Permanently shadowed regions (PSRs) at the lunar poles are potential reservoirs of frozen volatiles, and are therefore high-priority exploration targets. PSRs trap water and other volatiles because their annual maximum temperatures (40-100K) are lower than the sublimation temperatures of these species (i.e. H2O approx.104K). Previous studies using various remote sensing techniques have not been able to definitively characterize the distribution or abundance of ice in lunar PSRs. The purpose of this study is to search for signs of ice in PSRs using two complimentary remote sensing techniques: radar and visible images

Denevi, B. W.

Lawrence, S. J.

Mitchell, J. L.

Robinson, M. S.

Speyerer, E. J.

NASA Technical Reports Server

SEARCHING FOR WATER ICE AT THE LUNAR NORTH POLE USING HIGH-RESOLUTION 
IMAGES AND RADAR. J. L. Mitchell1, S. J. Lawrence2, M. S. Robinson1, E. J. Speyerer1, and B. W. Denevi3, 
1School of Earth and Space Exploration, Arizona State University, Tempe, AZ. (Julie.L.Mitchell@asu.edu), 2NASA 
Johnson Space Center, Houston, TX, 3Johns Hopkins University Applied Physics Laboratory, Laurel, MD. 
 
Introduction: Permanently shadowed regions 
(PSRs) at the lunar poles are potential reservoirs of 
frozen volatiles, and are therefore high-priority explo-
ration targets [1-4 and others]. PSRs trap water and 
other volatiles because their annual maximum tem-
peratures (40-100K, [1]) are lower than the sublima-
tion temperatures of these species (i.e. H2O ~104K) 
[1]. Previous studies using various remote sensing 
techniques have not been able to definitively character-
ize the distribution or abundance of ice in lunar PSRs. 
The purpose of this study is to search for signs of ice in 
PSRs using two complimentary remote sensing tech-
niques: radar and visible images. 
Background: Numerous PSRs were found to have 
circular polarization ratios (CPR) considered anoma-
lous because the CPR interior to the crater is high rela-
tive to the exterior; these CPR signatures could be the 
result of significant ice deposits in the crater [5]. How-
ever, the anomalous CPR signatures could also be the 
result of wavelength-scale blocks within the crater but 
not its ejecta. Here we assess the presence or absence 
of blocks associated with these radar-anomalous cra-
ters as an alternative to the ice hypothesis. The Lunar 
Reconnaissance Orbiter (LRO) Mini-RF instrument 
actively transmitted circularly-polarized S-band (12.6-
cm) radar waves [6]. The Lunar Reconnaissance Orbit-
er Camera (LROC) Narrow Angle Camera (NAC) has 
obtained long-exposure images of PSR interiors at the 
10+ m pixel scale [7, 8]. NAC images reveal PSR inte-
riors, allowing the identification of blocks (>20 m di-
ameter) to assess whether the radar signatures are like-
ly due to a rocky crater interior rather than ice. 
Methods: Water ice deposits and wavelength-scale 
blocks result in double-bounce backscatter of S-band 
radar waves. The m-chi decomposition [9] is used to 
produce a qualitative distribution of double-bounce 
backscatter detected by Mini-RF. The m-chi decompo-
sition separates radar returns into single-bounce 
backscatter indicative of features smooth at the wave-
length scale, volume scattering such as from randomly-
oriented rocks suspended in lunar regolith, and double-
bounce backscatter. Often m-chi decomposition com-
ponents are displayed as a colorized map, where sin-
gle-bounce backscatter is green, volume scattering 
blue, and double-bounce backscatter red [9] (Fig. 1). 
Radar alone cannot distinguish between young, 
fresh craters and older craters that contain water ice 
because both result in high double-bounce backscatter 
in their interiors. LROC long-exposure NAC images 
were used to allow a quantitative comparison of blocks 
between regions with potentially high concentrations 
of ice and regions definitively without ice. Long-
exposure images of PSRs have larger pixel scales than 
nominal-exposure products (10-20 m/px versus 0.5-1.0 
m/px nominal); therefore, only blocks larger than 20-
40 m in diameter can be detected. M-chi maps and 
block counts were produced for four CPR-anomalous 
craters and two CPR-normal craters at the north pole, 
and two CPR-anomalous equatorial craters (Table 1). 
 
Fig. 1. LROC NAC (left) and m-chi map (right) of polar CPR-
anomalous crater Rozhdestvenskiy N. 
Table 1. Craters of study. *Whipple was previously reported as pre-
Nectarian, but further analysis revealed it to be Imbrian. 
The standard LROC processing pipeline [7] and Cra-
terTools [10] were used to characterize block size-
frequency distributions (SFDs) at craters with blocks. 
The SFDs of small (<20 m) blocks in PSR-bearing 
craters were extrapolated using a power law [11]. 
SFDs allowed a comparison to be made between the 
numbers of small blocks in PSRs, outside of PSR-
bearing craters, and at fully lit equatorial craters. The 
slope of the log-log plot of the SFD, known as B from 
[11], allows a comparison of geologic/erosional pro-
cesses between sites that have the same composition. 
Polar CPR-anomalous craters could have B values dis-
tinct from equatorial CPR-anomalous craters, which 
cannot have ice due to high daytime temperatures. 
Results: Rozhdestvenskiy N showed double-
bounce backscatter in its interior and no detectable 
blocks (≥20 m diameter). Other craters where the pres-
Crater Lat (°N) 
Long 
(°E) CPR Age 
Rozhdestvenskiy N 84.04 203.73 Anom. Copernican 
Lovelace E 82.04 263.22 Anom. Imbrian 
Lovelace 82.08 250.49 Anom. Imbrian 
Whipple* 89.14 120.05 Anom. Imbrian* 
Main L 81.44 22.73 Normal Copernican 
Plaskett U 82.41 162.29 Normal Eratosthenian 
Byrgius C 21.17 295.49 Anom. Copernican 
Gardner 17.73 33.80 Anom. Imbrian 
https://ntrs.nasa.gov/search.jsp?R=20170001956 2019-08-31T17:51:37+00:00Z
ence of ice was proposed [3, Table 1] were dominated 
by single-bounce scattering with patches of double-
bounce backscatter and contained <0.2 total detectable 
blocks per km2. The low quantity of blocks is likely the 
result of resolution limitations since the majority of 
visible blocks in equatorial crater interiors were <15 m 
in diameter. Some craters showed an admixture of 
block sizes which produced equal parts double-bounce, 
single-bounce, and volume scattering; this combination 
of scattering results in a white color in m-chi maps. 
Of the eight craters included in this study, four had 
enough blocks in their interiors/PSRs and exteri-
ors/ejecta to allow meaningful extrapolation of SFDs. 
(Fig. 2). For all craters, the quantity of blocks was 
lower in the crater interior/PSR than the exterior. This 
trend is possibly an intrinsic property of lunar craters 
because it persists in both equatorial craters, which do 
not have ice, and in polar craters. Estimates of the 
number of small blocks in PSRs, produced by extrapo-
lating a power law (Fig. 3), showed no significant var-
iation between equatorial and polar block abundances 
with the exception of Whipple crater. In addition, there 
were no significant variations in B values between cra-
ters, showing that erosional processes likely do not 
vary significantly between craters with varying radar 
backscatter properties in these few cases. 
 
 
Fig. 2. SFDs of the interiors/PSRs and exteriors of equatorial and 
PSR-bearing lunar craters derived from this study. 
 
Fig. 3. Cumulative block distributions for small (<15 m) blocks. 
Pixel scale prevented direct counts of blocks <20 m, so a power law 
[11] was used to extrapolate their abundance. 
Discussion: Double-bounce backscatter, a sign of 
ice, was not exclusively observed at PSR-bearing cra-
ters, preventing a definitive identification of craters 
containing significant water-ice deposits. Rozh-
destvenskiy N had the highest double-bounce signature 
and no detectable blocks (20-m scale), and is therefore 
the most likely crater in this study to harbor ice depos-
its. However, a CPR enhancement in its interior (indic-
ative of water ice) has not been observed [12]. Other 
craters contained double-bounce backscatter in patch-
es, and these regions may be ice deposits, eroding 
slopes within PSRs, or hidden, sub-surface clasts. Low 
double-bounce backscatter does not necessarily pre-
clude the presence of water ice. Ice diffused within the 
regolith could be present but undetectable by current 
remote sensing methods. 
Because the interiors of young/fresh craters and 
older craters with ice could appear the same in radar, 
exterior block abundances were compared to estimate 
the likelihood of blocks in PSRs. All polar craters 
showed fewer blocks in their PSRs than exteriors; 
however, equatorial craters (Gardner and Byrgius C) 
showed the same trend, indicating that this is an inher-
ent property of craters and not a result of ice, lighting 
conditions, or other latitude-dependent factors. Block 
counts show similar block sizes and densities between 
CPR-normal polar and CPR-anomalous equatorial cra-
ters. There is no statistically significant variation in B 
values between all types of craters, preventing that 
metric from being a distinguishing factor between 
crater types. 
Conclusions: Most blocks observed in equatorial 
craters would be unresolvable in long-exposure NACs; 
therefore, blocks may still exist in craters classified 
here as block-free. SFDs did not reveal substantial 
differences between craters. The lack of consistent 
double-bounce backscatter in potential ice-bearing 
PSRs in this study indicates two possibilities: 1) not all 
lunar PSRs have water ice, or 2) there is water ice, but 
the physical state of the deposits is not detectable 
through this type of analysis. Future work should in-
clude m-chi maps and block counts of other polar 
CPR-anomalous craters to assess the presence of ice, 
including those at the south pole. 
References: [1] Paige, D. A. et al. (2010a) Sci., 
330, 479-482. [2] Sanin, A. et al. (2012) JGR, 117. [3] 
Lucey, P. et al. (2014) JGR-P, 119, 1665-1679. [4] 
Colaprete, A. et al. (2010) Sci., 330, 463. [5] Spudis, P. 
D., et al. (2013) JGR-P, 118, 2016-2029. [6] Raney, R. 
K. et al. (2011), IEEE, 99, 808-823. [7] Robinson, M. 
S. et al. (2010), Sp. Sci. Rev., 150, 81-124. [8] Koeber, 
S. D. et al. (2014) LPS XLV, Abst. # 2811. [9] Raney, 
R. K. et al. (2012) JGR-P, 117. [10] Kneissl, T. S. et 
al. (2011) Plan. Sp. Sci., 59, 1243-1254. [11] Hart-
mann, W. K. (1969) Icarus, 10, 201-213. [12] Fa and 
Cai (2012) JGR-P, 118, 1582–1608. 


Searching for Water Ice at the Lunar North Pole Using High-Resolution Images and Radar

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