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Annual thermal amplitudes and thermal detection of Southwestern U.S. Caves: Additional insights for remote sensing of caves on Earth and Mars

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  • U.S. Geological Survey
Conference Paper

Annual thermal amplitudes and thermal detection of Southwestern U.S. Caves: Additional insights for remote sensing of caves on Earth and Mars

ANNUAL THERMAL AMPLITUDES AND THERMAL DETECTION OF SOUTHWESTERN U.S.
CAVES: ADDITIONAL INSIGHTS FOR REMOTE SENSING OF CAVES ON EARTH AND MARS.
J. J.
Wynne
1,2
, T. N. Titus
1
, C. A. Drost
1
, R. S. Toomey III
3
and K. Peterson
4
,
1
U.S. Geological Survey, 2255 North Gemini Dr.,
Flagstaff, AZ 86001 (jut.wynne@nau.edu),
2
Merriam-Powell Center for Environmental Research, Department of Biological
Sciences, Northern Arizona University, Box 6077, Flagstaff, AZ 86011,
3
Mammoth Cave International Center for Science and
Learning, Mammoth Cave National Park, Box 7, Mammoth Cave, KY 42259, and
4
Department of Geography, University of
New Mexico, Albuquerque, NM 87131.
Introduction: Recent research has demonstrated
that cave-like features can be detected in the thermal
infrared on both Earth and Mars [1,2]. Caves are op-
timally detectable when the temperature contrast is
greatest between surface and cave entrance [1].
Our understanding of thermal behavior and ther-
mal detection of caves is still in its infancy [1,3]. Re-
solvability of caves via thermography is influenced by
several factors including cave volume, horizontal
length, depth from surface, percentage of rock ob-
structing the entrance, slope, aspect, geographic loca-
tion, elevation, topographic roughness, and geologic
substrate [1]. Currently, effects of these variables on
the thermal behavior of caves (and thus their detect-
ability) are not well understood. Detectability of caves
is also influenced by (i) the size of cave entrance vs.
the sensor’s spatial resolution, (ii) the precision of the
thermal measurements vs. the strength of the thermal
signal associated with the cave entrance, and (iii) the
viewing angle of the platform relative to the slope
trajectory of the cave entrance [1]. Consequently,
some caves are more easily detectable than others,
some will be detectable only at specific times of day,
while other caves may not be detectable at any time
[1,3].
Potential Importance of Martian Caves. (A) Caves
may be important in detecting evidence of extraterres-
trial life [4-6] because they offer protection from low
surface temperatures, unfiltered ultraviolet radiation
[4,5] and violent windstorms, which may degrade and
decompose organic materials. (B) A manned mission
to Mars will require access to significant H
2
O deposits
for drinking water, oxygen and liquid hydrogen fuel.
If water deposits exist, caves may provide the best
access to these resources [7]. (C) Future human explo-
ration and possible establishment of a permanent set-
tlement on Mars will require construction of living
areas sheltered from harsh surface conditions. Caves
with a protective rock ceiling would provide an ideal
environment where these shelters may be built [8].
The purpose of the work reported here is to further
our understanding of terrestrial cave thermal behavior,
particularly as it relates to detecting these features
using thermal remote sensing.
Results: We used thermistors to collect hourly
temperature measurements for at least one year at the
ground surface, entrance, and deep cave (dark zone) of
nine caves in the southwestern United States (seven
lava tube caves in western New Mexico and two lime-
stone caves in northern Arizona). For each cave, we
modeled temperature trends using Fourier analysis to
characterize thermal behavior, and line graphs to dis-
play temperature data to identify optimal times of de-
tection in the thermal infrared.
Figure 1: Temperature variation at Cathedral Cave, Arizona
with temperature trends over ~16 month period showing
surface (black dots), entrance (red line) and dark zone
(green line) temperatures and best-fit sine waves of surface
(solid blue line), entrance (blue dashed line) and dark zone
(blue dot-dashed line) are plotted.
Figure 2: Temperature variation at Braided Cave, New
Mexico with temperature data over ~14 month period. Color
schemes for temperature and best-fit sine waves are the
same as for Figure 1.
Study sites fell into three thermal behavior catego-
ries. Classic thermal behavior occurs when a cave is
in thermal equilibrium with the surface, and the deep
cave temperature is approximately equal to the annual
mean surface temperature. In pseudo-classic caves,
the mean annual temperature decreases from surface
Lunar and Planetary Science XXXIX (2008) 2459.pdf
to entrance and then levels off at the deep zone. Ice
cave behavior is consistent with a cave made of ice.
The annual mean temperature of the cave is near
freezing and the cave displays significant diurnal tem-
perature variations when the outside surface is snow
or ice covered.
Short-term data from two Chilean caves [1] pro-
vided insight into optimal times of day for detecting
these features in the thermal infrared. These longer
term data sets of southwestern U.S. caves enabled us
to identify broader windows for detection (i.e., seasons
and/or multiple times of day).
Figure 3: Temperature variation at Ice Cave, New Mexico
with temperature data over a ~13 month period. Color
schemes for temperature and best-fit sine waves are the
same as for Figure 1.
Discussion: Temperature data from these nine
caves provide much richer information on diurnal and
seasonal variation in cave and ground surface tem-
peratures. Analysis of the temperature patterns of
these caves contribute to a better understanding of
cave thermal behavior under a variety of conditions,
and how this behavior influences detectability of the
caves.
All of the New Mexico caves occurred within a
~20 km radius resulting in a high correlation of sur-
face temperatures across study sites. Differences in
surface temperature were related to the thermal inertia
of the surface material in which the data logger was
placed. Thermistors placed on solid rock surfaces
showed smaller diurnal and annual amplitudes while
sensors placed on unconsolidated soil showed larger
amplitudes. Temperatures measured at cave entrances
did not correspond well to physical characteristics of
either the cave or the surface, suggesting cave en-
trance data may reflect sensor placement within the
entrance rather than the actual thermal behavior of the
entrance.
Sensors placed within the dark zone provided tem-
peratures more representative of the cave interior.
This provided us with perhaps some of the best in-
sights into cave thermal behavior.
Classic thermal behavior occurs when the mean
deep cave temperature is approximately equal to the
annual mean surface temperature. We defined this
category as “classic” because previous research [e.g.,
9,10] suggests this is how caves behave thermally.
Only one of nine caves studied exhibited this behavior
suggesting this end member may be an outlier rather
than classic thermal behavior.
All caves had mean interior temperatures at least
10°C cooler than the surface mean. Four of the New
Mexico ice caves contained ice during most of the
year. This suggests these features may be in thermal
contact with a heat sink, such as large underground
ice deposits.
While this research lends additional support to the
viability of thermal detection of caves on both Earth
and Mars, it underscores there is still much to be
learned regarding cave thermal behavior. Our under-
standing of cave thermal behavior may be further im-
proved by (a) placing multiple sensors on the surface,
as well as within each cave entrance so that more ac-
curate thermal gradients from surface to dark zone can
be modeled, and (b) monitoring a larger number of
caves in different geographic and geologic regions to
better capture cave structure variability. These data
may then be used to develop thermal behavior simula-
tion models for Martian caves. This will ultimately
enable us to identify the range of conditions under
which caves are detectable in the thermal infrared,
thus improving our detection capabilities of caves on
both Earth and Mars.
Acknowledgements: Special thanks to J. Alford,
D. Billings, C. Gifford, T. Gilleland and D. Peterson
for data collection assistance, El Malpais National
Monument, Ice Caves Trading Company and Cathe-
dral Cave Preserve for study site access, and J. Blue,
G. Cushing, C. Gifford and R. Hayward for providing
comments on previous versions of this abstract. This
study was funded by NASA Exobiology grant
NNH04ZSS001N-EXB.
References: [1] Wynne, J. J. et al. (In Review) Diurnal
Thermal Behavior and Detection Techniques of Caves in the
Atacama Desert, Chile, Earth. Planet. Sci. Lett. [2] Cush-
ing, G. E. et al. (2007) GRL 34, L17201. [3] Rinker J. N.
(1975) Photogram. Eng. Remote Sensing 41, 1391-1400. [4]
Mazur, P. et al. (1978) Space Sci. Rev. 22, 3-34. [5] Klein,
H. P. (1998) JGR 103, 28463-28466. [6] Grin, E. A. et al.
(1998), LPS XXIX, Abstract #1012. [7] Baker, V. R. et al.
(1993) Ed. J. S. Lewis, Resources of Near-Earth Space
(University of Arizona Press, Tucson), p. 765-798. [8] Bos-
ton, P. J. et al. (2003), Grav. Space Biol. Bull. 16, 121-131.
[9] Cropley, J. B. (1965) Nat. Speleo. Soc. Bull. 27, 1-9.
[10] Pflitsch, A. and J. Piasecki (2003), J. Cave Karst Stud.
63, 160-173.
Lunar and Planetary Science XXXIX (2008) 2459.pdf
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... Contemporary work involved collecting and analyzing ground-based temperature measurements and thermal imagery [1,47,[57][58][59]. The leap in our ability to detect caves was largely attributed to higher instrument sensitivity, modern computing systems that make processor-intensive analytical techniques possible, and the availability of high accuracy and affordable ground-based meteorological instruments. ...
... [1], estimated optimal detection times and demonstrated the utility of ground-based temperature measurements for identifying imagery acquisition times for aircraft-borne missions. Wynne et al. [57] later improved our understanding of cave thermal behavior, proposing three endmembers of cave thermal behavior: Classic, pseudo-classic and ice cave. ...
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  • A Pflitsch
  • J Piasecki
Pflitsch, A. and J. Piasecki (2003), J. Cave Karst Stud. 63, 160-173.
In Review) Diurnal Thermal Behavior and Detection Techniques of Caves in the Atacama Desert
  • J J Wynne
Wynne, J. J. et al. (In Review) Diurnal Thermal Behavior and Detection Techniques of Caves in the Atacama Desert, Chile, Earth. Planet. Sci. Lett. [2] Cushing, G. E. et al. (2007) GRL 34, L17201. [3] Rinker J. N. (1975) Photogram. Eng. Remote Sensing 41, 1391-1400. [4]
  • P Mazur
Mazur, P. et al. (1978) Space Sci. Rev. 22, 3-34. [5] Klein, H. P. (1998) JGR 103, 28463-28466. [6] Grin, E. A. et al. (1998), LPS XXIX, Abstract #1012. [7] Baker, V. R. et al. (1993) Ed. J. S. Lewis, Resources of Near-Earth Space (University of Arizona Press, Tucson), p. 765-798. [8] Boston, P. J. et al. (2003), Grav. Space Biol. Bull. 16, 121-131.
  • J B Cropley
Cropley, J. B. (1965) Nat. Speleo. Soc. Bull. 27, 1-9.