Resolving the Spatial and Depth Dependent Hydrogen Distribution

46th Lunar and Planetary Science Conference (2015)
Resolving the Spatial and Depth Dependent Hydrogen Distribution within Lunar Permanently Shaded Regions Using the Lunar Polar Low-Altitude Neutron Experiment. David J. Lawrence1, Richard C. Elphic2, Richard S. Miller3, Patrick N. Peplowski1, 1Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
([email protected]); 2NASA Ames Research Center, Moffett Field, CA; 3University of Alabama in
Huntsville, Huntsville, AL, USA.
Introduction: Lunar permanently shaded regions
(PSRs) are fascinating environments. Their low temperatures (<100K), which have persisted for geologically long periods of time, permit trapping and sequestration of solar system volatiles. Predictions dating
back to the 1960s and 1970s proposed that lunar PSRs
would have enhanced water concentrations [1,2]. Subsequent spacecraft and Earth-based measurements using various techniques have provided evidence to support these predictions [e.g., 3,4,5]. However, the volatile content of lunar PSRs is substantially less than
what is seen within similar environments on Mercury
[6,7]. The reason for this difference is not understood.
The characteristics of PSRs and the processes that take
place in them have implications for a variety of topics
such as the origin and history of solar system volatiles
[8], synthesis of organic materials [9], and in-situ resources for human exploration.
Despite the fact that initial measurements of lunar
PSRs have been made, many aspects of PSRs are not
well understood. In particular, our knowledge of the
spatial distribution and depth dependence of hydrogen
concentrations at the lunar poles is tantalizingly incomplete. Here, we update a previous study [10] by
futher characterizing the extent to which a lowresource orbital mission can achieve significant improvements in our knowledge of the lunar polar hydrogen distribution. Improved knowledge of the polar
hydrogen spatial and depth distribution will provide
key input to studies of PSR volatile processes by isolating individual craters that may host enhanced hydrogen and can enable discrimination of various hypotheses that explain the formation and geologic history of the polar volatiles [11]. Data from such a mission will assist future landed missions that seek to target landing sites with volatile enhancements [12].
Hydrogen Measurements of Planetary Surfaces:
Polar hydrogen enhancements were first measured on
the Moon using the Lunar Prospector Neutron Spectrometer (LP-NS)[3]. The LP-NS was an omnidirectional detector whose spatial resolution was proportional to its orbital altitude (~30 km), and as a consequence its spatial resolution was sufficiently broad that
individual hydrogen enhancements were generally not
tied to specific PSRs. Nevertheless, combined measurements of epithermal and fast neutrons have been
used to show that bulk hydrogen enhancements in
Shackleton crater at the Moon’s south pole reach to the
surface [13], in contrast to other polar regions where
the hydrogen enhancements are likely buried by tens of
cm of dry soil [14].
To obtain higher spatial resolution measurements,
the Lunar Reconnaissance Orbiter (LRO) spacecraft
carried a collimated neutron detector known as the
Lunar Exploration Neutron Detector (LEND)[15],
which was planned to quantify hydrogen concentrations at a spatial resolution of 10 km near both lunar
poles. In light of the difficulties associated with improving orbital neutron measurements with neutron
collimation [16–20], we have investigated the benefits
of conducting a mission whose altitude over one of the
poles is significantly lower than that of the LP-NS
measurements. In this scenario, omnidirectional neutron measurements can provide improved the spatial
resolution proportional to the ratio of the respective
Lunar PLANE Mission: The Lunar Polar LowAltitude Neutron Experiment (PLANE) can be accomplished with simple neutron sensor on a small spacecraft and would make high spatial resolution measurements of the Moon’s south pole. Spatially resolved
bulk hydrogen concentrations are measured using epithermal neutrons; spatially-resolved depth-dependent
hydrogen concentrations are obtained with fast neutrons. A borated plastic scintillator makes measurements of both epithermal and fast neutrons [21].
The nominal mission scenario provides for lowaltitude (<20 km) passes over the lunar south pole with
higher-altitude apoapsis (~200 km) values over the
north pole to provide orbital stability. The spatial resolution performance of the Lunar PLANE mission is
driven by two primary factors: 1) altitude dependent
spatial footrprint; and 2) accumulation-time dependent
statistical sensitivity. To assess the statistical sensitivity, we use the likelihood ratio method that was designed for analysis of LP and LRO neutron data [19].
This method combines the expected accumulation time
with point-to-point statistical uncertainties over a given
spatial pixel size. Fig. 1A shows a polar map of relative epithermal neutron signal magnitudes that would
be measurable on 15 km sized pixels with a statistical
significance threshold of three sigma. A 30 ppm hydrogen sensitivity contour is shown, which illustrates
that highly sensitive hydrogen concentration measurements would be obtained poleward of 82ºS. Similar
sensitivities for fast neutrons on pixel sizes of 30 km
46th Lunar and Planetary Science Conference (2015)
are shown in Fig. 1B. The red contour line shows a
hydrogen sensitivity contour of 200 ppm. Because
epithermal and fast neutrons probe hydrogen deposits
to different depths, a combined analysis enables spatially resolved stratigraphy determination and/or depth
constraints to be placed on identified hydrogen enhancements [7,12]. We estimate that hydrogen concentrations of 200 ppm could be detected and quantified under a dry covering to depths of 25 cm with a
spatial resolution of 30 km.
The combined effects of spatial footprint and statistical precision are assessed using a simulated neutron
count rate map where we used the spatiallyreconstructed neutron count rate map of [20] as an
assumed ground truth. To simulate the neutron measurements for the Lunar PLANE mission scenario, we
use neutron transport simulations that have been validated for prior planetary missions [7]. Fig. 2A shows
the simulated count rate map and Fig. 2B shows the
measured map derived from LP-NS data. All of the
largest PSRs are spatially resolved, which is in contrast
to the measured LP-NS data that show a much broader
spatial resolution.
Conclusions: The Lunar PLANE mission concept
provides a high-heritage, low-risk means to significantly improve our knowledge of the spatial and depth
dependent hydrogen distribution at the Moon’s south
pole. With the performance presented here, we would
unambiguously identify the hydrogen concentrations
both inside and outside PSRs as well as provide spatially resolved hydrogen depth measurements over
large portions of the lunar south pole.
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Fig. 1. Contours of statistically significant neutron signal
reductions poleward of 80ºS using the Lunar PLANE accumulation times for a six month mission. A) Epithermal neutron signal reductions for 15 km sized pixels. All measurements inside the red contour indicates a hydrogen sensitivities better than 30 ppm. B) Fast neutron signal sensitivities
for 30 km sized pixels. All measurements inside the red contour indicates a hydrogen sensitivities better than 200 ppm.
Fig. 2. Epithermal neutron count rate maps poleward of 80ºS.
A) Simulated map of Lunar PLANE count rates where the assumed distribution is based on [20]. B) Measured LP-NS data.