As a forerunner to Chandrayaan-1 project, ISRO has initiated a Planetary Science and Exploration (PLANEX) programme with Ahmedabad(Gujarat) based Physical Research Laboratory(PRL) as its nodal center. The main aim of this programme is to enhance the scope of planetary research in the country, which was confined primarily to PRL at that time, and also formulate plans for ISRO’s long-term planetary exploration programme.

Several initiatives were taken up by PRL within the purview of the PLANEX project. These include activities such as yearly workshop to attract young people to planetary research, opportunity for such people to take up planetary research at PRL for extended durations, support for planetary research at various research Institution and Universities in the country and setting up of a National Facility for analysis of planetary materials.

These activities led to initiation of research activities in subjects related to planetary sciences in more then twenty academic centers in the country and also formation of Indian science teams for analysis of data to be obtained from all the Chandrayaan-1 payloads, including those from foreign countries. Several young people who have received training at PRL under the PLANEX project has been given permanent positions and one of them have taken very significant part in realizing the HEX payload, designed and developed at PRL, while two others will analyze the data from the imaging instruments (HySI, MMM, SIR-2 and TMC). There are others who will analyze data for HEX.

Prof. J. N. Goswami, current Director of PRL, is shouldering the responsibility of the Principal Scientist of Chandrayaan-1 mission and coordinating the activities of the eleven science groups. Four meetings of the Chandrayaan-1 Science Team, with participation of representative of all the payloads, has taken place during the last three years to chalk out science and data analysis plans for the mission.

PLANEX is also taking a lead in formulating the plans for ISRO’s future planetary exploration program. These efforts have led to recent approval for the Chandrayaan-2 mission and currently PLANEX is evaluating proposals from Indian scientists for possible Chandrayaan-2 payloads.

The High Energy X-ray (HEX) Payload for the Chandrayaan-1 Mission:

All solar system objects have its share of long-lived natural radioactivity, such as Uranium and Thorium, whose decay leads to release of photons of energies ranging from tens of kilo-electron volts (keV) to million electron volts (MeV). From the very beginning of planetary exploration, efforts have been made to detect these radiations emanating continuously from planetary surfaces to understand their evolution based on the inferred content of U and Th whose decay also provide internal heat during evolution of a planetary body.

Until recently, such detection was possible only for photons above 300 keV due to lack of suitable device for detection of low energy (50-300 keV) photons. Development of a new generation solid state detector, CZT (Cadmium-Zinc-Tellurium), has now opened up the possibility of exploring this energy region. The HEX experiment, designed and developed at the Physical Research Laboratory, in association with ISAC, Bangalore, will for the first time attempt detection of energetic photons in the energy range 30 to 270 keV emitted from U and Th present in the Moon using CZT detector. The primary aim of this experiment will be to quantify the proposed transport of volatiles (such as water) on the moon from sunlit hot regions to permanently shadowed polar region. The data obtained by HEX will also allow estimation of U and Th concentrations in lunar surface samples.

There is a global interest for future long-term exploration of the moon and particularly for setting up a human cum robotic base on it. At present the lunar polar region appears to be the most plausible site for such a base for two reasons. First there are mountains near the polar region nearly 5 km in height (e.g. Malapert Mountain near the South Pole), where sunlight is nearly perennial providing possibility of solar energy harnessing. Second, there are also permanently shadowed areas (e.g. base of Shackleton Crater near South Pole), with temperature much below zero degree, acting like a deep freezer, where there is a possibility of having water-ice mixed with lunar soils.

The sunlit areas of the Moon reach very high temperature in excess of 100 degree centigrade and conversely the shadowed region are like deep-freezers with temperature below minus hundred degrees. Thus any volatile atom or molecule (e.g. water molecule) present on the sunlit side of the moon will have sufficient energy to hop around and if it reaches a permanently shadowed cold region it will get permanently trapped. There are such regions near the lunar poles as the Sun-Moon angle does not allow sunlight to reach the base of the craters in the polar region. One therefore expects volatiles like water, brought in to the Moon by comet impacting on it or production of water molecule by solar wind interaction with lunar soils, to be concentrated in the permanently shaded polar region of the moon.

The HEX experiment will look for movement of the radionuclide Radon-222, which is a volatile and produced from decay of Uranium, on the lunar surface. Even though the overall concentration of Uranium in moon is low, its decay produces Radon that should also hop around and move to the permanently shadowed polar region leading to its elevated concentration before it eventual decay. The HEX experiment will identify the decay product of Radon that emits radiation with energy of ~45 keV, and an excess of this signal at pole compared to other areas of Moon will validate the hypothesis of volatile transport on Moon to permanently shaded polar region. This will give credence to possible presence of water ice mixed with lunar surface soils in the polar region. Another experiment on Chandrayaan-1 Mini-SAR will try to directly look for signal of water ice within the first meter of the permanently shadowed polar region.

A film on Chandrayan(India’s Moon Mission):