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DETERMINING LUNAR REGOLITH WATER CONTENT USING PERMITTIVITY
MEASUREMENTS WITH THE LUNAR VOLATILES SCOUT. C. Gscheidle1, S. Sheridan2, L. Richter3
and J. Biswas1, 1Institute of Astronautics, Technical University of Munich, Boltzmannstr. 15, 85748 Garching,
Germany (c.gscheidle@tum.de), 2School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA,
United Kingdom, 3OHB System AG, Manfred-Fuchs-Straße 1, 82234, Weßling, Germany.
Introduction: In recent year, missions to the
Moon have indicated that significant amounts of hy-
drogen and potentially water could be found there [1].
Especially the Lunar Crater Observation and Sensing
Satellite (LCROSS) has measured a water content of
5.6±2.9 w% for the Cabeus crater [2]. Nevertheless,
confirmation of these findings requires ground-truth
data. The Lunar Volatiles Scout (LVS), which is cur-
rently developed at the Technical University of Mu-
nich (TUM), can potentially provide this valuable
ground-truth data [3].
Lunar Volatiles Scout: The LVS is an instru-
mented drill designed for future mobile in-situ extrac-
tion, detection and analysis of lunar volatiles. A sche-
matic of the instrument is given in Figure 1. By heat-
ing the regolith to high temperatures, water and other
volatiles can be extracted and identified with an inte-
grated mass spectrometer. However, accurately deter-
mining the abundance of water in the regolith remains
difficult. To address this issue, a lunar regolith per-
mittivity measurement could be added to the LVS’ ca-
pabilities. In principal, the change in moisture content
can be detected by comparison of the LVS’ capaci-
tance between heating rod and drill shell before and
after heating the regolith. Both liquid water and water
ice have a significantly higher static relative permit-
tivity ( [4]) than both lunar regolith (
[5]) and vacuum ( ), which results in a
measureable change in capacitance when water has
been extracted from the probe volume. The usage of
permittivity measurements to analyze water content is
also included in ESA’s PROSPECT instrument pack-
age [6] and has been used for various applications
[7,8].
Theoretical Considerations: In this context, the
LVS functions a cylindrical capacitor with the heating
rod and drill shell as electrodes (see Figure 1). The
capacitance of the system is proportional to the inte-
rior material’s permittivity ε [9], which is a function
of the interior’s components individual permittivity.
Multiple mixing rules for the permittivity exist [10].
For simplicity, the ideal permittivity of a mixture
was chosen with being the volume
fraction of component [11]. The capacitance C of the
LVS system is then
()
with component height z, inner radius , outer radius
, and s denoting sand, w denoting water and v de-
noting vacuum (or air). When charging the capacitive
system with voltage , the ideal capacitor voltage
over time is described by
()
with being the time constant. Here, R is the
charging resistance. The water content can then be de-
termined by measuring the transient charging curve
and using Equation 1 and 2 to calculate .
Experimental Setup: The experiment setup is de-
picted in Figure 2. It consists of a concentric metal
tube and metal rod, which is hold in place by a non-
conductive 3D-printed structure. The rod is insulated
P1
P0
Ion Trap Mass Spectrometer
Reference Gas
System Orifice
Pressure
Sensors
Heating Rod
Augered
Drill Shell 100 mm
38 mm
Figure 1: LVS Schematic. Drill Shell (blue) and
Heating Rod (red) form the capacitive system.
Figure 2: Experimental Setup: LVS system con-
nected to read-out electronics and on precision
scale. Quartz sand and distilled water in the back-
ground.
towards the interior to prevent parasitic currents. The
interior of the tube can be filled with material of inter-
est. For simplicity and better availability, we used
quartz sand as analogue material for the pretests and
tracked the amount of water in the probe with a preci-
sion scale. Water content was chosen to be in the same
magnitude as the LCROSS measurements (see Table
1, [2]). Experiments with lunar regolith simulants are
planed.
The electronics consist of a commercial microcon-
troller charging the system with and reading a
buffered 10-bit analog input, which returns ADC val-
ues n. We used a charging resistance.
Experiments were conducted where the LVS sys-
tem was filled with different components. Four cases
have been selected to demonstrate functionality: (1)
empty, (2) filled with dry sand and (3) filled with wet
sand. For each experiment, ten measurements were
made. Figure 3 shows mean charging curves of the
system resembling the LVS. Additionally, exponen-
tial curves of form
have
been fitted to the data using a Nonlinear Least Square
method to get the time constants of the system.
Results: Mean values and standard deviations for
the time constants from selected experiments are
given in Table 1, alongside analytically calculated
values. Time constant correlates with both more mass
and higher water content. The experiments’ charging
curves also display distinct differences (see Figure 3).
A linear fit to measured over calculated time constants
returns a nearly unity slope and an offset of around
, possibly due to the measurement technique.
The linear dependency clarifies that the geometry of
the LVS can be adapted to function as capacitive sys-
tem with a dependency of the time constant to the ca-
pacitance of the contained material. This should in
principal hold true for moist lunar regolith. By meas-
uring relative change in capacitance instead of abso-
lute values, the system is also quite insensitive to ex-
ternal disturbances and uncertainties, such as geomet-
ric misalignments due to launch.
Additionally to measuring the water content, the
capacitance of the empty LVS system could be used
to determine the amount of regolith in the LVS. Reg-
olith and vacuum themselves have different permittiv-
ities which result in different capacities for different
filling heights. Experiments investigating this corre-
lation show promising results, while this relevant
property is otherwise hard to determine.
In general, the experiments show that the investi-
gated method works and that it can be applied to the
LVS. By measuring the permittivity before and after
the heating process, the LVS is able to determine the
amount of water in lunar regolith.
Additional Investigations: Nevertheless, there
are still major issues with the system, which are cur-
rently being analyzed with experiments and simula-
tions. Firstly, proper integration of in the LVS is being
investigated. Secondly, precise calibration of the sys-
tem with internal parasitic resistances and initial con-
ditions under thermal vacuum conditions can yield
better insight. The current read-out electronics are
neither flight ready nor capable of frequency depend-
ent permittivity measurements. Including analyses of
frequency behavior could also increase the science
yield of the instrument. Finally, the selection of the
mixing rules, especially with water ice, should be re-
vised and a reliable model for the permittivity of lunar
regolith are being developed to ease result interpreta-
tion.
References: [1] Crawford I. A. (2015) Progress
in Physical Geography: Earth and Environment, 39,
137-167. [2] Colaprete A. et al. (2010) Science, 330,
463–468. [3] Biswas J. et.al. (2020), Planetary and
Space Science, 181, 104826. [4] Uematsu M. and
Frank E. U. (1980) Journal of Physical and Chemical
Reference Data, 9, 1291-1306 [5] Chung D. H. et al.
(1972) Lunar and Planetary Science Conference Pro-
ceedings, 3, 3161-3172. [6] Sefton-Nash E. et al.
(2018) European Lunar Symposium [7] Soltani M.
and Alimardani F. (2012) Journal of Food Science
and Technology, 51, 3500-3504 [8] Lethuilier A.
(2016) PhD Thesis [9] Lehnert G. (2018) Elektromag-
netische Feldtheorie [10] Amooey A. A. (2013) Jour-
nal of Molecular Liquids, 108, 31-33 [11] Reis J.C.R.
et al. (2009) Chemical Physics, 11, 3977–3986
Figure 3: Preliminary Results: Charging Curves.
ADC read-out over time of the LVS system for dif-
ferent materials. ‘Empty’ and ‘Full’ refers to sand
filling, ‘Dry’ and ‘Wet’ to its water content with
the numbers indicating water content.
Table 1: Experiment data: Measured sand
mass and water content . Measured ()
and calculated () time constant for experi-
ments depicted in Figure 3.
Case
ms [g]
φw [%]
τm [µs]
τc [µs]
Empty
0
0
134±3
43
Dry
154
0
426±1
345
Wet 10
154
10
582±5
499
Wet 17
154
17
719±3
624