FORECAST WATER ICE RESERVES IN THE MOON POLAR REGIONS BY THE "LEND" DATA

Abstract

The study evaluates water ice reserves in the lunar polar regions using data from the LEND instrument on the LRO spacecraft.

Full text

FORECAST WATER ICE RESERVES IN THE MOON POLAR REGIONS BY THE “LEND” DATA. E. N.
Slyuta1, O. I. Turchinskaya1, O. S. Tretyukhina1, A. B. Sanin2, I. G. Mitrofanov2, M. L. Litvak2, 1 Vernadsky Institute
of Geochemistry and Analytical Chemistry of Russian Academy of Science, 119991 Moscow, Russia,
slyuta@geokhi.ru, 2 Institute for Space Research of Russian Academy of Science, 117997 Moscow, Russia.
Introduction: The first instrumental confirmation
of the possible presence of water ice in the lunar soil
was obtained from the LPNS neutron detector data on
board the Lunar Prospector spacecraft [1, 2]. The Lunar
Research Neutron Detector (LEND) aboard the LRO
spacecraft is a collimating spectrometer and provides
neutron imaging of the lunar surface with a resolution
of about 10 km from an orbit about 50 km high [3].
Based on the neutron count rate data, maps of the lunar
epithermal neutron flux with high spatial resolution
were created for the Northern and southern Polar
Regions of the Moon [4]
WEH content in the lunar soil: Estimates of the
water equivalent of hydrogen (WEH) in the study areas
were obtained from the relative variations in the neutron
count rate in these areas compared to a dry reference
area with the highest counts, which is considered
anhydrous [4]. The hydrogen content in dry reference
regions at the characteristic value of the neutron count
for these regions (2.36 cps) was taken to be 50 ppm,
which corresponds to 0.045 wt.% of the WEH [4].
Taking this correction into account, the neutron count
data in the northern and southern Polar Regions were
recalculated to the abundance of WEH in wt %, and a
map of the WEH distribution was constructed with a
resolution of about 5 km [4].
Water ice deposits: To identify anomalies with a
high content of WEH and delineate deposits with a
maximum content of water ice, points with a WEH
content of less than 0.1 wt.% were removed from the
data set. Filtering measurements by this criterion
reduced the number of points in the northern polar
region by 44.8%, and in the southern polar region by
45.6%. An array of data with a content of more than 0.1
wt.% in the ARCGIS system was processed using a
triangulation model, which allows you to create a
continuous surface from discrete values. The resulting
triangulation model was converted into a raster format
with a grid spacing of 625 m, which was superimposed
on a digital elevation model (DEM) of the Polar Regions
with the same resolution (Fig. 1).
After processing the resulting map in the ARCGIS
system, five discrete categories of WEH content were
outlined: V>0.1-0.2, VI>0.2-0.3, III>0.3-0.4, II>0.4-
0.5, I>0.5-0.53 wt.% (Fig. 2) and the areas occupied by
them were calculated (Table 1). Obviously, at the stage
of lunar exploration, the categories of water ice deposits
with an ice content of >0.3 wt.% are of greatest interest.
These categories of deposits were manually outlined
from pixel to pixel (Fig. 3, 4) and then the occupied area
of each of the deposits of these categories was
estimated.
Fig. 1. DEM map with a resolution of 625 m (gray
color) with overlaid converted WEH data in the range
of 0.3-0.4 wt.% with a similar resolution (blue color).
Dots show the distribution of WEH primary data.
Fig. 2. Distribution map of the distinguished categories
of WEH content in the Moon Polar Regions.
To estimate the predicted reserves of water ice, a
similar two-layer model of the distribution of the
epithermal neutron count rate in the lunar regolith [4, 5]
was used: a dry layer of regolith 10 cm thick on top, and
1089.pdf54th Lunar and Planetary Science Conference 2023 (LPI Contrib. No. 2806)

a uniform distribution WEH at a depth of 0.1–1 m. The
average soil density in a layer from 0.1 to 1.0 m is taken
equal to 1500 kg m-3 [6]. The average content of water
ice in the deposits for the assessment of reserves will be
taken equal to the minimum content in the selected
categories: I - 0.51 wt.%, II - 0.41 wt.%, III - 0.31 wt.%,
IV - 0.21 wt.% and V - 0.11 wt.%. The total probable
reserves in the North Polar Region are estimated at
about 8.8×108 tons, and in the South Polar Region about
7.6×108 tons (Table 1).
Table 1. Total predicted reserves of water ice in the
Moon Polar Regions
I-V
WEH, wt.%
S, km2
Ice, т
North Polar Region
II
0.41
126.6
700731
III
0.31
10785.2
45136062
IV
0.21
95649.2
271165482
V
0.11
380454.3
564974636
Total
881976911
South Polar Region
I
0.51
159.0
1094715
II
0.41
1 435.7
7946600
III
0.31
7 138.1
29872949
IV
0.21
86241.8
244495503
V
0.11
318705.9
473278262
Total
756688029
Fig. 3. Ice deposits with content >0.3 wt.% in the lunar
soil in the Moon North Polar Region.
Fig. 4. Ice deposits with content >0.3 wt.% in the lunar
soil in the Moon South Polar Region
In the northern Polar Region, 28 deposits of category
III and 1 deposit of category II were identified with an
area from 20 km2 to 1400 km2 and ice reserves from 85
tons to 6.5×106 tons (Fig. 3). In total, the predicted ice
reserves in the deposits in the Northern Polar Region are
estimated at about 4.6×107 tons. In the Southern Polar
Region, 21 deposits of category III, 3 deposits of
category II, and 2 deposits of category I with an area
from 20 to 1742 km2 and ice reserves from 83 to 7.3×106
tons were identified and delineated. In total, the
predicted ice reserves in South Polar Region deposits
are estimated at about 3.9×107 tons (Fig. 4).
Summary: The obtained estimates of the total
predicted water ice reserves in the both polar regions
according to the data of the LEND (1.6×109 tons) (Fig.
2, Table 1) agree quite well with the estimates obtained
earlier using the LPNS data (3×109 tons) [1]. The
estimate of probable reserves in the North Polar Region
(8.8×108 tons ) is also quite close to obtained earlier data
on the radar survey of the Chandrayan-1 Spacecraft
(6×108 tons) [7].
References: [1] Feldman W.C. et al. (1998) Science
281, 1496–1500; [2] Lawrence D. J. et al. (2006) J.G.R.
111(E08001); [3] Mitrofanov I.G. et al. (2010)
Sp.Sci.Rev. 150(1-4), 183–207; [4] Sanin A.B. et al.
(2017) Icarus 283, 20-30; [5] Litvak M.L. et al. (2016)
Plaet.Sp.Sci. 122, 53–65; [6] Slyuta E.N. (2014)
Sol.Sys.Res. 48(5), 330-353; [7] Spudis P.D. et al.
(2013) J.G.R. Pl. 118, 2016–2029.
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