A tale of two poles: Toward understanding the …...volatiles at the polar regions of the Moon and Mercury, J. Geophys. Res. Planets, 122,21–52, doi:10.1002/ 2016JE005167. Received

A tale of two poles: Toward understanding the presence,distribution, and origin of volatiles at the polarregions of the Moon and MercuryDavid J. Lawrence1

1The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA

Abstract The Moon and Mercury both have permanently shaded regions (PSRs) at their poles, whichare locations that do not see the Sun for geologically long periods of time. The PSRs of the Moon andMercury have very cold temperatures (<120 K) and as a consequence act as traps for volatile materials.Volatile enhancements have been detected and characterized at both planetary bodies, but the volatileconcentrations at Mercury’s poles are significantly larger than at the Moon’s poles. This paper documents thestudy of PSR volatiles at the Moon and Mercury that has taken place over the past 60 years. Starting withspeculative ideas in the 1950s and 1960s, the field of PSR volatiles has emerged into a thriving subfield ofplanetary science that has significant implications for scientific studies of the solar system, as well as futurehuman exploration of the solar system. While much has been learned about PSRs and PSR volatiles, manyfoundational aspects of PSRs are still not understood. One of the most important unanswered questionsis why the PSR volatile concentrations at the Moon and Mercury are so different. After describing the initialpredictions and measurements of PSR volatiles, this paper documents a variety of PSR measurements,summarizes the current understanding of PSR volatiles, and then suggests what new measurementsand studies are needed to answer many of the remaining open questions about PSR volatiles.

1. IntroductionPermanently shaded regions (PSRs) are locations on planetary bodies that do not see the Sun for geologicallylong periods of time and therefore have unique properties compared to other locations on planetary bodies.If PSRs exist on airless planetary bodies, they will have very cold temperatures (<~120 K) because they radiatedirectly to space with no source of heat input other than residual interior heat from the body itself and smallamounts of multibounce thermal photons from nearby sunlit locations. The low temperatures that exist forlong durations within PSRs can result in a variety of interesting effects. One of the most intriguing is thatvolatile materials, especially water ice, can become trapped within PSRs as a direct consequence of the coldtemperatures. Thus, while the residence time of volatiles for non-PSRs is short with timescales of days toweeks, the residence time of volatiles within PSRs can be geologically long (millions to billions of years).

The type examples of PSRs within the solar system are those that exist on the Moon and Mercury. The axes ofrotation for both these bodies are nearly perpendicular to their orbital plane around the Sun (1.5° for theMoon, 0.034° for Mercury, where 0° would be exactly perpendicular) [Siegler et al., 2015; Margot et al.,2012]. Because the Moon’s and Mercury’s rotational orientation has been stable for billions of years [Siegleret al., 2013, 2015], there are craters near the poles of both bodies that are sufficiently deep such that theirinteriors do not see the Sun, and they are therefore PSRs.

The existence of PSRs does not guarantee they will accumulate volatiles over time, but only makes such accu-mulation possible. There is a range of possible volatile sources that can include sources interior (endogenous)or exterior (exogenous) to the planet. Endogenous sources could be residual volatiles from ancient volcanismas well as more recent volatile releases or outgassing events [Crotts and Hummels, 2009]. There is a large vari-ety of exogenous sources that can include comets, asteroids, interplanetary dust particles, solar wind, andeven occasional giant molecular clouds that may pass through the solar system [Lucey, 2009]. In terms of theirtime of delivery, all sources can in principle be continuous and/or episodic. In a broader sense, it is now beingrecognized that “dry,” airless planetary bodies have a volatile transport system [Lucey, 2009; Lawrence, 2011],and when there are PSRs, such as on the Moon or Mercury, the PSRs are a key sink in such a transport system.

It is for these reasons (and others explained below) that PSRs have become a topic of intense study and inter-est within planetary science. Because PSRs are so different than other planetary environments, they contain a

LAWRENCE POLAR VOLATILES AT THE MOON AND MERCURY 21

PUBLICATIONSJournal of Geophysical Research: Planets

REVIEW ARTICLE10.1002/2016JE005167

Special Section:JGR-Planets 25th Anniversary

Key Points:• Permanently shaded regions (PSRs) atthe Moon and Mercury have beenstudied for over 60 years

• Mercury’s PSRs have significantlyhigher concentrations of volatiles thanthe Moon’s PSRs; the reason for thisdifference is not understood

• Future studies of PSRs at the Moonand Mercury will reveal muchimportant new information

Correspondence to:D. J. Lawrence,[email protected]

Citation:Lawrence, D. J. (2017), A tale of twopoles: Toward understanding thepresence, distribution, and origin ofvolatiles at the polar regions of theMoon and Mercury, J. Geophys. Res.Planets, 122, 21–52, doi:10.1002/2016JE005167.

Received 7 SEP 2016Accepted 6 OCT 2016Accepted article online 30 DEC 2016Published online 8 JAN 2017

©2016. The Authors.This is an open access article under theterms of the Creative CommonsAttribution-NonCommercial-NoDerivsLicense, which permits use and distri-bution in any medium, provided theoriginal work is properly cited, the use isnon-commercial and no modificationsor adaptations are made.

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wide range of fascinating effects, processes, and targets of scientific study. In addition to volatileenhancements, other interesting attributes about PSRs include unique surface charging and space plasmaphysics effects [Zimmerman et al., 2013; Jordan et al., 2015; Farrell et al., 2015], potentially distinctive geo-technical properties of the persistently cold and volatile-rich regolith [Schultz et al., 2010], and the possi-ble organic synthesis that may take place within PSR volatiles due to long-term cosmic ray bombardment[Crites et al., 2013]. Because of their unique nature, PSRs can be difficult to study, and even now in theearly 21st century there are many fundamental aspects of PSRs that are not understood. Nevertheless,current and future studies of PSRs hold great promise. PSRs are a significant scientific resource, not onlyfor what they can reveal about their host planetary bodies but because they have been trapping volatilesfor up to billions of years, they are a storehouse of solar system volatile materials, and they are thereforea resource for future studies of solar system history. As an analogy, it has been stated: “As Antarcticaconcentrates and stores solar system refractory materials in the form of meteorites, PSRs concentrateand store solar system volatiles.” (Originally heard by the author from Paul Lucey at a now, unrecalledplanetary science meeting.) Finally, for at least the Moon, the existence of volatile enhancements, andespecially water ice, can enable future human exploration to the Moon and elsewhere beyond in the solarsystem [Spudis, 2016].

The purpose of this paper is to review the scientific history of PSR volatiles by describing the early predictionsof PSR volatiles and the evidence that has accumulated over the last six decades that support these earlypredictions. The exploration of PSRs has been carried out in three main epochs as illustrated by the time linein Figure 1. These epochs are initial predictions (1952–1992), initial measurements (1992–2001), and detailedmeasurements (2002 to present).

A fundamental result that has emerged from these studies is that in spite of similar PSR environments, thequality and quantity of volatile enhancements at the Moon and Mercury are very different. At Mercury, thereis strong evidence frommany types of measurements that its PSRs contain large amounts of volatiles. In con-trast, while the Moon shows evidence of volatile enhancements within its PSRs, the volatile abundances aremuch less than at Mercury and appear nonuniform across different PSRs. Trying to understand theseMercury/Moon differences directly leads to trying to understand how the volatiles reached the PSRs and theirtime history within the PSRs. Much understanding has been gained, but many fundamental facts and

Figure 1. Timeline of PSR exploration for the (top) Moon and (bottom) Mercury. The initial suggestion of lunar polar vola-tiles was provided by Urey [1952] in 1952 and is not shown on this timeline.

Journal of Geophysical Research: Planets 10.1002/2016JE005167


properties of PSRs are not yet known. As a consequence, there is still significant information and data thatneed to be gathered about PSRs to enable further understanding. While some of these data can be obtainedremotely from orbital spacecraft, measurements will ultimately need to be acquired inside PSRs from thesurface of Mercury and the Moon.

The structure of this paper contains three main sections. First, in section 2 the timeline of Figure 1 is used asan outline to describe the three epochs of PSR exploration. Section 2 provides the bulk of the review, as thereis a wide and diverse history of predictions, measurements, and characterizations. Section 3 provides a briefreview of the current understanding of the origin and time history of PSR volatiles, especially in regard to howthis understanding relates to the differences between Mercury and the Moon. Finally, section 4 providessome perspective for the next 50 years by giving a brief summary of plans and thoughts for future studiesof PSR volatiles.

2. Timeline of PSR Exploration2.1. Initial Predictions

The initial predictions of volatile enhancements within PSRs were carried out sporadically for almost 40 yearsstarting in 1952. Harold Urey provided the first tangential suggestion that “condensed volatile substances”might be present at the Moon’s poles in his book “The Planets: Their Origin and Development” [Urey, 1952].Specifically, Urey stated in Chapter 2, pages 17–18:

“The moon has no detectable atmosphere and none could be expected because of the low gravita-tional field and the high temperature of its surface during the day. Near its poles there may be depres-sions on which the sun never shines, where condensed volatile substances might be present, but if theyever evaporated they would be rapidly lost”; (italics not in original)

The first study to investigate details of lunar polar water ice was carried out by Watson et al. [1961a]. Usinglunar nighttime temperatures of 120 K measured 30 years prior by Pettit and Nicholson [1930] and estimatesof possible water liberated from the surface, Watson et al. [1961a] suggested that there might be concentra-tions of water ice within PSRs. Soon thereafter,Watson et al. [1961b] presented more detailed calculations ofwater transport across the Moon and retention at the poles to show that water could indeed be trapped inpolar PSRs. In regard to sources, Watson et al. [1961b] primarily considered endogenous water from theMoon suggesting that external water from meteorites would be small compared to internal lunar watertrapped at the poles. While Watson et al. [1961b] did not make quantitative predictions of the possibleamount of polar water, Opik [1962] estimated that solar wind alone hitting the Moon could result in waterdeposits up to 100m at the lunar poles, a prediction that has not proven correct. Nearly 20 years after theseinitial studies Arnold [1979] made detailed predictions of the amount of ice within lunar PSRs by accountingfor various factors such as PSR lifetimes, temperatures, and areas, as well as volatile migration and possibleendogenic and exogenic sources. Arnold [1979] found that when these sources were considered, the totalmass of polar water ice could be in the range of 1016–1017 g. Arnold [1979] also predicted a water concentra-tion in the range of 1–10wt%, although this estimate was highly dependent on various parameters, such asarea of permanent shade and depth of water ice enrichment. It is interesting to note that the amount of waterice at Mercury’s poles estimated byMoses et al. [1999] and later by Lawrence et al. [2013] is 1016–1018 g, whichis within the same range estimated by Arnold [1979] for the Moon.

In contrast to the study by Arnold [1979], Lanzerotti et al. [1981] claimed that a significant accumulation ofwater ice was unlikely to be present in lunar PSRs; they estimated that water losses due to solar wind andEarth magnetospheric ions is comparable (106–108 protons/cm2/s) to the deposition estimated by Arnold[1979] of ~107molecules/cm2/s.

During this period, the Moon attracted the most attention of initial studies of polar volatiles. However,water ice at Mercury’s poles was briefly discussed in a paper by Thomas [1974]. This paper was focusedon the possibility of water within Mercury’s atmosphere but had the following statement: “If model ii[a model of Mercury’s atmosphere] prevailed on Mercury for a significant period in its history, ice couldhave accumulated in the polar regions in permanently shaded areas, as suggested for the Moon byWatson et al.” While some other studies parenthetically mentioned the possibility of volatiles within



Mercury’s poles [e.g., Kumar, 1976; Gibson, 1977], in general, Mercury was not a focus for studies of polarvolatiles during this time. In summary, during the years of 1952 to 1992, there were relatively few studiesof planetary PSRs. However, despite the few number of studies, the ones published demonstrate goodphysical insight and set the stage for the first round of planetary PSR measurements.

2.2. Initial Measurements of Polar Volatiles at Mercury and the Moon

The initial measurements that investigated polar volatiles at Mercury and the Moon took place during the lastdecade of the twentieth century using both Earth- and spacecraft-based instrumentation. The disparatemeasurement techniques of radar (Mercury and the Moon) and neutron spectroscopy (Moon) provided aninitial confirmation of volatile enhancements at both bodies. The radar and neutron measurements at theMoon were supported by illumination and topography studies from Earth-based radar and spacecraft-based measurements.2.2.1. Radar Measurements of Mercury and the MoonDespite the fact that most attention regarding polar volatiles was focused on the Moon, the first measure-ments of PSR volatile enhancements were acquired from Mercury using Earth-based radar. Water ice has dis-tinctive radar reflection properties such that it is highly radar reflective and it has a high circular polarizationratio (CPR) (i.e., same-sense to opposite-sense polarization of the radar echo) [Hapke, 1990], although othereffects such as rocky surfaces can cause high CPR (see below and section 2.3.1). Prior to measurements atMercury, these radar properties were used to characterize icy surfaces on Jupiter’s Galilean satellites[Campbell et al., 1978; Goldstein and Green, 1980; Ostro and Shoemaker, 1990], as well as Mars’ south polarice cap [Muhleman et al., 1991].

The first radar measurements of Mercury and interpretations of these measurements were reported in a trioof papers [Slade et al., 1992; Harmon and Slade, 1992; Paige et al., 1992] followed by a more detailed analysisand discussion by Butler et al. [1993]. The initial radar measurements described by Slade et al. [1992] andButler et al. [1993] (at a radar wavelength of 3.5 cm and a spatial resolution of 165 km) were carried out on8 August 1991 and 23 August 1991 from the Very Large Array (VLA) radio telescope in Socorro, NM, whichreceived signals transmitted from the 70m radio transmitter in Goldstone, CA. These data showed thatMercury’s north pole had a high CPR (Figure 2) indicative of water ice. In a complementary set of

Figure 2. Polarization ratios of the same-sense radar echo to the opposite-sense radar echo on the Mercury disk for eachof two experiments (figure from Slade et al. [1992]). (a) 8 August 1991 and (b) 23 August 1991. The boxed areas aroundthe north pole correspond to regions of more detailed analysis of the circular polarization ratios (figure courtesyScience magazine).



measurements conducted during1991 and 1992 from AreciboObservatory in Puerto Rico, Harmonand Slade [1992] reported radarreflection enhancements from bothpoles of Mercury that were consistentwith the VLA measurements. The factthat Arecibo data showed radar prop-erties at Mercury’s south pole (withinthe vicinity of the 150 km diameterChao Meng-Fu crater) similar to whatwas seen at Mercury’s north polefurther supported the idea thatmaterials unique to Mercury’s polarenvironment were responsible forthe detected radar properties [Butleret al., 1993]. The study of Paige et al.[1992] provided context for the radarmeasurements by showing that thelikely thermal environment withinPSRs at Mercury’s poles would becold enough to retain water ice for

geologically long periods of time. Butler et al. [1993] carried out a comprehensive analysis and interpretationof the radar data and thermal models and concluded that “ices (at least H2O) do exist at the poles of Mercurybut not in the form of totally exposed, uniform coverage ice caps. The ice deposits are at least tens of metersin depth, probably formed relatively rapidly in permanently shaded regions, and were subsequently coveredover by a shallow layer of dust or soil. Other ices may also collect in the same way and contribute ourobserved signal, depending on how cold it really is where the ices are being deposited.”

In a follow on study a couple of years later, Harmon et al. [1994] refined the analysis of the previously collectedradar data and obtained new data to show that the radar-bright enhancements were indeed within cratersnear Mercury’s polar regions, further supporting the idea that PSRs at both poles contained enhancedwater ice.

The first attempt to measure volatiles in the lunar polar regions was conducted from the Clementine space-craft that orbited the Moon for 71 days in 1994 [Nozette et al., 1994]. Instead of making a direct measure-ment of radar reflection, Nozette et al. [1996] carried out a bistatic measurement, where the radartransmitter was on the Clementine spacecraft and the radar signals were detected at Earth by the DeepSpace Network (DSN) antennae. The purpose of the bistatic measurement was to enhance possible reflec-tion effects from water ice and minimize effects caused by other surface features such as roughness and/orblocky material. Nozette et al. [1996] reported an enhancement of CPR for one orbit (orbit 234) over theMoon’s south pole, where there were likely PSRs, while other orbits not over likely PSRs showed no sucheffect (Figure 3). These results were interpreted as possibly due to water ice, though Nozette et al. [1996]stated that other effects such as surface roughness could in principle explain the measurement. In addition,due to the large footprint of the measurement, Nozette et al. [1996] stated that their measurement did notnecessarily imply there were large expanses of water ice but that the possible ice could be mixed aspatches within the polar regolith as “dirty ice.”

A number of other lunar radar studies followed. Stacy et al. [1997a] reported results from Earth-basedArecibo radar where large portions of the Moon’s south pole were imaged with radar backscatter andCPR data. Stacy et al. [1997a] found multiple radar enhancements associated with impact features, manyof which were in sunlit regions. Based on these results, they concluded that there was little evidence inthe Arecibo data for large expanses of ice at the Moon’s south pole. Subsequently, short reports byWeidenschilling [1997], Nozette et al. [1997], and Stacy et al. [1997b] made various points regarding thevalidity of the respective ice/no-ice claims based on the Clementine and Arecibo data. Simpson andTyler [1999] produced a detailed reanalysis of the Clementine bistatic radar data concluding that they

Figure 3. Circular polarization ratio (right circular polarization divided byleft circular polarization) from the Clementine mission (figure taken fromNozette et al. [1996]). Data are shown as a function of the bistatic angle βbetween the spacecraft, lunar surface, and Earth receiver for spacecraftorbit 234 for a 2.5° radius latitude band centered on the lunar south pole andfor orbit 235 for a 2.5° radius periodically illuminated band centered at 82.5°S,and for orbits 301 and 302, originating from a 2.5° radius band centeredat the north pole (figure courtesy of Science magazine).



could not reproduce the Nozette et al. [1996] results but that small amounts of water ice (1wt% H2O)were not inconsistent with the Clementine data. Finally, Nozette et al. [2001] reported a data integrationstudy of multiple data sets, concluding that their original claim of water ice enhancements at the Moon’ssouth pole was consistent with all data but that ambiguities still remained, which needed better data toresolve.2.2.2. Neutron Measurements of the MoonHydrogen abundances can be remotely measured on planetary surfaces using the technique of neutronspectroscopy [e.g., Feldman et al., 1991]. The principle of this technique starts with the fact that high-energy(~1GeV per nucleon) galactic cosmic rays (GCRs) hit airless planetary bodies and create spallation neutronsfrom the surface nuclei. These neutrons are emitted with energies, En, of ~1–10MeV and lose theirenergy (down to fractions of an eV) through various scattering processes. Planetary neutrons are typicallydivided into three energy ranges: fast (En> 0.5 MeV), epithermal (0.5 eV< En< 0.5 MeV), and thermal(En< 0.5 eV). The energy boundaries demarcate different types of neutron transport and lost processes.Hydrogen has the unique ability to moderate neutrons because hydrogen atoms and neutrons have thesame mass, which allows an efficient momentum transfer between the two. Of the three energy ranges,epithermal neutrons are most sensitive to the presence of hydrogen and are strongly depressed in the pre-sence of hydrogen within planetary materials.

Lingenfelter et al. [1961] first recognized the possibility of using neutrons to remotely measure hydrogen onplanetary surfaces. Feldman et al. [1991] later applied these ideas to the case of lunar PSRs and predicted thathydrogen abundances within the lunar PSRs could be measured with simple neutron sensors in orbit aroundthe Moon. One aspect of orbital neutron data that has particular relevance for measurements of polar hydro-gen abundances is that the spatial resolution of these data is roughly the altitude of the spacecraft makingthe measurement. This relatively broad spatial resolution is a consequence of the fact that the sensors gen-erally used to detect neutrons are omnidirectional detectors and observe neutrons out to the horizon seen bythe sensor (attempts to improve the spatial resolution have been made; see section 2.3.2).

The idea for using GCR-induced neutrons to measure hydrogen within the lunar PSRs was implemented onNASA’s Lunar Prospector (LP) mission [Binder, 1998], which orbited the Moon from 18 January 1998 to 19December 1998 at an altitude of 100� 20 km, and from 19 December 1998 to 31 July 1999 at an altitudeof 30� 15 km. Using the first 5 months of collected data from the Lunar Prospector Neutron Spectrometer(LP-NS), Feldman et al. [1998] reported count-rate decreases of epithermal neutrons at both lunar poles, whichprovided strong evidence for enhanced hydrogen abundances in and around the lunar PSRs. Assuming thatthe enhanced hydrogen reaches a depth of 2m, which is the depth of lunar regolith thought to be gardenedin 2 billion years [Arnold, 1979], Feldman et al. [1998] estimated that the total equivalent water at each polewas 3 × 109 t (or 3 × 1015 g). This value is roughly an order of magnitude less than the lower value predicted byArnold [1979]. Later studies using the full 18month data set refined the location and abundance of the lunarpolar hydrogen enhancements (Figure 4). With the higher spatial resolution data from the low-altitudeportion of the mission, Feldman et al. [2000] and Feldman et al. [2001] concluded that the hydrogen enhance-ments were generally located in and around PSRs, although the spatial resolution of the low-altitude data(30–45 km) [Maurice et al., 2004] was sufficiently broad to preclude identification of specific PSRs withhydrogen enhancements. At the south pole, which has larger PSRs, the neutron data were combined withmeasurements of PSR area (see section 2.2.3) to estimate that the hydrogen concentration within PSRs was1670� 890 ppm, or 1.5� 0.8wt% water equivalent hydrogen (WEH), if this hydrogen is in the form of waterice. The primary source of uncertainty in these estimates is uncertainties in the area of permanent shade andhowmuch of each PSR contains the enhanced hydrogen. At the north pole, where there are many small PSRs,Feldman et al. [2000] reported that the average hydrogen concentration over the full-area LP-NS footprintwas 100 ppm higher than the hydrogen seen at more equatorial latitudes. Based on various arguments fromlocation, abundance, and thermodynamics, Feldman et al. [2001] argued that the most likely form of theenhanced hydrogen was water ice.

In a response to the Feldman et al. papers, Hodges [2002] questioned the conclusions that the LP-NS datauniquely identified hydrogen at the lunar poles and suggested that abundance variations from otherelements (e.g., Si and Ca) could explain the epithermal neutron data. Lawrence et al. [2006] carried out a moredetailed modeling analysis than was done for the Feldman et al. studies and concluded that enhancedhydrogen abundances best explained the polar neutron data.



2.2.3. Illumination and Topography Studies of the Lunar PolesOne more collection of measurements related to the lunar poles in this time period of initial measurementswas that of topography and polar illumination. The Clementine spacecraft carried a laser altimeter fromwhich mostly global lunar topography measurements were obtained [Smith et al., 1997]. Due to the natureof the Clementine spacecraft’s orbit, direct topography measurements poleward of 82°N and 79°S werenot obtained [Cook et al., 2000]. Nevertheless, using long- and short-wavelength topography close to thesouth pole, it was inferred that possible crater-shaped PSRs were no larger than 80 km at the pole and likelyno larger than ~30 km for locations 2° off the pole [Zuber and Smith, 1997]. Data from Clementine imagesenabled the derivation of lunar topography maps using stereo imaging [Cook et al., 2000], although theresulting maps had a number of gaps.

The first detailed maps of lunar polar topography were obtained with Earth-based radar interferometry datawhere range differences between points on the Moon’s surface and two receivers on Earth were used toderive lunar elevation information [Margot et al., 1999]. With these topography data from both poles,Margot et al. [1999] used ray tracing algorithms to derive locations in permanent shade (Figure 5). One limita-tion to this technique is that locations on the Moon’s farside cannot be mapped, so PSR areas in unmappedareas were estimated based on modeling of expected topography. Margot et al. [1999] estimated areas of2650 km2 and 5100 km2 for the north and south poles, respectively. These values were used as input datafor estimates of total polar hydrogen from the LP-NS neutron data [Feldman et al., 2000, 2001]. Finally, usingClementine images taken at a range of local times, Bussey et al. [1999] quantified the illumination conditionsat the Moon’s south pole, and in a subsequent study [Bussey et al., 2003] found larger PSR areas of 7500 km2

and 6500 km2 at the north and south poles, respectively.2.2.4. Summary of Initial MeasurementsThe general conclusion from this first round of measurements at Mercury and the Moon is that the poles ofboth bodies have enhanced volatiles. However, the exact amount and form of the volatiles remained uncer-tain. While the evidence for water ice at Mercury’s poles was strong, alternate explanations for the radar datawere put forward to explain the radar data, ranging from enhanced sulfur abundances [Sprague et al., 1995]to unusual thermal effects in silicate materials [Starukhina, 2001]. For the Moon, arguments were made thatthe most likely form of enhanced hydrogen was water ice [Feldman et al., 2001], but due in part to the

Figure 4. Maps of epithermal neutrons for the north and south poles measured by the Lunar Prospector Neutron Spectrometer. Data are originally from Feldmanet al. [2001], and the figure is taken from Lucey et al. [2006] (figure courtesy of Reviews of Mineralogy and Geochemistry).



relatively small hydrogen concentrations, other studies suggested that the PSR hydrogen enhancementscould be explained by the long-term deposition of solar wind hydrogen [Crider and Vondrak, 2000;Starukhina and Shkuratov, 2000]. Thus, this situation set the stage for the large number of new Earth- andspacecraft-based measurements that are explored in the next section.

Figure 5. Locations of cold traps at the lunar (top) north and (bottom) south poles (figure from Margot et al. [1999]). Areasthat were visible to the radar and are in permanent shadow from solar illumination have been marked with white pixelvalues. Several other regions, which were not visible to the radar (that is, in radar shadow), have been depicted withgray pixels to indicate that they are predicted, on the basis of the topography of the surrounding terrain, to also be inpermanent shadow (figure courtesy of Science magazine).



2.3. Detailed Measurements of PSRs

Starting midway between 2000 and 2010, a large number of measurements and analyses of Mercurianand lunar polar regions were carried out. The primary measurements came from six spacecraft as well as addi-tional Earth-based data. Of the six spacecraft, one was flown by India (Chandraayan-1), one by Japan (Kaguyarenamed from SELENE), one by China (Chang’E-1), and the remaining were flown by NASA. The NASA mis-sions are the Lunar Reconnaissance Orbiter (LRO) [Chin et al., 2007], the Lunar Crater Observation SensingSatellite (LCROSS) [Colaprete et al., 2012], and the MErcury Surface, Space ENvironment, GEochemistry, andRanging (MESSENGER) mission [Solomon et al., 2007]. The data from the Earth-based measurements andsix spacecraft provided large amounts of new information about PSR environments and have provided asignificantly enhanced understanding of PSRs and their volatile enhancements. In addition, analyses andinterpretation of prior and newly collected data provided further understanding of PSRs. Because there is alarge number of PSR or PSR-related studies from 2006 to the present, only the highlights can be summarizedhere. The general categories of these studies are new results based on radar, neutron, and topography mea-surements, as well as new measurements of temperature, reflectance from different photon wavelengths,and an in situ impact measurement of a lunar PSR.2.3.1. New Information From Radar Measurements of Mercury and the MoonThis period of detailed measurements saw many types of radar measurements of both the Moon andMercury. For Mercury, a number of measurement campaigns from the Arecibo and Goldstone radio tele-scopes were carried out from 1999 to 2005. The results from these campaigns are summarized by Harmonet al. [2011]. Specifically, using improved measurements and analysis techniques, detailed radar maps wereproduced for both Mercury’s north and south poles (Figure 6). Based on these data, Harmon et al. [2011] con-cluded that many craters at both poles are completely covered with radar reflective materials, and manyother craters are partially covered. The 112 km diameter crater Prokoviev (86°N, 296°W) is a particularly dra-matic example of a partially covered radar-bright crater. Comparison of the 12.6 cm radar data from Arecibowith shorter wavelength (3.5 cm) data from Goldstone was interpreted by Harmon et al. [2011] to indicatethat for many locations, a dusty, less water-rich layer that is tens of centimeter thick covers pure water icelayers. Radar reflective signatures were detected in many small craters down to a latitude of 67°; the locationsof these craters are biased toward Mercury’s longitudinal cold poles. While providing more details, thesecomprehensive radar data continued to be consistent with the initial conclusions of Slade et al. [1992] andHarmon and Slade [1992] that Mercury’s PSRs are filled with water ice.

In contrast to the reasonably clear conclusions drawn from Mercury radar data, radar measurements of theMoon have continued to be more challenging to interpret. In 2006, Campbell et al. [2006] reported new radarreflection data of the Moon’s south pole using the Arecibo radio telescope. With particular attention toShackleton crater, which is located almost directly at the pole, Campbell et al. [2006] concluded that therewere no large expanses of water ice at Shackleton or elsewhere in the Earth-visible vicinity of the pole.Radar polarization signatures normally associated with ice were seen at various latitudes and often correlatedwith rocky surfaces. These results, however, did not rule out small amounts of water ice (<10wt%) dissemi-nated within grains in the lunar regolith.

Orbiting radar instruments were included on the payloads of the Chandraayan-1 and Lunar ReconnaissanceOrbiter (LRO) missions and acquired data from the lunar poles. Results from the Mini-SAR on Chandraayan-1and Mini-RF on LRO were published by Spudis et al. [2010] and Spudis et al. [2013], respectively (Figure 7).The conclusions from both these studies are that many craters across the Moon have high CPR valuesand that these high values can be due to either large amounts of surface roughness and/or the presenceof water ice. In both studies, two classes of high CPR craters were identified, where one class has highCPR both inside and outside the craters, whereas the other class, known as anomalous craters, has highCPR mostly inside the respective craters. A number of characteristics led Spudis et al. [2013] to conclude thatwater ice is the likely cause of the high CPR for the anomalous craters. First, most of these craters are alsoPSRs and could therefore host water ice. Second, a significantly larger number of these craters exist in thesmall-area polar regions than in the rest of the larger nonpolar area across the Moon, which suggests thatthere is something special about the polar regions to enhance the number of anomalous craters. Finally,application of radar scattering models [Thompson et al., 2011] suggests that water ice provides a good expla-nation of the polar anomalous craters compared to other types of craters where roughness could be thecause of the high CPR signatures.



In contrast to these results, separate studies investigated the possibility that factors other than water ice maybe causing the high CPR in the anomalous polar craters. Fa and Cai [2013] studied anomalous craters innonpolar regions, where water ice would not be present, and using topography data from these craters, theyconcluded that a relatively high rock abundance was likely responsible for their high CPR, a conclusion alsoreached by Spudis et al. [2013]. Based on additional statistical studies and a good match of their data with atwo-component mixed radar scattering model, Fa and Cai [2013] concluded that the high CPR withinanomalous polar craters might also be due to roughness. In a complementary study, Eke et al. [2014]analyzed Mini-SAR data and found that when these data were corrected for look-direction effects, theresulting CPR maps were significantly changed from the originally published data. With these corrected

Figure 6. Radar images of Mercury’s (a) south and (b) north polar regions (figure taken from Harmon et al. [2011]). For thesouth pole, themean radar illumination direction (arrow) is from the bottom and the dark blank area at the top is the regionthat is beyond the radar horizon (figure courtesy of Icarus).



data, Eke et al. [2014] showed that the CPR values did not correlate with surface temperatures as might beexpected if water ice was causing the high CPR values and that higher CPR values were found in craterswith steeper slopes, which may have more rocky surfaces. Thus, Eke et al. [2014] also concluded that otherfactors, such as surface roughness, may be responsible for some portion of the elevated CPR within polaranomalous craters.

Finally, in a different use of the Mini-RF instrument, Patterson et al. [2016] reported on the results of a bistaticmeasurement where radar signals from Arecibo were aimed at the Moon and detected at LRO, a reverse con-figuration of the original Clementine bistatic radar experiment of Nozette et al. [1996]. In this study, multiplelocations on the Moon, both polar and nonpolar, were observed at various bistatic phase angles between theradar source and receiver. For such a configuration, water ice is expected to show an “opposition surge” suchthat for very small bistatic angles, there should be a CPR enhancement. While a prior study did not findenhanced CPR within Cabeus crater [Neish et al., 2011], Patterson et al. [2016] found an opposition surgewithin sunlit portions of Cabeus crater nearby parts of the crater that are within permanent shade(Figure 8). Given the unique nature of the signature, Patterson et al. [2016] concluded that there might bewater ice beneath the surface where temperatures could be sufficiently cold to inhibit the release ofwater ice.2.3.2. New Information From Neutron Data at the Moon and MercuryAdditional analyses of prior neutron measurements and new neutron measurements from both the Moonand Mercury took place in the time period after 2006. As described in section 2.2.2, the primary limitationof the LP-NS data is that its spatial resolution was sufficiently broad to prevent a definitive identification of

Figure 7. The north pole area of the Moon in S-band zoom CPR images (blue is low; yellow and red are high) from Mini-RFshowing numerous, small “anomalous” craters (arrows) on the floor of Peary (78 km diameter; 88.6°N, 33°E) (figuretaken from Spudis et al. [2013]). The presence of “normal” fresh, young craters adjacent to the anomalous ones heresuggests that the high CPR is not derived from properties of the Peary crater floor material (figure courtesy of AmericanGeophysical Union).



hydrogen enhancements with specific PSRs. One way to provide higher spatial resolution information oflunar polar hydrogen measurements is to apply spatial deconvolution algorithms to the neutron data. Asequence of such analyses was carried out by Elphic et al. [2007], Eke et al. [2009], and Teodoro et al.[2010], where it was concluded (1) that the enhanced hydrogen at the lunar poles was preferentiallylocated within PSRs, (2) that the PSRs did not contain uniformly enhanced hydrogen, and (3) that theinferred hydrogen concentrations within PSRs ranged from 300 ppm to as high as a few weight percentWEH. In addition to these spatial deconvolution studies, Miller et al. [2014] carried out a combined analysisof LP epithermal and fast neutron data. Based on the knowledge that these different neutron energyranges are sensitive to different hydrogen burial depths, Miller et al. [2014] concluded that hydrogenenhancements of ~1wt% WEH at Shackleton crater were possibly located at the surface, whereas thehydrogen enhancements at other south polar locations were likely buried by tens of centimeters of driermaterial. Finally, Eke et al. [2015] carried out a global analysis of LP epithermal neutron data and found thatthe geometry of bowl-shaped craters in the size range of 20 to 60 km could cause a subtle enhancement ofepithermal neutrons. Based on this analysis, Eke et al. [2015] suggested that the inferred hydrogen abun-dances within the relatively deep Cabeus crater could have been underestimated at 1wt% WEH andmay be as high as ~4wt% WEH.

A second way to provide improved spatial resolution information of orbital neutron data is to use new detec-tion techniques. To this end, the LRO spacecraft carried the Lunar Exploration Neutron Detector (LEND)[Mitrofanov et al., 2010b], which used a collimator made from neutron-absorbingmaterials to narrow the fieldof view of a standard 3He neutron sensor. In principle, such a collimated sensor can provide measurementswith a spatial resolution that is up to a factor of 4 better than the LP-NS measurements. Initial results fromLEND were reported by Mitrofanov et al. [2010a] and claimed to have a spatial resolution small enough to

Figure 8. Plots of mean CPR (solid lines) versus bistatic angle (figure taken from Patterson et al. [2016]). Data are shownfor (a) highland materials, (b) radar-facing slopes, and (c) Cabeus floor materials sampled in five bistatic observationstargeting Cabeus. Uncertainty is represented by the 3σ standard deviation of the mean for the measurements in eachbistatic angle bin (dashed lines). (d) Summary plot shows results of all sampled regions (thick solid line is Cabeus) (figurecourtesy of Icarus).



resolve individual PSRs. As part of these results,Mitrofanov et al. [2010a] stated that hydrogen enhancementswere likely located both inside and outside PSRs and had concentrations in the range of 0.5 to 4.0wt% WEH,results generally consistent with the prior analyses of LP-NS data by Elphic et al. [2007], Eke et al. [2009],and Teodoro et al. [2010]. The spatial resolution performance claims of Mitrofanov et al. [2010a] andMitrofanov et al. [2010b] were disputed by Lawrence et al. [2010] and Lawrence et al. [2011a]. The disputeswere based on arguments that background epithermal neutrons which penetrate the collimator were sig-nificantly underestimated by the LEND analyses and that these uncollimated background neutrons werealso sensitive to hydrogen, but in an uncollimated manner. As a consequence, the Lawrence et al. studiesconcluded that the LEND spatial resolution was significantly broader than claimed by the Mitrofanov et al.studies. Following these initial reports, a large number of additional studies were published that provideddisparate conclusions of whether the LEND instrument either achieved [Mitrofanov et al., 2011, 2012;Boynton et al., 2012; Litvak et al., 2012; Sanin et al., 2012] or did not achieve [Eke et al., 2012; Miller,2012; Miller et al., 2012; Teodoro et al., 2014] its original spatial resolution claims. While this author con-tends the evidence conclusively shows that the LEND instrument did not achieve its original performancegoals and thus did not spatially resolve individual PSRs (as grounded upon multiple independent andcomplementary analysis techniques described in the above-referenced studies), to date, the respectiveclaims of the two different interpretations of LEND data have not reached a public uniform agreement.The interested reader is referred to the above mentioned papers and references therein for moreinformation.

The first neutron measurements at Mercury were conducted using the Neutron Spectrometer (NS)[Goldsten et al., 2007] on board the MESSENGER mission. The primary goal of these measurements wasto either confirm or refute the hypothesis that water ice is the dominant species present in Mercury’sPSRs. In contrast to the lunar neutron measurements that collected neutron data at low altitudes(30–100 km) relative to the Moon’s radius of 1737.4 km, the MESSENGER mission had a highly eccentricorbit about Mercury (2439.7 km radius), with orbit altitudes that ranged from 400 to ~20,000 km. Sincerobust planetary neutron measurements can only be made from distances of ~<1 planetary radius, theprimary MESSENGER NS measurements were made in Mercury’s northern hemisphere where the spacecrafthad altitudes in the range of 400–800 km. Even at these high altitudes, neutron measurements of Mercury’sPSRs were challenging, as the broad spatial resolution of these measurements (hundreds of kilometers)competed with the need to detect hydrogen signatures from craters with sizes generally less than 50 kmin diameter. Because of this broad spatial resolution relative to the size of the PSRs, the decrease in epither-mal neutron signal from 100wt% water ice within north pole PSRs at Mercury was expected to be a fewpercent or less [Lawrence et al., 2011b]. In comparison, the decrease in epithermal neutrons due to1wt% WEH at the Moon was around 7–10% [Feldman et al., 2000]. Nevertheless, based on the first10months of orbital data, Lawrence et al. [2013] reported a decrease in the polar epithermal neutron countrate that closely matched the expected count rate decrease if the north pole PSRs were filled with largeamounts (50–100wt% WEH) of water ice (Figure 9). In addition, measurements of higher-energy fastneutrons suggested that this water ice was likely buried beneath tens of centimeters of drier material,consistent with similar suggestions from Earth-based radar data [Harmon et al., 2011]. Thus, the Mercuryneutron data were consistent with the idea that Mercury’s radar-bright enhancements were most likelydue to water ice.2.3.3. Orbit-Measured Topography and Temperature InformationThe time period after the year 2006 saw the advent of many new types of measurements of the poles of theMoon and Mercury. These include topography and temperature measurements (discussed in this section), aswell as new reflectance measurements at a variety of wavelengths (discussed in section 2.3.4).

Lunar polar topography measurements were carried out by the Chinese Chang’E-1 [Ping et al., 2009], theJapanese Kaguya [Araki et al., 2009], and the U.S. LRO [Smith et al., 2010] spacecraft. These topography datawere used to construct models of polar illumination from which full-coverage PSR locations and areas werederived. Noda et al. [2008] used data from the Kaguya laser altimeter (LALT) instrument to obtain PSR areasthat were updated and within 20% of those of the partial-coverage results of Margot et al. [1999]. Full polarcoverage data from the LRO Laser Orbital Laser Altimeter (LOLA) were also used to construct illuminationmodels of PSRs at both lunar poles [Mazarico et al., 2011]. Of all these studies, Mazarico et al. [2011] foundthe largest PSR areas of 12,866 km2 for the north pole and 16,055 km2 for the south pole. Compared to the



prior studies of Noda et al. [2008] and Bussey et al. [2003],Mazarico et al. [2011] attributed these larger areas toa better identification of small craters (<10 km) in the north and a more regular digital elevation model (DEM)in the south. In general, these studies confirmed that PSRs in the north are smaller but more numerous,whereas the PSRs in the south are larger but fewer in number.

Direct temperature measurements were made of the lunar surface—and specifically the south pole—usingLRO’s Diviner instrument [Paige et al., 2010]. These measurements showed a wide range of temperatureswithin PSRs from ~120 K to less than 40 K (Figure 10). As a result, there are large expanses around theMoon’s south pole that are cold enough to trap water ice at the surface. Using these measured surfacetemperatures as a tie point, Paige et al. [2010] then derived a three-dimensional thermal model of theMoon’s south pole (Figure 10c) and identified depths down to 1m where water ice is stable (Figure 10d).These results showed that there are large areas of stability for subsurface water ice near the PSRs that canact as a “permafrost.” Siegler et al. [2016] reported updates of the water ice stability depths for the southpole, as well as the first water ice stability depths for the north pole. Paige et al. [2010] also showedthat the lunar PSRs are cold enough to trap other volatile species, such as sodium, mercury, and othercompounds (e.g., CO2). Such species were found in the first in situ measurements of lunar south pole material(section 2.3.5). The Chang’E-1 mission carried a four-channel microwave radiometer that obtained subsurfacetemperature measurements in the polar regions that are complementary to the Diviner measurements [Gongand Jin, 2012].

As with the different lunar spacecraft, NASA’s MESSENGER mission carried a laser altimeter—the MercuryLaser Altimeter (MLA)—from which topography measurements of Mercury were obtained. Because ofMESSENGER’s eccentric orbit, and the need for the MLA to obtain measurements from within ~1500 km ofthe surface, Mercury’s polar topography was only gathered from its northern hemisphere. The MESSENGERspacecraft did not have an instrument to directly measure surface temperature, but using the topographydata, a high fidelity thermal model was constructed for Mercury’s north pole (Figure 11) [Paige et al., 2013]grounded upon the same measurement-validated model of the Moon’s south pole. Based on these modeledtemperatures, a number of important new aspects of Mercury’s PSR’s were revealed. First, while Mercury’sPSRs are cold enough to trap water ice, they are nevertheless warmer than the Moon and do not reach thevery low temperatures seen at the lunar poles (<~50 K). Second, based on these thermal models, it was foundthat locations of water ice stability match well the locations of radar-bright regions, which further supportsthe idea that the radar-bright regions are composed of water ice. In an unexpected finding, differentlocations of surface and subsurface water ice stability match locations where the surface reflectance is anom-alously bright and dark, respectively (see section 2.3.4). Paige et al. [2013] interpreted these results to imply

Figure 9. (a) Measured (red) and simulated (black, blue) epithermal neutron count rates (figure taken from Lawrence et al. [2013]). Data are shown at Mercury’s northpole averaged over 2° wide latitude bins and plotted as a function of latitude. All corrections except for a radial Doppler effect have been applied to the data.Simulated count rates are shown for the cases of no hydrogen (black) and a thick surficial layer of 100% water ice (blue) in all radar-bright regions. The error barsdenote twice the measured standard deviation of the mean in each latitude bin. (b) Simulated and measured epithermal neutron count rates after correcting forthe radial Doppler effect, which is accomplished by normalizing to the simulation with no hydrogen (figure courtesy of Science magazine).



Figure 10. Maps of measured and model-calculated surface and subsurface temperatures in the lunar south polar region (figure taken from Paige et al. [2010]). Theouter circle on all maps is 80° south latitude. Observations were acquired between 6 September and 3 October 2009 as the Moon approached southern summersolstice. (a) Diviner-measured daytime bolometric brightness temperatures acquired between 11.4 and 13.6 h local time. (b) Diviner-measured nighttime bolometricbrightness temperatures acquired between 21.41 and 1.66 h local time. (c) Model-calculated annual average near-surface temperatures and the location of theLCROSS impact in Cabeus Crater. (d) Model-calculated depths at which water ice would be lost to sublimation at a rate of less than 1 kgm�2 per billion years.The white regions define the locations where water ice can currently be cold trapped on the surface, the colored regions define the upper surface of the lunarice permafrost boundary, and the gray regions define locations where subsurface temperatures are too warm to permit the cold trapping of water ice within 1m ofthe surface (figure courtesy of Science magazine).



that the bright regions are surface water ice and that the dark regions are composed of a less water-rich butmore carbon-rich (organic) material that may be a sublimation lag deposit left over from the original materialdeposited in the PSRs. Finally, MLA data have been used to place limits on the maximum thickness ofradar-bright deposits. Specifically, Talpe et al. [2012] examined the morphology and depth of many northpolar craters and found no detectable difference between craters hosting deposits and those not hostingdeposits. Based on this observation and the statistical limits of the analysis, Talpe et al. [2012] concluded thatthe radar-bright deposits have an upper limit thickness of <170m.2.3.4. Reflectance Measurements of PSRsReflectance measurements—passive and active—were made of PSRs at both the Moon and Mercury fromorbiting spacecraft. These measurements, some of which were not part of the original mission plans, haveyielded important new information about volatiles and PSR environments at both bodies.2.3.4.1. Reflectance Measurements: At the MoonAt the Moon, PSR reflectance measurements have been carried out in three broad categories: passivereflectance in the visible wavelengths, passive reflectance in ultraviolet (UV) wavelengths, and active (laser)measurements at the near-infrared (IR) wavelength of 1064 nm. The first direct passive image of a PSR wasreported by Haruyama et al. [2008], where the interior of Shackleton crater was imaged using the TerrainCamera on the Japanese Kaguya spacecraft (Figure 12). Haruyama et al. [2008] showed that there were noanomalous or unusual materials within Shackleton crater and concluded that there was no evidence forexposed water ice on its surface. In a follow-up study, Haruyama et al. [2013] used additional data from theKaguyamultiband imager to infer that bright features near Shackleton’s inner wall close to its rimmay be richin anorthosite as opposed to water ice, as was suggested from LRO LOLA reflectance data [Zuber et al., 2012](see more below). Even though surface ice was not detected at Shackleton, Haruyama et al. [2013] stated that1 to 2wt% water ice mixed within the soil at Shackleton [Miller et al., 2014] would be consistent withtheir observations. Visible wavelength images using the LRO Lunar Reconssaince Orbiter Camera (LROC) weretaken of the inside of multiple lunar PSRs [Koeber et al., 2014]. In contrast to the PSRs at Mercury [Chabot et al.,2014] (see also below), the lunar PSRs showed no anomalous features or terrains at visible wavelengths, butCPR-anomalous craters [Spudis et al., 2013] showed indications of being younger than their “nonanomalous”counterparts.

A second type of passive reflectance measurement was carried out in UV wavelengths using data from theLRO Lyman Alpha Mapping Project (LAMP) instrument. The LAMP instrument detects reflected UV photonsfrom interplanetary Lyman alpha radiation (121 nm) as well as from UV-bright stars. Using the first 18months

Figure 11. Maps of calculated surface and subsurface temperatures and water ice stability in the north polar region of Mercury, superposed on a shaded relief map ofMESSENGER topography (figure taken from Paige et al. [2013]). (a) Biannual maximum surface temperatures. (b) Biannual average temperatures at 2 cm depth(figure courtesy Science magazine).



of data accumulation of LAMP data, Gladstone et al. [2012] reported UVmeasurements of south pole PSRs andfound that all PSRs are darker compared to non-PSR regions and that most PSRs have stronger reflectance atlonger wavelengths (i.e., they are “redder”). From these results, Gladstone et al. [2012] inferred that thesurfaces of PSRs have higher porosities than non-PSR regions and that the preference for redder reflectancewas consistent with the presence 1 to 2wt% water frost at the top surface of the PSRs.

Hayne et al. [2015] carried out a more extensive modeling and analysis of both LAMP and polar temperaturedata measured by the Diviner instrument. In this study, Hayne et al. [2015] made a number of key observa-tions and conclusions regarding lunar PSR volatiles and environments. First, by separately delineating theUV spectra for temperatures above and below the water sublimation temperature of 110 K, Hayne et al.[2015] found that the UV spectral characteristics strongly changed for temperatures below 110 K, whichprovided compelling evidence for the presence of surface water frost. Second, based on the UV spectral char-acteristics within PSRs, Hayne et al. [2015] concluded that high porosities could not fully explain the PSRobservations. Third, a bimodal distribution of UV spectral water features at the south pole was found suchthat one population of PSR locations was grouped just below the water sublimation temperature of 110 K,and another, larger population of PSR locations had temperatures near 65 K. This distribution could beexplained by either a different type of temperature-dependent vertical mixing from impact gardening atlow temperatures and water-diffusion-based migration at temperatures near 110 K, and/or possible CO2

enhancements at the colder locations. This author notes, however, that if there are CO2 enhancements inthe few wt% range similar to the inferred water frost abundance [Hayne et al., 2015], and if such enhance-ments extended to centimeter-type depths (as do the inferred hydrogen enhancements based on neutrondata), then such CO2 enhancements would be detectable with thermal neutron data as has been done innonpolar locations at Mercury [Peplowski et al., 2016]. That such thermal neutron enhancements are notdetected in the vicinity of any of the cold PSR regions [Miller et al., 2014] argues against the CO2 hypothesis.Finally, Hayne et al. [2015] delineated a spatially heterogeneous distribution of water frost (Figure 13) and as aconsequence suggested that the destruction of water frost by impact gardening and space weathering isitself spatially heterogeneous.

Active reflectance measurements of lunar PSRs were made using the near-IR 1064 nm laser from LRO’s LOLAinstrument [Zuber et al., 2012; Lucey et al., 2014]. With these actively interrogated reflectance measurements,it was found that all PSRs have systematically higher reflectance values at 1064 nm than non-PSR regions[Lucey et al., 2014], with Shackleton crater showing a particularly high reflectance [Zuber et al., 2012; Luceyet al., 2014]. These high reflectances can be partially accounted for by mass wasting on steep-walled cratersrevealing new and brighter material. After accounting for steep-slope mass wasting, Lucey et al. [2014] ruledout enhanced porosity as the sole explanation for the increased reflectance but suggested that either

Figure 12. Inside Shackleton Crater at the Moon, a ~10.5 km radius crater near the lunar south pole (figure taken fromHaruyama et al. [2008]). (a) Image taken by the Kaguya Terrain Camera on 19 November 2007. The crescent in theimage is the portion of the crater’s inner wall illuminated by sunlight. The lunar south pole is on the upper left, indicatedby cross. (b) An enhanced image of Figure 12a. The permanently shadowed area inside Shackleton Crater is lit by scatteredlight from the illuminated portion of the upper wall. No deposit with a significant albedo anomaly is exposed on thefloor (figure courtesy of Science magazine).



enhanced water frost and/or reduced space weathering might explain the higher PSR reflectances. If surfacewater ice is responsible for the increased PSR reflectance, then possible abundances would range from 3 to14wt%; if reduced space weathering causes the enhanced reflectances, then the PSR regions would havebetween 50% and 80% less nanophase iron than mature lunar soil.2.3.4.2. Reflectance Measurements: At MercuryPassive and active measurements of Mercury’s PSRs were carried out with instruments on the MESSENGERspacecraft. Active reflectance measurements were made with the 1064 nm laser from MLA in a mannersimilar to the LOLA measurements of the Moon. Passive reflectance measurements were obtained usingMESSENGER’s Mercury Dual Imaging System (MDIS), which used a broadband clear filter (700 nm centralwavelength; 600 nm bandwidth) to gather images at pixel scales ranging from ~100m to almost as smallas 20m. The MDIS measurements characterized the PSR illumination conditions as well as imaged the perma-nently shaded portions of Mercury’s north pole craters.

As for the Moon, active reflectance measurements at Mercury are particularly useful for PSRs where there isno direct illumination from sunlight. The first reflectance measurements of Mercury’s PSRs were reported byNeumann et al. [2013]. In contrast to the Moon that showed moderate PSR brightness enhancements, theMLA data clearly revealed anomalous dark and bright regions in almost all the PSRs that were interrogated(e.g., Figure 14 shows dark regions). In general, these anomalous regions were located on poleward facingslopes and showed a high degree of spatial correlation with locations of radar-bright enhancements.When these data were compared with the polar thermal models [Paige et al., 2013], the optically brightregions (the most prominent being Prokofiev crater) were found in places where water ice is stable at the sur-face. A large number of dark regions were found in locations of thermal stability for subsurface water covered

Figure 13. Locations of anomalous UV albedo consistent with water ice (figure taken from Hayne et al. [2015]). Colorsindicate points with off-/on-band albedo ratio values >1.2 and Lyman α albedo <0.03, increasing from deep orange(1.2) to white (>3.2). The average Moon outside of the cold traps has a ratio of ~0.9. Ratio values in the range 1.2–4.0 areconsistent with water ice concentrations of ~0.1–2.0% bymass. If patchy exposures of pure water ice aremixed by area withdry regolith, the abundance could be up to ~10% (figure courtesy of Icarus).



by a layer (tens of centimeters thick) of less water-rich material. In addition, these locations were thermallystable at the surface for materials rich in carbon-bearing materials, many of which happen to have a darkreflectance. These thermal and reflectance results combined with the neutron and radar data made a verystrong case that Mercury’s PSRs were indeed filled not only with water ice but also more complex volatilematerials. In a recent study, Delitsky et al. [2016] suggested that the dark, carbon-rich material may be dom-inantly formed by energetic protons and electrons focused to the polar regions by Mercury’s magnetic field.

The first illumination maps of Mercury’s south [Chabot et al., 2012] and north [Chabot et al., 2013] poles wereconstructed using imagery from MESSENGER’s MDIS. These maps confirmed that all radar-bright regions arelocated within PSRs but that not all PSRs contain radar-bright materials. For craters farther than 10° from thepoles, the illumination maps confirmed the bias for radar-bright regions to be within PSRs at Mercury’s coldlongitudes of 90°E and 270°E rather than 0°E and 180°E. In a more extensive analysis of illumination condi-tions, Deutsch et al. [2016] found that ~45% of PSRs hosted radar-bright materials. Some of the craters lackingradar-bright materials are located within 10° of the pole (e.g., Sapkota and Burks; see Figure 15) and, as aconsequence, should have a similar thermal environment to nearby craters that do not host radar-brightdeposits. Deutsch et al. [2016] investigated the possibility that nonuniform coverage of the Earth-based radardata caused radar-bright materials to be “missed.” Simulations of the radar illumination conditions showedthat if these craters contained radar-bright materials, they should have been detected. This conclusion there-fore leaves open the possibility that whatever process deposited the volatile materials at Mercury’s north poledid so in a spatially nonuniform manner.

The interior of Mercury’s north polar PSRs were directly viewed using MDIS imagery in a manner similar to thePSR images of the Moon. In contrast to the Moon, Mercury’s PSRs show striking and distinctive features[Chabot et al., 2014, 2016]. Dark deposits identified by MLA were also easily identified as dark in the MDISimages (Figure 16). The center of Prokofiev crater shows up as bright in the image data as it did in theMLA data (Figure 17). Various textures within the deposits (e.g., small craters, and hills) do not appear tobe obscured by either the bright or dark deposits. As described by Chabot et al. [2014], the boundaries ofthe dark deposits are very sharp and distinct and, in most cases, are colocated with permanent shade bound-aries as well as radar-bright enhancements when present. In many instances, the dark regions are uniformlydark across large expanses, but as revealed in the highest spatial resolution data, there are sometimes smallerbright patches that appear to be associated with temperature variations within the larger dark regions. Theexistence of the underlying textures in dark and bright deposits indicates that these PSR deposits wereemplaced after the formation of such textural features. The existence of the sharp boundaries indicates thatthe deposits may be young, having been emplaced either in the geologically recent past and/or supplied byan ongoing process.2.3.5. Ground Truth Measurements of a Lunar PSRThe final set of measurements described in this section is from the LCROSS mission that flew in 2009[Colaprete et al., 2012]. The goal of the innovative LCROSS experiment was to investigate lunar PSRs byexercising a controlled impact into a PSR and observe the resulting effects. The LCROSS spacecraft wascolaunched with LRO on 18 June 2009 and carried a payload of cameras and reflectance spectrometers.The concept of operations was that the LRO Centaur upper stage shell, which flew to the Moon with the

Figure 14. Regional view of an area near Yoshikawa crater (81.2°N, 254°E), in polar stereographic projection (figure taken from Neumann et al. [2013]). Red circlesshow the outlines of six craters. (a) Maximum incident solar flux, as a percentage of the solar constant at 1 AU. (b) Radar cross section per unit area. Theprojected radar map [Harmon et al., 2011] has been shifted by 4 km to account for differences in projection and to achieve optimal registration with the MLA-basedmaps. Regions of interest are labeled. (c) MLA reflectance (colored dots) (figure courtesy of Science magazine).



Figure 15. Shadowed regions displayed on a mosaic of MESSENGER images from 80°N to 90°N in polar stereographic pro-jection (figure taken from Deutsch et al. [2016]). (a) Areas shadowed in all MDIS images are shown in cyan, radar-brightdeposits are in yellow, and areas with both shadow and radar-bright materials are in coral. Sapkota, Burke, and Yamadacraters are outlined in magenta. (b) Areas shadowed in the MLA model are shown in cyan, radar-bright deposits are inyellow, and areas with both shadow and radar-bright materials are in coral. Radar data are from Harmon et al. [2011](figure courtesy of Icarus).



Figure 16. Low-reflectance material in 31 km diameter Berlioz crater; the rim of the crater is outlined in pink. All images arein stereographic projection about the north pole; north is to the top (figure taken from Chabot et al. [2014]). (a) Radar-bright(yellow) [Harmon et al., 2011] and persistently shadowed (red) regions are collocated. (b) Illumination conditions ~20 hprior to acquiring the image in Figure 16c. (c) Wide-angle camera broadband image reveals low-reflectance material.(d) Low-reflectance area extends to the edge of the radar-bright and persistently shadowed regions. (e) Reflectance valuesmeasured by the Mercury Laser Altimeter (MLA) [Neumann et al., 2013]. (f) Calculated maximum surface temperatures[Paige et al., 2013] (figure courtesy of Geology).



LCROSS spacecraft after separating from the LRO spacecraft, would impact a PSR and be closely followed bythe LCROSS spacecraft. In the time between the impact of the Centaur and LCROSS spacecraft, LCROSS instru-mentation would make measurements of the impact and resulting ejecta plume. At the same time, it wasplanned that the orbiting LRO spacecraft would also make measurements of the impact event.

The LCROSS experiment was successfully carried out on 9 October 2009 when both it and the Centaur upperstage impacted into Cabeus crater at the Moon’s south pole (Figure 18). The initial results from the experi-ment were described by Colaprete et al. [2010], Gladstone et al. [2010], Hayne et al. [2010], and Schultz et al.[2010]. Spectral measurements of the ejecta plume detected the presence of water along with the presenceof a number of other volatile species, including light hydrocarbons, sulfur bearing species, and CO2 [Colapreteet al., 2010; Schultz et al., 2010]. The total amount of water detected using near-IR spectra was estimated to be155� 12 kg [Colaprete et al., 2010]. Based on this measurement and other information from the impact event,it was estimated that the water concentration at the impact site was 5.6� 2.9wt% H2O, which is larger thanthat estimated based on original values from neutron data [Feldman et al., 2001] but is closer to revised esti-mates of hydrogen enhancements within Cabeus crater [Eke et al., 2014]. Diviner observations of the impactevent showed a clear thermal signature, and based on these observations, Hayne et al. [2010] suggested thatup to 300 kg of water could have been liberated in the event, which is consistent with the LCROSS observa-tions. Finally, LRO LAMP also saw a clear signature from the impact event in UV wavelengths and inferred thepresence of a variety of species in the ejecta plume, including molecular hydrogen, carbon monoxide, andthe elements calcium, magnesium, and even mercury [Gladstone et al., 2010], which was predicted to be pre-sent in PSRs based on the presence of and expected thermodynamic behavior of mercury in lunar samples[Reed, 1999].

2.4. Interpretation and Explanation of PSR Volatiles

As is the case in any broad scientific topic, studies of interpretation and explanation interact contempora-neously with studies that make predictions and present measurement results. Thus, to provide context for

Figure 17. High-reflectance surface within Prokofiev crater, Mercury (figure taken from Chabot et al. [2014]). The rim of the 112 km diameter Prokofiev crater isoutlined in cyan. All images are in stereographic projection about the north pole, with 180°E to the top. (a) The radar-bright region (yellow outline) [Harmonet al., 2011] is located within the persistently shadowed region (red) determined from every available MDIS image. (b) Mercury Laser Altimeter reflectance values(color bar) at 1064 nm [Neumann et al., 2013] are higher in the radar-bright region. (c) A wide-angle camera (WAC) broadband image reveals an area of higherreflectance on the crater floor (pink box). (d) A second WAC broadband image acquired under different illumination shows the same higher-reflectance surface.(e) Expanded view of the area within the pink box in Figure 17c. (f) Expanded view of the area within the pink box in Figure 17d (figure courtesy of Geology).



the measurement investigations described in sections 2.2 and 2.3, broad categories of interpretation studiesare described. In mentioning specific papers, there is no attempt to provide an exhaustive description of allsuch studies; rather, the investigations mentioned here are illustrative of the types of interpretation studies ofPSR volatiles that have been conducted.

There are three broad categories of PSR volatile interpretation studies: the delivery of water and volatiles toPSRs, the stability of delivered volatiles within PSRs, and the modification of volatiles within the PSRs. Moseset al. [1999] provides a good type example of a study that investigates the delivery of various volatiles toMercury’s PSRs. In this study, Moses et al. [1999] investigated the delivery of water from interplanetary dustparticles, water-rich asteroids, and Jupiter family and Halley-type comets. It was concluded that all thesesources could possibly supply the estimated water to Mercury’s PSRs but that Jupiter family comets were par-ticularly promising because of their water content and orbital characteristics. The delivery of water to theMoon via comet impact has been studied through the simulation of the impact process and resulting timeevolution of the impact products [e.g., Ong et al., 2010; Stewart et al., 2011; Prem et al., 2015]. In some cases,it was determined that such impacts could create a transient, collision-dominated atmosphere around theMoon thus enhancing the amount of water that could reach the polar PSRs [Prem et al., 2015]. No such similarimpact-simulation studies have been carried out for Mercury. Other studies have investigated the migrationof water to lunar PSRs in the noncollisional, exospheric regime. A recent study byMoores [2016] suggests thatspatially heterogeneous water deposition in south polar PSRs may be a natural result of such water migration.

A study that investigated the stability of volatiles within PSRs was carried out in a classic investigation byVasavada et al. [1999]. This study derived thermal stability requirements for simple, flat-floored, bowl-shapedcraters at Mercury and the Moon. While actual craters are more complicated than those assumed byVasavada et al. [1999], this study nevertheless provided useful constraints from which to assess the volatile

Figure 18. Image from LCROSS visible camera showing sunlit ejecta about 20 s after impact of the LRO Centaur upper stageshell (figure taken from Schultz et al. [2010]). Inset shows a close-up with the direction of the Sun and the Earth (indicated byarrows). Asymmetry of the ejecta reflects, in part, the projected shadow over the crater (from the edge of Cabeus) andacross the ejecta cloud. Dotted circle represent the fields of view of the visible and near-infrared spectrometers (figurecourtesy Science magazine).



stability of real craters at actual locations on the Moon and Mercury. Additional studies have investigatedvolatile stability within Mercurian and lunar PSRs across geologic time periods [Siegler et al., 2011, 2013,2015]. These studies accounted for effects such as time-dependent obliquity, and the results suggest thereare fundamental differences in volatile stability between the Moon and Mercury due to such effects.

Finally, studies have investigated the modification of volatiles due to various surface and environmental pro-cesses. As an example, Crider and Vondrak [2003], Crider and Killen [2005], and Hurley et al. [2012] investigatedhow surface volatiles will be buried, destroyed, and spatially modified due to space weathering processeson the Moon and Mercury. As another example of volatile modification at the Moon, Siegler et al. [2016]extended prior volatile stability analyses to show that the current lunar-polar-hydrogen distribution maybe explained in part by the modification of a geologically old volatile deposit that has been systematicallychanged by time-dependent lunar obliquity changes due to lunar polar wander.

2.5. Summary of PSR Measurements

When all the PSR measurements from the Moon and Mercury are summarized (Table 1), a consistent pictureemerges that the poles of Mercury have large amounts of water ice, and the Moon’s poles, while enriched involatiles, have a much lower concentration of volatiles than Mercury. The radar data strongly support thisconclusion by showing large radar-bright enhancements at Mercury that are all within PSRs. In contrast, whileradar data from the Moon show evidence for water enhancements, these data do not show the same clearPSR-filling signatures that are seen at Mercury. Hydrogen measurements support this conclusion such thatall lunar neutron measurements indicate a hydrogen concentration no greater than a few wt% WEH withinPSRs, while hydrogen concentrations at Mercury’s PSRs are in the range of 50 to 100wt% WEH. The thermalenvironments at both bodies enable the retention of water ice and other volatiles. Imaging and reflectancemeasurements show clear differences. At the Moon, there is evidence for surficial frost of water and possiblyother species, but the presence of such frost is not uniform in all locations, and there is not uniform andunambiguous evidence that PSR lunar volatiles have been emplaced in large and thick contiguous areas.At Mercury, the reflectance data show clear evidence for bright and dark deposits that closely follow thermalstability for surface and subsurface water ice, respectively. Further, the clearly delineated reflectance featureswithin Mercury’s PSRs imply the presence of large and contiguous volatile deposits. Finally, even with theapparently smaller amount of water at the lunar PSRs, the in situ LCROSS data reveal the presence of acomplicated mixture of volatile materials. Based on the clear dichotomy between the nature and quantityof volatiles at the PSRs of Mercury and the Moon, a fundamental question that immediately arises is why isthere such a difference between the two bodies? While there are no clear answers yet to this question, poten-tial explanations can be offered, and these explanations are discussed in the next section.

3. Why Are the Moon and Mercury’s PSR Volatiles So Different?

Gaining understanding for why the lunar and Mercurian PSR volatiles are so different requires looking forspecific differences that can explain this dichotomy. While not exhaustive, broad categories of explanationare provided in Table 2. These explanations are not intended to cover every conceivable scenario, as thereare explanations that might not yet have been considered. Nevertheless, these explanations are intendedto be broad and illustrative and provide a summary of what might be possible. The four broad explanatorycategories are differences between the Moon and Mercury in delivery time of volatiles, type and

Table 1. Summary of Types of PSR Measurements at the Moon and Mercurya

Moon Mercury

Water (from radar) Spotty; not one-to-one correlated with PSRs. Fills PSRs and radar-bright regions always in PSRs.

Hydrogen <few wt% H2O; not one-to-one correlated with PSRs. 50–100wt% H2O; spatially correlated with PSRs.

Temperature Cold temperatures can trap volatiles. Cold temperatures can trap volatiles; slightly warmer than Moon.

Reflectance/imaging Consistent with frost in some places; not uniform withinall PSRs; no clear “deposits” within PSRs.

Consistent with surface water and carbon-rich organics; PSRsshow dark regions with sharp boundaries.

In situ Multiple volatile species; few wt% H2O. NA

aNA: not applicable.



composition of impactors, processes that operate on the Moon and Mercury, and the compositions of theMoon and Mercury themselves. It is likely that the complete explanation may involve some combination ofany number of these explanations.

Differences in delivery time. One straightforward explanation is that Mercury may have had a recent impact fillup its PSRs, whereas the Moon simply has not had any recent impacts. While this explanation might provide agood prima facie understanding of the Mercury/Moon difference, there is a sense that it is a “too easy” and adhoc explanation. In order for this to be a robust explanation, there needs to be multiple lines of independentevidence that would point to a recent delivery of volatiles to Mercury. It turns out there is such evidence. First,as described in section 2.3.4, Mercury’s PSR deposits are clearly delineated with sharp boundaries in thereflectance data. These boundaries suggest the deposits may be geologically young. No such features existon the Moon. Second, there is clear evidence that in many locations within Mercury’s PSRs, relatively purewater ice is covered by tens of centimeters of drier material. Given that pure water ice may be buried by driermaterial due to space weathering [Crider and Killen, 2005], this possible drier top layer suggests Mercury’sdeposits may be geologically young (<70Myr) [Lawrence et al., 2013]. Third, the relative purity of Mercury’spolar ice, especially compared to the Moon, suggests that it is a recent deposit, as modification processes willtend to dilute the water purity over time [Hurley et al., 2012]. Fourth, if a recent impact deposited the volatilesin Mercury’s PSRs, then one might expect to see evidence on Mercury’s surface of such an impact. A recentstudy by Ernst et al. [2016] suggests that a candidate source crater is Hokusai crater, which is located in thenorth central portion of Mercury (100 km diameter, 17°E, 58°N). Hoskusai crater is one of the youngest cratersonMercury, has an extensive ray system that stretches very large distances across Mercury, and has almost nosmall, superposed craters. Ernst et al. [2016] carried out a preliminary analysis of the type of impactor thatcould have created Hokusai and concluded it is not unreasonable for the Hokusai impactor to have deliveredthe necessary volatiles to Mercury’s poles. There is additional information from neutron-based compositionmeasurements that the Hokusai region may be enriched in lower mass materials indicative of a possible vola-tile enhancement [Lawrence et al., 2016a, 2016b]. In contrast to a recent delivery of volatiles to Mercury, at theMoon there is evidence for the ancient presence of enhanced hydrogen that has beenmodified by long-termobliquity changes [Siegler et al., 2016], which supports the idea that the Moon has not had a recent delivery oflarge amounts of volatiles.

While there is therefore some evidence to support a recent impact at Mercury and not at the Moon, morework, including analyses, modeling, and new measurements can be done to test this explanation. Specifictasks include the following: carry out new analyses of Mercury composition data, conduct better models ofcomet impacts at Mercury, and make in situ measurements at either or both bodies that could characterizethe composition and soil properties and compare these measurements with expectations for recent orancient volatile deposition.

Differences in impactors. The difference between the Moon’s and Mercury’s PSR water content may be due tothe original water content contained in different types of impactors, such that water delivered to Mercurymay be from high-water-content comets, while water delivered to the Moon is from lower-water-contentasteroids. While not directly applicable to the lunar PSRs, recent studies of trapped lunar water in Apollo

Table 2. Possible Explanations and Tests for Understanding the Difference Between the PSR Volatile Contents of Mercury and the Moon

Explanation Description How to Test?

Differences in delivery time Recent delivery to Mercury and not to the Moon. Find recent crater on Mercury; model recent comet impacts;characterize layering and stratigraphy.

Differences in impactors Mercury impacted by water-rich comet(s); Moonimpacted by less water-rich asteroid(s).

Measure isotopic abundances (D/H, oxygen andnitrogen isotopes).

Differences in processes Do slight temperature differences cause significant differencein processes that trap and remove volatiles? Does the

space environment cause different processes to operate?

Measure the composition within PSRs and three-dimensionalstratigraphy; correlate volatiles with temperature inside andoutside PSRs; characterize environmental effects by making insitu measurements of environmental factors (charged particles,

solar wind, etc.).

Differences in planetarycompositions

Can Mercury’s volatile-rich composition makesubstantial contribution to PSR volatile inventory?

Measure in situ compositions at Mercury; better understandMercury volatile composition and volatile release mechanisms.



volcanic glass samples have shown that this water is indistinguishable from that of bulk water in carbonac-eous chondrites [Saal et al., 2013]. Given that isotopic measurements of various species (e.g., D/H, 15N/14N)[Saal et al., 2013] can discriminate between these difference sources, a key test of this explanation is tomeasure these and other isotopic abundances within the PSRs at both the Moon and Mercury. If suchmeasurements could be made as a function of depth into the surface (down to tens of centimeters), thesedata could provide information for the time history of volatile deposition from different sources.

Differences in processes. There are different environmental factors operating on PSRs at the Moon andMercury, and as a consequence, there may be different processes that act within PSRs to modify the volatileconcentrations and distributions. As mentioned in section 2.3.3, the PSR temperatures in the lunar PSRs arenotably colder than those in Mercury’s PSRs. Thus, temperature-dependent processes, such as diffusionand/or thermal cycling [e.g., Schorghofer and Aharonson, 2014] may have significantly different effects onone body compared to the other. Environmental differences are not restricted to temperature, as the spaceenvironment is different between the two bodies. The Moon has no intrinsic magnetic field and only passesthrough the far portion of Earth’s magnetosphere for a portion of its orbit. In contrast, Mercury has a mag-netic field, which can shield the planetary surface from solar energetic particles, but is also the source ofinternally generated energetic particles [e.g., Lawrence et al., 2015a], some of which might modify PSR mate-rials [Delitsky et al., 2016]. Finally, Mercury’s closer distance to the Sun causes differences in the amount ofsolar wind and charged particles that reach its surface, as well as the number of impactors that can reachits orbit and hit the planet.

There is little understanding about how differences in these and other processes may result in different vola-tile contents at the two sets of PSRs. Additional understanding about such processes can be obtained bygathering new information about the three-dimensional hydrogen distribution at the lunar PSRs, and howthis distribution relates to the thermal environment. More studies to investigate charged particle effectscan be carried out. Finally, in situ measurements of various environmental factors (e.g., magnetic field,charged particles, solar wind, and hydrogen deposition and removal) would provide important constraintson the level to which differences in processes might play a role in the overall difference between the two setsof PSRs.

Differences in planetary compositions. Until the MESSENGER mission, it was not considered that differences incomposition between the Moon and Mercury might help to explain the difference between the PSR volatilecontent of the two bodies. However, MESSENGER compositional measurements show that Mercury is moreenriched in volatile elements (K, S, Na, Cl, and C) than was expected for a supposedly volatile-depleted body[e.g., Peplowski et al., 2011, 2014, 2016; Nittler et al., 2011; Evans et al., 2015]. In addition, a geologic featurenow known as hollows was discovered to be ubiquitous across Mercury’s surface. Hollows are relatively small(less than tens of kilometers across), geologically young, rimless depressions that are mostly located withinimpact features [Blewett et al., 2011]. The best current explanation for the existence of hollows is that theyare formed through the relatively rapid loss of volatile material [Blewett et al., 2013]. These observations ofMercury therefore suggest it may be possible that some portion of Mercury’s PSR volatiles could be due tothe release of volatile material that finds its way to the poles and gets trapped. Interestingly, this idea issimilar to the original idea proposed by Watson et al. [1961b] of lunar endogenous material supplying waterto the lunar poles. In contrast to Mercury, the Moon has low abundances of volatile materials and thereforewould not likely contribute large concentrations of endogenous volatile material to the lunar PSRs.

The idea that endogenous volatiles might supply a substantial portion of water to Mercury’s PSRs is currentlyonly a speculation; thus, much work needs to be done to test if it is a viable hypothesis. One argument againstthis idea is that an endogenous delivery of volatiles would need a special means to keep the PSR water rela-tively pure, which may not be easy for a presumably continuous process that takes place over long periods oftime. Progress for testing this idea can be made by better understanding the nature of Mercury’s volatilematerials, how they are released, and then how they are trapped and possibly accumulated within PSRs.As with the other explanations, in situ composition measurements, both within and outside PSRs, wouldprovide key constraints and enable tests of specific hypotheses.

Combination of explanations. The final explanation for the difference between the Moon and Mercury may ofcourse be some combination of different explanations given above. For example, the water at Mercury mayhave been supplied by a recent comet impact, and no such comets may have hit the Moon in the geologically



recent past. Further, even if there is recent and/or ongoing volatile deposition, different processes could stillbe affecting the resulting deposits in different ways [Delitsky et al., 2016]. So while there is informationpointing at all these explanations as possible explanations, we currently do not have enough informationto definitively resolve why there is the difference between the PSRs at the Moon and Mercury.

4. PSR Exploration in the Next 50 Years

Much has been learned in the first half century of PSR exploration. The field started with a few speculativestudies and has reached a well-bounded understanding of the nature of PSR volatiles. As is almost alwaysthe case when initial speculation and prediction gives way to “finding things out,” reality is more complicated(and interesting) than could have been envisioned at the beginning. So it is with studies of PSR volatiles.

While a broad understanding is now known, there are still many basic and fundamental aspects of PSRs andPSR volatiles that are still not understood. The nature of the soil and layering (mechanical, detailed composi-tion) are still largely unknown. Models have been generated and predictions have been made aboutprocesses that operate within PSRs, but actual knowledge of such processes is limited. The major questionof why the PSR volatiles are so different at the Moon and Mercury is still not resolved.

Predictions about what might take place in the next 50 years of PSR exploration is challenging at best, as ithas been noted “Prediction is very difficult, especially about the future” (This quote has been attributed tovarious people, but often it has been stated as a quote from Niels Bohr.). Nevertheless, there are broad cate-gories of future exploration that will likely be accomplished in the next 50 years. Below is a brief survey ofthese categories.

The next spacecraft scheduled to go to Mercury is the Bepi Colombo mission, which is being jointly built andoperated by the European Space Agency (ESA) and the Japanese Space Agency JAXA [Benkhoff et al., 2010].Bepi Colombowill orbit two spacecraft aroundMercury, and the ESA-built Mercury Planetary Orbiter will orbitMercury with increased global coverage compared to MESSENGER, especially of Mercury’s south pole, whichwas not studied up close by MESSENGER due to its eccentric orbit with periapsis in the northern hemisphere.One instrument that can make a new type of measurement of Mercury’s PSRs is the Mercury Radiometer andThermal Infrared Spectrometer (MERTIS) [Hiesinger et al., 2010], which will obtain measurements of Mercury’ssurface in the spectral range of 7–40μm. MERTIS will enable direct measurements of the PSR thermal envir-onment as well as characterize the sulfur content and mineralogy of Mercury’s surface including the PSRregions. Neutron data from Bepi Colombo should provide the first characterization of hydrogen abundancesof Mercury’s south polar PSRs [Mitrofanov et al., 2010c].

At the Moon, a number of new orbital measurements could be accomplished that would provide key newinformation. Measurements of hydrogen concentrations with improved spatial resolution can be made byflying neutron sensors at very low altitudes (<10 km) near PSRs to isolate the hydrogen concentrations toindividual PSRs. A pathfinder mission to make this measurement from a CubeSat platform is currentlyplanned [Hardgrove et al., 2015]. A full-coverage, high-statistics measurement that could carry out high spatialresolution measurements, as well as provide spatially resolved, hydrogen burial-depth information, likelyrequires a larger instrument and more spacecraft resources than can be provided by a CubeSat [Lawrenceet al., 2015b]. New multiwavelength spectral data at the Moon, either active or passive, could provide addi-tional details about volatiles and frost within lunar PSRs.

A new leap in knowledge will require landed measurements from within a PSR. Such measurements areessential but challenging. One of the biggest challenges is the need to operate a landed spacecraft in the verycold PSR environment, which is difficult for engineering (power, thermal, and mechanical) and operationalreasons. At Mercury, there is the additional challenge of safely landing a spacecraft in the deep gravitationalwell at Mercury’s location near the Sun. Nevertheless, such missions would reveal fundamentally new infor-mation about PSRs (composition, stratigraphy, and processes) that would likely challenge and expand ourcurrent understanding of PSRs.

While there are no currently planned PSR-landed missions, NASA is currently studying a lunar polar rovercalled Resource Prospector that would carry out investigations in sunlit regions near south pole PSRs[Elphic et al., 2015]. While the RP rover will not be designed to survive in large, deep PSRs, it may still inves-tigate small shaded regions in which enhanced volatiles might be present and PSR-like process might be



operating. There are also reports that the Russian and Chinese space agencies are planning lunar polar mis-sions, although details of PSR-specific missions are unclear. In any case, an in situ PSR mission (or seriesof missions) will likely be accomplished by some or multiple space agencies and will provide significantchanges in our understanding of PSR volatiles.

Finally, the Moon and Mercury do not necessarily contain the only PSRs in the solar system but are onlythe best known examples. A recent study has reported that the asteroid Ceres has craters near its polesthat may approximate PSRs [Schorghofer et al., 2016]. Given that Ceres is much more volatile rich thaneither the Moon or Mercury, it likely represents another end-member of PSR environments with interestingand unique qualities.

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AcknowledgmentsThe author would like to thankDr. William C. Feldman for generouslyproviding me with the initial opportu-nity at Los Alamos National Laboratoryto carry out work in this exciting field ofPSR volatiles; I am exceedingly gratefulfor the long and fruitful relationship(both scientific and personal) withDr. Feldman. The material in this paperwas originally presented as a seminar inNovember 2014 at Brown University,which was supported by the NASA SolarSystem Exploration Research VirtualInstitute (SSERVI). The work to completethe paper was supported by differentgrants from NASA including theMESSENGER Participating Scientistprogram (grant NNX08AN30G), theNASA Lunar Advanced Science andExploration Research program (grantNNX13AJ61G), and a NASA SSERVIgrant to Johns Hopkins UniversityApplied Physics Laboratory (grantNNA14AB02A). All data presented inthe paper either reside in the NASAPlanetary Data System and/or can befound via the referenced papers. Theauthor thanks Wenzhe Fa and RichardElphic for providing helpful reviews thatimproved the scope and quality of thismanuscript.

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A tale of two poles: Toward understanding the …...volatiles at the polar regions of the Moon and Mercury, J. Geophys. Res. Planets, 122,21–52, doi:10.1002/ 2016JE005167. Received