Cold traps are locations on the Moon that are shielded from sunlight where volatiles such as water could accumulate and persist against sublimation for geologic timescales. We model how long it takes accumulating craters to produce and then obliterate sub-kilometer scale cold traps. Sub-meter cold traps are extremely ephemeral, evolving in and out of existence over less than a few thousand years; however, larger 100 m to 1 km-scale cold traps may persevere for geologic timescales and preserve a record of the volatile history of the Moon.

Neutral exospheres of large airless bodies consist of atoms or molecules on ballistic trajectories. An import example is the lunar water exosphere, thought to transport water to cold traps. In anticipation of future observational measurements, the theory of thermalized surface-bounded gravitationally-bound exospheres is further developed. The vertical density profile is calculated using thermodynamic averages of an ensemble of ballistic trajectories. When the launch velocities follow the Maxwell-Boltzmann Flux distribution, the classical density profile results. For many other probability distributions, including thermal desorption from a vertical wall, the density diverges logarithmically near the surface. Hence, an exosphere resulting from thermal desorption from a rough surface includes a ground-hugging population that appears to be colder than the surface. Another insight derived from the thermodynamic perspective is that cold traps can be interpreted in terms of the frostpoint of the water exosphere, if the long-term average of the pressure of the exosphere is considered. Ice in lunar caves is long-lasting only if the cave interior is below the cold trap temperature threshold.

Water ice is expected to be trapped in permanently cold regions near the lunar poles. Other ices (super-volatiles) are trapped at lower temperatures, close to the lowest temperatures measured within the lunar permanently shadowed regions (PSRs). Here, the thermal stability of solid carbon dioxide in the south polar region is determined by analysis of 11 years of temperature measurements by Diviner, a radiometer onboard the Lunar Reconnaissance Orbiter. Sublimation rates averaged over a draconic year are far lower than peak sublimation rates. Small spatially contiguous pockets of CO2 ice stability are found in the craters Amundsen, Haworth, de Gerlache, and others, over a cumulative area of roughly 200 km(2). The LCROSS probe impacted one of those pockets and released CO2, serving as validation of the thermal stability calculations. Future surface missions can utilize this highly localized resource for the production of fuel, steel, and biological materials. Plain Language Summary Carbon-bearing species would be essential for sustained robotic or human presence on the Moon, for use in rocket fuel and biological materials. Various volatiles can be cold-trapped in permanently shadowed craters near the lunar poles. The existence of carbon dioxide cold traps has previously been surmised, but the required temperatures are near the lowest surface temperatures that have been reliably measured. Extensive and improved analysis of 11 years of orbital surface temperature measurements establishes the existence of carbon dioxide cold traps on the Moon, which potentially host high concentrations of solid carbon dioxide. Large CO2 cold traps are rare, however, and the geographic concentration of the resource will have policy implications. Key Points Time-dependent sublimation rates for CO2 are calculated based on 11 years of Diviner temperature measurements Extensive data analysis establishes the existence of carbon dioxide cold traps in the south polar region of the Moon Solid carbon dioxide is expected to be highly localized

A transient lunar atmosphere formed during a peak period of volcanic outgassing and lasting up to about similar to 70 Ma was recently proposed. We utilize forward-modeling of individual lunar basaltic eruptions and the observed geologic record to predict eruption frequency, magma volumes, and rates of volcanic volatile release. Typical lunar mare basalt eruptions have volumes of similar to 10(2)-10(3) km(3), last less than a year, and have a rapidly decreasing volatile release rate. The total volume of lunar mare basalts erupted is small, and the repose period between individual eruptions is predicted to range from 20,000 to 60,000 years. Only under very exceptional circumstances could sufficient volatiles be released in a single eruption to create a transient atmosphere with a pressure as large as similar to 0.5 Pa. The frequency of eruptions was likely too low to sustain any such atmosphere for more than a few thousand years. Transient, volcanically induced atmospheres were probably inefficient sources for volatile delivery to permanently shadowed lunar polar regions.

A time-dependent simulation of the argon-40 exosphere of the Moon shows that the semiannual oscillation of argon detected by the neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer spacecraft is consistent with adsorptive respiration in seasonal cold traps near the lunar poles. The magnitude of the oscillation requires that high-energy adsorption sites on soil grain surfaces at polar latitudes be as free of water contamination as soils at low latitudes. This requirement is met by the combination of two generally ignored water removal mechanisms: solar wind bombardment of exposed adsorption sites and the serpentinization reaction of water with olivine. The significance of these processes is supported by the lack of evidence of water in Lunar Atmosphere and Dust Environment Explorer data, which, in turn, establishes an upper bound for exospheric transport of water to polar traps at less than 10(14) molecules/Ga. Plain Language Summary The neutral mass spectrometer on the Lunar Atmosphere and Dust Environment Explorer spacecraft recorded a gradual rise and then fall of atmospheric argon-40 that is consistent with a 140-day segment of a semidraconic oscillation. The semidraconic oscillation of argon is important because its existence has harsh implications for the accumulation of water in polar cold traps. Simulations show that the existence of the oscillation implies respiration of argon atoms in seasonal cold traps near both poles, which in turn requires that polar soil grain surfaces have significant areas of high-energy adsorption sites that are not contaminated by water molecules. The paper argues that such extreme cleanliness can be explained by two previously ignored processes. One is surface scouring by solar wind bombardment of the lunar surface, which leads to escape or scatter to lower latitudes. The other is sequestration of water by the reaction of olivine with water, a process that is known as serpentinization. Coupled with meteoritic gardening, these process are capable of removing water from the lunar atmosphere faster than current estimates of water sources. This conclusion does not preclude the assimilation of water in permanent traps, but it severely reduces the amount of water available for assimilation.

The long-term stability of water ice at cold traps depends on subsurface temperature and regolith thermophysical properties. Based on Chang'E-2 microwave radiometer data, we have inverted attenuation coefficient, thermal gradient and instantaneous temperature profiles at permanently shaded craters (Cabeus, Haworth and Shoemaker) on the Moon's south pole. The nonuniformity of the inverted attenuation coefficient within the craters reflects the inhomogeneous thermophysical properties of regolith. In addition, thermal gradient decreased significantly from the crater walls to the bottoms, which may be caused by scattered sunlight, internal heat flux and earthshine effect. Considering continuous supplement of water ice (with volumetric fraction 0-10%) at cold traps, it changes subsurface thermophysical properties but has little effect on thermal gradient. We also assumed that abundant ice (10%) mixed with regolith, the inversion results showed that the maximum difference of diurnal temperatures between wet and dry regolith were no more than 0.5 K. That is, the effect of water ice on subsurface thermal behavior can be neglected. (c) 2016 Elsevier Ltd. All rights reserved.

Temperature regime at the LCROSS impact site is studied. All detected species in the Cabeus crater as well as CH4 and CO clathrate hydrates except H-2, CO, and CH4 are stable against evaporation at the LCROSS impact site. CO and CH4 can be chemisorbed at the surface of the regolith particles and exist in the form of clathrate hydrates in the lunar cold traps. Flux rates of delivery of volatile species by asteroids, micrometeoroids, O-rich, C-rich, and low-speed comets into the permanently shadowed regions are estimated. Significant amounts of H2O, CO, H-2, H2S, SO2, and CO2 can be impact-produced during collisions between asteroids and O-rich comets with the Moon while CH3OH, NH3 and complex organic species survive during low-speed comet impacts as products of disequilibrium processes. C-rich comets are main sources of CH4, and C2H4. (c) 2012 COSPAR. Published by Elsevier Ltd. All rights reserved.

The scientific objectives of neutron mapping of the Moon are presented as 3 investigation tasks of NASA's Lunar Reconnaissance Orbiter mission. Two tasks focus on mapping hydrogen content over the entire Moon and on testing the presence of water-ice deposits at the bottom of permanently shadowed craters at the lunar poles. The third task corresponds to the determination of neutron contribution to the total radiation dose at an altitude of 50 km above the Moon. We show that the Lunar Exploration Neutron Detector ( LEND) will be capable of carrying out all 3 investigations. The design concept of LEND is presented together with results of numerical simulations of the instrument's sensitivity for hydrogen detection. The sensitivity of LEND is shown to be characterized by a hydrogen detection limit of about 100 ppm for a polar reference area with a radius of 5 km. If the presence of ice deposits in polar cold traps is confirmed, a unique record of many millions of years of lunar history would be obtained, by which the history of lunar impacts could be discerned from the layers of water ice and dust. Future applications of a LEND-type instrument for Mars orbital observations are also discussed.

[1] Both steady and episodic sources have been proposed as sources of hydrogen observed by Lunar Prospector in association with the regions of permanent shadow at the poles of the Moon. Either source could supply significant quantities of water to the poles. However, space weathering processes affect the retention of water in the cold traps. We investigate those effects by simulating the evolution of a column of regolith in the region of permanent shadow over time. We determine the hydrogen concentration as a function of depth using a Monte Carlo model of discrete impacts and of delivery from the solar wind. We treat the delivery, sublimation, sputtering, and very small scale impacts as continual processes. Comparing the amount of water delivered to the poles to the amount remaining after space weathering, we find a retention efficiency of 5.6%. The retention efficiency of the polar cold traps is adequate for preserving volatile deposits over long periods of time. The average hydrogen concentration in the regolith column is 4100 ppm in the top meter after 1 Gyr. This is a saturation level in the regolith. Increasing the amount of time deepens the enriched layer but does not lead to increased concentrations. In 1 Gyr, about 1.6 m of the regolith is gardened. Therefore the top meter, which is probed by the neutron spectroscopy technique, has reached steady state in the simulations. The 4100 ppm saturation level is about half of the amount of hydrogen inferred from the Lunar Prospector neutron data.