. Volatiles at the Lunar South Pole: mission to  AMUNDSEN CRATER

1971: APOLLO 14 (NASA)

Apollo 14 was the eighth manned mission in the Apollo programme and the third mission to land on the Moon, touching down on 5 February 1971. The crew were Commander Alan B. Shepard, Jr, Stuart A. Roosa (Command Module Pilot) and Edgar D. Mitchell (Lunar Module Pilot). After landing at the destination for Apollo 13 - Shepard and Mitchell took two moon walks, adding new seismic studies to the by now familiar Apollo experiment package, and using a "lunar rickshaw" pull cart to carry their equipment...

 
 

 
 

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 South Pole

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

The region poleward of 80°S is almost entirely within the South Pole-Aitken (SPA) basin, 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).

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 the 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.

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 physical properties of the extremely cold regolith.

 

A mission needs to access areas where the NRC’s science goals can be  addressed. These goals are (1) to determine the compositional state and distribution of volatiles in lunar polar regions, (2) to determine the sources for lunar polar volatiles, (3) to understand the transport, retention, alteration, and loss processes that operate on volatile materials at PSRs, (4) to understand the physical properties of the extremely cold polar regolith, and (5) to determine what the cold polar regolith reveals about the ancient solar environment .

From these criteria, the research has been limited to sites with temperatures of 54–130 K (below the sublimation point of various volatiles) and where slopes and the local geology reach NRC's goals. Thus, the focus of any measurements will be on the 9% of the Amundsen’s interior that is in permanent shadow.

VOLATILES AT THE LUNAR SOUTH POLE: MISSION TO AMUNDSEN CRATER* 

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

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.

The Amundsen crater is centered at 84.6°S, 85.6°E. Its diameter is about 100-km in the form of a complex crater with heavily terraced walls and prominent central peaks.

Amundsen has been formed in the late Nectarian (~3.92–3.85 Ga), though its floor is somewhat younger (Imbrian, ~3.85–3.8 Ga). 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. 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) set amongst diverse geologic units. 

Moreover, one mission can land on a relatively flat crater floor in the sunlight, where solar power is available, and then make brief traverses into shadowed regions. 

The study identified two landing sites (A and B) on the floor of Amundsen crater that are lit up to ~25% of a lunation.  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) 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 also helped define station locations. 

The above illustration show an enhanced view of recommended sites in the South Polar region satisfying Science Goal 4c: Understant the transport, retention, alteration, and loss processes that operate on volatile materials at Permanetly Shaded Lunar Regions.

The Area A (83.93°S, 90.45°E) consists of six science stations, all with elevated hydrogen levels (~110– 123 ppm), navigable slopes (< 15°), and with temperatures ranging from ~23–100 K and averaging ~40–50 K. Stations A1 and A6 address three of the five NRC science goals (b, c, and e), while 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 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 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. 

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 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.

 
 

The Permanently Shadowed Regions (PSRs) in the South Pole and the longitude/latitude grid. Credit: lroc.asu.edu/NASA 

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.

 

THE LUNAR POLES MAY BEAR WITNESS TO THE VOLATILE FLUX OVER THE LATTER PART OF SOLAR SYSTEM HISTORY

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.

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.

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