Localized pyroclastic deposits (LPDs) are low-albedo accumulates of pyroclastic material with distinct positive topographic signatures that are found dominantly along highland-mare boundaries. Previous workers hypothesized that LPDs represent products of a lunar equivalent of Vulcanian-style eruptions, based in part on the observation that some of the deposits in Alphonsus Crater have large vent volumes in comparison with their deposit volumes, indicating a low proportion of juvenile material in the deposits. The objective of this study is to better understand eruption mechanisms by determining how the proportion of juvenile material, as calculated using deposit and vent volumes, varies among LPDs in Alphonsus Crater and elsewhere on the Moon using contemporary data and methods. Deposit and vent volumes for 23 LPDs from eleven sites were calculated by differencing current and modeled pre-eruption surfaces using digital terrain models (DTMs) derived from Lunar Reconnaissance Orbiter Camera Narrow Angle Camera (LROC NAC). Results show that LPDs have a wide range of juvenile proportions, many of which are more juvenile-rich than previously thought. Additionally, there is a positive relationship between juvenile material proportion and deposit volume and thickness, and a positive relationship between juvenile volume and dispersal area. LPDs also bear a broad range of thinning profiles which span a range of multiple eruption types on Earth. These findings, along with previous studies employing spectroscopic analysis of these deposits, indicate there is greater diversity among LPDs in composition and morphometry than previously understood, and that previously published simplified Vulcanian models may apply only to the deposits containing the least amount of juvenile material, with all others perhaps requiring a combination of multiple eruptive mechanisms. Furthermore, dynamic model results suggest that the most widespread lunar deposits in this study were formed by magma containing 2000-5000 ppm of dissolved volatiles, consistent with recent estimates via melt inclusion analysis, but contrary to long-held ideas that the Moon was largely degassed during its formation. (C) 2020 Elsevier B.V. All rights reserved.

Lunar localized pyroclastic deposits are low albedo deposits with areas 2500 km(2)), and (4) provide useful parameters for future volcanological modeling. From this study, we find that: (1) localized pyroclastic deposits exhibit low relief structures, (2) the surface rock abundance and circular polarization ratio of localized pyroclastic deposits display a wide range of values (0.2-0.5% and 0.3-0.6, respectively), (3) the glass abundance of localized pyroclastic deposits vary between 0 and similar to 80 wt.%, (4) there are four types of localized pyroclastic deposits based upon the surface rock abundance and glass abundance parameters, (5) pyroclastic deposits within the same floor-fractured crater tend to have similar properties, and (6) localized pyroclastic deposits are diverse with respect to regional pyroclastic deposits, but a subset of localized pyroclastic deposits have similar physical and mineralogical properties to regional pyroclastic deposits. (C) 2016 Elsevier Inc. All rights reserved.

We find evidence for hydrated material in the lunar regolith using albedo protons measured with the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO). Fluxes of these albedo protons, which are emitted from the regolith due to steady bombardment by high energy radiation (Galactic Cosmic Rays), are observed to peak near the poles, and are inconsistent with the latitude trends of heavy element enrichment (e.g., enhanced Fe abundance). The latitudinal distribution of albedo protons anti-correlates with that of epithermal or high energy neutrons. The high latitude enhancement may be due to the conversion of upward directed secondary neutrons from the lunar regolith into tertiary protons due to neutron-proton collisions in hydrated regolith that is more prevalent near the poles. The CRaTER instrument may thus provide important measurements of volatile distributions within regolith at the Moon and potentially, with similar sensors and observations, at other bodies within the Solar System. (C) 2016 Published by Elsevier Inc.

Oppenheimer crater is a floor-fractured crater located within the South Pole-Aitken basin on the Moon, and exhibits more than a dozen localized pyroclastic deposits associated with the fractures. Localized pyroclastic volcanism on the Moon is thought to form as a result of intermittently explosive Vulcanian eruptions under low effusion rates, in contrast to the higher-effusion rate, Hawaiian-style fire fountaining inferred to form larger regional deposits. We use Lunar Reconnaissance Orbiter Camera images and Diviner Radiometer mid-infrared data, Chandrayaan-1 orbiter Moon Mineralogy Mapper near-infrared spectra, and Clementine orbiter Ultraviolet/visible camera images to test the hypothesis that the pyroclastic deposits in Oppenheimer crater were emplaced via Vulcanian activity by constraining their composition and mineralogy. Mineralogically, we find that the deposits are variable mixtures of orthopyroxene and minor clinopyroxene sourced from the crater floor, juvenile clinopyroxene, and juvenile iron-rich glass, and that the mineralogy of the pyroclastics varies both across the Oppenheimer deposits as a whole and within individual deposits. We observe similar variability in the inferred iron content of pyroclastic glasses, and note in particular that the northwest deposit, associated with Oppenheimer U crater, contains the most iron-rich volcanic glass thus far identified on the Moon, which could be a useful future resource. We propose that this variability in mineralogy indicates variability in eruption style, and that it cannot be explained by a simple Vulcanian eruption. A Vulcanian eruption should cause significant country rock to be incorporated into the pyroclastic deposit; however, large areas within many of the deposits exhibit spectra consistent with high abundances of juvenile phases and very little floor material. Thus, we propose that at least the most recent portion of these deposits must have erupted via a Strombolian or more continuous fire fountaining eruption, and in some cases may have included an effusive component. These results suggest that localized lunar pyroclastic deposits may have a more complex origin and mode of emplacement than previously thought. (C) 2016 Elsevier Inc. All rights reserved.

The upper 25-100 nm of the lunar regolith within the permanently shaded regions (PSRs) of the Moon has been demonstrated to have significantly higher surface porosity than the average lunar regolith by observations that the Lyman-alpha albedo measured by the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) is lower in the PSRs than the surrounding region. We find that two areas within the lunar south polar PSRs have significantly brighter Lyman-alpha albedos and correlate with the ejecta blankets of two small craters (<2 km diameter). This higher albedo is likely due to the ejecta blankets having significantly lower surface porosity than the surrounding PSRs. Furthermore, the ejecta blankets have much higher Circular Polarization Ratios (CPR), as measured by LRO Mini-RF, indicating increased surface roughness compared to the surrounding terrain. These combined observations suggest the detection of two craters that are very young on geologic timescales. From these observations we derive age limits for the two craters of 7-420 million years (Myr) based on dust transport processes and the radar brightness of the disconnected halos of the ejecta blankets. (C) 2015 Elsevier Inc. All rights reserved.

We utilize surface temperature measurements and ultraviolet albedo spectra from the Lunar Reconnaissance Orbiter to test the hypothesis that exposed water frost exists within the Moon's shadowed polar craters, and that temperature controls its concentration and spatial distribution. For locations with annual maximum temperatures T-max, greater than the H2O sublimation temperature of similar to 110 K, we find no evidence for exposed water frost, based on the LAMP UV spectra. However, we observe a strong change in spectral behavior at locations perennially below similar to 110 K, consistent with cold-trapped ice on the surface. In addition to the temperature association, spectral evidence for water frost comes from the following spectral features: (a) decreasing Lyman-alpha. albedo, (b) decreasing on-band (129.57-155.57 nm) albedo, and (c) increasing off-band (155.57-189.57 nm) albedo. All of these features are consistent with the UV spectrum of water ice, and are expected for water ice layers >similar to 100 nm in thickness. High regolith porosity, which would darken the surface at all wavelengths, cannot alone explain the observed spectral changes at low temperatures. Given the observed LAMP off-band/on-band albedo ratios at a spatial scale of 250 m, the range of water ice concentrations within the cold traps with T-max < 110 K is similar to 0.1-2.0% by mass, if the ice is intimately mixed with dry regolith. If pure water ice is exposed instead, then up to similar to 10% of the surface area on the 250-m scale of the measurements may be ice-covered. The observed distribution of exposed water ice is highly heterogeneous, with some cold traps <110 K having little to no apparent water frost, and others with a significant amount of water frost. As noted by Gladstone et al. (Gladstone, G.R. et al. [2012]. J. Geophys. Res.: Planets 117(E12)), this heterogeneity may be a consequence of the fact that the net supply rate of H2O molecules to the lunar poles is very similar to the net destruction rate within the cold traps. However, an observed increase in apparent H2O abundance with decreasing temperature from similar to 110 K to 65 K suggests that destruction of surface frosts by impact gardening and space weathering is spatially heterogeneous. We find a loosely bimodal distribution of apparent ice concentrations with temperature, possibly due to competition between vertical mixing by impact gardening and resupply of H2O by vapor diffusion at sites -110 K. Finally, we cannot rule out the possibility that the colder population of ice deposits is in fact primarily carbon dioxide ice, although peak temperatures of similar to 65 K are slightly higher than the usual CO2 sublimation temperature of similar to 60 K. (c) 2015 Elsevier Inc. All rights reserved.

The Lunar Exploration Neutron Detector (LEND) onboard the Lunar Reconnaissance Orbiter (LRO) detects a widespread suppression of the epithermal neutron leakage flux that is coincident with the pole-facing slopes (PFS) of the Moon's southern hemisphere. Suppression of the epithermal neutron flux is consistent with an interpretation of enhanced concentrations of hydrogen-bearing volatiles within the upper meter of the regolith. Localized flux suppression in PFS suggests that the reduced solar irradiation and lowered temperature on PFS constrains volatility to a greater extent than in surrounding regions. Epithermal neutron flux mapped with LEND's Collimated Sensor for Epithermal Neutrons (CSETN) was analyzed as a function of slope geomorphology derived from the Lunar Orbiting Laser Altimeter (LOLA) and the results compared to co-registered maps of diurnally averaged temperature from the Diviner Lunar Radiometer Experiment and an averaged illumination map derived from LOLA. The suppression in the average south polar epithermal neutron flux on equator-facing slopes (EFS) and PFS (85-90 degrees S) is 3.3 +/- 0.04% and 4.3 +/- 0.05% respectively (one-sigma-uncertainties), relative to the average count-rate in the latitude band 45-90 degrees S. The discrepancy of 1.0 +/- 0.06% between EFS and PFS neutron flux corresponds to an average of similar to 23 parts-per-million-by-weight (ppmw) more hydrogen on PFS than on EFS. Results show that the detection of hydrogen concentrations on PFS is dependent on their spatial scale. Epithermal flux suppression on large scale PFS was found to be enhanced to 5.2 +/- 0.13%, a discrepancy of similar to 45 ppmw hydrogen relative to equivalent EFS. Enhanced poleward hydration of PFS begins between 50 degrees S and 60 degrees S latitude. Polar regolith temperature contrasts do not explain the suppression of epithermal neutrons on pole-facing slopes. The Supplemental on-line materials include supporting results derived from the uncollimated Lunar Prospector Neutron Spectrometer and the LEND Sensor for Epithermal Neutrons. Published by Elsevier Inc.

The polar regions of the Moon and Mercury both have permanently shadowed environments, potentially capable of harboring ice (cold traps). While cold traps are likely to have been stable for nearly 4 Gyr on Mercury, this has not been the case for the Moon. Roughly 3 +/- 1 Gya, when the Moon is believed to have resided at approximately half of its current semimajor axis, lunar obliquities have been calculated to have reached as high as 77 degrees. At this time, lunar polar temperatures were much warmer and cold traps did not exist. Since that era, lunar obliquity has secularly decreased, creating environments over approximately the last 1-2 Gyr where ice could be stable (assuming near current recession rates). We argue that the paucity of ice in the present lunar cold traps is evidence that no cometary impact has occurred in the past billion years that is similar to the one(s) which are thought to have delivered volatiles to Mercury's poles. However, the present ice distribution may be compatible with a cometary impact if it occurred not in today's lunar thermal environment, but in a past one. If ice were delivered during a past epoch, the distribution of ground ice would be dictated not by present day temperatures, but rather by these ancient, warmer, temperatures. In this paper, we attempt to recreate the thermal environments for past lunar orbital configurations to characterize the history of lunar environments capable of harboring ice. We will develop models of ice stability and mobility to examine likely fossil remains of past ice delivery (e.g. a comet impact) that could be observed on the present Moon. We attempt to quantify when in the Moon's outward evolution areas first became stable for ice deposition and when ice mobility would have ceased. (c) 2014 Elsevier Inc. All rights reserved.

H2O (v = 0) and O(P-3(J=2,1,0)) desorbates were measured with resonance-enhanced multiphoton ionization following 157-nm irradiation of amorphous solid water (ASW) deposited on a lunar mare basalt. Both H2O photodesorption and O(P-3(J)) photodissociation products of ASW were studied in the attempt to better understand the competition between photodesorption and photodissociation of water in the condensed phase on a lunar surface. The oxygen atom time-of-flight (TOF) spectrum was measured as a function of spin-orbit state, H2O exposure, and 157-nm irradiation time. Maxwell-Boltzmann distributions with translational temperatures of 10,000 K, 1800 K, 400 K, and 89 K fit the four TOF components. For high H2O exposures, diffusion out of pores in the lunar substrate made a large portion of the O(P-3(J)) signal appear to be sub-thermal. Water depletion cross sections were measured at exposures between 0.1 and 10 Langmuir (1 L = 10(-6) Torr s). These cross sections decreased with increasing coverage and matched previously measured cross sections from a lunar impact melt breccia. Additionally, non-resonant ionization was employed to detect vibrationally excited water indirectly through its fragments. The OH+ fragment of H2O (v*) and the O(P-3(J)) photodissociation product increased in intensity during prolonged irradiation as hydroxyl groups accumulated on the surface and then recombined. For an initial exposure of 5 L H2O, after reaching maximum signal, the cross sections for H2O (v*) and O((3)P4(2)) depletion were measured to be 1.2 x 10(-19) cm(2) and 6.7 x 10(-20) cm(2), respectively. (c) 2014 Elsevier Inc. All rights reserved.

Low energy secondary ions ejected by the solar wind are an important component of tenuous exospheres surrounding airless bodies, since these ions carry information on the planetary surface composition. In this work we examine the dependence of secondary-ion abundance, as a function of energy and mass, on surface composition. The surface compositions of two Apollo soils (10084 and 62231) and a synthetic Corning glass lunar simulant were measured with X-ray photoelectron spectroscopy and correlated with the spectra of secondary-ions ejected from the same soils by 4 key He ions. XPS spectra for lunar soils show that the surface compositions are similar to the bulk, but enriched in Fe and 0, while depleted in Mg and Ca. 4 keV He irradiation on the lunar soils and a glass simulant preferentially removes 0 and Si, enriching the surface in Al, Ti, Mg, and Ca. Secondary-ion species ejected from the Apollo soils by 4 keV He include: Na+, Mg+, Al+, Si+, Ca+, Ca++, Ti+, Fe+, and molecular species: NaO+, MgO+ and SiO+. Secondary ion energy distributions for lunar soil 10084 and 62231 rise rapidly, reach a maxima at similar to 5 eV for molecular ions and Na+, similar to 7.5 eV for Fe+, and similar to 10 eV for Mg+, Al+, Si+, Ca+ and Ti+, then decrease slowly with energy. We present species-dependent relative conversion factors for the derivation of atomic surface composition from secondary-ion count rates for 4 keV He irradiation of lunar soils 10084 and 62231, as well as the Corning glass lunar simulant. (c) 2014 Elsevier Inc. All rights reserved.