A subsurface depocenter in the South Polar Layered
Deposits of Mars
J. L. Whitten1 , B. A. Campbell1 , and G. A. Morgan1
1Smithsonian Institution, Washington, District of Columbia, USA
Abstract The South Polar Layered Deposits (SPLD) are one of the largest water ice reservoirs on Mars, and
their accumulation is driven by variations in the climate primarily controlled by orbital forcings. Patterns of
subsurface layering in the SPLD provide important information about past atmospheric dust content,
periods of substantial erosion, and variations in local or regional deposition. Here we analyze the SPLD
using SHAllow RADar (SHARAD) sounder data to gain a unique perspective on the interior structure of the
deposits and to determine what subsurface layers indicate about the preserved climate history. SHARAD data
reveal a major deviation from the gently domical layering typical of the SPLD: a subsurface elongate
dome. The dome most likely formed due to variations in the accumulation of ice and snow across the cap,
with a higher rate occurring in this region over a prolonged period. This SPLD depositional center provides
an important marker of south polar climate patterns.
1. Introduction
The south polar region of Mars is covered with a deposit of layered H2O ice, CO2 ice, and dust known as the
South Polar Layered Deposits (SPLD) (Figure 1). This layering is interpreted to record the climate history of
Mars over at least the last 7–100 Myr (the SPLD surface has been dated to between 7.25 ± 3.6 and
14.5 ± 7.2 Ma [Herkenhoff and Plaut, 2000] and 30–100 Ma [Koutnik et al., 2002]) and, as a result, images of
the SPLD have been studied extensively [e.g., Murray et al., 1972; Howard et al., 1982; Herkenhoff and
Murray, 1990; Byrne and Ivanov, 2004;Milkovich and Plaut, 2008]. Variations in the dust content of the subsur-
face layers may indicate changes in ice stability or dust loading. Correlations between orbital parameters such
as obliquity, eccentricity, and precession, and layer structure and composition (e.g., H2O ice versus CO2 ice)
may reveal the cause and effect relationship between these orbital parameters and the presence and
morphologic characteristics of the polar layered deposits [Murray et al., 1973; Toon et al., 1980;
Thomas et al., 1992]. The SPLD is interpreted as almost entirely water ice [Nye et al., 2000; Titus et al., 2003;
Bibring et al., 2004; Zuber et al., 2007; Plaut et al., 2007], but the discovery of significant CO2 reservoirs
[Phillips et al., 2011] has dramatic implications for paleo-atmospheric pressure regimes and the recent climate
of Mars [e.g., Bierson et al., 2016].
Several studies have analyzed the subsurface layering exposed in troughs and at the edge of the SPLD to char-
acterize its overall internal structure. Byrne and Ivanov [2004] analyzed the elevation of outcrops in the top few
hundredmeters of the uppermost Bench Forming Layer (BFL) sequence to determine the overall shape of the
SPLD subsurface layers. This layer sequence has a diagnostic pattern, including four morphologically distinct,
bench-forming units. The locations of BFL outcrops indicate that the overall shape of the SPLD is a gently dip-
ping domewith its apex in the center of Australe Mensa (86.88°S, 357.24°E). The dip angle of the layering sug-
gests that the SPLD extent has not changed substantially from when the BFL sequence was emplaced. A later
analysis of the SPLD internal structure by Milkovich and Plaut [2008] revealed three distinct layer sequences,
from top to bottom: the Bench Forming Layer, Promethei Lingula Layer (PLL), and the Inferred Layer (IL). An
outcrop of the lowermost IL sequence was only observed in one Thermal Emission Imaging System
(THEMIS) visible image, but its regional extent was implied by elevation variations of the BFL and PLL
sequences [Byrne and Ivanov, 2004; Milkovich and Plaut, 2008] not associated with basal topography [Plaut
et al., 2007]. This analysis of THEMIS image data confirmed previous results that the SPLD layers are dome
shaped, but that the areal extent of each layer sequence differs; the BFL and IL more closely follow the current
extent of the south pole residual cap (SPRC), compared with the PLL, which is found throughout the SPLD.
Previous studies relied on images of outcrops within troughs and at the edges of the SPLD. Radar sounder
data provide a unique perspective, enabling a view into the subsurface to trace radar echoes from
WHITTEN ET AL. DEPOCENTER UNDERNEATH THE SLPD, MARS 8188
PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2017GL074069
Key Points:
• We identify the first subsurface dome
in the South Polar Layered Deposits
using SHARAD data
• Calculations indicate that the dome is
a physical structure that is offset to the
east from Australe Mensa
• The location of the dome indicates the
major H2O snow/ice depositional
center has recently oscillated in extent
and location
Supporting Information:
• Supporting Information S1
Correspondence to:
J. L. Whitten,
whittenj@si.edu
Citation:
Whitten, J. L., B. A. Campbell, and G. A.
Morgan (2017), A subsurface depocenter
in the South Polar Layered Deposits of
Mars, Geophys. Res. Lett., 44, 8188–8195,
doi:10.1002/2017GL074069.
Received 5 MAY 2017
Accepted 7 AUG 2017
Accepted article online 10 AUG 2017
Published online 31 AUG 2017
©2017. American Geophysical Union.
All Rights Reserved.
reflecting interfaces throughout the
deposit and produce reliable layer and
sequence correlations [Milkovich et al.,
2009; Christian et al., 2013]. Here we use
data from the SHAllow RADar (SHARAD)
on the Mars Reconnaissance Orbiter
(MRO) to characterize the internal struc-
ture of the SPLD and to identify a feature
very different from the simple view of a
single regional domical layer structure.
We then consider possible causes of this
observed feature, including influences
from basal topography, ice composition
(H2O versus CO2), and snow/ice deposi-
tional centers.
2. Data and Methods
SHARAD sounder data are used to inves-
tigate the internal structure of the SPLD
to better understand the unit deposi-
tional history. The SHARAD instrument
aboard the MRO spacecraft has a
10 MHz bandwidth centered at 20 MHz
[Seu et al., 2004, 2007]. Measurements
are displayed as radargrams, with time
delay on the vertical axis and spacecraft
position on the horizontal axis (Figure 2). After synthetic aperture processing the horizontal, or along-track,
resolution is 0.3–1.0 km. The vertical resolution in free space is 15 m; in geologic materials the vertical range
resolution improves, dependent upon the material dielectric properties. SHARAD has a vertical resolution of
~8.4 m in water ice with a dielectric constant (ε0) of 3.15. Exposures of subsurface layers in troughs indicate
that individual layers are on the order of 10 m thick and layer groups, or packets, are >100 m thick [Byrne
and Ivanov, 2004; Milkovich and Plaut, 2008]. Thus, it is difficult to observe individual layers in SHARAD, but
layer packets are resolvable.
In the SPLD, the radar signal is often attenuated to the point that it does not reveal some/all of the underlying
interfaces (Figure S1 in the supporting information). In particular, the radar signal undergoes a diffuse
Figure 2. Radargram of a subsection of SHARAD track 496301. This water ice depth-corrected radargram was processed
using the incoherent summing technique [Campbell et al., 2015], which enhances subsurface layer visibility. Compare
the shape of the layers that define the dome structure (inside white box) with the expected horizontal layering in
Promethei Lingula (PL). The white horizontal bar above the SPLD indicates the approximate location of CO2 ice [Bierson
et al., 2016]; solid white shows where CO2 ice is mapped, and dashed white shows the location of off-nadir CO2 ice
that may influence the position of radar echoes. Orange lines in inset trace reflector boundaries and show variation
in layer thickness.
Figure 1. The South Polar Layered Deposits and the location of the sub-
surface elongate dome. The dome (white box) is identified in the SPLD
(white tone overlay), just east of the residual ice cap (black tones). Red
line denotes the location of SHARAD track 496301 (Figure 2). Dashed
black circle is the rim of the Prometheus Basin (~875 km in diameter
[Condit and Soderblom, 1978]). MOLA 128 ppd (~0.5 km/pixel) gridded
topography [Smith et al., 2001] over hillshade basemap.
Geophysical Research Letters 10.1002/2017GL074069
WHITTEN ET AL. DEPOCENTER UNDERNEATH THE SLPD, MARS 8189
backscattering process that causes a bright background echo known informally as “fog” [Campbell et al.,
2014, 2015]. Campbell et al. [2015] developed an incoherent summing technique to increase the signal-to-
noise ratio of SHARAD radargrams available in the Planetary Data System in order to more readily trace
subsurface radar echoes [Campbell et al., 2015] (Figure S1). In this technique, radar echoes from tracks that
overlap within 1 km of the ground point of a reference track are summed. The greatest improvement in
the signal-to-noise ratio of the radargrams occurs in regions closest to the MRO coverage gap around the
pole. In this study, 918 SHARAD radargrams were processed using this incoherent summing technique in
order to map the extent of subsurface reflectors in the SPLD (Figure 2).
In order to evaluate the structure of the SPLD and trace internal layers, the radargrams were depth-corrected
[e.g., Putzig et al., 2009; Phillips et al., 2011] using a real dielectric constant (ε0) of 3.15 for the subsurface, a typi-
cal value of pure water ice under Martian conditions [Johari, 1976; Grima et al., 2009]. It is plausible that
another material, such as CO2 ice, is present and contributing to the vertical position of the radar reflectors.
For example, if a water ice depth correction was applied to a column of material with a substantial amount of
CO2, the subsurface layers are expected to deform upward due to the lower real dielectric constant for CO2
(~2.1 [Simpson et al., 1980; Pettinelli et al., 2003]). Therefore, if the water ice depth correction produced non-
horizontal surfaces in the radargrams analyzed, the amount of CO2 and H2O ice required to move the vertical
position of a convex upward reflector into line with other horizontal reflectors was calculated.
3. Non-horizontal Layering in the SPLD
Analysis of 918 SHARAD radargrams corrected for the speed of light in H2O ice indicates that substantial local
variations in the layer structure of the SPLD occur to the east of Australe Mensa, just east of the SPRC. This
structure, interpreted as an elongate dome, is evident in 381 radargrams (Figure S2 and Table S1). Most of
the SPLD structure, identified by radar echoes interpreted to result from interfaces between ice layers and
more concentrated dust layers, is near-horizontal. Close to this thickest part of the deposit, however, there
is a region where the layering traces a convex upward or domical cross section (Figures 2 and 3). The radar
interfaces exhibit a convex upward shape from the surface down to the last detectable reflector (Figure 2);
the elongate dome is defined as the entire series of convex upward reflectors visible in the radargrams. In
some radargrams the radar reflectors are visible for large horizontal distances, and in other SHARAD track
orientations the series of reflectors composing the dome are visible for smaller horizontal distances. This posi-
tive relief feature is observed in single-track radargrams, but is most apparent in those processed using an
incoherent summing technique to add echoes from overlapping tracks (Figure S1). There are other smaller,
nonhorizontal features in the SPLD; here we focus on the largest detected.
The large convex upward structure has a maximum diameter of ~210 km, measured from its largest horizon-
tal extent in the radargrams, and its crest, or highest point on the uppermost reflector, is ~200–300 m below
the SPLD surface. The mean difference between a near-horizontal layer (e.g., Figure 4, right hand side of
radargrams) and the dome crest is ~280 m (for a dielectric permittivity ε0 = 3.15, consistent with H2O ice
[Johari, 1976; Grima et al., 2009]) (Figure 3). The morphology of the structure changes with depth; it evolves
from a double-crested feature to a single-crested feature with increased elevation (Figure 2), though the
same sequence or spacing of the elongate dome radar reflectors is observed in all of the SHARAD radargrams.
This change in morphology is most apparent in the southernmost SHARAD tracks, which are tangent to the
polar coverage gap. There is no evidence for unconformities or layers pinching out near this positive relief
feature. Several of the structure-tracing layers continue hundreds of kilometers across the SPLD, from the side
of the structure downward into regions of near-horizontal layering (Figure 4). In cross section, the subsurface
layers along the domical structure subtly change in thickness, being thickest toward the crest of the feature
and thinning downslope as the radar reflectors approach horizontal (Figures 2 and 3). Measurements of radar
time delay indicate that the layers can be up to 20 m thicker at the crest compared with the descending
slopes of the convex upward structure (Figure 2, orange lines).
When viewed together, all of the identified positive relief features traced in cross section form a 3-D elongate
dome structure (Figure 3), centered at approximately 87.0°S 49.3°E. The center of the elongate dome may be
even closer to the geographic south pole, but due to the polar coverage gap resulting from the MRO orbit
inclination, it is difficult to precisely measure the southern extent of the feature. Integrating from the base
Geophysical Research Letters 10.1002/2017GL074069
WHITTEN ET AL. DEPOCENTER UNDERNEATH THE SLPD, MARS 8190
of the cap [Plaut et al., 2007], the elongate dome has an approximate minimum volume of ~2 × 104 km3, or
~1.25% of the SPLD.
4. Formation of the Elongate Dome
There are several possible formation mechanisms for this large elongate dome, including the presence of
thick CO2 ice layers, deflection of the ice by basal topography or another preexisting structure (i.e., ancient
remnant ice cap), or an ice/snow depositional center. Here we assess each potential mode of formation
and justify our proposed mechanism.
In the SHARAD data, there is the potential for convex upward features to result from an error in the applied
depth correction [Phillips et al., 2011]. Only water ice is assumed for the applied depth correction here. If
Figure 3. The subsurface elongate dome. (a) A 3-D view of the subsurface dome, interpolated from 24 SHARAD radargrams,
offset from the surface for clarity. Surface elevation from MOLA. (b) Aerial view of the subsurface elongate dome (see
white box from Figure 1). Black dashed box corresponds to the data shown in Figure 3a. Red lines show the location of
Figures 3c and 3d. MOLA hillshade overlain by interpolated dome topography; color scale for the dome topography is the
same in Figures 3a and 3b. Close view of the domical cross sections identified within the summed depth-corrected
radargram tracks: (c) 786301 and (d) 3588501.
Geophysical Research Letters 10.1002/2017GL074069
WHITTEN ET AL. DEPOCENTER UNDERNEATH THE SLPD, MARS 8191
instead there is CO2 ice above the apparently curved layers, its lower real dielectric constant (ε0 = 2.1 [Simpson
et al., 1980; Pettinelli et al., 2003]) would create a false convex upward shape. Comparison with maps of the
distribution of massive near-subsurface CO2 ice [Phillips et al., 2011; Bierson et al., 2016] indicates that the
elongate dome is not located directly beneath any identified CO2 deposit. CO2 ice attenuates the radar
signal less than H2O ice and dust, resulting in increased radar echo strengths for underlying reflectors
[Phillips et al., 2011]: there is no enhancement of the SHARAD radar signal below the detected dome
(Figure 2). Additionally, the lack of signal enhancement in Mars Advanced Radar for Subsurface and
Ionospheric Sounding (MARSIS) radargrams, collected at lower frequencies (e.g., 5 MHz), further confirms
the absence of CO2 ice deposits above the observed SPLD elongate dome (Figure S3).
To further ensure that there is not a CO2 ice reservoir, the thickness of CO2 required to deflect the uppermost
layers in the observed elongate dome away from the elevation of the surrounding horizontal layers was cal-
culated [Phillips et al., 2011]. Generally, the thickness of CO2 required to align these uppermost layers horizon-
tally would equal or exceed the amount of material that is physically above them by 50–200 m. Even if the
thickness calculations were uncertain within 100 m, a substantial and rapid change in the vertical composi-
tion of the SPLD is unlikely. There would also need to be an abrupt horizontal compositional change, from
hundreds of meters of CO2 ice above the observed dome to hundreds of meters of H2O ice. Owing to the
difficulty of this compositional structure, the SPLD elongate dome is interpreted as a physical structure,
not an artifact of the data processing.
Basal topography, or remnants of a former ice sheet, may have formed the elongate dome. Displacement of
the BFL layer from its expected elevation near a section of the buried Prometheus Basin rim demonstrates
that basal topography can affect the position of subsurface layers, while not affecting the surface topography
[Byrne and Ivanov, 2004]. Prometheus Basin (~875 km in diameter) [Condit and Soderblom, 1978] formed prior
to the emplacement of the SPLD and has created a predictable change in the basal relief. Approximating the
basin rim structure by a circle shows that it continues beneath the SPLD, with its southernmost extent near
the geographic south pole (Figure 1). The predicted location of the Prometheus Basin rim and the elevated
BFL outcrops [Byrne and Ivanov, 2004] do not, however, intersect the region of the elongate dome; the
elongate dome is located interior to both of these features.
Plaut et al. [2007] produced amap of the SPLD basal interface using 60MARSIS orbits. The southern half of the
Prometheus Basin rim is vaguely identifiable, where the basal interface of the SPLD near 90°E is topographi-
cally lower (~0.5 km) than the rest of the basal interface. Immediately below the elongate dome there is a
positive relief feature in the basal topography. However, based on the paucity of distinct MARSIS basal
Figure 4. Continuous subsurface interfaces from the elongate dome to the horizontal layer packets. Subsection of depth-corrected SHARAD radargrams 579502 (top
row) and 3622801 (bottom row) showing the lack of unconformities. White arrows denote the approximate location of the dome crest centerline, and colored lines
trace two separate radar interfaces (which are not the same in these two radargrams); a dotted pattern indicates a lower confidence in the location of the layer
position. The 3622801 blue line corresponds to the radar reflector mapped in Figure 3. Images are 360 km across; images are ~940 m (Figure 4, top row) and ~790 m
(Figure 4, bottom row) high.
Geophysical Research Letters 10.1002/2017GL074069
WHITTEN ET AL. DEPOCENTER UNDERNEATH THE SLPD, MARS 8192
reflector detections in this region of the SPLD (Figure S3), we interpret this topographic feature to be an inter-
polated artifact, caused by a single debatable data point [Plaut et al., 2007, their Figure 2]. If the basal interface
did increase in elevation below the dome, it should be observed in multiple MARSIS tracks, since other lower-
lying basal reflectors are detected elsewhere in the SPLD. This is not observed. Thus, neither MARSIS nor
SHARAD data provide clear evidence for the presence of preexisting topography, either basal topography
or a remnant buried ice sheet, that may affect the SPLD subsurface layer structure.
The sculpting of the elongate dome through differential erosion can also be ruled out as a formationmechan-
ism due to a lack of observed unconformities in the subsurface layers (Figures 2 and 3). If the high central
topography of the ridge crest of the elongate dome was the result of erosion, the layers would end abruptly
at the edge of the erosional remnant and lateral unconformities would occur at its edges. Individual layers or
layer sequences can be traced across the dome, continuing uninterrupted into the more horizontal layering
in the thickest regions of the SPLD (Figure 4). Based on the above discussion, we favor the ice/snow deposi-
tional center formation hypothesis for the detected SPLD dome.
5. The SPLD Record of the Past Martian Climate
Prior analysis of SHARAD sounder data of the north polar layered deposits (NPLD) indicates that there was a
shift in the location of a north polar snow/ice depocenter early in the cap formation. The infilling of a buried
chasma and burial of the elongate dome below Gemina Lingula, and the elevation distribution of other
paleosurfaces compared with the current NPLD surface topography provide evidence for these nonuniform
depositional patterns [Putzig et al., 2009; Holt et al., 2010]. A similar change in depocenter(s) is plausible for
the SPLD.
While the current Martian topography influences the south polar climate [Richardson and Wilson, 2002;
Colaprete et al., 2005], migration of depocenters in the south polar region is supported by a comparison of
the position of the highest point on the SPLD plateau, Australe Mensa, the most recently deposited H2O
(and dust) layer sequence (WRAP) [Smith et al., 2016], and the elongate dome (Figure S4). Australe Mensa
is at the highest elevations of the SPLD and thus has experienced the most accumulation. Its center is
~200 km west of the elongate dome and in order to be at higher elevations than the dome today, the
Australe Mensa region would have experienced substantially more ice and/or snow deposition. The WRAP
is the most recent accumulation of H2O ice, and the thickest part of this deposit is centered at 81.5°S, 25.5°E
(Figure S4). It is offset to the northeast from both the elongate dome and Australe Mensa by ~300 km.
In addition, the south polar accumulation zone has decreased in size between the emplacement of the WRAP
and the BFL [Milkovich and Plaut, 2008]. This observation demonstrates how the most recent climate in the
south polar region has not been stable during the lifetime of the SPLD (7–100 Myr [Herkenhoff and Plaut,
2000; Koutnik et al., 2002]). Comparison of the areal extent of the identified layer sequences, IL, PLL, BFL,
and WRAP [Milkovich and Plaut, 2008; Smith et al., 2016], further emphasizes the changing size of the south
polar accumulation zone and oscillations of its center position (Figure S4). This observed concentration of
recent deposition between 270°E and 90°E thus further supports the hypothesis that the elongate dome is
a former ice/snow depositional center.
Further, the variation in layer thickness across the elongate dome supports the hypothesis that it represents a
region of increased deposition; a layer sequence at the peak of the dome is up to 30 m thicker than the same
layer in the region of horizontal layering. Water ice mounds or domes are observed elsewhere on Mars, such
as within impact craters in the north polar region, and are hypothesized to form from local increases in
deposition and decreases in ablation [Brown et al., 2008; Conway et al., 2012; Brothers and Holt, 2016].
Formation of an elongated dome in the SPLD by enhanced local accumulation is thus plausible.
The subsurface elongate dome indicates that the SPLD was not deposited as a near-horizontal “layer cake.”
Instead, the emplacement of the SPLD involved uneven deposition of ice and dust, with local depocenters
accumulating relatively more material than other regions. The size and vertical extent of the elongate dome
suggests a relatively long-lived depositional region that would have been controlled by the climate. The fact
that the uppermost and lowermost layer packets (BFL and IL) have approximately the same areal extent
[Milkovich and Plaut, 2008] (Figure S4), which differs from the PLL sequence and the WRAP, suggests that
the IL does not have to be an erosional remnant of the earliest SPLD materials or another ancient ice
Geophysical Research Letters 10.1002/2017GL074069
WHITTEN ET AL. DEPOCENTER UNDERNEATH THE SLPD, MARS 8193
sheet. Rather, the IL may represent the maximum depositional extent of the fledgling SPLD when deposition
was more similar to today.
After an erosional hiatus, the larger areal extent and sequence thickness of the PLL means it would have
formed during a period of higher accumulation rates; increased accumulation is associated with periods of
minimal polar insolation and obliquity variations over the last 10 Myr [Levrard et al., 2007]. The presence of
SPLD materials within the craters Elim and Katoomba (43.6 km, 80.17°S, 96.80°E, and 51.2 km, 79.01°S,
127.81°E, in diameter, respectively) hint that the SPLD, specifically the PLL layer sequence that corresponds
to these intracrater materials, may have been modestly more extensive in the recent past [Byrne and
Ivanov, 2004; Rodriguez et al., 2015]. The BFL sequence was subsequently deposited during a period of time
when the zone of accumulation decreased in areal extent. Lastly, the WRAP was deposited, offset from the
SPLD center and covering an even smaller area than the BFL. This smaller extent indicates a further decrease
in the zone of accumulation in the south polar region of Mars.
6. Conclusions
The detection of non-horizontal SHARAD radar echoes that form an elongate dome beneath the SPLD sur-
face, coupled with the gradual decrease in the area covered by the zone of accumulation, displays the evolu-
tion of the south polar climate over the last >100 Myr. The elongate dome formed during a period of
enhanced local deposition within the SPLD. Comparisons of the areal extent of specific polar units (IL, PLL,
BFL, andWRAP) with the location of the subsurface dome reveal the oscillatory (in extent and center location)
behavior of the ice/snow depocenter in the south polar region of Mars.
References
Bibring, J.-P., et al. (2004), Perennial water ice identified in the south polar cap of Mars, Nature, 428, 627–630, doi:10.1038/nature02461.
Bierson, C. J., R. J. Phillips, I. B. Smith, S. E. Wood, N. E. Putzig, D. Nunes, and S. Byrne (2016), Stratigraphy and evolution of the buried CO2
deposits in the Martian south polar cap, Geophys. Res. Lett., 43, 4172–4179, doi:10.1002/2016GL068457.
Brothers, C. T., and J. W. Holt (2016), Three-dimensional structure and origin of a 1.8 km thick ice dome within Korolev Crater, Mars, Geophys.
Res. Lett., 43, 1443–1449, doi:10.1002/2015GL066440.
Brown, A. J., S. Byrne, L. L. Tornabene, and T. Roush (2008), Louth crater: Evolution of a layered water ice mound, Icarus, 196, 433–445,
doi:10.1016/j.icarus.2007.11.023.
Byrne, S., and A. B. Ivanov (2004), Internal structure of the Martian south polar layered deposits, J. Geophys. Res., 109, E11001, doi:10.1029/
2004JE002267.
Campbell, B. A., G. A. Morgan, N. E. Putzig, J. W. Holt, and R. J. Phillips (2014), Properties of the Mars polar layered deposits from radar
sounding, Intl. Conf. Mars 8, abstract 12350.
Campbell, B. A., G. A. Morgan, N. E. Putzig, J. L. Whitten, J. W. Holt, and R. J. Phillips (2015), Enhanced radar visualization of structure in the
south polar deposits of Mars, Lunar Planet. Sci. Conf. 46, abstract 2366.
Christian, S., J. W. Holt, S. Byrne, and K. E. Fishbaugh (2013), Integrating radar stratigraphy with high resolution visible stratigraphy of the
north polar layered deposits, Mars, Icarus, 226, 1241–1251, doi:10.1016/j.icarus.2013.07.003.
Colaprete, A., J. R. Barnes, R. M. Haberle, J. L. Hollingsworth, H. H. Kieffer, and T. N. Titus (2005), Albedo of the south pole on Mars determined
by topographic forcing of atmospheric dynamics, Nature, 435, 184–188, doi:10.1038/nature03561.
Condit, C. D., and L. A. Soderblom (1978), Geologic map of the Mare Australe area of Mars, USGS Misc. Invest. Ser. Map, I-1076.
Conway, S. J., N. Hovius, T. Barnie, J. Besserer, S. Le Mouélic, R. Orosei, and N. A. Read (2012), Climate-driven deposition of water ice and the
formation of mounds in craters in Mars’ north polar region, Icarus, 220, 174–193, doi:10.1016/j.icarus.2012.04.021.
Grima, C., W. Kofman, J. Mouginot, R. J. Phillips, A. Hérique, D. Biccari, R. Seu, and M. Cutigni (2009), North polar deposits of Mars: Extreme
purity of the water ice, Geophys. Res. Lett., 36, L03203, doi:10.1029/2008GL036326.
Herkenhoff, K. E., and B. C. Murray (1990), Color and albedo of the South Polar Layered Deposits on Mars, J. Geophys. Res., 95, 1343–1358,
doi:10.1029/JB095iB02p01343.
Herkenhoff, K. E., and J. J. Plaut (2000), Surface ages and resurfacing rates of the Polar Layered Deposits on Mars, Icarus, 144, 243–253,
doi:10.1006/icar.1999.6287.
Holt, J. W., K. E. Fishbaugh, S. Byrne, S. Christian, K. Tanaka, P. S. Russell, K. E. Herkenhoff, A. Safaeinili, N. E. Putzig, and R. J. Phillips (2010), The
construction of Chasma Boreale on Mars, Nature, 465, 446–449, doi:10.1038/nature09050.
Howard, A. D., J. A. Cutts, and K. R. Blasius (1982), Stratigraphic relationships within Martian polar cap deposits, Icarus, 50, 161–215,
doi:10.1016/0019-1035(82)90123-3.
Johari, G. P. (1976), The dielectric properties of H2O and D2O ice Ih at MHz frequencies, J. Chem. Phys., 64, 3998–4005, doi:10.1063/1.432033.
Koutnik, M., S. Byrne, and B. Murray (2002), South Polar Layered Deposits of Mars: The cratering record, J. Geophys. Res., 107(E11), 5100,
doi:10.1029/2001JE001805.
Levrard, B., F. Forget, F. Montmessin, and J. Laskar (2007), Recent formation and evolution of northern Martian polar layered deposits as
inferred from a global climate model, J. Geophys. Res., 112, E06012, doi:10.1029/2006JE002772.
Milkovich, S. M., and J. J. Plaut (2008), Martian South Polar Layered Deposit stratigraphy and implications for accumulation history, J. Geophys.
Res., 113, E06007, doi:10.1029/2007JE002987.
Milkovich, S. M., J. J. Plaut, A. Safaeinili, G. Picardi, R. Seu, and R. J. Phillips (2009), Stratigraphy of the Promethei Lingula, south polar layered
deposits, Mars, in radar and imaging datasets, J. Geophys. Res., 114, E03002, doi:10.1029/2008JE003162.
Murray, B. C., L. A. Soderblom, J. A. Cutts, R. P. Sharp, D. J. Milton, and R. B. Leighton (1972), Geological framework of the south polar region of
Mars, Icarus, 17, 328–345, doi:10.1016/0019-1035(72)90004-8.
Geophysical Research Letters 10.1002/2017GL074069
WHITTEN ET AL. DEPOCENTER UNDERNEATH THE SLPD, MARS 8194
Acknowledgments
We wish to thank the two anonymous
reviewers for their helpful comments
and suggestions. The SHARAD instru-
ment was provided to NASA’s Mars
Reconnaissance Orbiter (MRO) mission
by the Italian Space Agency (ASI), and its
operations are led by the DIET
Department, University of Rome “La
Sapienza” under an ASI contract. This
work was funded under JPL contracts to
the MRO-SHARAD science team. The
authors thank the MRO mission team
and the SHARAD instrument team for
their efforts in acquiring the radar
sounder data set. All data supporting
these results can be found in the sup-
porting information and NASA’s
Planetary Data System.
Murray, B. C., W. R. Ward, and S. C. Yeung (1973), Periodic insolation variations on Mars, Science, 180, 638–640, doi:10.1126/
science.180.4086.638.
Nye, J. F., W. B. Durham, P. M. Schenk, and J. M. Moore (2000), The instability of a south polar cap on Mars composed of carbon dioxide, Icarus,
144, 449–455, doi:10.1006/icar.1999.6306.
Pettinelli, E., G. Vannaroni, A. Cereti, F. Paolucci, G. Della Monica, M. Storini, and F. Bella (2003), Frequency and time domain permittivity
measurements on solid CO2 and solid CO2-soil mixtures as Martian soil simulants, J. Geophys. Res., 108(E4), 8029, doi:10.1029/
2002JE001869.
Phillips, R. J., et al. (2011), Massive CO2 ice deposits sequestered in the South Polar Layered Deposits of Mars, Science, 332, 838–841,
doi:10.1126/science.1203091.
Plaut, J. J., et al. (2007), Subsurface radar sounding of the South Polar Layered Deposits of Mars, Science, 316, 92–95, doi:10.1126/
science.1139672.
Putzig, N. E., R. J. Phillips, B. A. Campbell, J. W. Holt, J. J. Plaut, L. M. Carter, A. F. Egan, F. Bernardini, A. Safaeinili, and R. Seu (2009), Subsurface
structure of Planum Boreum from Mars Reconnaissance Orbiter Shallow Radar soundings, Icarus, 204, 443–457, doi:10.1016/j.
icarus.2009.07.034.
Richardson, M. I., and R. Wilson (2002), A topographically forced asymmetry in the martian circulation and climate, Nature, 416, 298–301,
doi:10.1038/416298a.
Rodriguez, J. A. P., et al. (2015), New insights into the Late Amazonian zonal shrinkage of the martian south polar plateau, Icarus, 248,
407–411, doi:10.1016/j.icarus.2014.08.047.
Seu, R., D. Biccardi, R. Orosei, L. V. Lorenzoni, R. J. Phillips, L. Marinangeli, G. Picardi, A. Masdea, and E. Zampolini (2004), SHARAD: The MRO
2005 shallow radar, Planet. Space Sci., 52, 157–166, doi:10.1016/j.pss.2003.08.024.
Seu, R., et al. (2007), SHARAD sounding radar on the Mars Reconnaissance Orbiter, J. Geophys. Res., 112, E05S05, doi:10.1029/2006JE002745.
Simpson, R. A., B. C. Fair, and H. T. Howard (1980), Microwave properties of solid CO2, J. Geophys. Res., 85, 5481–5484, doi:10.1029/
JB085iB10p05481.
Smith, D. E., et al. (2001), Mars Orbiter Laser Altimeter: Experiment summary after the first year of global mapping of Mars, J. Geophys. Res.,
106, 23,689–23,722, doi:10.1029/2000JE001364.
Smith, I. B., N. E. Putzig, J. W. Holt, and R. J. Phillips (2016), An ice age recorded in the polar deposits on Mars, Science, 352, 1075–1078,
doi:10.1126/science.aad6968.
Thomas, P., S. Squires, K. Herkenhoff, A. Howard, and B. Murray (1992), Polar deposits on Mars, in Mars, pp. 767–795, Univ. Arizona Press,
Tucson.
Titus, T. N., H. H. Kieffer, and P. R. Christensen (2003), Exposed water ice discovered near the South Pole of Mars, Science, 299, 1048–1051,
doi:10.1126/science.1080497.
Toon, O. B., J. B. Pllack, W. Ward, J. A. Burns, and K. Bilski (1980), The astronomical theory of climatic change on Mars, Icarus, 44, 552–607,
doi:10.1016/0019-1035(80)90130-X.
Zuber, M. T., R. J. Phillips, J. C. Andrews-Hanna, S. W. Asmar, A. S. Konopliv, F. G. Lemoine, J. J. Plaut, D. E. Smith, and S. E. Smrekar (2007),
Density of Mars’ South Polar Layered Deposits, Science, 317, 1718–1719, doi:10.1126/science.1146995.
Geophysical Research Letters 10.1002/2017GL074069
WHITTEN ET AL. DEPOCENTER UNDERNEATH THE SLPD, MARS 8195