector with high sensitivity and energy resolution, and a new generation Cd-Zn-Te (CZT) solid state array detector is used in this experiment. Long time integration will be required to detect the emission because of the significant lunar continuum background and weak signal strength. The various sub-systems of the HEX flight payload and test results from ground calibration are described in this article. HEX will be the first experiment aimed at detecting low energy (<300 keV) gamma ray emission from a planetary surface.

The presence of volatiles near the lunar poles is considered. The chemical composition of a lunar atmosphere temporarily produced by comet impact is analyzed during the day and night. C-rich and long-period comets are insufficient sources of water ice on the Moon. O-rich short-period cornets deliver significant amounts of H2O, CO2 SO2, and S to the Moon. An observable amount of polar hydrogen can be delivered to the Moon by a single impact of all O-rich short-period comet with diameter of 5 kin in the form of water ice. The areas where CO2 and SO2 ices are stable against a thermal sublimation are estimated to be around 300 and 1500 km(2,) respectively. If water ice exists in 2 cm top regolith layer, CO2 and SO2 ices can be stable in the coldest parts of permanently shaded craters. The delivery rate of elemental Sulfur near the poles is estimated to be 10(6) gyr(-1). The sulfur content is estimated to be as 41 high as 1 wt% in the polar regions. The SELENE gamma-ray spectrometer can detect sulfur polar caps oil the Moon if the sulfur Content is higher than 1 wt %. This instrument can check the presence of hydrogen and minerals with the unusual chemical composition at the lunar poles.