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Exploring Organic Environments in the Solar System
The Moon is of interest to the study of organic environments for two very different reasons:
The lunar surface as a witness plate. That is, it is a location that provides for long-term integration of collected material and thus might have sampled other carbonaceous asteroids that are not present in recent meteorite collections; and
The lunar surface as the abode of special microenvironments. The lunar materials studied to date come from the Moon’s equatorial regions, and these areas are not typical of all lunar environments.
The second possibility is of considerable potential interest, and the remainder of this section is devoted to its discussion.
Permanently shadowed regions exist at both lunar poles. As long ago as 1961, Watson, Murray, and Brown suggested that the extremely low temperatures experienced in these locations, less than some 50 K, would act as cold traps for volatile material impacting the lunar surface.8 Thus, for example, water and other volatile materials—derived from comets, asteroids, meteorites, or interplanetary dust particles impacting the Moon’s surface or, alternatively, created during the reduction of lunar regolith by H- ions from the solar wind—could freeze out on grains in the polar regions and, in principle, persist for considerable periods of time.9 Such informed speculation has been supported by the subsequent detection of hydrogen concentrations in the lunar polar regions with the neutron spectrometer on the Lunar Prospector spacecraft.10 That is readily, but not definitively, explained as ice deposits.
The possibility of water ice deposits at the lunar poles raises the issue of the presence of other volatiles, including organic volatiles, since the likely sources of the water, particularly from comets, may also be abundant sources of organic materials. Given a source of raw materials and the availability of likely energy sources (e.g., from cosmic rays and interstellar ultraviolet radiation), it is reasonable to ask if organic synthesis is actively occurring at the lunar poles.
The irradiation of carbon-, hydrogen- and oxygen-bearing ices by ultraviolet radiation or cosmic rays can lead to the synthesis of organic compounds. Similarly, organics may be formed at the lunar poles by the action of the solar wind on the ice there in the same way that they are formed in ice on interstellar dust particles. The radicals formed by the radiation may react with the inorganic carbonaceous condensates to generate simple organic compounds (see in Chapter 2, in the section “The Interstellar Medium,” the subsection “The Synthesis of Interstellar Molecules”).11
Instruments on NASA’s forthcoming Lunar Reconnaissance Orbiter (LRO), scheduled for launch in 2008, will directly address questions relating to polar ices. These instruments include the following:
The Lunar Exploration Neutron Detector (LEND), which will map the flux of neutrons from the lunar surface to create 5-km-resolution maps of the hydrogen distribution and characterize the surface distribution and column density of near-surface water ice deposits;
The Diviner Lunar Radiometer Experiment, which will map the temperature of the entire lunar surface at 300-m horizontal scales to identify cold traps and potential near-surface and exposed ice deposits; and
The Lyman-Alpha Mapping Project (LAMP), which will observe the entire lunar surface in the far ultraviolet to search for exposed surface ices and frosts in the polar regions and will provide subkilometer-resolution images of permanently shadowed regions at the lunar poles.
None of these instruments, nor those on other planned lunar orbiters, such as India’s Chandrayaan 1, China’s Chang’e, or Japan’s Selene or Lunar-A, will directly address key questions surrounding the putative existence of organic materials at the lunar poles. Indeed, it is not clear that the definitive detection and study of lunar organics are possible within the current generation of remote-sensing instruments. It is possible that the secondary payload on LRO’s launch vehicle, the Lunar Crater Observation and Sensing Satellite (LCROSS), may return spectroscopic evidence of the presence of organic materials in the Moon’s polar regions, but it is likely that the study of lunar organics is more appropriately addressed by a lander mission.