VOLATILES IN THE INTERCRATER HIGHLANDS OF THE LUNAR NORTH POLE
Source: C. E. Roberts and al. - 43rd Lunar and Planetary Science Conference (2012) PDF1215
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.
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 theIntercrater 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.
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 diameter, 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; ), 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.
The Permanently Shadowed Regions (PSRs) in the South Pole and the longitude/latitude grid. Credit: lroc.asu.edu/NASA
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.
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.
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.
Right figure: 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.
Each area consists of 3 to 5 geologic stations located within a 10 km radius (for extravehicular activity safety reasons; ) of a flat (≤ 1° slopes) landing site. All stations can be reached following rover-accessible paths (slopes <20°; ), 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.
LUNAR SOUTH POLE
Above: 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].
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.
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.
On the 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].
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.
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.
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.
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.
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.
The National Research Council (NRC) identified five science objectives as being essential to understanding lunar polar volatiles . 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.
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.
One of the Science Goal of interest is to Understand the physical Properties of the Extremely 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 found also that the extremely 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.
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.
Collection of a rake sample on the Apollo 16 mission (Apollo 16 photograph AS16–116–18690). Credit: NASA
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.
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.
In the below table, the sublimation point of various substances at lunar surface pressures. The sublimation temperature values come from Zhang and Paige (2010).
Respectively, it is showing the 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
Source & 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