. The potential for volatiles in the Intercrater Highlands of the lunar North Pole

. Volatiles at the Lunar  South Pole: A case stydy for an mission to  AMUNDSEN CRATER

Apollo 14: Mission to Fra Mauro 1971 NASA 3rd Moon Landing Alan Shepard

"Astronauts: Alan B. Shepard, Stuart A. Roosa, and Edgar D. Mitchell Launch date: January 31, 1971 Includes the early problem of docking the Command and Lunar Modules, landing on the Moon, experiments package, climb up Cone Crater, onboard experiments, scenes in the NASA JSC's Lunar Receiving Laboratory, and commentaries by noted scientists. AWARDS: Chris Certificate, Columbus Film Festival, 1971 * Golden Eagle, Council on International Nontheatrical Events (CINE), 1971". Credit: Jeff Quitney

. ROSCOMOS gives OK to LUNA-25 (LUNA-GLOB)
. A Landing Site for Russia's Luna-Glob
. Tycho Crater
. ASTROBOTIC will revolutionize the Moon
. SHAKLETON ENERGY is on the line of the Moon

. The CISLUNAR SPACE ECONOMY of UNITED LAUNCH ALLIANCE (ULA)
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Coordination Group (ISECG)
. The China Chang'e-4 mission to the Moon will be Historical!
. Renewed focus on the Moon
. The Chinese Chang'e-5 mission will return samples from the Moon
MOON EXPRESS: Expanding earth's economic and social sphere to the Moon 
. ROCKET LAB is ready to make business
ISPACE - Expand our planet. Expand our future
. SpaceIL of Israeli, Team INDUSSYNERGY MOON INTERNATIONAL Team HAKUTO
. The Lunar Orbiter Laser Altimeter (LOLA)
. Landing sites: Pit crater/lava tubesPolar RegionsSouth Pole - Aitken BasinMarius Hills, the Aristarchus plateau, Rima BodeGruithuisen Domes & Moscoviense
. Landing sites: the Orientale basin, the Schrödinger basin, Irregular Mare Patches/Ina Caldera, Magnetic Anomalies and Swirls, Compton-Belkovich Volcanic Deposit (CBVD) and P60 Basaltic Unit
. Exploring the Moon surface - SOON

 
 

Lunar North Pole

New Reduced Data Records (RDRs) available as part of the 32nd Planetary Data System (PDS) release include two versions of the polar illumination maps for each pole. They include this time-weighted north pole illumination map that extends from 88ºN to the lunar north pole at 90ºN, as well as other new products. These maps show how much sunlight specific locations receive over the course of a lunar year. Areas that are nearly white are almost always in the sunlight, while black areas are Permanently Shadowed Regions (PSRs).  Credit: NASA/GSFC/Arizona State University.

Rovers and instruments that could explore these regions would normally rely on solar power and batteries for power. Even in places near the poles that receive some sunlight, it is necessary to understand how much sunlight they receive so that a rover does not accidentally become stuck in the dark long enough to run down its batteries. For more information on polar rover traverses, see Lunar Exploration: Planning the Next Steps.

 

Lunar SouthPole

The spin axis of the Moon is titled less than two degrees, thus the lighting conditions at the poles are always extreme. The long deep shadows give the impression that the polar regions are unusually rough and thus dangerous. However, the topography is no different than that found near the equator. So, polar landings should be relatively safe, even in permanently shadowed regions (PSRs), if the spacecraft is equipped with active landing sensors (lidar or radar). The permanent shadows are formed by topographic highs, and even though these high points are often narrow, they are very inviting landing spots because many are illuminated for most of the lunar year. Future surface missions to the lunar poles will likely utilize these highly illuminated regions, where solar power is abundant, as jumping off points to determine what volatiles (water ice, methane, etc.) PSRs may hold, and how these volatiles can be utilized as resources for future human exploration. For now, you can explore the high-resolution oblique view of the illuminated rim of Shackleton crater near the South Pole, on the right.

Left: spectacular oblique view of the rim of Shackleton crater (21 km diameter, 89.66°S, 129.20°E). While no location on the Moon stays continuously illuminated, three points on the rim remain collectively sunlit for more than 90% of the year. These points are surrounded by topographic depressions that never receive sunlight, creating cold traps that can capture ices, NAC M1224655261LR [NASA/GSFC/Arizona State University].

 
 

Reference: A Global Lunar Landing Site Study to Provide the Scientific Context for Exploration of the Moon / Edited by David A. Kring and Daniel D. Durda / Study Prepared by Members of the LPI-JSC Lunar Exploration Summer Intern Program / LPI-JSC Center for Lunar Science and Exploration / A Member of the NASA Lunar Science Institute / Copyright  2012 / LPI Contribution No. 1694
About sites selection proposed to reach... 

Science Concept 4: The Lunar Poles Are Special Environments That May Bear Witness to the Volatile Flux Over the Latter Part of Solar System History

To complete the landing site selections, authors have used the Spatial Analyst Tools in ArcMap. To proceed, they imported relevant layers and pre-processed it to obtain new data sets. The data were combined according to different criteria for each Science Goal in order to find suitable sites for each one. At the end, results were overlapped to find suitable sites. Post-processing analysis for each result led to selection of the highest priority sites.

At Outer Space Economy - Moon, we explain some of those.

The era of lunar exploration began in the late 1950s with the Soviet Luna and American Pioneer missions, and continued into the 1970s with the Ranger, Zond, Lunar Orbiter, Surveyor, and Apollo programs. After the 1970s, there was a hiatus in lunar exploration until the 1990 launch of the Japanese Hiten spacecraft. Since that time, there has been a renewed widespread interest in returning to the Moon, and in the past two decades, American, Japanese, European, Chinese, and Indian spacecraft have begun to return large amounts of new data on the lunar polar regions.

GeneralMoon facts

Average Distance from Earth: 238,855 | 384,400 kilometers

Orbit and Rotation Period: 27.32 Earth Days

Equatorial Radius: 1,079.6 miles | 1.737.5 kilometers

Mass: 0.0123 of Earth's (a bit more than 1 percent)

Gravity: 0.166 of Earth's (If you weigh 100 pounds (45 kilograms) on Earth, you'd weight 16.6 pounds (7.5 kilograms) on the moon)

Temperature Range: -414 to 253 degrees Farenheit (-248 to 123 degrees Celsius)

The moon’s iron-rich inner  is about 149 miles, or 240 km, in radius. It is surrounded by a liquid iron shell of 56 miles (90 km) thick, and a partially 93 miles (150 km) molten layer of a thickness.

The mantle extends from the top of the partially molten layer to the bottom of the moon’s crust. It is most likely made of minerals like olivine and pyroxene, which are made up of magnesium, iron, silicon and oxygen atoms.

The crust has a thickness of about 43 miles (70 km) on the near-side hemisphere and 93 miles (150 km) on the far-side. It is made of oxygen, silicon, magnesium, iron, calcium and aluminum, with small amounts of titanium, uranium, thorium, potassium and hydrogen.

Source: https://moon.nasa.gov/about/in-depth/

A - North Pole

In general, the North Pole is characterized by an high density of small Permanent Shadowed Regions (PSRs). At the PSRs, a high number of small sites can be found in the area between Hermite and Peary craters, as well as on the northern wall of Hermite, Rozhdestvensky, and Rozhdestvensky W craters.

Small areas cover the northern half of Peary crater floor as well as its wall.

B - North Pole

One of th Science Goal of interest is to Understand the physical Properties of the Extremly Cold Polar Regolith, as well as the Volatile. It was found that, at the Poles, the Volatile content varies in the range of  ~0.1 to 1.8kg per 0.1 kg/m3 of H, and ~1 to 20 g per 0.1kg/m3 of H (noble gases).

Scientists founad also that the extremly low temperatures (50–70 K) can have a drastic effect on regolith properties such as bulk density and conductivity. And, the subsurface temperature profiles is likely different due to the temperature of the surrounding material.

C - South Pole

In the South pole, sites of interest are located above the 84° latitude. In this region, the largest site is on the northeastern part of the floor of Amundsen crater. Within the smaller craters in Faustini, Haworth, and de Gerlache craters, small sites are concentrated in an elevated area between Haworth and Nobile craters.

The Amundsen crater could be an excellent choice of landing because their sites are within a PSR and is surrounded by warmer regions that are mostly illuminates. With a Central Peak and walls that have slopes shallower than 25°, it allow easy exploration.

D - South Pole

In the South pole regions showing above, a small concentration of sites are visible mostly on the floors of medium-to-large craters. Those craters are Amundsen, Faustini, Haworth, and de Gerlache craters, as well as the plains west of Shoemaker.  

The highest priority for landing sites is to have a maximum annual temperature of ≤54 K , usually within PSRs.  At this point, it will be possible to study of physical properties of extremely cold regolith.

The most generally accepted lunar geologic chronology is the one established by Wilhelms (1987). This chronology divides lunar history into five main epochs: the pre-Nectarian (>3.92 Ga), the Nectarian (3.92 to 3.85 Ga), the Imbrian (3.85 to 3.2 Ga), the Eratosthenian (3.2 to 0.8 Ga), and the Copernican (<0.8 Ga) (Fig. 1.9). Only the lower Imbrian and earlier time boundaries are known with any accuracy, because of the Apollo and Luna samples. The later periods are based on relative stratigraphy of surface features and the boundaries are approximate.

In the lunar science community, volatiles are defined as chemical elements and compounds that become unstable and vaporize, sublimate, or are otherwise mobilized at low temperatures. Lunar volatile elements can be divided into two groups, vapor-mobilized and solar-wind-implanted. The latter group includes H, C, N, and the noble gases (He, Ne, Ar, Kr, Xe). Solar-wind-implanted volatiles are most likely to be found in permanently shadowed regions (PSRs) at the lunar poles (Haskin et al., 1991) due to "cold trapping", though the implantation process is globally homogeneous.

Table at right give the sublimation point of various substances at lunar surface pressures. The sublimation temperature values come from Zhang and Paige (2010).

Chemical Formula, Name, Sublimation Temperature(K) & Sublimation Temperature(°C)

N2: Nitrogen, 16.20, -257.0/ CO: Carbon monoxide, 18.20, -255.0/ Ar: Argon, 19.50, -253.7/ CH4: Methane, 22.00, -251.2/ Kr: Krypton, 24.50, -248.6/ Ar-6H2O: Argon clathrate, 28.90, -244.3/ Xe: Xeon, 36.10, -237.1/ H2S: Hydrogen sulfide, 50.60, -222.6/ CO2: Carbon dioxide, 54.30, -218.9/ SO2: Sulfur dioxide, 62.30, -210.9/ NH3: Ammonia, 65.50, -207.7/ C5H12: Penthane, 73.60, -199.6/ HCN: Hydrogen cyanide, 80.50, -192.7/ C7H8: Toluene, 87.60, -185.6/ NH4SH: Ammonium hydrosulfide, 96.10, -177.1/ H2O: Water, 106.6, -166.6/ S: Sulfur, 201.5, -71.65

Vapor-mobilized elements are transferred from solid materials to a coexisting vapor-phase at moderate temperature. This group includes S, Cu, Zn, As, Se, Ag, Cd, In, Te, Hg, Tl, Pb, Bi, and the halogens (F, Cl, Br, I). The vapor-mobilized volatiles are mostly added by small impactors, and so should be deposited fairly uniformly throughout the lunar regolith (Haskin et al., 1991).

At the poles, the regolith has been extensively gardened and reworked and is consequently extremely fine-grained. Such grain sizes provide greater surface area for volatile adsorption. The volatile-rich upper layers of the polar regolith are then buried at greater depths by small impact processes. This preserves the volatiles by protecting them from atomic particle bombardment.

 
 

The geology of the region poleward of 80°N is mostly composed of crater fields approaching geometric saturation. Since some craters are at most a few kilometers in diameter, some contains several large (D > 70 km) flat-floored, such as Peary. No craters lying entirely the North of the 80° latitude line exhibit central peaks. The terrain in the study area on the nearside is mostly Imbrian-aged basin and plains material. On the farside, it is mostly Nectarian basin material and ancient pre-Nectarian cratered terrain, with dense crater clustering (USGS, 2009a).

Because the Moon's axis of rotation is nearly perpendicular to the plane of its orbit around the Sun, with an axis inclination ~1.5°, the Sun appears always at or near the horizon in polar regions. The very small obliquity causes some high topographic features near the lunar poles with a constant illumination, while some  depressions are permanently shaded from sunlight. These regions may act as "cold traps" for volatiles migrating across the lunar surface. When Volatiles enter PSRs, they are unable to escape due to extremely low temperatures.

The potential for volatiles in the Intercrater Highlands of the lunar North Pole *

Lunar polar volatiles can provide clues about solar and planetary evolutionary processes . In addition, local sources of water and other volatiles on the Moon may enable in-situ resource utilization (ISRU) for future permanent human bases. To address these science and exploration issues, we need to identify lunar landing sites where the distribution and physical properties of volatiles can be studied.

The National Research Council (NRC) identified five science objectives as being essential to understanding lunar polar volatiles [1]. These were to (a) determine the compositional state and distribution of volatiles in lunar polar regions, (b) determine the sources for lunar polar volatiles, (c) understand the transport, retention, alteration, and loss processes that operate on volatile materials at permanently shadowed regions (PSRs),  (d) understand the physical properties of the extremely cold polar regolith, and (e) determine what the cold polar regolith reveals about the ancient solar environment.

To determine where the NRC’s objectives can be met by human and/or robotic surface missions, a survey of both polar regions was conducted. Here, the focus is on the

Intercrater Polar Highlands region (IPH) near the lunar North Pole, which contains landing sites where all five NRC science objectives can be addressed. To study the region further and to evaluate specific landing sites within it, we integrated observations from the Lunar Prospector, the Lunar Reconnaissance Orbiter, and the Chandrayaan-1 spacecraft (see at the right)with existing USGS geologic maps. The outcome of their data integration is represented in Figure above.

Centered at 285°E, 89°N, the IPH is ~4000 km2 and consists of rugged, hummocky terrain bounded by Peary, Rozhdestvenskiy, and Hermite craters. The IPH is relatively homogeneous due to its age and near-complete crater saturation. The majority of the region has been characterized as pre-Nectarian (~4.60–3.94 Ga) and Nectarian (~3.94–3.86 Ga) in age, which is bounded by highly subdued pre-Nectarian crater material. Within the IPH there are also a few young craters of Erastosthenian age (3.2–1.1 Ga), namely Hermite A.

FIGURE: Enhanced view of recommended sites in the north polar region,

The IPH contains abundant PSRs that are smaller, but more numerous, than those at the South Pole.  PSRs within the IPH are typically <10 km in diamter, occupying small topographic lows that are often degraded simple craters. These regions have not been illuminated by the Sun in at least 2 Ga and, thus, act as ‘cold traps’ for volatiles migrating from other regions on the lunar surface.  Orbital measurements indicate that the IPH region has enhanced hydrogen abundances (>150 ppm; [6]), low average temperatures (<54 K), and high circular polarization ratio (CPR) values. The Mini-SAR CPR data suggest that ice may be heterogeneously distributed within many small craters near the North Pole and be at least tens of wavelengths (∼2–3 m) thick.

The authors have identified two sites that can provide access to areas with overlapping science objectives and that have slopes suitable for rover operations.

In the Area A, located at 258°E and 88.5°N, 3 science stations are possible. In the station A1, because temperatures don't exceed 54 K, it provides an opportunity to sample the regolith that could contain volatile compounds with sublimation temperatures above 54 K (CO2, SO2, NH3, C5H12, HCN, C7H8, H2O, and S). This station exhibits some of the highest hydrogen abundances in the IPH region (>155 ppm), further supporting the potential for hydrogen compounds within the regolith at this location. The low CPR values observed at A1 would permit ground truth for the global CPR dataset, which could prove to be immensely useful for locating subsurface ice in the future.

At the station A2, there is an opportunity to sample volatiles with sublimation temperatures >50 K (like H2S and the compounds listed for A1), as the maximum annual temperature is ~50 K at this location. Station A2 is located within a degraded simple crater and would enable access to deeper deposits from the crater floor. Enhanced hydrogen abundances (>154 ppm) are observed at A2, as well as high CPR values (suggestive of subsurface water ice).

The station A3 is like the station A2 in that it has similar hydrogen abundances, and would provide an opportunity to sample volatiles that sublimate above 50 K. On the other hand, A3 exhibits medium CPR values, in contrast with A2. The hills near the southern edge of area A experience ~35–45 % illumination over four full lunar cycles and have the greatest solar power collection potential for a mission to the IPH.

Figure 1: Case study for the IPH near the lunar North Pole. Science areas A (red) and B (yellow) include a landing site (stars), 3–5 stations (dots), and 10-km exploration radius (large empty circles). PSRs are in dark blue, while areas fulfilling all science objectives are in light blue.

In the Area B, located at 273°E and 89°N, stations exhibit hydrogen abundances of 152 ± 1 ppm and temperatures (<49 K maximum annual) similar to stations in A, so they provide the same potential volatile sampling opportunities. All five B stations have minimum annual temperatures <20 K (with B1 and B4 being <15 K). The station B4 is within a steep-walled (20°–35°) crater, en abling the sampling of the crater rim/wall material, and providing access to deeper deposits from the crater floor (similar to A2).  As with area A stations, B stations sample a range of CPR values.

Each area consists of 3 to 5 geologic stations located within a 10 km radius (for extravehicular activity safety reasons; [10]) of a flat (≤ 1° slopes) landing site. All stations can be reached following rover-accessible paths (slopes <20°; [11]), and are located in areas where all science objectives can be addressed. Multiple core samples (to depths of 3 m) collected in a grid-like fashion would address science objective (a) locally, as well as objectives (d) and (e) due to the stations' low annual average temperatures of ~31–39 K. Sampling regolith to depths of 3 m is required to verify potential subsurface water ice as suggested by high CPR values at some stations. Samples collected at stations situated in variable terrains (e.g. crater rim/floor) partially address objectives (a) and (c). To address science objectives (b) and (c), ion and particle collectors could be set up to asses s volatile flux at the boundary of the PSR in which each station resides. In order to fully address science objective (c), regolith core samples could be collected not only at each station, but also outside and on the borders of their respective PSRs.

There are many ways that volatiles can move across the lunar surface, including electrostatic levitation, random ballistic walk, thermal diffusion, impact vaporization, and impact gardening.

In the colder temperature regime (maximum annual temperatures < 54 K), our result is generally
characterized by small locations, particularly in the Intercrater Polar Highlands (IPH) between Hermite,
Peary, and Rozhdestvenskiy craters. Sites also exist on the bases and walls of large craters, such as in
Hermite, Peary, and Lenard craters.
For the warmer (90–130 K) temperature range, a similar pattern exists, but with more sites present (Fig.
4.13). Larger areas exist in the floors and walls of moderately-sized craters such as Lovelace,
Rozhdestvenskiy U, and Nansen F and A. Rozhdestvenskiy U is notable also for its high hydrogenabundances and the large area matching Science Goal 4c selection criteria.

Enhanced view of recommended sites in the North polar region satisfying Science Goal 4c: UNDERSTAND THE TRANSPORT, RETENTION, ALTERATION, AND LOSS PROCESSES THAT OPERATE ON VOLATILE MATERIALS AT PERMANENTLY SHADED LUNAR REGIONS

* C. E.  Roberts and al. - 43rd Lunar and Planetary Science Conference (2012) PDF1215

Chandrayaan-1

The Indian Space Research Organization launched the Chandrayaan-1 craft in 2008, which was equipped with a VIS/NIR spectrometer, the Moon Mineralogy Mapper (M3). This instrument detected absorption features indicative of hydroxyl (both lone OH and possibly H-OH, or H2O) near 2.8 to 3.0 μm at the surface (upper 1-2 mm of regolith), with the 3.0 μm feature appearing strongest at high latitudes (Pieters et al., 2009). Chandrayaan-1 also obtained polarimetric radar data with the instrument Mini-SAR, which measured coherent backscatter levels in various craters on the lunar surface. Specifically, it returned information on the ratio of radar signals returned with right- versus left-circular polarization, known as the Circular Polarization Ratio (CPR). Most high CPR readings covered an area including both a crater‟s interior and surrounding regions, indicating a rough or fresh surface. However, there were some anomalous craters in PSRs with elevated CPR values in crater interiors without elevated values exterior to the crater, consistent with the presence of water ice at depth in these craters (Spudis et al., 2010).

Artist's rendition of Chandrayaan-1 in lunar orbit (image credit: ISRO)

The Mini-SAR instrument onboard Chandrayaan-1 also found what is believed to be water ice at the poles (Spudis et al., 2010). This radar instrument, emitting a circular polarized signal, allowed the calculation of the circular polarization ratio (CPR), which is the ratio of the signal received in the same sense as transmitted to that received in the opposite sense. A high CPR value both interior and exterior to a crater is believed to be the result of a rough surface, indicative of a fresh crater.

Spudis et al. (2010) suggest that a high CPR value found only inside a crater is consistent with the presence of water ice at depths up to 2–3 m. According to their results, ice would be heterogeneously distributed within many, but not all, small craters near the North Pole.

Because the IPH region lacks substantial topographic highs, temperatures remain consistently cold (~23–54 K average annual temperatures), and there is little direct sunlight. Models suggest the entire region experiences ~0–25% illumination over a period of four full (18.6 earth-year) cycles (Mazarico et al., 2011). While this is suboptimal for generation of solar power during a mission, the situation can be improved by installing solar panels in nearby well-lit areas or mounted on ~10-meter-high masts. Alternatively, radiogenic power sources could be used. Because of the patchwork nature of the PSRs in the IPH, landing can occur in sunlit regions (attractive for solar power) and short drives can then access the PSRs for measurements and sample collection.

The Neutron Spectrometer onboard Lunar Prospector mapped the hydrogen content over the entire Moon for the top 50 cm of regolith and showed a clear enhancement at the lunar poles (Feldman et al., 1998). However, the Neutron Spectrometer data alone is not sufficient to distinguish between hydrogen and different hydrogenated volatile species (Feldman et al., 2000). Feldman et al. (2001) later found that the largest concentrations of hydrogen at the poles coincide with Permanently Shadowed Regions (PSRs). They concluded that a significant portion of the hydrogen detected at the poles was most likely to be in the form of water molecules.

Pieters et al. (2009) later analyzed Moon Mineralogy Mapper (M3) spectra and found distinct absorption bands at 2.8 and 3.0 μm that could be attributed to OH and/or H2O present in the top 1–2 mm of the lunar surface. The strongest absorption features were found near the poles in cooler regions and seemed to coincide with several fresh craters in feldspathic terrain. They observed a general lack of correlation with previous neutron measurements, suggesting that the formation and retention of OH and H2O are continuous surficial processes. Cheek et al. (2011) believe that, in the region they studied, the space weathering process increased the concentration of adsorbed OH and H2O.

Spacecrafts used for exploration

Clementine

In 1994 a backscatter radar experiment was performed to remotely detect the presence of ice at depth under the lunar surface. Radio waves were beamed from Clementine into polar areas, reflected from the surface, and received on Earth by the Deep Space Network. One orbit was directly over a PSR at the South Pole, and it produced coherent backscatter consistent with the presence of water ice (Spudis et al., 1998).

Lunar Reconnaissance Orbiter-LRO

New LRO data are also greatly enhancing our understanding of lunar volatiles. These data sets include high-resolution imagery, topography, temperature, and hydrogen data. We have used the most current existing versions of these data sets at the time this study was conducted. In 2009, the Centaur stage of LRO‟s Atlas V rocket was impacted into Cabeus crater, chosen because it was a PSR at the South Pole with elevated hydrogen levels. A shepherding satellite flew through the ejecta plume and observed spectra that corresponded with the presence of the volatile compounds H2O, OH, H2S, SO2, C2H4, CO2, CH4, CH3OH.

Rendering of Lunar Reconnaissance Orbiter (LRO) in orbit (image credit: http://lroc.sese.asu.edu)

Lunar Prospector

A neutron spectrometer was flown on Lunar Prospector in 1998, and measured epithermal-, thermal-, and fast-neutron fluxes. Maps of the epithermal- and fast-neutron fluxes were produced, and depressions in epithermal-neutron fluxes were observed at both poles within PSRs. However, no measureable depression in fast neutrons was observed at the poles. This data is consistent with deposits of hydrogen in the form of water ice at depths of ~50 cm under the lunar regolith at PSRs (Feldman et al., 1998).

The Lunar Prospector Neutron spectrometer has found those hydrogen concentrations at the Pole

The South polar region has ~46% more sites than the North. Its almost 4,000 km2 more coverage permit it to reach Science Goals targeted. This is the reason why, the south polar region is preferred.

At right, enhanced view of recommended sites in the south polar region satisfying Science Goal 4c: UNDERSTAND THE TRANSPORT, RETENTION, ALTERATION, AND LOSS PROCESSES THAT OPERATE ON VOLATILE MATERIALS AT PERMANENTLY SHADED LUNAR REGIONS

Diagram showing some important temperatures for Science Goal 4d, on the right, with other temperatures provided for context on the left.

South Pole

The region poleward of 80°S is almost entirely within the South Pole-Aitken (SPA) basin (Mest et al., 2011), the largest and oldest discernible impact basin on the Moon. The study area is dominated by heavily degraded impact craters ~20–100 km in diameter. The large craters are mostly flat-floored, with the exception of two craters that exhibit central peaks. Fresh, bowl-shaped craters are rare, although Shackleton is a prime example of a crater of this type. The terrain at the South Pole is mostly pre-Nectrain basin and crater material, but also includes some Nectarian and Orientale basin materials (USGS, 2009a).

*K. D. Runyon and al. - 43rd Lunar and Planetary Science Conference (2012) PDF1619

A mission needs to access areas where the NRC’s science goals can be  addressed. These goals are (a) to determine the compositional state and distribution of volatiles in lunar polar regions, (b) to determine the sources for lunar polar volatiles, (c) to understand the transport, retention, alteration, and loss processes that operate on volatile materials at PSRs, (d) to understand the physical properties of the extremely cold polar regolith, and (e) to determine what the cold polar regolith reveals about the ancient solar environment .

From these criteria, we limited our search to sites  with temperatures of 54–130 K (below the sublimation point of various volatiles) where the slopes and local geology facilitate the study of all the goals. Thus, the focus of any measurements will be on the 9% of Amundsen’s interior that is in permanent shadow, but will be facilitated by the power available in the remainder of the crater.

SCIENCE GOAL 4E: DETERMINE WHAT THE COLD POLAR REGOLITH REVEALS ABOUT THE ANCIENT SOLAR ENVIRONMENT

When lunar regolith is heated, it releases volatile elements as gases, notably particles that had been implanted in that regolith by the solar wind. In deeper regolith, these particles may be quite old, meaning that they were implanted at a different time in our Sun‟s evolution.

Solar wind particles implanted in polar regolith and preserved in PSRs would help fill an important temporal gap in our knowledge of the Sun. Through time, lunar obliquity has varied up to 77° (Siegler et al., 2011), though the Moon‟s obliquity has been relatively constant over the past 2 Ga (Siegler et al., 2011a; 2011b, Ward et al., 1975, Arnold et al., 1979). It follows that modern-day PSRs have likely been shadowed for roughly the past 2 Ga, with the oldest PSRs being those closest to the poles. Thus, the epoch of solar history measureable at the lunar polar regions extends from the present back to around 2 Ga. According to Lal et al. (1991) meteorite and lunar records can reveal solar history for the first 0.5 Ga of solar system history and the last <10 Ma, but times between these are not readily interpretable.

Science Goal 4e can be addressed in most extremely cold regions (<54 K). The map of recommended
sites required that areas be PSRs and have the coldest minimum annual temperatures, though we included cold non-PSR regions in order to observe diurnal effects on SW content, especially regarding noble gas gain and loss.

North polar region

Figure 4.20 shows a close-up view of the recommended landing sites to address Science Goal 4e at the north polar region. The recommended sites are found on the wall and floor of large craters, having a diameter larger than 50 km, such as Rozhdestvenskiy W, Lovelace, Hermite, Nansen F, and Plaskett V. They are also found on smaller craters such as Rozhdestvenskiy K, Lenard, Houssay, Hinshelwood, and Whipple, as well as in smaller craters on the floor of large craters like Rozhdestvenskiy, Hermite, Peary, Rozhdestvenskiy W, Florey, and Byrd. The Intercrater Polar Highlands also contain many small interesting sites.

Above, the Permanently Shadowed Regions (PSRs) in the North Pole and the longitude/latitude grid. Credit: lroc.asu.edu/NASA  CLICK ON IT TO SEE BIGGER

South polar region

Many sites also can address Science Goal 4e in the south polar region. Figure 4.21 shows a close-up view of the recommended landing sites there, found in large craters such as Cabeus, Cabeus B, Amundsen, Idel'son, Shoemaker, Haworth, and Nobile as well as in smaller craters such as Faustini, Wiechert, Wiechert U, Wiechert P, Sverdrup, de Gerlache, and on the floor of Skackleton.

The coldest temperatures measured on the Moon are below 23 K (Paige et al., 2011), which puts them in the same temperature range as the stability of xenon (Xe), krypton (Kr), and argon (Ar) (Zhang et al., 2010). This means that there is a possibility that noble gases could be frozen out of the lunar exosphere in these places and possibly sequestered in the regolith, at least for short periods of time. Because the lowest average temperatures on the Moon are around 38 K (Paige et al., 2011), long-term solid-state storage of noble gases seems unlikely, but periodic „trap-and-release‟ cycles may be more common. This provides an interesting analog for periodic comets, which may undergo similar temperature fluctuations as they change distance from the Sun.

There are very few sites where all goals overlap in the north polar region. Generally, these are patchy areas related to degraded crater morphologies, particularly in the Intercrater Polar Highlands (IPH) near the geographic North Pole. The largest continuous region of Science Goal overlap is in Lenard crater (D = 48 km). Other notable areas include sites on the floor of Peary crater, and on the southern walls of Rozhdestvenskiy W, Hermite, and Peary craters. There are more areas on the North Pole than at the South Pole that can address all five Science Goals simultaneously.

Enhanced view of recommended sites in the north polar region satisfying Science Goal 4e.

Enhanced view of recommended sites in the south polar region satisfying Science Goal 4e.

In the south polar region, many sites can be found in the bottoms of smaller craters within larger craters. This is the case for Faustini, de Gerlache, Haworth, Shoemaker, and some of the near-polar highlands/intercrater areas. The largest Science Goal overlap site at the South Pole is Amundsen crater (D = 105 km), which is also a convenient site because there are a range of morphologies present on the crater floor, allowing contrasting areas with different physical or thermal properties.

VOLATILES AT THE LUNAR SOUTH POLE: A CASE STUDY FOR A MISSION TO AMUNDSEN CRATER* 

Amundsen crater, centered at 84.6°S, 85.6°E, is a ~100-km-diameter complex crater with heavily terraced walls and prominent central peaks (Fig. 1).

Amundsen formed in the late Nectarian (~3.92–3.85 Ga), though its floor is somewhat younger (Imbrian, ~3.85–3.8 Ga) [4]. The crater floor has relatively low slopes (< 5º), which are attractive for landing and rover operations. Amundsen sits on the southern limb of the South Pole-Aitken (SPA) basin (D ~ 2500 km), the oldest and largest discernible impact crater on the lunar surface [e.g., 5]. Because of the great depth of SPA (~13 km), Amundsen’s mineralogy is likely intermediate between mafic and felsic, due to mixing of lower crustal or upper mantle material with feldspathic highlands material [6]. As Amundsen is a complex crater, it likely contains a broad range of geologic units, such as impact melts or volcanic material on the crater floor, bedrock outcrops on the crater wall, slumped material from high on the crater’s wall or rim, and uplifted basement rocks in the central peak. There are also many smaller (km-scale) craters in various states of degradation on Amundsen’s floor.

Amundsen is a particularly attractive landing site due to its sizeable permanently shadowed regions (PSRs, which may thermally trap volatiles) and its relatively high hydrogen abundances (~100 to ~150 ppm [7]) set amongst diverse geologic units.  Moreover, one can land on a relatively flat (safe) crater floor in sunlight, where solar power is available, and then make brief traverses into shadowed regions.  To illustrate how a mission to Amundsen can be used to address the NRC’s goals for volatile exploration [1], we discuss two areas of interest, with suggested landing sites and science stations in each.

We identified two landing sites (A and B) on the floor of Amundsen crater that are lit up to ~25% of a lunation [9].  Those sites provide access to stations within PSRs while providing a base of operations in an illuminated region.  Having the ability to establish stations in both sunlit regions and adjacent PSRs also has several scientific advantages.  The stations outside of PSRs can serve as experimental controls for the processes that affect volatile distribution within PSRs.  Contrasts between the two regions can also be used to evaluate transport mechanisms.  Remotely observed circular polarization ratios (CPR) [10] also vary around both landing sites, providing an opportunity to groundtruth the global data set and test the effects of ground ice and surface roughness on those CPR values. Temperatures derived from the Diviner radiometer [11] also helped define station locations. 

Fig. 1.Amundsen crater, showing PSRs (dark blue), sites where all five science goals can be met simultaneously (light blue), proposed science stations (circles), and proposed landing sites (stars). Radii of 10 and 20 km from the landing sites shown as solid & dashed lines. Base map is LRO/WAC/LOLA shaded relief.

Area A (83.93°S, 90.45°E) consists of six science stations, all with elevated hydrogen levels (~110– 123 ppm [7]), navigable slopes (< 15°) [9], and with temperatures [11] ranging from ~23–100 K and averaging ~40–50 K. A1 and A6 address three of the five NRC science goals (b, c, and e), while stations A2 to A5 address them all. The maximum temperature (Tmax) distribution places constraints on stations’ expected volatile abundance based on volatiles’ sublimation points. Volatile sublimation temperatures [7] near the Tmax of the stations include CO2 and hydrogen sulfide (Stations A2 to A5); water and ammonium hydrosulfide (A1); and toluene (A6).

These stations explore the distribution of volatiles in several geologic sites that will have variable regolith properties and potentially cavities for icy deposits, while also providing access to geology that address other NRC [1] goals. Station A1 is in a small PSR amidst an interesting complex of overlapping, asymmetric simple craters. A2 is on flat terrain while A3 and A4 are on degraded crater rims, though the latter is also near fresh craters and their ejecta. A5’s location on a debris slump will allow sampling of a range of lithologies, particularly from higher on Amundsen’s wall and rim, while A6 is on the terraced wall. All but Station A6 are within the 10 km astronaut walk-back safety zone. 

Area B (83.82°S, 87.53°E) consists of seven science stations; all have elevated hydrogen levels (between ~98–125 ppm [7]), slopes < 6° [9], and temperatures [11] ranging from ~23–239 K with an average of ~37–73 K. Stations B1 and B2 address none of the science goals directly but serve as controls.  Stations B3, B6, and B7 address all five of the NRC goals, while stations B4 and B5 address goals b, c, and e. Area B also allows sampling in various geologic regimes. Station B1 is in a diurnal region at the base of Amundsen’s central peak. B2 is in a small PSR on the bottom of a small, fresh crater while B3 is on the flat Amundsen floor very near the terraced walls. Stations B4 and B5 are on different debris slumps, allowing for sampling of stratigraphically higher and laterally diverse material. B6 samples a simple crater on Amundsen’s terrace. Station B7 also samples the terrace, though in a location that also satisfies all five NRC goals.

Amundsen crater is a prime area for studying lunar volatiles. Its geologic diversity, elevated hydrogen  abundances, cold PSRs adjacent to warmer diurnal regions, and overall accessibility make it an appealing and interesting target for future lunar missions from both science and mission planning perspectives. This is in contrast to the more limited science opportunities at Shackleton crater, which has steeper walls, simpler geology, and whose interior is entirely in permanent shadow.

Above, the Permanently Shadowed Regions (PSRs) in the South Pole and the longitude/latitude grid. Credit: lroc.asu.edu/NASA   CLICK ON IT TO SEE BIGGER

THE LUNAR POLES ARE SPECIAL ENVIRONMENTS THAT MAY BEAR WITNESS TO THE VOLATILE FLUX OVER THE LATTER PART OF SOLAR SYSTEM HISTORY.

Science Goal 4a: Determine the compositional state (elemental, isotopic, mineralogic) and compositional distribution (lateral and depth) of the volatile component in lunar polar regions.

While the lateral distribution of LP NS epithermal neutrons could only be measured to the nearest half degree (Feldman et al., 2000), subsequent noise-reduction analysis coupled with assumptions about the coverage of shadowed areas has offered a potentially higher resolution maps (Elphic et al., 2007; Eke et al., 2009). These maps appear to show that the observed hydrogen signal could best be explained by concentrated water ice on the floors of certain shadowed craters, specifically Shackleton, de Gerlache, Faustini, and Cabaeus.

Surface investigation of this distribution should therefore be focused chiefly on small-scale variation within polar craters.

Thermal diffusive modeling shows that water ice, under certain thermal regimes, will migrate downward in to the regolith (Crider and Vondrak, 2003b; Schorghofer and Taylor, 2007). As the ice migrates downward, micrometeorite impacts will heat and churn the top few centimeters, leaving a mostly desiccated top layer. This layer may not be totally dry, however, as certain hydrates created in the regolith could survive at or nearer to the surface (Cocks et al., 2002). In addition, since the downward diffusion increases with temperature, the maximum column-inegrated water ice density may at surfaces with temperatures between 110–120 K (Schorghofer and Taylor, 2007). Low surface density molecular or atomic hydrogen in the regolith, however, would not necessarily be confined in the upper few centimeters.

High dynamic-range optical observations of the interior of Shackleton crater show that its floor is similar in albedo and texture to the walls, confirming that any ice must be in the subsurface (Haruyama et al., 2008). Determining the vertical distribution of polar volatiles, therefore, requires the direct measurements of volatile contributions and compositions with depth down at least a few centimeters into the regolith, and up to one meter. This investigation needs to be preformed a several different sites, preferable with different surface temperatures and illuminations.

The best sites to send surface investigations are therefore ones that can address as many of these questions as possible in the shortest spatial distance (see Figs. 8.26–8.31). Shackleton is the best studied, as its rim is on the south pole. According to Kaguya LALT altimetry, its floor is 4 km below its rim, with an average grade of approximately 25% on its walls. It is therefore close to a potential outpost location, but would require careful access planning.

Fasutini is less often cited, but according to the analysis of Elphic et al. (2007), should have a similar hydrogen content to Shackleton, but with an increasing gradient across it floor from north to south (Fig. 8.30). Shoemaker is similar, but with potentially lower hydrogen (Elphic et al., 2007). While these craters are further from the south pole, the gradient in Faustini would be an excellent location for an overland traverse to measure volatile concentration and depth as a function of mean surface temperature.

There is also an unnamed crater between Shackleton and Faustini that also appears to have a significant amount of hydrogen, and has much shallower slopes than any of the named craters. Cabaeus and de Gerlache both show higher hydrogen than Shackleton in the Elphic et al. (2007) model, correlating well with the terrain model in Noda et al. (2008), which shows relatively large craters on the floors of both major craters. These floor craters are protected from sunlight reflected off the main craters‟ rims, and are thus potentially the richest areas in volatiles. Cabaeus has a larger hydrogen signal, but is more distant from the pole than de Gerlache.