Venting, Collapsing, and Roughening
Erode the Martian Residual South Polar Cap
1
Buhler ,
1
Ingersoll ,
Premise
Dark Fans
1
The martian Residual South Polar Cap (RSPC,
Fig. 1) comprises 1-10 m thick deposits of CO2
ice on top of the (mostly H2O ice) martian
south polar cap that persist throughout the
entire martian year. The thicker deposits are
known to exhibit quasi-circular pits and a
polygonal fracture pattern on their upper
surface, thus becoming rougher with age [1,2].
The roughening of the surface is thought to
lead to the formation of pits [3], but this has
not yet been demonstrated observationally.
We detail morphological features seen on the
SPRC in HiRISE images, and describe a process
of CO2 sublimation via venting, collapsing,
and surface roughening that is capable of
explaining the erosive morphology of the
RSPC based on these observations.
Collapsing
B2/B8
C1
South Pole
100 km
We observe that roughness on the surface of
mesas is exploited to make pits. We observe
the formation of three different types of pits:
(i) quasi-circular pits with flat floors (Fig. 8,
Col. A), (ii) crescentic pits with sloped floors
(Fig. 8, Col. B), and linear pits (Fig. 9). Context
for Column A is in the upper left panel of Fig. 8.
Context for Column B is in the lower left panel
of Fig. 8. We only observe quasi-circular and
linear pits forming in thicker mesas. However,
we observe crescentic pits in both thick (>1 m
thick) and thin (<1 m thick) mesas (Fig. 10). We
also observe that smooth depressions do not
evolve steep walls (Fig. 10). We thus infer that
there is a characteristic roughness scale on
which catastrophic surface erosion will take
place [cf. 3].
B7
87°
1. Map of RSPC labelled with units found in our study
locations, using the convention of Thomas et al., 2016 [4].
65°, MY 29 LS 269
Dark fans with areas ~10 m2 appear exclusively
near the edges of CO2 deposits (Fig. 2),
appearing no later than mid-spring (LS 232).
The long axis of the fan is typically oriented
perpendicularly with respect to the edge of the
CO2 deposit, the narrowest width of the fan is
closest to the CO2 deposit, and the fan is
typically darkest in the narrow region close to
the CO2 deposit (Fig. 3). Dark fans are also
occasionally apparent on the upper surface of
the mesas (Fig. 4). We interpret the fans as
evidence of entrained dust in a pressurized
gas flow venting from the interior of the
mesas, similar to the fans seen in ‘Spider
Terrain’ [5]. We hypothesize that the
pressurized gas is formed by sublimation of
CO2 ice within the mesas.
64°, MY 30 LS 272
64°, MY 29 LS 290
3
Head
High
Low
50 m
8
Column A Column B
71°, MY 28 LS 238
6
9
71°, MY 28 LS 237
MY 28 LS 238
20 m
MY 28 LS 323
High
Low
MY 29 LS 264
50 m
67°, MY 29 LS 249
MY 29 LS 314
data
gap
50 m
MY 30 LS 210
71°, MY 28 LS 238
MY 30 LS 344
50 m
65°, MY 31 LS 275
MY 31 LS 221
High
Low
MY 31 LS 350
MY 32 LS 336
50 m
50 m
50 m
66°, MY 28 LS 285
71°, MY 28 LS 237
Low
Low
High
Low
fan
High
100 m
20 m
10 m
4. Fans draped on mesa (Unit B7).
2. Fan formation is densest in Unit B7. 3. Typical fan morphology (Unit A1).
Illumination direction indicated by sun symbol, followed by solar incidence angle, Mars Year (MY), and Solar Longitude (LS).
Surface Texture
7
73°, MY 29 LS 314
The upper surfaces of mesas exhibit two
50 m
84°, MY 31 LS 347
Time 4
Time 3
5. Two images of the same area taken under nearly identical
illumination conditions. Left: No crack (arrows). Solar
incidence angle 65°, phase angle 63°, and emission angle 2.1°.
Right: 34 sols later, crack is visible. Solar incidence angle 66°,
phase angle 66°, and emission angle 0.5°.
6. T1. The upper surface of a mesa is smooth. T2. A depression
with an angular boundary is now apparent. T3. 4 sols later,
the outline of the depression is different (circled areas). T4. 1
year later the depression is no longer visible, but cracks are
apparent at the locations of ridges in panels B and C (arrows).
4
High
Time 2
84°, MY 30 LS 346
50 m
66°, MY 29 LS 290
Low
50 m
Abstract #2550
3
2
83°, MY 30 LS 344
Time 1
High
km2-scale
Pit Formation
A0/Un
A1
5
Cracks (Fig. 5) and 10
to 1
depressions on the upper surface of CO2 mesas
(Fig. 6) are apparent across many regions of
the RSPC. The depressions have relief of ~10
cm (determined with shadow measurements)
and the border of the depressions evolve
rapidly, which we interpret as evidence of
vertical subsidence. We interpret both the
cracks and depressions as the surface
manifestation of internal structural failure of
the CO2 mesas due to sublimation of CO2
within the mesas, which is then vented out,
creating the dark fans (Figs. 2-4).
Conclusions
2
Fassett ,
Peter B.
A. P.
B. L.
C. I.
J. W.
1California Institute of Technology, 2Mt. Holyoke College, 3Brown University
Contact: [email protected]
m2 -
1
Ehlmann ,
50 m
Crescents
Crescent-shaped pits are found in both thick
(>1 m) and thin (<1 m) mesas. We observe that
crescents form differently depending on the
thickness of the mesa. In thinner mesas,
roughness along the edge of mesas is exploited
to form crescents (Fig. 11, a thin mesa). In
thicker mesas, crescents exploit linear cracks
(Fig. 8, Col. B, Unit A1). In thicker mesas,
crescentic pits form steep walls and have
efficient areal erosion, while crescents in
thinner mesas do not form steep walls nor
have efficient areal erosion (Fig. 10). We note
that different units have a preference for
differently shaped pits. For example, Unit B7
develops quasi-circular pits (Fig. 2) while Unit
A1 develops crescentic pits with smooth ramps
(Fig. 12).
scales of texture (Fig. 7, a thick mesa):
wider troughs (red arrows) and narrower
ridges (blue arrows). We observe narrow
ridges forming as cracks that evolve and
invert their topographic relief (process not
shown). This may be due to an infilling of
the cracks with seasonal CO2, creating a
resistant deposit, or another unknown
process. We only observe the wide troughs
on thicker (presumably older) mesas. We
do not observe the thicker troughs form,
but hypothesize that the thicker troughs
record internal structural failures that have
become reinforced over time.
High
Low
10 m
11
10
20 m
Low
High
66°, MY 28 LS 285
66°, MY 28 LS 290
68°, MY 29 LS 245
66°, MY 31 LS 254
78°, MY 32 LS 211
80°, MY 32 LS 337
12
crescents
100 m
smooth
79°, MY 30 LS 334
Thick
71°, MY 28 LS 238
Thin
Low
Very Thick
83°, MY 32 LS 237
We describe RSPC features that have not been detailed before: (i) dark fans associated with CO2 mesas, (ii) linear and areal collapses on
the upper surfaces of mesas, and (iii) the initiation of new pits. We thus infer a process that is capable of explaining the previously noted
increase in roughness of the upper surface of the mesas as they become older: namely, internal sublimation (indicated by dark fans)
leads to collapse and subsidence of the CO2 mesas, which in turn provides nucleation sites for the ubiquitous pits of the RSPC.
100 m
References
[1] Thomas, P.C. et al. (2005) Icarus, 174, 535-559.
[2] Byrne, S. and Ingersoll, A.P. (2003) Science, 299,
1051-1053. [3] Byrne, S. et al. (2008) 39th LPSC, 1011. [4] Thomas, P.C. et al. (2016) Icarus, 268, 118130 [5] Piqueux, S. (2003) JGR, 108, 1-9