Stratigraphy of Promethei Lingula, south polar layered deposits, Mars, in radar and imaging data sets

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114,, doi: /2008je003162, 2009 Stratigraphy of Promethei Lingula, south polar layered deposits, Mars, in radar and imaging data sets S. M. Milkovich, 1 J. J. Plaut, 1 A. Safaeinili, 1 G. Picardi, 2 R. Seu, 2 and R. J. Phillips 3 Received 9 April 2008; revised 20 November 2008; accepted 10 December 2008; published 10 March [1] The south polar layered deposits (SPLD) of Mars have been studied through imagery for decades. Now, two subsurface sounding radar instruments have collected data: the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument on Mars Express and the Shallow Radar (SHARAD) instrument on Mars Reconnaissance Orbiter are observing the SPLD at multiple frequencies (1.8 5 and 20 MHz, respectively). Both instruments detect subsurface reflections in the Promethei Lingula region of the SPLD. MARSIS detects up to three reflections within Promethei Lingula, in addition to detecting the basal contact between the SPLD and the underlying plains. SHARAD detects 10s of reflections without penetrating to the base of the SPLD. MARSIS reflections likely correlate to the boundaries of packets of reflections in the SHARAD data; whatever change in composition of the SPLD that causes the SHARAD reflections to occur in packets may also be the source of the MARSIS reflections. In several locations, SHARAD reflections can be compared directly with images of layers; an individual reflection corresponds to 3 7 layers in images at resolutions of 6 m/pixel. Reflection surfaces are calculated using the radar observations and extrapolated to the SPLD margins where they are compared to the stratigraphy observed in images. We find that the MARSIS reflections, and thus the packet structure within the SHARAD observations, are likely related to distinctive groups of layers rather than individual layers. The radar data sets allow us to confirm several predictions concerning the interior of the SPLD from stratigraphic studies of images, including that most of the layers extend throughout the region and that they decrease in elevation toward the margin of the SPLD. Citation: Milkovich, S. M., J. J. Plaut, A. Safaeinili, G. Picardi, R. Seu, and R. J. Phillips (2009), Stratigraphy of Promethei Lingula, south polar layered deposits, Mars, in radar and imaging data sets, J. Geophys. Res., 114,, doi: /2008je Introduction 1.1. South Polar Layered Deposits [2] The south polar layered deposits (SPLD) are kilometers thick water ice rich deposits found at the south pole of Mars. The SPLD have been studied extensively by a number of imaging instruments, including Mariner 9 [Murray et al., 1972], Viking [e.g., Thomas et al., 1992], Mars Global Surveyor [e.g., Malin and Edgett, 2001], Mars Odyssey [Kolb and Tanaka, 2006; Milkovich and Plaut, 2008], and recently Mars Reconnaissance Orbiter [Herkenhoff et al., 2008]. From these observations, the following inferences have been drawn: visibility of individual layers of the SPLD, exposed on the walls of the troughs and scarps that cut into the deposit, are due to varying amounts of dust mixed in with the ice [e.g., Thomas et al., 1992] or to changes in the 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. 2 INFOCOM Department, La Sapienza University of Rome, Rome, Italy. 3 Department of Space Studies, Southwest Research Institute, Boulder, Colorado, USA. Copyright 2009 by the American Geophysical Union /09/2008JE surface texture which in turn may be related to variations in physical properties such as ice grain sizes [e.g., Milkovich and Head, 2006]. Individual layers are continuous over hundreds of kilometers [Kolb and Tanaka, 2006] and certain stratigraphic sequences of layers can be found over a thousand kilometers apart [Milkovich and Plaut, 2008]. Layers are not horizontal, but neither do they precisely follow the overall topography of the deposit; in some places they are best described as a parabolic dome [Byrne and Ivanov, 2004]. While the details of the formation process of the layers remains unknown, it is thought that the varying ice to dust ratios of individual layers is related to the effect of quasiperiodic orbital variations on the Martian climate and thus on the material accumulated at the polar regions [e.g., Cutts and Lewis, 1982]. The presence of unconformities implies several episodes of removal of SPLD material followed by further accumulation [Kolb and Tanaka, 2006; Milkovich and Plaut, 2008]. Craters on the surface of the deposits suggest surface ages of Ma [Herkenhoff and Plaut, 2000; Koutnik et al., 2002; Plaut, 2005], though there is evidence that the upper surface of the SPLD in places is exposing different portions of the stratigraphic sequence and thus may vary in age [Milkovich and Plaut, 2008]. 1of21

2 Figure 1. (a) MOLA topography of the south polar region of Mars with locations of Promethei Basin, Promethei Chasma, Australe Sulci, and Promethei Lingula (box, location of Figure 1b) indicated. (b) Locations of radar data analyzed within the Promethei Lingula region. Black lines indicate MARSIS orbits while white lines indicate SHARAD orbits. Numbers identify orbit numbers. White circles indicate location of THEMIS images in Figures Base map is MOLA shaded relief. [3] The Promethei Lingula region of the SPLD ( E, Figure 1) is a lobe of the deposit that extends into the Prometheus Basin and is bounded by Chasma Australe, a major reentrant, on one side and Promethei Chasma on the other. Away from the deposit margins, few troughs or scarps are observed within the majority of the northern portion of Promethei Lingula. The southern portion where Promethei Lingula joins with the rest of the SPLD contains several troughs and scarps, including the canyon system Australe Sulci. Investigation of Promethei Lingula in visible wavelengths indicates that the same sequence of layers can be found throughout the region [Milkovich and Plaut, 2008] and that the region has seen three major episodes of accumulation separated by two periods of erosion [Kolb and Tanaka, 2006] Radar Sounders [4] Recently, two radar instruments have begun collecting information about the subsurface of the polar regions. The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) sounding radar onboard ESA s Mars Express emits a 1 MHz bandwidth pulse and operates in 4 bands centered at 1.8, 3.0, 4.0, and 5.0 MHz. The radargrams shown here are from the 4.0 and 5.0 MHz band. The along-track footprint of the radar is 5 10 km after onboard synthetic aperture processing. Radar processing includes phase distortion and ionospheric delay corrections. The cross-track footprint is km, and the vertical resolution in free space is 150 m [Picardi et al., 2004]. MARSIS is able to penetrate through the stack of layers and obtain reflections off of the contact with the basement material. The strength of this basal reflection indicates that the material of the SPLD is almost pure water ice [Plaut et al., 2007]. Shallow Radar (SHARAD) on NASA s Mars Reconnaissance Orbiter operates at a central frequency of 20 MHz with a 10 MHz bandwidth. The along-track footprint is km after onboard processing and the cross-track footprint is 3 7 km. The vertical resolution in free space is 15 m [Seu et al., 2004]. SHARAD also penetrates the stack of layers although it does not reach the basement material [Seu et al., 2007]. [5] The fact that MARSIS penetrates to the base of the SPLD but SHARAD does not could be due to several factors. Although the bulk composition of the SPLD is most likely water ice, the internal structure (such as fracturing or inhomogeneity distribution) can cause significant propagation losses at higher-frequency bands. The wavelength of SHARAD at 20 MHz is 4 times shorter than MARSIS s 5 MHz and as a result structures on the order of few meters could cause severe propagation loss in SHARAD observations while remaining almost transparent to MARSIS. There may also be a transmission loss effect from surface roughness at scales comparable to the radar wavelength. For a given surface roughness, SHARAD would be more susceptible to this than MARSIS. This effect would be seen in the strength of the surface reflection: if the surface echo is dim then the radar has been scattered by the rough surface but if the reflection is bright, the surface is smooth and the propagation loss is due to effects from the interior of the SPLD. [6] Both radars detect subsurface reflections at multiple depths within the SPLD in several locations [Plaut et al., 2007; Milkovich and Plaut, 2007; Milkovich et al., 2007; Seu et al., 2007], but most clearly in Promethei Lingula. These reflections could be caused by individual layers or by packets of layers [Nunes and Phillips, 2006], or they could be interference effects such as multiples and reverberations related to the scale of the SPLD layering relative to the radar wavelengths [Plaut et al., 2007]. [7] In this analysis, we describe the stratigraphy of Promethei Lingula as seen by both the radar instruments and seek to answer the question: can radar reflections be correlated to layers within images? Reflecting layers within ice deposits have been observed with radar sounders on Earth [e.g., Bailey et al., 1964; Robin et al., 1969]. Research 2of21

3 Figure 2. MARSIS radargrams. In each radargram, horizontal scale bar is 50 km long and vertical scale bar represents 250 m depth in water ice. (a) Orbit (b) Graph of reflections within orbit (c) Orbit (d) Graph of reflections within orbit (e) Orbit (f) Graph of reflections within orbit (g) Orbit (h) Graph of reflections within orbit (i) Orbit (j) Graph of reflections within orbit (k) Orbit (l) Graph of reflections within orbit (m) Orbit (n) Graph of reflections within orbit of21

4 Figure 2. (continued) 4of21

5 Figure 3. SHARAD data from Promethei Lingula. (a) Radargram from orbit (b) Radargram from orbit Note dark zones of no reflections that break radar stratigraphy into packets. The pole is toward the left and the edge of the deposits is toward the right in both radargrams. into the source of these reflections has focused on density variations due to pore spaces or bubbles, conductivity variations due to acidity changes caused by volcanic aerosols and alkaline dust layers, and variations in ice grain orientation as the source of these reflections [e.g., Dowdeswell and Evans, 2004, and references therein]. While reflections correlating to dusty layers are observed within terrestrial ice sheets, they tend to be due to the ph of the dust influencing the acidity (and therefore the conductivity) of the ice [e.g., Taylor et al., 1993; Eisen et al., 2003] rather than the amount or mineral composition of the dust. Lab experiments on the radar response of ice-dust mixtures suitable for Martian conditions are underway [Heggy et al., 2007]. [8] To determine which of the above physical changes with depth may be important for radar studies of the SPLD it is necessary to consider the known physical properties of the Martian ice deposits and the limitations of the data sets available. Major density variations in terrestrial ice are confined to the upper few 100 m of an ice sheet where firn is present and are thus not applicable to the SPLD. However, volcanic aerosols and variations in grain orientation can be found at depth in ice sheets and may be present within the SPLD as well. Theoretical studies indicate that at frequencies of tens to 100 MHz both mechanisms can cause radar reflections; at lower-frequencies reflections are predominately due to changes in acidity, while at higherfrequencies reflections are predominately due to changes in the ice fabric [Fujita and Mae, 1994]. While MARSIS frequencies are within the range where reflections are predominately caused by conductivity variations due to changes in ice acidity, a layer of ice with the correct grain orientation, if it exists, may cause a reflection detectable by SHARAD. We currently have no information about the ice grain orientation or chemical composition (i.e., acidity of the ice or the presence of small amounts of clathrates) of the SPLD, and so operate under the assumption that fluctuations in ice to dust ratios cause large enough changes in dielectric constant to produce a radar reflection. Modeling by Nunes and Phillips [2006] of layered basaltic dust and ice indicates that dusty layers will cause reflections at both SHARAD and MARSIS frequencies but the resulting reflections may be different for each radar. A sequence of multiple dusty layers may cause a single radar reflection in either SHARAD or MARSIS, and multiple reflections in SHARAD may correlate to a single MARSIS reflection. [9] It is important to note that the radargrams shown throughout display radar returns in time and not distance; conversion to depth below the surface requires assumptions about the material the radar is passing through. We calculate depth assuming a real dielectric constant for pure ice of 3.0. Laboratory experiments at 245 K measure a real delectric constant of for pure ice [Heggy et al., 2007]; Martian polar temperatures are colder than this, which, if it changes the value at all, would lower the dielectric constant. The radargrams in this paper are given vertical scale bars calculated with this dielectric. However, they only apply to the portion of the radargram below the surface reflection corresponding to the SPLD; above the surface the radar pulse is traveling through free space and the dielectric constant for ice does not apply. Since the distances scale by the inverse of the square root of the dielectric constant, small variations in material (i.e., dust content) will not have a profound effect on the depths calculated here. For example, an ice sample with 25% basaltic dust by mass fraction has a dielectric constant of 3.4 at 245 K [Heggy et al., 2007] which corresponds to less than a 10% change in depth 5of21

6 Figure 4. (a) Radargrams from orbits 2431 and 2413, crossing SHARAD orbits, combines to form a single radargram illustrating commonality in layer depths. The vertical white line indicates where the two radargrams cross and is also the location of each radargram closest to the pole. Reflections match and are continuous between the radargrams. (left) Orbit (right) Orbit (b) Graph of reflections with depth from each radargram in Figure 4a, the y axis is the location of the crossing point. compared to that in pure ice. We find this acceptable considering the resolutions of the many data sets used in this analysis. [10] In addition, even on Earth where ground penetrating radar observations of ice sheets can be directly compared to ice cores with their wealth of in situ data, including variations in chemistry, density, grain size, grain orientation, and electrical conductivity with depth, it is difficult to clearly and unambiguously identify the sources of radar reflections [e.g., Eisen et al., 2003]. Efforts to do the same with the Martian radar observations are further complicated by our poor knowledge of the chemistry of the SPLD and the fine-scale distribution of dust with depth, and even our imperfect understanding of how an individual layer within the SPLD forms and which processes have affected the layer since formation. The work reported here represents a first step at integrating the radar observations with our current understanding of the stratigraphy, internal structure, and history of the SPLD. 2. MARSIS Observations [11] Seven orbits of MARSIS data were analyzed; their locations are shown in Figure 1. Radargrams for specific orbits were selected for their clarity (containing clear multiple subsurface reflections) and their distribution, in order to cover a wide range of locations within Promethei Lingula. Reflections must be observed in two frequency bands (4 and 5 MHz) in order to be included for analysis. In some locations within the SPLD, the depth and brightness of a reflection sometimes varies with frequency. These reflections may be caused by interference effects due to the scales of the SPLD layering and the radar wavelength [Plaut et al., 2007], where a downward propagating wave 6of21

7 Figure 5. Local unconformity in SHARAD radargrams. In each radargram, the poleward direction is to the left. (a) Orbit No unconformity is observed. (b) Orbit Dashed lines outline wedge of reflections that may be an unconformity. (c) Orbit Dashed lines outline wedge of reflections that may be an unconformity. (d) Orbit Dashed lines outline wedge of reflections that may be an unconformity. (e) Orbit Dashed lines outline wedge of reflections that may be an unconformity. (f) Orbit No unconformity is observed. (g) Locations of radargrams in Figures 5a f. Shaded region indicates location of wedge of reflections that may be an unconformity. and an upward reflective wave constructively interfere to create a standing wave and thus an observed radar return. The interference pattern (depth to reflection) would vary with frequency [Farrell et al., 2008]. Such a phenomenon is not observed in any of the radargrams used for this analysis, nor for those used in an analysis of the nearby head of Chasma Australe [Farrell et al., 2008]. [12] Up to three internal (i.e., not surface or basal) reflections are observed in the MARSIS data (Figure 2); these reflections extend for distances between 10 and 200 km. In radargrams where all three reflections are observed, the uppermost reflection does not extend for the same distance horizontally as the lower two. Some reflections are continuous along the entire radargram (e.g., orbits 2783 and 2645) while others fade in and out (e.g., orbits 2711 and 2506). Reflections where orbits cross are at the same elevations in each radargram (within 100 m, the vertical resolution of the data) and the reflections in the remaining orbits are at similar elevations; we therefore conclude that the same reflectors are being observed in each orbit. Thus, MARSIS observes three significant reflective surfaces within Promethei Lingula. Each surface generally trends with the basal reflection rather than with the surface topography, and tends to have a slight dip toward the margin of the deposits over the length of the observed reflection. 3. SHARAD Observations [13] Between 20 and 50 internal reflections can be seen in the SHARAD radargrams (the locations of each orbit analyzed are white lines in Figure 1, and examples of the radargrams are in Figure 3). Radargrams are labeled with a mode number, which identifies which portion of the orbit they are from. Many radargrams show a subsurface echo immediately following the surface echo; the second reflection is an artifact of the processing involved in the creation of the radargrams. The radargrams analyzed here display a structure where 8 10 reflections form groups or packets 7of21

8 Figure 6. Local unconformity in SHARAD radargram. (a) Orbit White box indicates location of Figure 6b. (b) Closer look at a portion of the radargram containing truncating reflections (indicated by arrows). (c) Location of orbit Highlighted region corresponds to area where truncated reflections in Figure 6b are observed. separated by nonreflective zones. Many packets are further divided into groups of 2 4 reflections bounded by brighter reflections. Individual reflections are parallel to subparallel and can be traced in a single radargram over distances of several hundred kilometers with a few notable exceptions described below. SHARAD does not detect a strong basal reflection; instead, the lower reflections are faint and discontinuous, and occur at depths of 1 km. This is due to attenuation of the radar signal [Seu et al., 2007]. [14] Comparisons of crossing orbits, such as those of 2431 and 2413 in Figure 4, reveal that reflections in each radargram are at the same elevation. Neighboring but noncrossing orbits display the same packet structure with nonreflective zones occurring at comparable elevations (e.g., Figure 5). This implies that the same reflections can be found in multiple orbits, consistent with reflections caused by internal layering. The regional stratigraphy as observed by SHARAD contain a lower sequence of near-horizontal reflecting surfaces extending hundreds of kilometers throughout the Promethei Lingula region underneath a sequence of reflections that gently dip toward the northern margin of the SPLD. [15] A few reflections, however, are not continuous. As described by Milkovich et al. [2007] and Seu et al. [2007], several orbits (including 2202, 2624, 3890, and 4312, Figure 5) contain a number of reflections that form a wedge of SPLD that narrows and ultimately pinches out toward the SPLD margin. Within this wedge, reflections that make up this wedge cannot be seen to truncate against another reflection. Additional reflections overlie much of this wedge of SPLD, and are oriented at an angle with respect to the underlying layers. These reflections also are subparallel to each other and are truncated at the surface of the SPLD. Seu et al. [2007] proposed that these features represent a buried angular unconformity, but since no reflection is seen to truncate against another reflection the authors did not rule out an unusual depositional history such as shifting patterns of accumulation near a trough structure. [16] We revisit these features and examine their broader geological context to determine if a stronger case can be made that they are an angular unconformity (Figure 5). Available orbits as of April 2008 reveal that the wedge is restricted to E; neither orbit 4734 to the west nor orbit 4330 to the east contains reflections that disappear, although both contain reflections that truncate with the surface. The wedge itself is 200 m thick and 100 km long at its largest location, and gets thinner to the west. As of April 2008, no similar wedge of reflections has been observed elsewhere within the SPLD despite thorough SHARAD coverage of the deposits. We therefore search this area for additional unconformities or other unusual features. [17] Chasma Australe cuts through to the base of the SPLD to the west, but the wedge does not extend to the walls of the chasma. We conclude that this feature is unlikely to be related to Chasma Australe. The region E lies at the mouth of Australe Sulci, a broad canyon feature cutting 500 m into the SPLD and examined in both imaging and topography data by Koutnik et al. [2005] and Kolb and Tanaka [2006]. Koutnik et al. [2005] described a number of linear, subparallel grooves on the surface, tens of meters deep and hundreds of meters long. The authors interpreted these features to be indicative of recent, but not ongoing surface erosion due to katabatic winds being channeled through the canyon. Kolb and Tanaka [2006] investigated the layer stratigraphy exposed in the canyon walls and concluded that an earlier episode of erosion occurred in this region followed by further accumulation as demonstrated by layers draped over underlying SPLD. On the basis of the evidence for several episodes of erosion concentrated in this region of Promethei Lingula and the lack of a narrowing wedge of reflections elsewhere, we conclude that the features observed in these radargrams 8of21

9 Figure 7. Comparison of MARSIS 2783 and SHARAD (a) Locations of radargrams. (b) Elevations of all reflections in MARSIS 2783 (gray line with circles) and major reflections in SHARAD 2202 (black line with diamonds). Error bars on MARSIS data indicate range of elevations corresponding to the resolution of the instrument. The uppermost surface is from MOLA data at the location of each orbit. (c) SHARAD 2202 radargram. Lines trace reflections that most closely correspond to MARSIS reflections; note that these are not the MARSIS reflections directly projected onto the SHARAD radargram. The pole is toward the left and the edge of the deposits is toward the right in the radargram. are most likely to be an angular unconformity and probably caused by erosion related to, although not likely directly caused by, the proximity of Australe Sulci, located immediately to the south (Figure 5). The history within the radargrams thus is: initial deposition of a near-horizontal layer sequence, followed by erosion at an angle, with renewed deposition on the angled surface and finally erosion to the current surface topography. The lack of reflections clearly truncated by other reflections may be due to layers being eroded to thicknesses below the resolution of the radar. [18] Additional unusual reflections are seen in a thicker region of SPLD at the southwest portion of Promethei Lingula, near 108 E, 84 S. Reflections in orbit 3328 (Figure 6) are truncated in several locations, forming z-like patterns that are likely due to truncating layers, which in turn indicate a complex history of accumulation and erosion in the upper portions of the SPLD. Orbits on either side and less than 25 km away do not contain such clear truncations, implying that this may be a local phenomenon. [19] Overall, SHARAD observations within the Promethei Lingula region indicate that layers generally dip toward the northern margin of the SPLD. In the stratigraphy we identify a large unconformity and several smaller unconformities, implying at least three episodes of widespread accumulation separated by local erosion. Despite this complex history, individual reflections can be traced for several hundred kilometers and across multiple radargrams. 4. MARSIS-SHARAD Comparisons [20] As the SHARAD and MARSIS reflections are both caused by the same sequence of layers within the SPLD, understanding how these data sets relate to each other may ultimately provide insight into the nature of the SPLD. We take the first steps in such an analysis by looking for features in the SHARAD radargrams that are consistently 9of21

10 Figure 8. Comparison of MARSIS 2783 and SHARAD (a) Locations of radargrams. (b) Elevations of all reflections in MARSIS 2783 (gray line with circles) and major reflections in SHARAD 2413 (black line with diamonds). Error bars on MARSIS data indicate range of elevations corresponding to the resolution of the instrument. The uppermost surface is from MOLA data at the location of each orbit. (c) SHARAD 2413 radargram. Lines trace reflections that most closely correspond to MARSIS reflections; note that these are not the MARSIS reflections directly projected onto the SHARAD radargram. The pole is toward the left and the edge of the deposits is toward the right in the radargram. at similar depths to the MARSIS reflections, and assume that such features are caused by the same portion of the SPLD layer sequence. [21] We compare reflections observed by MARSIS to those observed by SHARAD in seven MARSIS orbits and fifteen SHARAD orbits by comparing the elevations of reflections in neighboring and crossing orbits shown in Figure 1. Each MARSIS reflection was included in this analysis; however only the brightest SHARAD reflections were included. Each frame (the vertical bar of pixels representing the response to an individual radar pulse) of a MARSIS radargram that contains an internal reflection was measured, while 20 frames in each SHARAD radargram were measured. The number and location of these frames was selected to best describe the variations in depth of reflections. The entire SHARAD radargram was not included because of the size of the data set (up to 50 reflections traceable in many cases over more than 1000 frames per radargram). The elevations of each reflection were calculated by measuring the time delay between the surface and the reflection in question, converting to distance in pure ice using a dielectric constant of 3.0, and subtracting this from the elevation of the surface as measured by Mars Orbiter Laser Altimeter (MOLA). The difference in wavelength between the two data sets means a difference in resolution, which is a source of error in this comparison. In each comparison, the reflections from MARSIS and SHARAD orbits that either are roughly parallel (Figures 7 and 8) or are crossing (Figures 9 11) were plotted on the same axes. For parallel orbits, measurements from the entire radargram for both radars were included. For crossing orbits only the data from the points at which the orbits crossed were included. [22] Figures 7 and 8 show the results of comparing several neighboring, roughly parallel orbits. In both comparisons, the upper two MARSIS reflections follow SHARAD reflections closely, while the lowest MARSIS reflection trends with another SHARAD reflection. The SHARAD radargram in Figures 7c and 8c has the reflections 10 of 21

11 Figure 9. Comparison of MARSIS 2711 and SHARAD (a) Locations of radargrams. (b) Elevations of all reflections in MARSIS 2711 (gray line with circles) and major reflections in SHARAD 2202 (black line with diamonds). Error bars on MARSIS data indicate range of elevations corresponding to the resolution of the instrument. The uppermost surface is from MOLA data at the location of each orbit. (c) SHARAD 2202 radargram. Lines trace reflections that most closely correspond to MARSIS reflections; note that these are not the MARSIS reflections directly projected onto the SHARAD radargram. The pole is toward the left and the edge of the deposits is toward the right in the radargram. that thus correspond to the MARSIS reflections highlighted (note that these are not the MARSIS reflections directly plotted over the SHARAD radargram). Figures 9 11 show the results of comparing crossing orbits. In Figures 10 and 11 in particular, three MARSIS orbits are compared to a SHARAD orbit. In each case, all three MARSIS orbits intersect at the same SHARAD reflection for the upper and middle MARSIS reflections, while the lowest MARSIS reflection is close to the region where the SHARAD reflections are difficult to detect because of signal attenuation. Again, the SHARAD radargram in Figures 10c and 11c has the reflections that correspond to the MARSIS reflections highlighted. [23] Through a comparison of the radargrams in Figures 7c, 8c, 9c, 10c, and 11c, it becomes apparent that each SHARAD radargram has the same reflections highlighted as corresponding to MARSIS reflections. The MARSIS reflections seem to occur near the dark regions in the SHARAD radargrams which break the SHARAD stratigraphy into packets. In particular, the first SHARAD reflection after the uppermost dark region corresponds to the upper MARSIS reflection, the final SHARAD reflection before the second major dark region corresponds to the middle MARSIS reflection, and a SHARAD reflection surrounded by dark zones toward the base of the radargram corresponds to the lower MARSIS reflection. We conclude that the MARSIS reflections correlate with these bands within the SHARAD stratigraphy.however,wedonotattachanymajorsignificance to the fact that one MARSIS reflection appears to correlate to the SHARAD reflection at the top of a band and another to the SHARAD reflection at the bottom of a band given the difference in resolution between the two radars. [24] What is the significance of these dark bands? One possibility is that they indicate that the ice is cleaner (i.e., contains less dust) than the surrounding layers, and thus are more transparent to the radar. The transition to dustier ice may therefore produce a large enough change in dielectric properties to cause strong SHARAD reflections and a 11 of 21

12 Figure 10. Comparison of SHARAD 4286 and MARSIS orbits 2945, 2506, and (a) Locations of radargrams. (b) Elevations of all reflections in MARSIS radargrams (gray line with circles) and major reflections in SHARAD 4286 (black line with diamonds). Error bars on MARSIS data indicate range of elevations corresponding to the resolution of the instrument. The uppermost surface is from MOLA data at the location of each orbit. (c) SHARAD 4286 radargram. Lines trace reflections that most closely correspond to MARSIS reflections; note that these are not the MARSIS reflections directly projected onto the SHARAD radargram. The pole is toward the left and the edge of the deposits is toward the right in the radargram. MARSIS reflection. There are certainly other radar dark bands within the SHARAD radargrams that do not correlate with features within the MARSIS radargrams; these may represent clean ice layers that do not have enough of a contrast in dust content with their neighbors to produce a MARSIS reflection. MARSIS basal reflections indicate that the SPLD contains less than 10% dust [Plaut et al., 2007]. 5. Radar-Image Comparisons 5.1. Surface of Promethei Lingula [25] Many of the SHARAD radargrams contain reflections that are truncated at the surface of the SPLD. This allows us to directly compare the individual reflections with layers exposed by topographic variations on the Promethei Lingula surface. Reflections in four SHARAD orbits examined in this way intersect the surface at locations of the SPLD where layers are seen in Thermal Emission Imaging System (THEMIS) 35 m/pixel visible images which lie along the SHARAD ground track. Layers in each image correlate with an individual reflection; an example is shown in Figure 12. Along SHARAD orbit 4312, it is possible to compare not only with THEMIS V but also Mars Orbiter Camera (MOC) image R11/03432, at 6 m/pixel (Figure 13). At higher resolution, it can been seen that what appears to be a single layer at THEMIS scales is actually multiple (3 7) layers eroding in groups, resulting in a stairstepped topographic profile. The underlying physical properties of the layers causing this erosional behavior may also be the cause of a radar reflection. For example, a layer or group of layers that erode more easily than those above and below may have a higher dust to ice ratio and therefore a different dielectric response, resulting in a radar reflection Margin of Promethei Lingula [26] The majority of layers within Promethei Lingula are exposed on the margins of the deposit. It is impossible in most places, however, to directly compare them to the radar as in the previous section because individual reflections within a radargram are not continuous to the margin of the SPLD. Therefore we must extrapolate from the radar observations of the interior to the deposit margins where we can compare them to the optical images. 12 of 21

13 Figure 11. Comparison of SHARAD 3297 and MARSIS orbits 2945, 2506, and (a) Locations of radargrams. (b) Elevations of all reflections in MARSIS radargrams (gray line with circles) and major reflections in SHARAD 3297 (black line with diamonds). Error bars on MARSIS data indicate range of elevations corresponding to the resolution of the instrument. The uppermost surface is from MOLA data at the location of each orbit. (c) SHARAD 3297 radargram. Lines trace reflections that most closely correspond to MARSIS reflections; note that these are not the MARSIS reflections directly projected onto the SHARAD radargram. The pole is toward the left and the edge of the deposits is toward the right in the radargram. [27] By comparing crossing orbits and using stratigraphic correlations between orbits, a three dimensional picture of the internal structure of the SPLD as observed by radar can be assembled. These correlations allow us to interpolate a surface to fit the measured elevations of SHARAD reflections corresponding to the MARSIS reflections and then extrapolate this surface to the margins of Promethei Lingula. The inverse distance weighting tool within ESRI s ArcGIS program (with power parameter between 2 and 3) was used to perform this analysis on the 15 SHARAD orbits shown in Figure 1. Inverse distance weighting was chosen as it is one of the simplest interpolation algorithms and gives greater importance to the data points closest to the interpolated surface location. While data from orbits containing an unconformity were used, only reflections from above or below the unconformity were used; orbits containing these reflections and no unconformity were included as well. [28] The three resulting surfaces are found in Figure 14. In addition, each map displays the individual SHARAD measurements used to calculate the surfaces and the outline of the SPLD according to the mapping of Kolb and Tanaka [2001]. The upper reflector has 540 ± 40 m relief (Figure 14a), the middle reflector 420 ± 35 m relief (Figure 14b), and the lowest reflector has 360 ± 30 m relief (Figure 14c). Each reflector trends lower in elevation as it approaches the margin of the SPLD. [29] At the margin of Promethei Lingula the elevations of the surfaces can be compared to the sequence of exposed layers as observed in images registered to the MOLA gridded topography. THEMIS 17 m/pixel images were selected for comparison because they are a good compromise between resolution (allowing variations in layers to be seen) and spatial coverage (allowing enough of the surface to be seen in a single image that features such as craters can be used to confirm the alignment between images and topography). MOC images, with much higher resolutions, are often misaligned and usually lack features useful for this purpose. 13 of 21

14 Figure 12. Comparison of reflections intersecting with the surface and layers exposed on the surface. (a) THEMIS visible V image (36 m/pixel). White solid line indicates ground track of SHARAD radargram. (b) Radargram of SHARAD 3869 coaligned with THEMIS image in Figure 12a. The pole is toward the left and the edge of the deposits is toward the right in the radargram. Dashed quasi-horizontal line indicates lowest reflection that intersects with the surface. Dashed vertical lines indicate where reflections intersect with the surface. Note correspondence with layers exposed in Figure 12a. [30] Three images were selected for comparison on the wall of Chasma Australe (locations of images are marked in Figure 1). Results of the comparison between the extrapolated reflector surfaces and the images can be seen in Figures We look for what sequence of layers is found at the same elevation (or depth) range as the extrapolated reflector surface and consider which of these layers is likely to cause a reflection. Given the finely layered nature of the SPLD in images, it would be possible to find an individual layer at any depth that might correspond with an individual radar reflection. Therefore we must look for significant changes in layering style, such as going from thin layers to thick, or layers that are eroding in a different manner than their neighbors, to look for correlations with significant radar reflections (in this case, the SHARAD reflections that we propose are correlated with the MARSIS reflections). [31] Along the Chasma Australe wall, all three images contain the same sequence of layers, as shown by the stratigraphic correlations of Milkovich and Plaut [2008] and shown in Figure 18. Furthermore, the packets of layers that correspond to the depth of the calculated reflector surfaces are the same in each image. In particular, a sequence of dark, thin layers near the bottom of the layer stratigraphy in this region correlate to the lowermost calculated reflector. A shift in layering appearance between a region where individual layers tend to be subtle and hard to distinguish and a region of thicker (40 m), bright, knobby, and therefore erosion resistant, layers may correlate to the upper calculated reflector, while the middle calculated reflector may be a region of the SPLD where an erosionresistant layer is surrounded by dark, thin layers. Individual SHARAD reflections likely are due to groups of layers between these transitions. A closer look at these layers is shown in Figure 19. We conclude that the same layers in each image correlate with the calculated reflector surfaces, which in turn correspond to both the MARSIS reflections and changes from multiple reflections to no reflections in SHARAD. Therefore, these features in the radar data sets correlate with distinctive groups of layers within the SPLD. 6. Conclusions [32] Comparisons of neighboring and crossing orbits indicate that MARSIS reflections correlate to the boundaries of packets of reflections in the SHARAD data; whatever change in composition of the SPLD that causes the SHARAD reflections to occur in packets may also be the source of the MARSIS reflections. The MARSIS reflections also are at depths corresponding to distinctive groups of layers in the optical images. These layers are likely distinctive because of variations in dust content from their neighbors, which would also cause variations in dielectric properties and therefore could be a source for the reflections. The stair-stepped topography of some layers eroding in groups on the surface of Promethei Lingula correlates with individual SHARAD reflections; we interpret this to imply that compositional variations (i.e., dust content) are 14 of 21

15 Figure 13. Comparison of reflections intersecting with the surface and layers exposed on the surface. (a) THEMIS visible V image (36 m/pixel). White solid line indicates ground track of SHARAD radargram. White box indicates location of Figure 13c. (b) Radargram of SHARAD 4312 coaligned with THEMIS image in Figure 13a. The pole is toward the left and the edge of the deposits is toward the right in the radargram. Dashed quasi-horizontal line indicates lowest reflection that intersects with the surface. Dashed vertical lines indicate where reflections intersect with the surface. Note correspondence with layers exposed in Figure 13a. (c) Subframe of V providing a closer look at the layers exposed on the surface. White boxes indicate locations of Figure 13d (upper box) and Figure 13e (lower box). (d) Subframe of MOC R11/03432 (6 m/pixel). Sun from lower right. (e) Subframe of R11/ Note that each layer in the THEMIS image corresponds to several layers in the MOC image eroding in a stairstepped fashion. 15 of 21

16 Figure 14. Reflectors calculated from SHARAD measurements that correlate to internal (nonbasal) MARSIS reflections. (a) Upper reflector, (b) middle reflector, and (c) lowest reflector. Dots indicate SHARAD measurements. 16 of 21

17 Figure 15. Comparison of calculated reflectors and image stratigraphy exposed in THEMIS visible image V (17 m/pixel). (a) Profile along A A 0 with MOLA topography of the surface (thin black line), upper (light gray line), middle (dark gray line), and lower (thick black line) calculated reflections from the subsurface. (b) Profile along B B 0 with MOLA topography of the surface (thin black line), upper (light gray line), middle (dark gray line), and lower (thick black line) calculated reflections from the subsurface. (c) Subframe of V Lines indicate locations of profiles in Figures 15a and 15b. Black crossing lines correspond to calculated reflector elevations as determined from parts A and B. White lines indicate error range in elevations, calculated from the standard deviation of the residuals of the inverse distance weighting calculation. 17 of 21

18 Figure 16. Comparison of calculated reflectors and image stratigraphy exposed in THEMIS visible light spectrometer (VIS) image V (17 m/pixel). (a) Profile along A A 0 with MOLA topography of the surface (thin black line), upper (light gray line), middle (dark gray line), and lower (thick black line) calculated reflections from the subsurface. (b) Profile along B B 0 with MOLA topography of the surface (thin black line), upper (light gray line), middle (dark gray line), and lower (thick black line) calculated reflections from the subsurface. (c) Subframe of THEMIS VIS image V Lines indicate locations of profiles in Figures 16a and 16b. Black crossing lines correspond to calculated reflector elevations as determined from parts A and B. White lines indicate error range in elevations, calculated from the standard deviation of the residuals of the inverse distance weighting calculation. 18 of 21

19 Figure 17. Comparison of calculated reflectors and image stratigraphy exposed in THEMIS VIS image V (17 m/pixel). (a) Profile along A A 0 with MOLA topography of the surface (thin black line), upper (light gray line), middle (dark gray line), and lower (thick black line) calculated reflections from the subsurface. (b) Profile along B B 0 with MOLA topography of the surface (thin black line), upper (light gray line), middle (dark gray line), and lower (thick black line) calculated reflections from the subsurface. (c) Subframe of THEMIS VIS image V Lines indicate locations of profiles in Figure 17a and 17b. Black crossing lines correspond to calculated reflector elevations as determined from parts A and B. White lines indicate error range in elevations, calculated from the standard deviation of the residuals of the inverse distance weighting calculation. Figure 18. Stratigraphic correlations of layer sequences in images from Figures ((left) V , (middle) V , and (right) V ). The three THEMIS images are aligned so that major layers can be seen to correlate although the locations of the images do not overlap; the lefthand image is 100 km away from the right-hand image. The floor of Chasma Australe can be seen at the base of each image; the vertical relief from the floor of Chasma Australe to the top of the layers is 700 m. Scale bar in each image represents 1 km horizontal distance. Upper, middle, and lower calculated reflectors, indicated by the lines as described in Figures 17 19, crop out at consistent regions within the stratigraphic sequence in each image. The dashed box indicates the location of Figure of 21

20 Figure 19. A closer look at the layers corresponding to the upper, middle, and lower calculated reflectors. MOC image E10/02641 (3 m/pixel). Vertical lines indicate range of elevations corresponding to error bars on the calculated reflections. Sun from the right. causing both changes in the mechanical properties of layers and radar reflections. [33] These results are all based on simple, first-order assumptions about the physical properties of the SPLD, including that the bulk SPLD are pure ice with a dielectric constant of 3 and that all features within the radargrams can be explained with changes in dust to ice ratio. Further analysis of this region with the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) 18 m/pixel data may prove fruitful. CRISM is onboard MRO and currently operating at Mars; it will enable the generation of maps of water ice grain size, dust particle size, and dust concentration for the surface exposures of the layers [Green et al., 2007]. These parameters are thought to have an effect on the physical properties of the layers. For example, a layer containing some amount of dust may absorb more insolation and thus sublimate faster than a pure ice layer [e.g., Milkovich and Head, 2006]. [34] The continuity of both the SHARAD and the MARSIS reflections, as well as their correlations with the layers in images, indicate that the stratigraphic sequence of SPLD exposed along the Promethei Lingula margin extends throughout the region. Reflections, and thus layers that cause them, are not horizontal, but are higher toward center of deposit and lower toward the margins, although they do not precisely mirror the topography of the SPLD surface but in some locations actually intersect with the surface. A buried angular unconformity is confined to a region near Australe Sulci. An additional unconformity may exist in a second part of Promethei Lingula, indicating that multiple episodes of significant erosion have occurred in local regions within Promethei Lingula during SPLD history. [35] Previous analyses of the stratigraphy and structure of the SPLD in the Promethei Lingula region have been based on imagery and topographic data [Kolb and Tanaka, 2006; Milkovich and Plaut, 2008]. These studies have identified correlatable layer sequences across this entire region [Milkovich and Plaut, 2008] and identified unconformities within the stack of layers [Kolb and Tanaka, 2006]. Layers generally dip gently toward the northern margin of Promethei Lingula [Kolb and Tanaka, 2006;Milkovich and Plaut, 2008]. All of these observations are consistent with our conclusions from the radar data. [36] Kolb and Tanaka [2006] propose a formation history for this region that includes three major stages of accumulation, designated as units Planum Australe 1a, Planum Australe 1b, and Planum Australe 2, separated by two periods of intense erosion, with Australe Sulci and other canyon features forming in the second major erosional period (between Planum Australe 1b and Planum Australe 2). The buried unconformity near Australe Sulci in the SHARAD radargrams (Figure 5) may therefore be associated with the first erosional period (between Planum Australe 1a and 1b), allowing for further accumulation (Planum Australe 1b) before the formation of Australe Sulci. It is not clear in the radar data outside this region where the boundaries of these units should be. [37] The results of this initial comparison between radar and imaging data sets are broadly consistent with earlier analysis of SPLD stratigraphy and provide some insight into the source of the radar reflections but are hampered by the many uncertainties in our understanding of the properties of the SPLD themselves. There are many outstanding questions concerning both the radar data and the region as a whole: What is the true behavior of the layers toward the deposit margin? Is the dust itself causing radar reflections, or is it causing acidity changes in the ice? Why are reflections not seen over the same horizontal distance in both MARSIS and SHARAD? What drove the localized erosion in the Australe Sulci region where we observe an unconformity? As additional imagery and radar data continue to be collected and as fine-scale compositional data begins to be available for this region, these questions can be revisited. 20 of 21