Geothermal heat flow is obtained as a product of the two separate measurements of geothermal gradient in, and thermal conductivity of, the vertical soi/rock/regolith interval penetrated by the instrument. Heat flow measurements are a high priority for the geophysical network missions to the Moon recommended by the latest Decadal Survey [I] and previously the International Lunar Network [2]. The two lunar-landing missions planned later this decade by JAXA [3] and ESA [4] also consider geothermal measurements a priority

Hedlund, M.

Nagihara, S.

Taylor, P. T.

Zacny, K.

NASA Technical Reports Server

COMPACT, DEEP-PENETRATING GEOTHERMAL HEAT FLOW INSTRUMENTATION FOR LUNAR 
LANDERS S. Nagiharai, K. Zacny', M. Hedlund', and P. T. TaylOr', iDepartment of Geosciences, Texas Tech 
University, Lubbock, TX 79409 (seiichi.nagihara@ttu.edu), 'Honeybee RoboticS, Pasadena, CA 91103, ]Goddard 
Space Flight Center, Greenbelt, MD 20711. 
Introduction: Geothermal heat flow is obtained as 
a product of the two separate measurements of geo-
thermal gradient in, and thermal conductivity of, the 
vertical soiVrockiregolith interval penetrated by the 
instrument. Heat flow measurements are a high priority 
for the geophysical network missions to the Moon rec-
ommended by the latest Decadal Survey [I] and previ-
ously the International Lunar Network [2]. The two 
lunar-landing missions planned later this decade by 
JAXA [3] and ESA [4] also consider ·geothermal 
measurements a priority. 
One of the difficulties associated with heat flow 
measurement on the MOOD is that it requires excava-
tion of a relatively deep (-3 m) hole in order to avoid 
the long-term changes in lunar -surface thermal envi-
ronment affecting the subsurface temperature meas-
urements [2]. Such changes may be due to the 18.6-
year-cylele lunar precession [5, 6], or may be initiated 
by presence of the lander itself [7]. Therefore, a key 
science requirement for a lunar heat flow instrument 
for the future lunar missions is to penetrate 3 minto 
the regolith and to measure both thermal gradient and 
thennal conductivity. Engineering requirements ·are 
that the instrument itself has minimal impact on the 
subsurface thermal regime and that it must be a low-
mass and low~power system like any other science 
instrumentation on planetary landers. It would proba-
bly be impossible. to meet the engineering require-
ments, if the instrument utilizes a long probe driven 
into the ground by a rotary or percussive drill. 
Here we repon progress in the authors' effort to 
develop a new, compact lunar heat flow instrumenta-
tion that meets all of these science and engineering 
He gas is required for excavating 0.6 m in 22 seconds 
[9]. The near-vacuum envirolWlent of the lunar surface 
maxi~izes the mechanical force of the gas jet. 
t: Co.mposire 
Stem with-10 
inleWared RIDs 
to.. tbrtmal 
gradirnt 1nN-
suremen.t 
2:Penelraling 
Conr with The .. -
.... 1 Conductivity 
Needle 
3:: Drive Spool 
4: Pnrumatic 
Spool 
5: duid~ Bushing 
and8ru5hes 
requirements. _ _ . __ ."' ____ . _ . . ___ __ __ " _ ___ . _ _ . __ .. _ _ .. _ _ "_. _ _ _ - - _ .. _--- .~. J J ~ J. 
J . ~ .. - ---- -· -The P roboscis Excavation System: The recently 
developed 'proboscis' excavation system [8] can large-
ly meet the low-power, low-mass, and the depth re-
quuements. The excavation system utilizes a stem 
which winds out of a pneumatically driven reel and 
pushes its conical tip into the regolith. Simultaneously, 
gas jets, emitted from the cone tip, loosen and blow 
away the soil (Fig. 1). In its current design, the stem is 
primarily made of glass fiber for its mechanical 
strength and relatively low thermal conductivity. He-
lium gas is used for the jet, because it is commonly 
available for planetary landers in pressurizing the pro-
pellant tank. Lab tests using an earlier proboscis mod-
el in a vacuum chamber have shown that only 8 g of 
Figure 1: Top: A conceptual drawing of the proposed 
heat flow instrumentation attached to a leg of a lunar 
lander. Bottom: More detailed schematics of the major 
components of the heat flow system. 
The In-situ Thermal Conductivity Probe At-
tached to the Proboscis Excavation System: A typi-
cal thermal conductivity probe used for terrestrial soil 
samples (the so-called 'needle probe') consists of a 
thin metal tube of - 2-mm diameter and -S-cm length, 
which contains a linear electric heater along its length 
and a temperature sensor (e.g .• thermistor) at its center. 
When the probe is inserted into the soil, it heats up and 
https://ntrs.nasa.gov/search.jsp?R=20120015991 2019-08-30T22:52:21+00:00Z
monitors the temperature increase [10]. The measure-
ment theory requires that the length of the probe is 
much greater than its diameter and that the probe is 
made of highly conductive material. In such a config-
uration, one can assume that the heat diffuses away 
through the soil dominantly in the radial direction from 
a line heat source, and that temperature of the probe is 
always the same as that of the soil in contact with the 
probe. Then, the thermal conductivity can be an alge-
braic function of the heat input and the logarithmic rate 
of the temperature rise: 
Q dOn t) 
K=-·-- (1) 
4rr dT 
where K is the thermal conductivity, Q is the heat gen-
erated per unit length of the probe, T is the tempera-
ture, and t is the time. 
The thermal conductivity probe for the new heat 
flow system is attached to the tip of its penetrating 
cone (Fig. 2). In order not to diminish the excavation 
efficiency, the probe is short (I-cm). The probe has a 
diameter of3-mm in order to insure good thermal con-
tact with powdery regolith materials in vacuum, and 
for mechanical strength. The penetrating cone in its 
current design is made of a low-conductivity plastic in 
order to thermally insulate the probe from the rest of 
the instrument. The short needle contains a platinum 
wire-wound resistance temperature detector (RTD), 
and a thin heater wire which wraps around the cylin-
drical ceramic casing of the RTD. 
During a deployment, when the penetrating cone 
reaches one of the depths targeted for thermal conduc-
tivity measurement, it stops blowing gas, and the stem 
pushes the short probe into the yet-to-be excavated, 
undisturbed bottom-hole soil. Then, it begins heating 
and monitors the temperature rise. When, the meas-
urement is complete, the system resumes excavation. 
RTD 
r 
, ":: caU 
-cigur~ 2: Photographs of the prototype thermal con-
ductivity probe. 
Thermal Conductivity Experiments: A prototype 
of the 'short thermal conductivity probe (Fig. 2) has 
been tested with lunar regolith simulant JSC-IA placed 
in a vacuum chamber for various air pressures. The 
container of the simulant was large enough to accom-
modate two probes inserted, -8 cm apart. One was the 
new short probe and the other was a standard thermal 
conductivity probe (Decagon KD2 Pro) with 2.4-mm 
diameter and 10-cm length. The two probes were far 
enough apart to allow simultaneous heating experi-
ments. Data from the latter probe yielded thermal 
conductivity of JSC-IA as a function of chamber pres-
sure (Fig. 3). 
0.3 
" 
bl.~ 
! 
~ 0.2 
~ 
-6:°.15 t. 
c 
8 0.1 :.!' __ 
-;; '. E ' ! 0.05: 
I- . 
o . 
• 
• 
• 
."-
! -
, - - - - - - - -Pressure (Torr) 
Figure 3: Thermal conductivity of the JSC-lA lunar 
simulant measured for a range of chamber pressures. 
The simulant was well compacted before the meas-
urements. 
,. .• :.c, ..• ,. , 
..... e. D .~. " ... ' . " . ..,iI , + heater on Neater off ~ 
2G 30 40 
Time [min.) -0 o 10 so 
Figure 4: Temperature records from 6 heating tests of 
the short probe shown in Fig. 2. Each curve represents 
one test result conducted at a fixed chamber pressure. 
The thermal conductivity of the regolith at that pres-
sure is noted for each of the curves dra\VIl. 
For each set of heating experiments at a fixed pres-
sure, the short probe was heated for 30 minutes (Fig, 4) 
with a constant power of SO mW. The length/diameter 
ratio of the short needle is not large enough to allow 
direct application of the standard needle probe tech-
nique (Eq. 1) . However, it can be seen that, for each 
heating experiment, there- is a linear relationship be~ 
tween the temperature and the natural log of time after 
-4 minutes of heating (Fig. 5): 
T = C In t + To (2) 
where To is the initial temperature and C is a constant. 
If this were for a standard, long needle probe, C is 
equivalent to QI(4nK). For the short probe, such rela-
tionship does not necessarily hold. However, it gives 
hope that the logarithmic rate of temperature increase 
(C) for the short probe may be inversely proportional 
to the thermal conductivity of the medium being meas-
ured. In other words, the relationship between these 
two quantities may be similar to Eq. 1. We have ob-
tained the product of C and the thermal conductivity 
obtained 'by the standard probe (K) for each set of ex-
periments (Table I). The CK values for pressures less 
than 20 Torr Or thermal conductivity values of 0.1 
W/IDK are remarkably similar. Within the low pres-
sure, low themtal conductivity range, C and K are in-
deed inversely proportional. Therefore, it is possible to 
uniquely determine the thermal conductivity of the 
medium from knowledge of the temperature increase 
with time, if the CX value has been pre-determined for 
the probe by a series of calibration experiments. 
16 ."-'.- -_ .• _,.. ... . ----,--.-.. . -... . ,,- - _ . .. _..,..._._. --.. -· ....... -----.---1 
M.nP9tlm~ 
............. I i'" ••••• , 
i 12t a •• ••••••••• .. •• 0.055 W/mK -------------·'1: 
~ . .................................. ! 
.a 10r.· .•.•.. 
i .............. ~W~~~.WJWW~~.~J
! 8C~W~~ .· · · · .. ······~··W?·~·~!.~~··JJJJ
6 ~ •••••••• ... ~·.!~.~.~mlR _ j t·· .•••.. 
! . 0190 WlmK ! 
4iJ.~J. . I 
u U ~T ~a U 6 ~ U U U U 
Ln (tlmo) 
Figure 5: Temperature records from the short probe 
heating tests replotted in the natural log time scale. 
Time is in seconds. The slope of each line corresponds 
to the parameter C described in the text. 
Table 1: ex values obtained from the heating tests of 
b Ii 'f£ K I the short pro e or JSC-l A at dl erent va ues. 
Pres. 2 5 10 20 50 100 
K 0.039 0.055 0.081 0.109 0.157 0.190 
CX 0.146 0.146 0.147 0.146 0.136 0.126 
Uruts for pressure and thermal conducllV1ty are Torr 
and W IIDK, respectively. 
Discussion and Conclusions: In-situ thermal con-
ductivity of IUDar regolith has been previously reported 
to be 0.009 to 0.013 W/IDK at Apollo 15 and 17 sites 
[II]. In our lab experiments, we were not able to low-
er the chamber pressure below 2 Torr to duplicate the 
condition on the Moon. It is still noteworthy that, at 
the lowest thermal conductivity values achieved for the 
JSC-I A simulant, the short probe yielded the best per-
formance. For thermal conductivities between 0.039 
W/IDK and 0.109 W/IDK, the CXvalues were constant 
(Table I) . Within this range, it is possible to obtain the 
thermal conductivity simply as: 
K = 0.146 x d~;tF (3) 
Given how constant C'K is in this range (Table I), it 
may be possible to determine thermal conductivity 
within:!: 0.001 W/IDK. Whether or not this relation-
ship holds at lower pressures needs to be test in future 
studies. 
The use of empirically obtained C'K is very similar 
to the approach taken by the investigators of the Apol-
lo Heat Flow Experiments [12]. Their 50-cm long, 
2.54-diameter probe was heated only at short (-2.5 
cm) sections for thermal conductivity measurements. 
The conveotional line heat source model CEq. I) was 
not applicable. The investigators empirically obtained 
CX for their probes by carrying out a series of lab ex-
periments. 
Prior to the present study, two types of compact in-
situ thermal conductivity systems were proposed for 
low-mass lunar robotic missions. One was a burton-
shaped device containing a heater-RTD assembly, im-
bedded in the outer casing of a bullet-shaped penetra-
tar (O.8-m length and 0.15-m diameter) dropped from a 
Lunar-orbiting spacecraft [13]. The other was a heat-
er-RTD assembly built into the casing of a 'mole' self-
hammering system deployed from a lander [14 , 15]. 
Either of these previous approaches has . difficulty 
in achieving high accuracy in thermal conductivity 
measurement, mainly because-- temperature measured 
by its RID is heavily influenced by that of the instru-
ment body to which the sensOr is built/attached. The 
instrument body has a much larger heat capacity and 
thermal inertia than the temperature sensor itself. The 
instrument body is 2 or 3 orders of magnitude more 
thermal conductive than lunar regolith. Therefore, 
temperature felt by the senSOr may be closer to that of 
the instrument body reacting to the self heating than 
that of the soil/regolith in contact. 
The present design of inserting a small, low-heat-
capacity probe into regolith significantly reduces the 
thermal inertia problem. Also, in this design, the probe 
is relatively insulated from the rest of the instrument 
body. This way, the RTD senses the temperature of 
the regolith more accurately and responds more quick-
ly to temperature changes. It has further advantage in 
that the small probe causes less mechanical disturbance 
to the regolith than the penetrator (free-falling into the 
regolith) or the mole (hammering and compacting the 
soil) does. FinaUy, the small probe does not require as 
much heater power in making a thermal conductivity 
measurement, because its heat capacity is much less. 
Acknowledgments: This work is supported by the 
Planetary Instrument Definition and Development pro-
gram of NASA. 
References: [I] National Research Council (2011) 
pub# 13117. [2] Cohen, B. A. et al. (2009) ILN Final 
Report. [3] Tanaka et al. (2012) LPSC XXXXlII 1651. 
[4] ESA Lunar Lander Team (2012) Exploration Sci-
ence Requirements Document. [5] Wieczorek, M. A. 
and Huang, S. (2006), LPSC XXXVII, 1682. [6] Saito, 
Y. et al. (2006), Bull. Japanese Soc. Planet. Sc. 16, 
158- 164. [7] Kiefer, W.S. (2011) Planet Space Sc., 60, 
155-165. [8] Zacny, K. et al. (2011) LEA G 2028. [9] . 
Zacny et al. (2009) LPSC XXXX 1070. [10] 
Beardsmore, G. R. and Cull, J. P. (2001) Crustal Heat 
Flow, Cambridge Univ. Press. [II] Langseth er al. 
(1976) LPSC VJJ3143-3171. [12] Langsetetal. (1972) 
Apollo 15 Prelim. Sc. Rep. 11-1-11-20 . [13] Mizutani 
et al. (2003), Adv. Space Res. 21, 2315-2321. [14] 
Seweryn et al. (2008) LPSC XXXIX 1957 [IS] Spohn et 
al. (2012) LPSC XXXXIII 1445. 


Compact, Deep-Penetrating Geothermal Heat Flow Instrumentation for Lunar Landers

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