Mapping and volume calculations methodology In this ...

Mapping and volume calculations methodology In this paper, we have documented a major SPLD retreat stage following spiral trough development, and which led to extensive polar ice removal in Australe Scopuli and Sisyphi Planum. We have estimated that spiral trough formation resulted in the removal of ~7000 km3 from the polar plateau (Table 1). In contrast, the latest stage of SPLD resurfacing, which could be younger than 10 million years, appears to have resulted in the removal of ~ 58,000 km3. By comparison the Great Lakes contain ~23,000 km3, which is enough water to cover the 48 contiguous United States to a uniform depth of ~3 m. The trough volumes were calculated below conformable surfaces modelled from depression rim elevations derived from our vector mapping (Fig. S1a), which was performed using Esri Arc GIS (Geographic Information System). The basemap in our GIS project included a Mars Global Surveyor Mars Orbiter Laser Altimeter (MOLA) 115 m/pixel digital elevation model (DEM), as well as derived greyscale and color-coded hillshade mosaics and a 10-m-interval contour map (for example, Fig. S1a, which shows only 100-m contours). As required, spot values and elevation profiles were also locally checked to assist in the mapping. Where the 115 m/pixel DEM topography was not available (in northern portions of Ultima Lingula and Cavi Angusti), we utilized 230 m/pixel DEMs for our mapping. We modelled three separate capping surfaces that serve as proxies to Late Amazonian preerosional SPLD topography. To this purpose, the vector polygons representing the margins of mapped troughs were first converted to points (Fig. S1b). The rim points were “pushed” through the MOLA DEM to extract elevation attributes and generate TINs (triangulated irregular networks; Fig. S1c), which are then transformed to raster DEMs of the reconstructed topography (Fig. S1d). Our volumetric calculations were performed by subtracting the mapped eroded

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topography from the reconstructed paleo-surfaces, which produced a difference value for each pixel representing the eroded material depth (in meters) per pixel (Fig. S1e and S1f). The pixel area is multiplied by the sum of depths to derive total erosional volumes. Several small zones occur within the differential DEM that were subsequently assigned as null pixels. This occurs where topography rises above the modelled capping surface, for example where there is a buttelike feature nested within the depression, or otherwise where the modelled capping surface slices through “material” above the modelled surface (black arrows in Fig. S1f). These pixels are easily nullified as they are assigned negative values after subtracting the capping and original DEMs; they are omitted from the total volume determinations. We generated three capping DEMs: • One for the combined upper and lower spiral features. • One for troughs located in Australe Scopuli. There are only a few upper trough occurrences located within southeast Australe Scopuli system and their position above the ~2800 m elevation. Due to their very limited spatial occurrence for purposes of volume determinations we have grouped them together with the more abundant troughs in Australe Scopuli below this elevation. • One defining the pre-erosional surface in Sisyphi Planum. Spiral troughs in the north polar plateau are thought to have undergone poleward migration (Howard, 1978; Smith and Holt, 2010). However, detailed geologic mapping suggests that those in the south pole formed by in-situ dissection (Kolb and Tanaka, 2001). In our calculations, we assume that the margins of the mapped depressions outline pre-existing conformal surfaces, which is also in accordance with the finding that the upper SPLD strata appear to broadly conform to the domical shape of Planum Australe (Byrne and Ivanov, 2004).

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Sisyphi Planum includes multiple subtle depressions nested within broad plains, as well as deep curvilinear troughs that locally mark the Sisyphi scarp, providing evidence of regional in-situ SPLD erosion. Our assumption here is that prior to large-scale regional erosion, the polar plateau extended to the northernmost margin of highly eroded SPLD materials in Sisyphi Planum (Fig. 2a). In order to reconstruct the pre-erosional surface of this region, we generated a capping DEM that extended from the upper margin of the Sisyphi scarp to our mapped margins of eroded SPLD materials in Sisyphi Planum. Table S1. Estimates of SPLD erosional volumes resulting during spiral trough formation (Late Amazonian stage I) and the development of the Australe and Sisyphi scarps. Eroded area (km2)

Eroded volume (km3)

Spiral troughs

85,148

7,002

Australe Scopuli troughs

85,242

14,215

101,121

43,754

186,363

57,969

271,511

64,971

Erosional stage Late Amazonian (I)

Erosional feature

Late Amazonian (II) Sisyphi Planum

Late Amazonian (II) subtotal TOTAL SPLD EROSION

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Supplementary Figure 1. Sample area within Australe Scopuli showing key steps to determine eroded volumes from SPLD trough features. (a) Vector mapping extents of select troughs within Australe Scopuli (blue zones) and 100-m-interval contours (blue lines) on MOLA 115 m/pixel digital elevation model (DEM) hillshade base. Inset maps shows selected zone. (b) Trough rim points. Each point is attributed with its coincident MOLA DEM value. (c) TIN surface generated from elevation points; trough extents superposed for reference. (d) Raster DEM generated from TIN, which approximates pre-erosional 'capping' surface representing minimally eroded trough volumes. (e) Clipped capping surface. (f) Modelled capping surface minus the current DEM, indicating depth below cap in meters. Depths are multiplied by trough areas to derive eroded volumes. Black arrows indicate nullified zones where differential DEM values were negative. indicating where current features protrude above the capping surface (for example, at butte-like features). Crater counting methodology Impact crater populations were determined and analyzed using CraterTools (Kneissl et al., 2011) and Craterstats (Michael and Neukum, 2010), respectively. The counting area within Cavi Angusti includes the typical morphologies observed in the region and was chosen based on (1) available CTX coverage and quality, as well as (2) the fact that the average crater size-frequency distribution is well-represented in the region (i.e., if there were only one large impact crater, it would not have been included in the counting area as this sized crater would have altered the derived model age). The largest seven bins are aligned along an isochron. This particular crater size-frequency distribution is well-suited to derive an absolute model because of the large number of bins aligned along the isochron (except the largest bin, which is slightly above the isochron but within the statistical error bars). At the smaller diameter end, the curve deviates

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from the isochron, which essentially means that smaller diameter craters are not well-preserved in this environment or, more likely, have been modified (eroded or buried) by a particular geological process. The largest bin is included in the fit range because we interpreted this crater (only one crater in this bin) to be part of the CSFD (crater size-frequency distribution) that postdates the outcrop. There is no indication that this crater is modified in a way that it could have been interpreted to be part of an older, underlying unit. Also, if this crater was excluded from the fit range, the resulting model age would be 110 Ma, about 10 Myr less than in the current fit (and still within the statistical error of this method). Cited references Byrne, S., Ivanov, A. B., 2004. Internal structure of the Martian south polar layered deposits. Journal of Geophysical Research. 109. Howard, A. D., 1978. Origin of the stepped topography of the Martian poles. Icarus. 34. Kneissl, T., Van Gasselt, S., Neukum, G., 2011. Map-projection-independent crater sizefrequency determination in GIS environments – new software tool for ArcGIS. Planet. Space Sci. 59, 1243-1254. Kolb, E. J., Tanaka, K. L., 2001. Geologic History of the Polar Regions of Mars Based on Mars Global Surveyor Data II. Amazonian Period. Icarus. 154, 22-39. Michael, G. G., Neukum, G., 2010. Planetary surface dating from crater size-frequency distribution measurements: partial resurfacing events and statistical age uncertainty. Earth and Planetary Science Letters. 294, 223-229. Smith, I. B., Holt, J. W., 2010. Onset and migration of spiral troughs on Mars revealed by orbital radar. Nature. 465, 450-453.

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