Sixth Mars Polar Science Conference (2016)
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Thermal Inertia Dependent Late Spring Albedo in South Polar Gullies. C. P. Mount1 and P. R. Christensen2,
Arizona State University, 1151 S. Forest Ave., Tempe, AZ, 85287, [email protected], [email protected].
Introduction: Approximately 25% of the Martian
atmosphere condenses and then sublimes at the poles
annually, making the CO2 cycle a dominant driver in
the current global climate [1-3]; because the global
climate and atmospheric circulation depend on the CO2
cycle, we must examine modern CO2 processes to understand the climate history of Mars. The CO2 cycle is
controlled by the polar energy balance which depends
critically on the emissivity and albedo of the surface,
which varies during spring and across the polar caps. It
is therefore important to understand the underlying
processes governing CO2 emissivity and albedo.
Temporal and spatial variations in springtime albedo of seasonal deposits has been well documented [49] and is thought to be caused primarily by depositional mode, grain size, and contaminants [4-9]. Most studies have focused on cap-wide and/or regional scale
variations [4-9], where depositional modes can vary
widely. However, little work has been done on the role
ice-free surface properties have on grain size evolution
nor at the small scale where depositional modes are
likely more uniform. We hypothesize that ice-free surface properies are, in part, responsible for driving albedo variations in seasonal ice, particularly on the
small scale. Here we investigate the dependence of
springtime CO2 albedo on ice-free thermal inertia.
Observations: Four gully sites were identified between -67°N and -72°N that exhibit similar temporal
albedo evolution and late spring albedo variations.
Three of the sites are situated on polar pit scarps and
one is on a crater wall. In early spring the surface at all
sites has almost no contrast, with a nearly homogeneous brightness. This changes radically by late spring.
Figure 1 displays High Resolution Science Experiment (HiRISE) red channel images of the four sites in
late southern spring. The fan material at the end of the
gullies is bright in each, while the floors of the pits and
crater are characteristically darker than the fan material. Figure 1a is the Type example of these gully sites
and is the focus on the preliminary results discussed
herein.
Figure 2a is colorized Lambert albedo of Figure 1a.
The HiRISE albedo is subject to an absolute error of
±30% [10], but offers superior spatial resolution for
small-scale analysis. Albedos vary between 0.20 and
0.45, with the scarp being the lowest, the fan material
being the brightest, and the pit floor being intermediate. Interesting low albedo features are labeled A1 –
A5.
Figure 1. Late spring HiRISE images of four gully sites
identified to exhibit similar albedo behavior. a) Polar pit
gully at 1.35°E, -68.4°N; Ls~226° b) Polar pit gully at
3.15°E, -71.2°N; Ls~245°. c) Crater wall gully at 264°E, 70.8°N; Ls~237°. d) Polar pit gully at 346°E, -69.4°N;
Ls~237°.
To understand the level of error associated with the
HiRISE albedo, we constructed a synthetic instrument
response function to approximate the red channel (0.55
– 0.85 μm) to first order and deconcolve a Compact
Reconaissance Imaging Spectrometer for Mars
(CRISM) Full Resolution Targeted (FRT) spectral im-
Sixth Mars Polar Science Conference (2016)
age, taken at the same Ls, to HiRISE albedo (Figure
2b). This analysis will be useful when analyzing albedo
of HiRISE scenes not accompanied by CRISM spectra.
Figure 2b is given the same color stretch as 2a to show
that the CRISM albedos are less extreme than HiRISE
albedos.
Figure 2c is a THermal EMission Imaging System
(THEMIS) ice-free thermal inertia (TI) image of the
same scene as Figure 1a. The locations of the low albedo features from 2a are also indicated on this image.
The thermal inertia ranges from ~300 to 600 J m-2 K-1
s-1/2, with high TI units at the scarp and albedo features,
low TI units in the pit floor and the plains above the
scarp, and moderate TI units at the fans. A1 has the
lowest albedo of the gully site as well as the highest
thermal inertia. This is likely due to the high boulder
count in the gully alcoves. A2 is a layer of blocky bedrock. This accounts for the relatively high thermal inertia. A3 is an east-facing hillslope and A4 is a northfacing hillslope. Why these have high thermal inertia is
unknown. A5 is also a blocky layer of bedrock.
Results: The discrepancy in the extrema of the
CRISM and HiRISE albedos likely stems from subpixel mixing. A FRT pixel is the size of 72 HiRISE
pixels, thus there is significant spatial averaging. This
accounts for the less extreme albedo variations in the
CRISM albedo, as the larger pixel size shifts values
closer to the average and suppresses variations. A spatial deconvolution of the HiRISE albedo to CRISM
resolution is required to show this, but to first order,
the albedos are nearly identical. This gives high confidence to HiRISE derived albedos.
It is unclear whether the low albedo features A1 –
A5 are due to complete sublimation or rather modification of seasonal ice. Ice may be present but either, thin
or patchy. This maybe be a cause of spring sublimation
or could be related to excess heat stored in the high TI
units during fall deposition. If any ice is present it
could also be highly transparent. High TI implies high
thermal conductivity, thus high TI would be conducive
to conducting away heat. This could inhibit fracturing
and foster annealing [11]. Positive feedback could lead
to highly transparent late spring deposits. Comparison
with summertime albedos could assist in discerning
whether ice is present as would spectral analysis using
CRISM. Finally ice depths could be estimated if sufficient boulders are present [12]
Whether the low albedo/high TI anit-correlation is
present the three gully sites is still unknown. Investigation of the other three sites is necessary. If the correlation holds for the other sites, observations should be
taken to a larger scale.
Conclusions: Preliminary results at one gully site
suggest that late spring albedo and ice-free thermal
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inertia are anti-correlated. Units with high thermal inertia have low albedo in late spring, while units of low
thermal inertia are higher in albedo. However, gully fan
material itself is of moderate thermal inertia yet retains
the highest albedo in the region throughout spring.
Thus, we conclude that while ice-free thermal inertia
does appear to affect springtime albedo, it is likely a
second-order effect. Other factors such as slope or
slope azimuth may also be responsible for the observed
variations.
Figure 2. a) Colorized HiRISE lambert albedo of Figure 1a.
Low albedo features are labeled A1 – A5. b) CRISM FRT
lambert albedo deconvolved to HiRISE bandpass of Figure
1a. To first order the HiRISE albedo and CRISM albedo are
identical. c) THEMIS ice-free thermal inertia of same scene
in Figure 1a. Low albedo features from Figure 2a are labeled
and appear to be correlated with high thermal inertia units.
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