Lunar Polar Probe showing boom for gamma-ray spectrometer. It can be argued however that the availability of generous quantities of water can greatly enhance the quality of life and ease of function within a large permanent base. People require recreation and relaxation and the ability to provide such apparently “frivolous” amenities as swimming pools, decorative ponds and fountains, plenty of water for baths and showers should not be overlooked as a morale factor. Similarly, generous amounts of water for washing, cleaning, sewage transport, portable radiation shielding, and other uses can greatly facilitate base operations. On Earth, water is a major component in many manufacturing processes. It can function as a cooling or heating agent or heat transfer medium and as a diluter, carrier, or reagent in a variety of chemical processes. The hydrogen and oxygen which can be obtained from water are equally useful as chemical reagents in various chemical operations. Examples include use of hydrogen in reducing ores and oxygen in the combustion of waste products into harmless oxides. While a variety of innovative techniques have been devised for carrying out industrial processes without water, the ready availability of water will make it much easier to apply well understood techniques now used on Earth to Lunar operations. Mission Concept The primary instrument which is usually considered for a mission of this type is the gamma-ray spectrometer. This instrument depends for its function on the detection of gamma rays which are emitted from all bodies in the solar system. The original source of the energy is cosmic rays which penetrate the mass and in decelerating deposit energy in the mass in the form of orbital electrons excited to a higher than normal energy state. When the electrons decay back to their normal state the energy is emitted as a gamma ray. The energy of the gamma ray is characteristic of the specific atom from which it is emitted. Therefore, if the gamma rays can be detected and sorted according to energy then it is possible to derive what atoms in what percentage make up the mass from which the rays are being emitted. Several problems arise inherently from the nature of the gamma-ray spectrometer. The number of gamma rays given off depend upon the intensity of the incident cosmic radiation. Since this is relatively low, the emitted gamma ray flux is also low. Since this is a characteristic of the universe, there is nothing which can be done about it except to accumulate data over a long period of time. Common materials can be detected reasonably quickly while less common ones require long integration times. Another problem is the lack of discrimination. Since all objects in the solar system are bombarded by cosmic rays, all objects give off gamma rays and therefore will be detected by the gamma-ray spectrometer. This means that the instrument is not only measuring the elemental composition of the astronomical body in question but also of the spacecraft which brought it and even of itself. In order to distinguish the signal from the target planet the instrument must be calibrated by reading this background signal while it is in deep space far from the planetary body. Even this does not completely solve the problem since the process of burning propellant to enter orbit will change the background because of the removal of the propellant mass. While all this complicates the issue, careful mission design and data analysis can still provide a useful result. Because of its sensitivity to background radiation from the spacecraft and because the more sensitive detectors must be maintained at a low temperature (as low as 90 °K) the gamma ray spectrometer will most probably be mounted on a boom which will place it some two meters from the spacecraft. This remote placement aids both problems since gamma ray intensity decreases with distance from the source. (For a point source the rate of decrease is as the square of distance however typically the spacecraft will be too large and too close to appear as a point source.) The thermal requirement is aided by the fact that the detector is isolated from the heat generated by electrical activity in the spacecraft. By careful thermal isolation from the spacecraft, prevention of energy from sunlight, earthlight or moonlight from reaching the detector, and design of a radiator to reject any internally generated heat to space it is probably possible to cool the detector to the desired temperature without active refrigeration. This is desirable from considerations of reliability and weight reduction. If necessary refrigeration of the detector can be provided by a variety of means. The most reliable is probably thermoelectric cooling since it requires no moving parts or consumable substances although fairly inefficient. Even given the complications discussed above, the gamma-ray spectrometer appears to be the instrument best suited to detection of water and other volatiles in the lunar polar regions. It is quite sensitive to hydrogen and many of the other lighter elements which would be expected to constitute deposits of volatiles. Because gamma rays are emitted from the bulk of the material, elemental abundances are sensed to some depth beneath the surface (limited by self shielding of the gammas by the material itself), usually about half a meter. Thus even if a layer of dust overlies the volatiles they will still be detected if within a half meter of the surface. This would seem probable for cold trapped materials. Given adequate observing time, the gamma-ray spectrometer can also provide an abundance map for a variety of other elements. Table 2 shows sensitivity of the instrument to the elements. TABLE 2 Minimum Detection Concentration Material Concentration Water 0.7% Hydrogen 0.08% Oxygen 0.5% Carbon 1.0% No other instrument capable of operating from orbital altitudes can provide elemental data equivalent to that available from the gamma-ray spectrometer. Because of the lack of atmosphere and the large area to be surveyed, orbital exploration is the only practical approach. Therefore, any mission dedicated to the discovery of cold trapped polar volatiles will include a gamma-ray spectrometer regardless of other instrumentation. The second half of this paper will appear in the March/April issue of the UPDATE. REFERENCES 1. A Primer in Lunar Geology, Ronald Greeley and Peter Schultz editors, NASA Ames Research Center, 1974 2. “On the Possible Presence of Ice on The Moon,” Kenneth Watson, Bruce C. Murray, and Harrison Brown, J. Geophysical Res. 66, p.1598, May 1961 3. “Water on the Moon,” James R. Arnold, Univ, of Calif. San Diego, presented at Seventh Annual Lunar Science Conference, 16 March 1976 4. “The Behavior of Volatiles on the Lunar Surface,” Kenneth Watson, Bruce C. Murray, J. Geophysical Res. 66, p.3O33, Sept. 1961 5. “Prospects for Finding the Most Valuable Potential Resource on the Moon: Water,” P. M. Muller, Jet Propulsion Laboratory, presented at Seventh Annual Lunar Science Conference, 16 March 1976
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