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

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We present a proposed technique for identifying martian caves using THEMIS infrared images.
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EVALUATION OF A PROPOSED TECHNIQUE FOR IDENTIFYING MARTIAN CAVES IN THEMIS
INFRARED IMAGES. K. B. Newcomer
1
, J. Moersch
1
, N. A. Cabrol
2
, E. Grin
2
, J. J. Wynne
2,3
, and M. Chojnacki
1
.
1
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996 (kbell20@utk.edu),
2
SETI Carl Sagan Center, 189 N. Bernardo #100, Mountain View, CA 94043,
3
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
suggestive.
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
-2
K
-1
s
-1/2
). 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.
References:)
[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
-2
K
-1
s
-1/2
.
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
-2
K
-1
s
-1/2
.
VIS Image
ID
Center
Latitude
Center
Longitude
Incidence
Angle
(Max)
V07946019
8.48 °N
162.60°E
77.540
V10456019
20.41
123.82
67.399
V11478007
15.74
161.83
71.590
V12041006
21.18
121.69
71.686
V12066009
21.55
120.57
71.638
V12877003
21.71
125.51
74.162
V14225010
22.93
120.07
83.794
V14462024
19.97
122.98
85.830
a b
c
c
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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... 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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