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Evaluation of a proposed technique for identifying Martian caves in THEMIS infrared images


Abstract and Figures

We present a proposed technique for identifying martian caves using THEMIS infrared images.
Content may be subject to copyright.
, J. Moersch
, N. A. Cabrol
, E. Grin
, J. J. Wynne
, and M. Chojnacki
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996 (,
SETI Carl Sagan Center, 189 N. Bernardo #100, Mountain View, CA 94043,
Colorado Plateau Research Station
and Department of Biological Sciences, Northern Arizona University, Flagstaff, AZ 86011, Northern Arizona
University, Flagstaff, AZ 86011.
Introduction: Caves can provide shelter against the
inhospitable surface conditions on Mars, protecting
not only possible past life, but also potentially
protecting humans in future exploration missions [1].
The key to being able to utilize caves on Mars begins
with finding them. Images of cave-like features from
various visible-wavelength cameras unfortunately do
not show sufficient detail to unequivocally identify
caves. Here, we evaluate a potential technique for
identifying cave skylights on Mars in orbital thermal
infrared (IR) images, which relies on the hypothesis
that these features should not exhibit the same diurnal
thermal behavior as the surrounding land surface.
Background and Method: Cushing et al. [2] have
identified seven possible skylight entrances into
Martian caves on the volcano Arsia Mons, dubbed
the “Seven Sisters,” using IR images from the Mars
Odyssey Thermal Emission Imaging System
(THEMIS). These features were found to exhibit
diurnal temperature variations that were smaller than
their surrounding surfaces [2].
Here, we expand on this observation in an
attempt to define a systematic method for locating
caves in THEMIS night IR image data. Our method
is based on the hypothesis that the geologic materials
in cave openings may not exhibit typical diurnal
thermal patterns because their temperatures are
influenced by additional factors, such as contact with
potentially large volumes of air below the surface.
Thermophysical models for Mars (e.g., [3]) typically
assume horizontal surfaces, with temperatures solely
controlled by insolation, the albedo, and the thermal
inertia of the surface materials. In practice, the results
from such models are constrained with measured
quantities to derive the thermal inertia of the surface:
surface temperature measurements (e.g., from
THEMIS IR images), albedos (from visible
observations), and the observing circumstances are
used to select from a family of different possible
model-generated diurnal thermal curves associated
with different thermal inertias.
Figure 1 shows an example family of different
modeled diurnal thermal curves from [4] for a set of
surfaces that differ only in their thermal inertias. To
the extent that the model accounts for all relevant
physics, the temperature of a typical homogeneous,
horizontal Martian surface would be expected to
follow a single diurnal curve because the thermal
inertia of a surface should not change on that
timescale. If there are factors influencing the
temperature of the surface that are not accounted for
in the model - for example, contact with a reservoir
of subsurface air inside a cave (either cold-trapped in
the winter or warm-trapped in the summer) - it is
possible that the temperature of the affected surface
might not follow a single diurnal curve predicted by
the model. Put another way, if temperature
measurements of the same surface from two different
times of day yield two different thermal inertias, this
would be taken as evidence of anomalous thermal
behavior, suggestive of unaccounted factors
influencing the surface temperature. Identification of
such anomalous thermal behavior by itself would not
be sufficient to identify caves because other factors
not included in this model (e.g., diurnal
sublimation/condensation of volatiles) exist.
However, taken together with photogeologic
evidence for features possibly associated with cave
entrances, such thermal behavior would be very
Figure 1: Sample diurnal thermal curves for different
thermal inertia surfaces that are otherwise identical
(adapted from [4]).
To test the proposed method of cave detection,
THEMIS IR images of candidate cave features taken
at two different times during predawn hours were
used to calculate thermal inertia images using the
model of [3] in the jENVI THEMIS analysis software
suite (which also takes into account seasonal
differences in the observations and atmospheric
42nd Lunar and Planetary Science Conference (2011) 2739.pdf
conditions). The two thermal inertia images were
then registered to each other and subtracted to create
a differenced thermal inertia image. Surfaces that
conform to the model will report approximately the
same thermal inertia at both times, leading to a value
near zero in the differenced image (within the
uncertainties of measurement, quantified by [5] as
total ~65 J m
). Surfaces that have
temperatures influenced by non-standard factors
could have anomalously bright or dark areas in the
differenced image because the thermophysical model
would report two different
thermal inertias at the two times of day.
Table 1
Data: The proposed technique was applied to three
of the Seven Sisters targets (Abby, at 240.54° E,
6.713°S, and Nikki, at 240.55°E, 6.708°S, and
Wendy, 240.32°E, 7.84°S) where sufficient quality
data were available at two pre-dawn times, as a check
of the method where cave identifications are
relatively well-established. A recent survey of over
40,000 THEMIS visible images [6] produced a list of
54 targets with morphological evidence potentially
related to caves. Eight of these targets (Table 1) have
good quality THEMIS night IR images at two pre-
dawn times, which were also processed in the method
described above.
Results: The example thermal inertia difference
image in Figure 2 (containing the Abby and Nikki
features from the Seven Sisters [2]) show no bright or
dark! areas that stand out against the background at
the geographic locations of the features. The putative
cave features analyzed from the list of [6] also have
thermal inertia differences that were no different than
the surrounding surface material. For all features
examined, the proposed cave-related features all but
disappear into the surrounding terrain (Figure 3).
Discussion: The failure of our proposed method for
identification of cave-related features using thermal
inertia anomalies has several possible explanations,
including: 1) The features examined so far are not
true caves, but rather pit craters, fissures, or other
shallow features without deep reservoirs of trapped
air; 2) Some of the features may be true caves with
deep reservoirs of trapped air, but it has insufficient
thermal mass and/or airflow to strongly influence the
temperature of geologic materials at the putative
entrances; 3) The uncertainties associated with
calculation of thermal inertias are larger than the
magnitude of the proposed effects.
Some of the candidates of [6] with the best
photogeologic evidence for features associated with
possible caves could not be analyzed due to the
insufficient quality/timing of the THEMIS IR
coverage. We will continue to test the method as
better data of these targets are acquired by THEMIS.
[1] Wynne, J. J. et al., (2008) EPSL, 272, 240-250.
[2] Cushing, G. et al., (2007) GRL, 34, L17201. [3]
Mellon, M. T. et al., (2000) Icarus, 148, 437-455. [4]
Putzig, N. E. et al., (2007) Icarus, 191, 68-94. [5]
Fergason R. L. et al. (2006) JGR, 111, E12004. [6]
Cabrol, N. et al., (2009) LPS XV, Abstract #1040
Figure 2. Images for of proposed caves Abby and Nikki
from [2]: a) Visible THEMIS image V17716001; b) Night IR
THEMIS image I15151011; c) Thermal inertia difference
image made using image in b) and image I01309002. For
scale, individual pixels seen are in b) and c) are 100m.
Units of tiu are J m
Figure 3. (a) Visible image of candidate cave or cavity
feature (THEMIS Image V07946019) from [6]. (b) Thermal
inertia images of same region from THEMIS Image
I07703013. (c) Difference of thermal inertias from THEMIS
images I07703013 and I18910003. For scale, individual
pixels seen are in b) and c) are 100m. Units of tiu are J m
VIS Image
8.48 °N
a b
42nd Lunar and Planetary Science Conference (2011) 2739.pdf
... Since the evidences were obvious but not concrete, in 2011 an attempt by Newcomer et al. (2011) was made to model the different surface behaviours and to apply this model to observational data. Their idea was that caves unlike any other surface feature would not undergo the detected diurnal thermal patterns (Newcomer et al. 2011). ...
... Since the evidences were obvious but not concrete, in 2011 an attempt by Newcomer et al. (2011) was made to model the different surface behaviours and to apply this model to observational data. Their idea was that caves unlike any other surface feature would not undergo the detected diurnal thermal patterns (Newcomer et al. 2011). A surface inertia model for different surfaces with different properties was established (see figure 1.7) (Newcomer et al. 2011). ...
... Their idea was that caves unlike any other surface feature would not undergo the detected diurnal thermal patterns (Newcomer et al. 2011). A surface inertia model for different surfaces with different properties was established (see figure 1.7) (Newcomer et al. 2011). Although the various rock and surface types vary, their overall behaviour remains very similar (Newcomer et al. 2011). ...
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Human missions to Mars generate great potential in exploring the red planet with emphasis on the search for the development of microbial life during its wet history and the question whether Earth as a living planet is unique or whether life will be able to develop everywhere in the universe if the conditions are given at some point in time. Crewed research missions to other planetary bodies are difficult and expensive, but not impossible. Several space agencies and Mars societies and organisations nowadays undertake the effort of conducting Mars analogue research, i.e. testing hardware, software and procedures on Earth in a Mars-like environment in order to offer perfect working tools and subsystems once a human mission to Mars will launch. The Austrian Space Forum (ÖWF) is one of these organisations and has developed its own Mars space suit simulator called Aouda.X. During yearly analogue missions, hardware improvements, new applications as well as scientific procedures and planning mechanisms are tested. On future Mars missions, time and resources will be very limited. In order to ensure a maximum of scientific research within these constraints, a detailed and well-thought-through Mission and Activity Planning is of significant importance. Developing a method on how to properly plan these activities and on how to allocate for possible complications is a basic need. This work summarises and elaborates on the author's work as one of the two Flight Planners for the Dachstein Mars Simulation 2012, conducted in the Giant Ice Cave in Upper Austria under the lead of the ÖWF. The personal experiences and data gained regarding the optimisation of science via proper scheduling mechanisms, which are presented here, can help to further improve the planning process not only for analogue missions on Earth, but also for a future Mars mission.
... Cave entrances typically appear as warm features in thermal imagery acquired at night and cool features in midday imagery (e.g., [1,36,47,48]) because cave entrances are generally characterized by smaller diurnal temperature changes than the surrounding surface rock. Deep interior cave temperatures are typically stable (e.g., [49,50]) due to diurnal surface temperatures, which are dampened via thermal conduction of the geologic substrate [51,52]. ...
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Since the initial experiments nearly 50 years ago, techniques for detecting caves using airborne and spacecraft acquired thermal imagery have improved markedly. These advances are largely due to a combination of higher instrument sensitivity, modern computing systems, and processor intensive analytical techniques. Through applying these advancements, our goals were to: (1) Determine the efficacy of methods designed for terrain analysis and applied to thermal imagery; (2) evaluate the usefulness of predawn and midday imagery for detecting caves; and (3) ascertain which imagery type (predawn, midday, or the difference between those two times) was most informative. Using forward stepwise logistic (FSL) and Least Absolute Shrinkage and Selection Operator (LASSO) regression analyses for model selection, and a thermal imagery dataset acquired from the Mojave Desert, California, we examined the efficacy of three well-known terrain descriptors (i.e., slope, topographic position index (TPI), and curvature) on thermal imagery for cave detection. We also included the actual, untransformed thermal DN values (hereafter "unenhanced thermal") as a fourth dataset. Thereafter, we compared the thermal signatures of known cave entrances to all non-cave surface locations. We determined these terrain-based analytical methods, which described the "shape" of the thermal landscape hold significant promise for cave detection. All imagery types produced similar results. Down-selected covariates per imagery type, based upon the FSL models, were: Predawn-slope, TPI, curvature at 0 m from cave entrance, as well as slope at 1 m from cave entrance; midday-slope, TPI, and unenhanced thermal at 0 m from cave entrance; and difference-TPI and slope at 0 m from cave entrance, as well as unenhanced thermal and TPI at 3.5 m from cave entrance. Finally, we provide recommendations for future research directions in terrestrial and planetary cave detection using thermal imagery.
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1] Seven possible skylight entrances into Martian caves were observed on and around the flanks of Arsia Mons by the Mars Odyssey Thermal Emission Imaging System (THEMIS). Distinct from impact craters, collapse pits or any other surface feature on Mars, these candidates appear to be deep dark holes at visible wavelengths while infrared observations show their thermal behaviors to be consistent with subsurface materials. Diameters range from 100 m to 225 m, and derived minimum depths range between 68 m and 130 m. Most candidates seem directly related to pit-craters, and may have formed in a similar manner with overhanging ceilings that remain intact.
Thermal inertia derivation techniques generally assume that surface properties are uniform at horizontal scales below the footprint of the observing instrument and to depths of several decimeters. Consequently, surfaces with horizontal or vertical heterogeneity may yield apparent thermal inertia which varies with time of day and season. To investigate these temporal variations, we processed three Mars years of Mars Global Surveyor Thermal Emission Spectrometer observations and produced global nightside and dayside seasonal maps of apparent thermal inertia. These maps show broad regions with diurnal and seasonal differences up to 200 tiu at mid-latitudes (60° S to 60° N) and 600 tiu or greater in the polar regions. We compared the seasonal mapping results with modeled apparent thermal inertia and created new maps of surface heterogeneity at 5° resolution, delineating regions that have thermal characteristics consistent with horizontal mixtures or layers of two materials. The thermal behavior of most regions on Mars appears to be dominated by layering, with upper layers of higher thermal inertia (e.g., duricrusts or desert pavements over fines) prevailing in mid-latitudes and upper layers of lower thermal inertia (e.g., dust-covered rock, soils with an ice table at shallow depths) prevailing in polar regions. Less common are regions dominated by horizontal mixtures, such as those containing differing proportions of rocks, sand, dust, and duricrust or surfaces with divergent local slopes. Other regions show thermal behavior that is more complex and not well-represented by two-component surface models. These results have important implications for Mars surface geology, climate modeling, landing-site selection, and other endeavors that employ thermal inertia as a tool for characterizing surface properties.