Necessary, but perhaps not sufficient, conditions for organic synthesis are the presence of a feedstock of organic elements, an energy source to drive chemical reactions, and sufficient mobility of ions to allow reactions to take place. The lunar poles almost certainly possess all of these necessary conditions. In 1961 Watson and coworkers (1) showed that the low obliquity of the Moon (1.5 degrees) would allow topographic lows in the lunar polar regions to be permanently shadowed from sunlight. The lack of sunlight and permanent exposure to the 3 kelvin background of space should force the shadowed regions at the poles to achieve extremely low temperatures. Watson and coworkers reasoned that these low temperatures would cold-trap any volatile chemicals which propagate through the tenuous lunar exosphere, for example, water introduced during the impact of a comet. Arnold (2) alerted the scientific community to this largely overlooked paper and subsequently various aspects of this notion have been studied in the intervening decades, including the longevity of the current low obliquity (probably greater than 2 by)(3); the expected temperature using models of increasing sophistication which show that temperatures below 50 kelvin are reasonable in the shallow subsurface (4,5,6); transport of water and other species from impact sources across the Moon which show that significant fractions of volatiles can make it to the cold traps (e.g. water at ~5%) (7,8); and loss mechanisms from the cold traps themselves which suggest that slow, continuous sources such as water-bearing micrometeorites probably are not stable against loss, but large episodic sources such as comet impact may allow significant retention of volatiles (9,10). Lunar volatiles can have originated from a variety of sources. Over time the lunar surface has swept up volatiles from the Sun, comets, asteroids and other meteoritic material, and episodically has been bombarded with interstellar grains and ions during passage through the cores of giant interstellar molecular clouds (Figure 1).

It was the discovery of hydrogen concentrations at the lunar poles by the neutron spectrometer carried by the Lunar Prospector mission which promoted these sophisticated speculations to the realm of fact (11). Neutron spectroscopy is highly sensitive to the presence of hydrogen because the mass of the H-nucleus (a proton) is nearly identical to that of the neutron, so hydrogen acts as an excellent neutron moderator. Neutrons are produced in the lunar surface by galactic cosmic rays interacting with nuclei in lunar materials. This interaction produces high-energy neutrons (> several hundred keV) which are then scattered and slowly lose energy with each nuclear scatter. The effectiveness of the hydrogen nucleus at removing energy from a neutron ("moderating") means that in areas of high hydrogen concentration, neutrons of intermediate energy will be depleted relative to the low-hydrogen surroundings. Maps of the upward flux of the intermediate energy neutrons (epithermal neutrons) show that the lunar poles are deficient in these neutrons, indicating the presence of a strong moderator. The most effective and cosmically abundant moderator is the element hydrogen, so the lunar polar epithermal neutron deficiency is attributed to a relatively high concentration of hydrogen at the poles.

The phase that is the vector of this hydrogen is unknown, but the popular presumption has been that the hydrogen is borne by water ice. This is not necessarily the case. The general hydrogen enhancement in the polar regions is consistent with hydrogen implanted by the solar wind, which is inhibited in diffusing out of lunar grains by low polar temperatures. The highest concentrations of hydrogen observed are barely consistent with this hypothesis; this is the maximum amount of H that can be forced into a silicate lattice. The Lunar Prospector data are of relatively low spatial resolution (60 km pixel size) so it is likely that the observed concentrations are actually due to smaller areas of much higher concentrations which are inconsistent with solar wind implantation. However, in the context of this paper, a solar wind source for polar hydrogen is not a problem for enabling lunar organic synthesis because the solar wind is a prolific source of C, and N in addition to H as will be discussed below.

Figure 1. Sources ranging from solar to interstellar have emplaced volatiles on the lunar surface.

An experiment carried by Apollo 17 mission makes the pure solar wind hypothesis rather unlikely. The Lunar Atmosphere Composition Experiment (12), placed on the lunar surface by Schmitt and Cernan, used a mass spectrometer to measure the species present in the extremely tenuous lunar atmosphere. The experiment showed dramatic increases in the abundance of masses 15-16, 28 and 44 just before lunar dawn, increasing to instrument saturation as the Sun rose. This indicated the presence of gaseous species cold trapped on the lunar night side, expelled by solar heating. These masses correspond to methane or atomic oxygen, diatomic nitrogen or carbon monoxide, and carbon dioxide respectively. Because these species are condensable, but highly mobile during the lunar day, they must be exchanging with the lunar polar cold traps. The ultimate source of this gas is unknown, but could be due to chemical reactions between formerly implanted solar wind ions mobilized during micrometeorite impact, or the tapping of a polar volatile reservoir of uncertain provenance.

So both direct (Lunar Prospector) and indirect (LACE) evidence suggests that the lunar polar cold traps contain volatiles, and these volatiles are derived from sources which contain biogenic elements. Given the hypothesized, and reasonable, low temperatures of the polar cold traps, what energy sources are available to drive organic synthesis?

Laboratory experiments have repeatedly shown that both hard UV and energetic protons can stimulate organic synthesis (13). UV irradiation of simple mixtures of C, H, O and N-bearing (CHON) ices such as water and CO₂, (even without N-bearing ice) produces simple organic molecules. Similar experiments using high energy protons, analogous to galactic cosmic ray protons, also result in the production of simple organics. Both these energy sources are available at the lunar poles. The lunar surface, even the polar surface, is illuminated by scattered interstellar Lyman a UV radiation, and also by galactic cosmic ray protons.

Finally, irradiation of ices produces chemically active radicals, but low temperatures clearly inhibit ion mobility. Many synthesis experiments require some amount of temperature cycling of samples from very low (tens of kelvins or less) to higher temperatures to allow reactions to take place. The more sophisticated thermal models of the poles shows that the upper 20 cm of the lunar regolith experiences significant temperature excursions owing to indirect illumination of the permanently shadowed surface by light bouncing off crater walls or other topographic highs. The lunar surface is also stirred by micrometeorites, and larger meteorites, so icy material has the opportunity to be exposed to a range of depths and hence temperatures and temperature variations. Finally, the length of time available for ion diffusion, perhaps billions of years, may allow reactions even at low stable temperatures.

The lunar poles demonstrably contain deposits of H, and this hydrogen is almost certainly accompanied by the other biogenic elements. The polar deposits are exposed to radiations that are demonstrated to be able to drive organic synthesis. And the polar surface has either experienced sufficient temperature excursion to allow chemical reactions to proceed, or have provide sufficient time even for low temperature organic chemistry. The rest of this paper will examine some of these issues more closely.

Figure 2. The lunar volatile system. Volatiles are input from the various extralunar sources and most material is directly lost to space. A portion arrive and are trapped at the poles on timescales ranging from minutes (if lost to sputtering), to hundreds of millions of years (if sequestered by burial).

The actual nature of the polar volatile deposits is a function of the time history of the sources, and the rates and efficiencies of transport and loss mechanisms (Figure 2). While the poles are "illuminated" by all of the potential sources, transport of volatiles from anywhere on the lunar surface allows the poles to tap the entire Moon for volatiles. The estimates of the area of permanent shadow based on radar imaging suggest that the cold traps cover only about one hundredth of one percent of the lunar surface (14), so transport mechanisms need not be very efficient to allow material impacting the Moon away from the cold traps to dominate the polar inventory. In fact, they are relatively efficient and several percent of the material originally emplaced will enounter a cold trap assuming ballistic trajectories are followed in the collisionless lunar atmosphere. Loss mechanisms are also efficient, both en route to the cold traps (hence the loss of ~95% of material) via thermal Jeans' escape and ionization and sweeping, and in the cold traps via sputtering by solar wind and erosion by UV. Comparison of model volatile input rates from various sources has led to the conclusion that existing steady-state sources are not stable against losses. In other words, erosion of ice by sputtering and UV is faster than accumulation by transport from meteoritic sources. Sputtering and UV cannot outpace large episodic inputs, most notably comets, however. A comet would deposit a large amount of water ice and probably other volatiles essentially instantaneously at the poles, probably via a temporarily thick collisional atmosphere (15). (Incidentally, such a thick atmosphere might transiently warm polar deposits, both driving organic chemistry, but possibly purging the most volatile species.) Thick ice deposits, subject to relatively rapid loss via sublimation and the mechanisms cited, would be partially sequestered from loss by impact of larger meteorites, which would bury portions of the "icecap" under their ejecta blankets, protecting them from further losses. This could give rise to a situation where a core tube driven into a permanently shadowed region would reveal a pattern of alternating horizons of ice and regolith.

Not inconsistent with this view are observations of the lunar atmosphere. Notably, though interesting and relevant mass peaks were detected by LACE, no peaks corresponding to water or hydroxyl were detected, and dedicated searches for these species have been conducted using UV spectroscopy from Earth and by Apollo have been unsuccessful. On its face these observations are troubling given to dominance of a cometary volatile source by water ice. On the other hand, the mass peaks detected all correspond to highly volatile species, which are clearly more susceptible to thermal processes. If the polar deposits have a significant component of cometary volatiles buried at shallow levels at temperatures in the vicinity of 80-100K, water ice will be quite inactive but carbon dioxide, methane and carbon monoxide will have very high vapor pressures and tend to escape. The presence of these species may indicate a relatively youthful deposit. On the other hand, these gases can be produced from chemical reactions among solar wind gases liberated by micrometeorite impact, though again the lack of water vapor is puzzling.

The solar wind is a feasible source for the deposit as intimated in the introduction. Apollo soils contain on the order of 100 micrograms per gram of soil of H, C, and N (16). These elements are released on heating of lunar soil to fairly high temperatures (>500 degrees) which is consistent with high noontime equatorial temperatures experienced by lunar soils (>350 degrees Celsius). At the poles presumably more solar wind gas can be retained, but because thermal diffusion is inhibited by low temperatures, and by retention of deeply implanted gas which was subsequently released by micrometeorite heating. The released and cold-trapped gas will concentrate on grain surfaces where it will be available for chemical reactions. The general excess of H in the polar regions is 3x, and is as high as 30x in some locations. Thus, scaling from equatorial lunar soils, concentrations of solar wind gas containing C, H and N in soils can be approximately 1 part per thousand in bulk, and far higher concentrations on grain surfaces. If most of the excess gas is present on grain surfaces rather than distributed through the bulk (solar wind gas in equatorial soil is always concentrated on grain boundaries), C-, H- and N-rich rinds on grains could be many microns thick.

Thus, observations support two scenarios: horizons of (probably) cometary ice layered between slabs of lunar soil, or a general concentration of C, H and N on grain surfaces distributed throughout the soil. In either case, a rich feedstock of material is present for organic chemistry.

We have cited UV and galactic proton radiation as potential sources to drive lunar organic chemistry, but the latter is greatly favored. UV radiation effects are confined to the very surface, where chemistry is directly in competition with erosion by the UV and by sputtering. In contrast, the galactic protons are emplaced at a range of depths up to a meter below the lunar surface. Below the optical surface grains are shielded both from the constructive and destructive effects of both UV and solar wind ions. At depths below a few centimeters, lunar soil is protected from possible high daytime temperatures driven by multiple scattering from nearby topographic highs. Any CHON-bearing volatiles or volatile assemblages are relatively protected, but are immersed in the proton flux. As cited above, experiments have shown that proton irradiation is a productive energy source for organic synthesis from pure simple ices.

The ideas presented here are no more than a plausibility argument for the presence of indigenous lunar organics in the polar microenvironment. If organics are present in the lunar poles, they represent an opportunity to field-test models of organic synthesis which are proposed for comets and interstellar clouds which in turn are proposed to have been important in providing organic material to the early Earth. No other location in the solar system is so (relatively) convenient for this type of study. To confirm this plausibility would require organic analysis in the cold traps themselves. The possible presence of organics could probably not justify an unmanned scientific mission to the lunar poles, but the scientific importance of lunar polar volatile sources which range from solar to interstellar, and the continuing interest in polar water for future space resources may provide that justification. This paper suggests that experiments conducted by such a mission should include organic detection instruments.

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