IEEE - Aerospace and Electronic Systems - March 2020 - 57

Hardgrove et al.

Figure 1.
Top plot shows simulated lunar neutron energy spectra varying
with weight percent of water-equivalent-hydrogen (WEH). The
bottom plot shows the fractional reduction in epithermal neutron
counts with WEH wt%.

LunaH-Map will directly observe the epithermal neutron
contributions from within and outside PSRs. These epithermal neutron maps will contribute to our understanding of the geologic evolution of volatiles on the Moon
and can inform future landed missions to the lunar South
Pole that are focused on finding volatiles for resources.
Understanding the bulk, small-scale hydrogen abundances on the Moon has the potential to reveal the nature of
PSRs elsewhere in the Solar System, as similarly shadowed regions on Mercury have been observed to contain
substantially more volatile than those on the Moon [21].
The source of hydrogen near the lunar surface is not
well understood, although several methods for its production and redistribution to the poles have been proposed [6], [7]. Hydroxyl and H2O can be implanted
onto airless planetary surfaces by the solar wind, and
these H-bearing phases can then migrate toward the
poles by impacts or other processes, becoming trapped
in regions of permanent shadow near the lunar poles
[24]. The Moon Mineralogy Mapper (M3) on the Indian
spacecraft Chandrayaan-1 [29], the Visible and Infrared
Mapping Spectrometer (VIMS) on Cassini [4] and the
High Resolution Instrument-Infrared spectrometer on
Deep Impact [36] have all reported evidence for the
presence of OH/H2O in mid-latitude regions on the
MARCH 2020

Moon based on the presence of a 3.0 mm absorption
feature. While these observations clearly demonstrate
that the Moon is not as dry as we once thought, the
source of this water is still open to debate. Implantation
of solar wind H and impacts by comets and water-rich
asteroids may have played a role at various times
throughout the geologic history of the Moon [24].
Neutron spectroscopy is a powerful tool and has
become a standard in planetary science because the
energy distributions of thermal (<0.4 eV) and epithermal
neutrons (0.4-105 eV) that reach the detector are highly
dependent upon the hydrogen content of the top meter of
a planetary surface (Figure 1). LP was the first planetary
mission to carry a neutron spectrometer, but many have
followed, including Mars Odyssey, the MErcury Surface,
Space ENvironment, GEochemistry, and Ranging mission, LRO, the Dawn mission to Vesta and Ceres and the
Dynamic Albedo of Neutrons instrument on the Mars
Curiosity Rover ([9], [13]; Boynton et al., 2004; [17],
[26], [27], [31]). The LunaH-Map Miniature Neutron
Spectrometer (Mini-NS) is the first planetary mission to
use CLYC (Cs2LiYCl6:Ce), an inorganic scintillator with
elpasolite crystal structure for neutron detection [16]. The
CLYC detectors are packaged into an array making up a
relatively large surface area (200 cm2). The high efficiency of CLYC for epithermal neutrons, coupled with
the low-spacecraft periapse at the Moon's South Pole will
enable LunaH-Map to create maps of hydrogen within
$5 degrees of the pole that include contributions from
within the lunar South Pole PSRs. Figure 1 shows the
strong sensitivity of epithermal neutrons to hydrogen content. The primary science goal of the LunaH-Map mission
is to evaluate the uniformity of hydrogen across the lunar
South Pole. The Mini-NS has been designed with enough
sensitivity to map hydrogen with good statistical confidence ($20% relative) at levels as low as 0.6% WEH
($600 mg/g H) at spatial scales 15 km2. Preliminary
analyses based on the LunaH-Map science orbit demonstrate the contribution of PSRs to the epithermal neutron
signal from a set of low-altitude passes over the lunar
South Pole. For a set of five orbit tracks, Figure 2 shows a
preliminary assessment of the contributions from PSRs
(red) to the observed epithermal neutron count rate. For
the small ($kilometer) PSR labeled, the contribution of
the PSR to the neutron count rate reaches $60%.

MISSION OVERVIEW
The LunaH-Map spacecraft is equipped with a low-thrust
ion propulsion system, gimbaled solar arrays, three reaction wheels, a star tracker, an X-Band radio, a command
and data handling system, a power control system, and
a miniature neutron spectrometer (Duncan C., 2015,
Cheung, K.M., 2015). After deployment, LunaH-Map will

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