Shackleton Crater Research Articles

Enhanced hydrogen at the lunar poles: New insights from the detection of epithermal and fast neutron signatures

The Lunar Prospector (LP) and Lunar Reconnaissance Orbiter (LRO) orbital neutron spectroscopy data sets represent a unique comprehensive multimission lunar resource. A rigorous statistical approach has been used to (re)analyze neutron data from both missions to provide new details regarding the relationships between the individual detector data sets, as well as a new evaluation of enhanced hydrogen deposits at the lunar poles. Using the multimission epithermal neutron data set, we find water ice distributed broadly across the poles, but showing evidence for a dependence on topographical features. A footprint averaged water equivalent hydrogen (WEH) abundance of 106 ± 11 ppm at each pole, with maxima of 131 and 112 ppm at the north and south poles, respectively, is derived from the epithermal neutron data. We also report the first definitive detection of a fast neutron signature consistent with an enhanced hydrogen hypothesis. These data suggest a highly localized distribution, consistent with Shackleton Crater, corresponding to a footprint averaged WEH abundance of 194 ± 11. If confined to this crater, this abundance yields a localized deposit of ∼0.7% WEH. Details of the analysis approach are presented along with spatial distribution maps showing the intriguing enhanced hydrogen deposits at the lunar poles.

LANDSAFE: LANDING SITE RISK ANALYSIS SOFTWARE FRAMEWORK

Abstract. The European Space Agency (ESA) is planning a Lunar Lander mission in the 2018 timeframe that will demonstrate precise soft landing at the polar regions of the Moon. To ensure a safe and successful landing a careful risk analysis has to be carried out. This is comprised of identifying favorable target areas and evaluating the surface conditions in these areas. Features like craters, boulders, steep slopes, rough surfaces and shadow areas have to be identified in order to assess the risk associated to a landing site in terms of a successful touchdown and subsequent surface operation of the lander. In addition, global illumination conditions at the landing site have to be simulated and analyzed. The Landing Site Risk Analysis software framework (LandSAfe) is a system for the analysis, selection and certification of safe landing sites on the lunar surface. LandSAfe generates several data products including high resolution digital terrain models (DTMs), hazard maps, illumination maps, temperature maps and surface reflectance maps which assist the user in evaluating potential landing site candidates. This paper presents the LandSAfe system and describes the methods and products of the different modules. For one candidate landing site on the rim of Shackleton crater at the south pole of the Moon a high resolution DTM is showcased.

An upper limit for ice in Shackleton crater as revealed by LRO Mini‐RF orbital radar

Although diverse measurements have indicated H+, OH−, or H2O species in the lunar polar regions, pinpointing its location, form, and abundance in specific reservoirs has proven elusive. Here we report on the first orbital radar measurements of Shackleton crater near the lunar south pole. Mini‐RF observations indicate a patchy, heterogeneous enhancement in CPR (circular polarization ratio) on the crater walls whose strength decreases with depth toward the crater floor, a result that is most consistent with a roughness effect due to less mature regolith present on the crater wall slopes. However, the results do not rule out a modest ice contribution, and an upper limit of ∼5–10 wt% H2O ice (up to 30 vol.%) present in the uppermost meter of regolith is also consistent with the observations.

Constraints on the volatile distribution within Shackleton crater at the lunar south pole

Shackleton crater is nearly coincident with the Moon's south pole. Its interior receives almost no direct sunlight and is a perennial cold trap, making Shackleton a promising candidate location in which to seek sequestered volatiles. However, previous orbital and Earth-based radar mapping and orbital optical imaging have yielded conflicting interpretations about the existence of volatiles. Here we present observations from the Lunar Orbiter Laser Altimeter on board the Lunar Reconnaissance Orbiter, revealing Shackleton to be an ancient, unusually well-preserved simple crater whose interior walls are fresher than its floor and rim. Shackleton floor deposits are nearly the same age as the rim, suggesting that little floor deposition has occurred since the crater formed more than three billion years ago. At a wavelength of 1,064 nanometres, the floor of Shackleton is brighter than the surrounding terrain and the interiors of nearby craters, but not as bright as the interior walls. The combined observations are explicable primarily by downslope movement of regolith on the walls exposing fresher underlying material. The relatively brighter crater floor is most simply explained by decreased space weathering due to shadowing, but a one-micrometre-thick layer containing about 20 per cent surficial ice is an alternative possibility.

基于"嫦娥一号"激光测高数据的月球极区光照条件研究

The illumination condition of the lunar polar regions can be used to analyze the choice of ideal landing sites and to conduct the investigation of possible water ice deposits. The laser altimeter data obtained by 'Chang/E-1' can provide high accuracy digital elevation models (DEM) for polar illumination model. In this study, we establish the mathematical model to study the polar illumination condition, by combining 'Chang' E-1' altimeter data and known parameters related to lunar orbit. The calculation period of illumination rate is 19 years, which is longer than the node precession period (18. 6 years). In this paper, the horizon method is used instead of the ray-tracing method, and the results show: (1) We don't find the areas that receive permanent illumination; (2) There are regions that are in constant darkness; (3) The edge of the craters can be found to be as continuous illumination region in the north summer or the south summer; (4) The Shackleton crater is one of the main targets as landing sites of the lunar rover.

Illumination conditions of the lunar polar regions using LOLA topography

We use high-resolution altimetry data obtained by the Lunar Orbiter Laser Altimeter instrument onboard the Lunar Reconnaissance Orbiter to characterize present illumination conditions in the polar regions of the Moon. Compared to previous studies, both the spatial and temporal extent of the simulations are increased significantly, as well as the coverage (fill ratio) of the topographic maps used, thanks to the 28 Hz firing rate of the five-beam instrument. We determine the horizon elevation in a number of directions based on 240 m-resolution polar digital elevation models reaching down to ∼75° latitude. The illumination of both polar regions extending to ∼80° can be calculated for any geometry from those horizon longitudinal profiles. We validated our modeling with recent Lunar Reconnaissance Orbiter Wide-Angle Camera images. We assessed the extent of permanently shadowed regions (PSRs, defined as areas that never receive direct solar illumination), and obtained total areas generally larger than previous studies (12,866 and 16,055 km 2, in the north and south respectively). We extended our direct illumination model to account for singly-scattered light, and found that every PSR does receive some amount of scattered light during the year. We conducted simulations over long periods (several 18.6-years lunar precession cycles) with a high temporal resolution (6 h), and identified the most illuminated locations in the vicinity of both poles. Because of the importance of those sites for exploration and engineering considerations, we characterized their illumination more precisely over the near future. Every year, a location near the Shackleton crater rim in the south polar region is sunlit continuously for 240 days, and its longest continuous period in total darkness is about 1.5 days. For some locations small height gains (∼10 m) can dramatically improve their average illumination and reduce the night duration, rendering some of those particularly attractive energy-wise as possible sites for near-continuous sources of solar power.

Advanced modeling of optimal low-thrust lunar pole-sitter trajectories

One immediate and cost-effective solution for lunar south pole coverage is a low-thrust pole-sitter spacecraft. When the spacecraft's fuel is nearly expended, it can also be used as part of a larger constellation that might require more time to deploy. This study examines the feasibility of the lunar pole-sitter from a dynamical standpoint. The model includes the effects of Earth oblateness, solar wind, gravity perturbations, and shadowing on a 500 kg spacecraft equipped with an NSTAR thruster. Lunar librations are also incorporated into path constraints on elevation angle and altitude from a potential ground station at the Shackleton crater. The solutions utilize multiple phases, including transfer-out from the International Space Station and spiral-down into a stable lunar orbit once the pole-sitting period is completed. Computation of the trajectory is based on a 12th-order collocation scheme. A direct transcription routine maximizes the time in the coverage phase, increasing the time span from 387 to 451 days. During this period, the minimum elevation angle relative to the Shackleton crater site is 6°.

Illumination conditions of the south pole of the Moon derived using Kaguya topography

We have used the Kaguya laser altimeter-derived topography to conduct a comprehensive study of the illumination conditions at the Moon’s south pole. We have determined, by comparing simulated and actual Clementine images, that the Kaguya topography can be used to generate realistic illumination conditions. We generated an average illumination map for the year 2020 for the lunar south pole region. From this we identified the areas that receive the most illumination. The place receiving the most illumination (86% of the year) is located close to the rim of Shackleton crater at 88.74°S 124.5°E. However two other areas, less than 10 km apart from each other, are collectively lit for 94% of the year. We found that sites exist near the south pole that are continuously lit for several months during summer. We were also able to map the locations and durations of eclipse periods for these areas. Finally we analyzed the seasonal variations in lighting conditions, from summer to winter, for key areas near the south pole. We conclude that areas exist near the south pole that have illumination conditions that make them ideal candidates as future outpost sites.

Lack of Exposed Ice Inside Lunar South Pole Shackleton Crater

The inside of Shackleton Crater at the lunar south pole is permanently shadowed; it has been inferred to hold water-ice deposits. The Terrain Camera (TC), a 10-meter-resolution stereo camera onboard the Selenological and Engineering Explorer (SELENE) spacecraft, succeeded in imaging the inside of the crater, which was faintly lit by sunlight scattered from the upper inner wall near the rim. The estimated temperature of the crater floor, based on the crater shape model derived from the TC data, is less than approximately 90 kelvin, cold enough to hold water-ice. However, at the TC's spatial resolution, the derived albedo indicates that exposed relatively pure water-ice deposits are not on the crater floor. Water-ice may be disseminated and mixed with soil over a small percentage of the area or may not exist at all.

Concerning the dissipation of electrically charged objects in the shadowed lunar polar regions

NASA recently suggested the construction of a lunar outpost at the south pole near the rim of Shackleton crater. While there are a number of advantages to such a base, the region will have periods of time when there is limited or no solar illumination ‐ thereby reducing photoelectric and solar wind plasma currents compared to most of the lunar dayside. As a consequence of this reduction in environmental currents, we find that human systems charged by contact electrification with the regolith (e.g., roving, excavation) will have increased difficulty in removing accumulated electric charge. This situation is especially true within the cold, shadowed regions adjacent to the terminator (such as within Shackleton crater itself) where there are essentially no photoelectric currents, vastly reduced plasma currents (due to the local wake) and a highly‐reduced regolith conductivity. In essence, there is no pathway for accumulated charge to “leak away” or dissipate, thereby creating an electrostatic hazard. Calculated dissipation timescales are found to be ∼1 millisecond in the weakly sunlit terminator region and dayside but could approach 100's of seconds in the “current‐starved” shadowed regions.

Age makes Moon crater attractive site for lunar base

Dating of Shackleton crater suggests it may offer supply of ice.

Geology of Shackleton Crater and the south pole of the Moon

Using new SMART‐1 AMIE images and Arecibo and Goldstone high resolution radar images of the Moon, we investigate the geological relations of the south pole, including the 20 km‐diameter crater Shackleton. The south pole is located inside the topographic rim of the South Pole‐Aitken (SPA) basin, the largest and oldest impact crater on the Moon and Shackleton is located on the edge of an interior basin massif. The crater Shackleton is found to be older than the mare surface of the Apollo 15 landing site (3.3 Ga), but younger than the Apollo 14 landing site (3.85 Ga). These results suggest that Shackleton may have collected extra‐lunar volatile elements for at least the last 2 billion years and is an attractive site for permanent human presence on the Moon.

Models of the distribution and abundance of hydrogen at the lunar south pole

Permanently shadowed locations at the lunar poles are potential sites for significant concentrations of cold‐trapped volatiles, including water ice. Hydrogen enhancements are seen at the poles, but the physical form, abundance and distribution of this hydrogen remains controversial. Using a pixon‐based image reconstruction algorithm to effectively improve spatial resolution, we derive maps of the lunar south polar water‐equivalent hydrogen concentration that are fully consistent with the orbital neutron measurements, with abundances greater than 0.5 wt% in some permanently shadowed locations. This is much greater than the highest solar wind hydrogen abundance in returned lunar samples, and may indicate ice between regolith grains. If the hydrogen distribution is inhomogeneous within a permanently shadowed crater, then even higher abundances are implied. In Shackleton crater, for example, the derived count rates are consistent with 10% of the crater floor area having 20‐wt% water‐equivalent hydrogen, and the remainder at 0.25 wt%.

No evidence for thick deposits of ice at the lunar south pole

Shackleton crater at the Moon's south pole has been suggested as a possible site of concentrated deposits of water ice, on the basis of modelling of bi-static radar polarization properties and interpretations of earlier Earth-based radar images. This suggestion, and parallel assumptions about other topographic cold traps, is a significant element in planning for future lunar landings. Hydrogen enhancements have been identified in the polar regions, but these data do not identify the host species or its local distribution. The earlier Earth-based radar data lack the resolution and coverage for detailed studies of the relationship between radar scattering properties, cold traps in permanently shadowed areas, and local terrain features such as the walls and ejecta of small craters. Here we present new 20-m resolution, 13-cm-wavelength radar images that show no evidence for concentrated deposits of water ice in Shackleton crater or elsewhere at the south pole. The polarization properties normally associated with reflections from icy surfaces in the Solar System were found at all the observed latitudes and are strongly correlated with the rock-strewn walls and ejecta of young craters, including the inner wall of Shackleton. There is no correlation between the polarization properties and the degree of solar illumination. If the hydrogen enhancement observed by the Lunar Prospector orbiter indicates the presence of water ice, then our data are consistent with the ice being present only as disseminated grains in the lunar regolith.

Regolith properties in the south polar region of the Moon from 70-cm radar polarimetry

The south polar region of the Moon contains areas permanently shadowed from solar illumination, which may provide cold traps for volatiles such as water ice. Previous radar studies have emphasized the search for diagnostic polarization signatures of thick ice in areas close to the pole, but near-surface regolith properties and regional geology are also important to upcoming orbital studies of the shadowed terrain. To study regional regolith variations, we collected 70-cm wavelength, 450-m resolution, dual-circular polarization radar data for latitudes 60–90° S using the Arecibo and Greenbank telescopes. The circular polarization ratio, μ c , is sensitive to differences in rock abundance at the surface and up to tens of m below the surface, depending upon the regolith loss tangent. We observe significant variations in μ c , attributed to changes in the surface and subsurface rock population, across the south polar highlands. Concentric haloes of low polarization ratio surrounding Hausen, Moretus, and other young craters represent rock-poor ejecta layers. Values of μ c up to ∼1 occur in the floors and near-rim deposits of Eratosthenian and Copernican craters, consistent with abundant rocky ejecta and/or fractured impact melt. Enhanced μ c values also correspond to areas mapped as Orientale-derived, plains-forming material [Wilhelms, D.E., Howard, K.A., Wilshire, H.G., 1979. USGS Map I-1162], and similar polarization properties characterize the permanently shadowed floors of craters Faustini and Shoemaker. Small areas of very high (>1.5) circular polarization ratio occur on shadowed and seasonally sunlit terrain, and appear to be associated with small craters. We suggest that regolith in low-lying areas near the south pole is characterized by a significant impact melt component from Orientale, which provides a source for excavation of the block-rich ejecta around small craters observed in this and earlier radar studies. The lower portion of the interior wall of Shackleton crater, permanently shadowed from the sun but visible from Earth, is not significantly different in 70-cm scattering properties from diurnally/seasonally sunlit areas of craters with similar morphology.

Integration of lunar polar remote‐sensing data sets: Evidence for ice at the lunar south pole

In order to investigate the feasibility of ice deposits at the lunar south pole, we have integrated all relevant lunar polar data sets. These include illumination data, Arecibo ground‐based monostatic radar data, newly processed Clementine bistatic radar data, and Lunar Prospector neutron spectrometer measurements. The possibility that the lunar poles harbor ice deposits has important implications not only as a natural resource for future human lunar activity but also as a record of inner solar system volatiles (e.g., comets and asteroids) over the past billion years or more. We find that the epithermal neutron flux anomalies, measured by Lunar Prospector, are coincident with permanently shadowed regions at the lunar south pole, particularly those associated with Shackleton crater. Furthermore, these areas also correlate with the β = 0 circular polarization ratio (CPR) enhancements revealed by new processing of Clementine bistatic radar echoes, which in turn are colocated with areas of anomalous high CPR observed by Arecibo Observatory on the lower, Sun‐shadowed wall of Shackleton crater. Estimates of the extent of high CPR from Arecibo Observatory and Clementine bistatic radar data independently suggest that ∼10 km2 of ice may be present on the inner Earth‐facing wall of Shackleton crater. None of the experiments that obtained the data presented here were ideally suited for definitively identifying ice in lunar polar regions. By assessing the relative merits of all available data, we find that it is plausible that ice does occur in cold traps at the lunar south pole and that future missions with instruments specifically designed to investigate these anomalies are worthy.