Geomorphology 121 (2010) 22–29

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Geomorphology

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Transverse Aeolian Ridges on : First results from HiRISE images

James R. Zimbelman ⁎

Center for and Planetary Studies, MRC 315, National Air and Space Museum, Smithsonian Institution, Washington, D.C. 20013-7012, United States article info abstract

Article history: Three images obtained by the High Resolution Imaging Science Experiment (HiRISE) were analyzed for the Received 11 July 2008 information they could provide regarding Transverse Aeolian Ridges (TARs) on Mars. TARs from five locations Received in revised form 23 February 2009 in a HiRISE image of the floor of Ius show remarkably symmetric (cross-sectional) profiles, with Accepted 26 May 2009 average slopes for the entire feature of ~15°; these results apply to TARs that span an order of magnitude in Available online 2 June 2009 wavelength and a factor of 6 in height. A HiRISE image of Gamboa in the northern lowlands shows low albedo sand patches b2 m high that are covered with sand ripples, surrounded by larger TAR-like Keywords: fi Sand ripples that are similar in pro le to surveyed granule ripples on Earth. TARs in a HiRISE image from Terra Sirenum, in the cratered southern highlands, are comparable in height to those in Ius Chasma, but many have Ripple tapered extensions that are more consistent with them being erosional remnants rather than the result of Granule ripple extension of the TAR by from the tapered end. The new observations generally support a reversing Topography transverse dune origin for TARs with heights ≥1 m, and a granule ripple origin for TAR-like ripples with Transverse dune heights ≤0.5 m. Published by Elsevier B.V.

1. Introduction of the saltating grains (p. 62). He further argued that the characteristic path represented a length scale determined by the velocity gained “Transverse Aeolian Ridge” (TAR) is a general term proposed for from the , from which he inferred that ripple wavelength must be linear to curvilinear aeolian features that could be the result of either the physical manifestation of the characteristic path of wind-driven dune or ripple processes (Bourke et al., 2003). These features are sand (Bagnold, 1941, p. 150). Sharp (1963) argued that because ripples nearly always oriented with crests perpendicular to wind directions begin as small-amplitude, short-wavelength forms, and change that could result from topographic confinement, as on the floor of a wavelength as they evolve to steady state, Bagnold's characteristic valley or linear depression (Fig. 1). As summarized below, TARs were path concept was suspect. Through geometric arguments, Sharp first described from Mars Orbiter Camera (MOC) images, but since (1963) proposed that ripple wavelength depends on ripple amplitude November of 2006, aeolian features of various kinds are abundant in and the angle at which saltating grains approach the bed, controlled the remarkable images being returned by the High Resolution Imaging by the length of the zone shadowed from significant impact and by Science Experiment (HiRISE) camera (e.g., Bridges et al., 2007). This wind velocity. report presents observations of TARs derived from three HiRISE Anderson (1987) developed an analytical model for the initiation images that improve on our knowledge about these distinctive wind- of sand ripples produced by perturbations on a flat sand surface. Using related features, as first reported in a presentation at the workshop on the “splash function” concept derived from experiments by Unger and “Planetary : A record of climate change”, which was held in Haff (1987), Anderson (1987) modeled the relatively low-velocity Alamogordo, New Mexico (Zimbelman, 2008). ejection of grains caused by the impact of faster-moving saltating grains in a process termed ‘reptation’, from the Latin for “to crawl”,to 2. Background distinguish this mode of sand transport from , suspension, and impact creep. Numerical modeling including saltation and 2.1. Sand ripples and granule ripples reptation (Anderson and Haff, 1988) showed that small, fast-moving ripples overtake and merge with larger, slower forms, resulting in the Bagnold (1941) described a “characteristic path” for well sorted, growth of the mean wavelength and a decline in the dispersion of wind-blown sand grains, which he argued was the result of wavelengths. Reptation and saltation are now incorporated into momentum removed from the wind at the height reached by most continuum analytical models developed for study of aeolian features on Earth and Mars (e.g., Momiji et al., 2000; Yizhaq et al., 2004; Yizhaq, 2005). ⁎ Tel.: +1 202 633 2471; fax: +1 202 786 2566. When aeolian sand is poorly sorted, the coarse grains become E-mail address: [email protected]. concentrated at the crests of features Bagnold (1941,p.153–6) termed

0169-555X/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.geomorph.2009.05.012 J.R. Zimbelman / Geomorphology 121 (2010) 22–29 23

2.3. Dunes and TARs on Mars

Dunes were first clearly identified on Mars in Mariner 9 images through comparison with terrestrial dunes (McCauley et al., 1972; Cutts and Smith, 1973). Viking Orbiter images led to many reports about dunes, but primarily for the barchans and barchoid ridges in the extensive north polar (e.g., Cutts et al., 1976; Breed et al., 1979; Tsoar et al., 1979; Lancaster and Greeley, 1990). Most Martian dunes appear to be transverse in orientation, with crescentic barchans the dominant dune type in the north polar erg (Greeley et al., 1992). Viking resolution was insufficient to indicate slip faces except for the largest dunes of the north polar erg (Tsoar et al. 1979). Zimbelman (1987) examined all Viking Orbiter images with a resolution b10 m/ pixel; one fourth of these highest resolution images included patches of aeolian lineations, essentially independent of the image location on Mars. The Mars Orbiter Camera (MOC) greatly improved on the Viking resolution, providing images of up to 1.5 m/pixel resolution (Malin and Edgett, 2001). MOC revealed a wealth of aeolian features across the planet, and its images clearly resolved slip faces on some dunes Fig. 1. Transverse Aeolian Ridges (TARs) on the floor of Nirgal Vallis, Mars. TARs are N150 m in width. The term ‘Transverse Aeolian Ridge’ (TAR) was first fl oriented with crest axes perpendicular to trend of the valley oor, with wavelengths of proposed by Bourke et al. (2003) for aeolian features on Mars where from 30 to 100 m. Portion of MOC image E02-02651, 27.8° S, 316.7° E, 2.8 m/pixel, NASA/ JPL/MSSS. slip faces are not observed directly (e.g., Fig. 1), to preserve both ripple and dune options for their origin. A systematic examination of MOC images within a longitudinal sector (~240° to 180°E) from the south to ‘ridges’ rather than ripples, arguing that the coarse grains are moved by the north poles documented the latitudinal distribution of TARs the impact of saltating sand (also called ‘impact creep’, Greeley and (Wilson and Zimbelman, 2004); equatorward of ~60° , TARs Iversen,1985, p.17). Sharp (1963) described ‘granule ripples’ consisting are relatively abundant, most with wavelengths of 20 to 60 m, of a surface concentration of grains N1 mm, roughly equivalent to whereas TARs are much less frequent in the polar regions. Balme et al. Bagnold's ‘ridges’; Sharp concurred with Bagnold that impact creep, (2008) and Shockey and Zimbelman (2007) extended the survey of along with significant deflation, was the dominant process of motion in MOC images that show TARs to longitudinal sectors ~180° from that of granule ripples. Granule ripples can grow to great size, attaining the Wilson and Zimbelman (2004) study, obtaining results for TAR wavelengths exceeding 20 m and heights of over 60 cm (Bagnold,1941, distributions that are comparable to those from the earlier study. TARs p.155), although they are more commonly observed in the wavelength therefore are relatively ubiquitous throughout the equatorial and mid- range of ~3 to 10 m with heights of ~25 cm (Sharp, 1963; Zimbelman of Mars. and Williams, 2005). The granule coating tends to be concentrated at The Mars Exploration Rovers (MERs) Spirit (Squyres et al., 2004a) the surface, with the interior of the bedform comprised primarily of and Opportunity (Squyres et al., 2004b) have provided a wealth of sand with only occasional granules; the granules are most thickly new information about aeolian bedforms and sediments. The MERs accumulated at the crest of the ripple (Sharp, 1963). have documented the presence of granule ripples at both sites, as well as broad deposits comprised primarily of basaltic sand (Greeley et al., 2.2. Ripples versus dunes 2004, 2006; Sullivan et al., 2005, 2008). The High Resolution Imaging Science Experiment (HiRISE) now provides the capability to obtain Bagnold (1941) described important distinctions between ripple orbital images of Mars with a spatial resolution of as good as 25 cm/ and dune processes; dunes usually display a slip face dominated by pixel (McEwen et al., 2007). Initial results from HiRISE provided avalanche processes (p. 189) while ripples have the coarsest material important new insights into aeolian deposition and on Mars, collected at the crests with the finest material in the troughs, such as multiple length scales observed for aeolian features at many whereas the opposite is true for dunes (p.145). Normal sand ripples locations (Bridges et al., 2007). HiRISE images will not resolve (as described in Section 2.1) are the smallest member in a hierarchy individual particles that comprise aeolian landforms on Mars. The of aeolian landforms. Wilson (1972a,b) documented three distinct obscuring effect of the pervasive Martian dust makes it difficult to scales for aeolian landforms; ripples (wavelength 0.01 to 10 m), derive particle size information for individual landforms through dunes (wavelength ~10 to 500 m), and draas (wavelength ~0.7 to infrared remote sensing, except at locations where dust deposition is 5.5 km). Note that Wilson's work includes both (small) sand ripples inhibited by the activity of large sand deposits (e.g., Fenton, 2005; and (large) granule ripples in the features he called ‘ripples’. Regions Fenton and Mellon, 2006). The only confirmation of particle size for of overlap exist between small dunes and large ripples, but the Martian aeolian features currently comes from rovers that can place a sediment size of a 10 m dune is typically b0.2 mm while surface Microscopic Imager directly onto the feature (e.g., Greeley et al., 2004; sediments on a ripple of 10 m wavelength are N1mm,andno Herkenhoff et al., 2004). observed transitional features occur between these two groups (Wilson, 1972a). Therefore, particle size is an important determinant 3. Ius Chasma of which class a 10-m-scale aeolian landform belongs to; this 10 m size is comparable to the smallest features initially identified as TARs The first publicly released full resolution HiRISE image (TRA_ on Mars (Wilson and Zimbelman, 2004). Features at all three scales 000823_1720, centered at 7.7°S, 279.5°E, in aerocentric coordinates) can be present at one location, but they are built by differing wind demonstrated that 25 cm/pixel (map-projected) resolution was obtain- intensities and durations (Wilson, 1972b), consistent with the ideas able from the mapping orbit. This first image was of the floor of Ius of Sharp (1963), who stated that each form reaches its own quasi- Chasma, part of the extensive Valles Marinaris canyon system (Fig. 2). equilibrium state; i.e., ripples do not grow to become dunes, nor do The image revealed intricate layering exposed in the deposits of the dunes grow into draas. canyon floor, along with several prominent patches of TARs. Nearly all of 24 J.R. Zimbelman / Geomorphology 121 (2010) 22–29

Fig. 2. First publicly released full-resolution HiRISE image of Mars, which includes numerous TARs scattered across the floor of Ius Chasma. A through E indicate the locations of TARs where profiles were obtained from simplified photoclinometry (see Table 1 and Fig. 3). Browse version of HiRISE image TRA_000823_1720, 7.7° S, 279.5° E, 25 cm/pixel map- projected resolution, taken 9/29/06, NASA/JPL/U of A. the TARs in the Ius Chasma image have a ‘simple’ crest-ridge plan view The slope within 2 m of either side of the crest in Fig. 3 is 24°; even morphology, following the classification scheme of Balme et al. (2008). considered as an upper limit, this value is less the angle of repose TARs were measured at five locations in the Ius Chasma image (letters in expected for a slip face. The TAR profiles are more symmetric than Fig. 2) by collecting the pixel values along lines perpendicular to the local profiles measured across several terrestrial aeolian features ranging crest orientation of the TARs. The measured profile lines typically were from normal sand ripples through granule ripples to stabilized and within ~30° of the solar illumination azimuth, and the solar incidence angle was 30° from the horizon. The pixel values along each line were used to generate a photoclinometric profile through two simplifying assumptions: 1) the albedo (visual reflectivity) of the materials is assumed to be constant along the line (that is, the line does not cross albedo boundaries), and 2) the photometric properties of the surface materials are considered to be Lambertian, where reflected brightness is solely dependent upon the solar incidence angle to the surface (Zimbelman and Williams, 2008). Non-Lambertian aspects of Martian materials (i.e., Minneart photometry) can cause profiles derived using Lambertian photometry to produce slopes that are slightly (less than a few percent change) steeper than reality (R. Kirk, pers. comm.), but still the overall profile shape is correctly reproduced in a Lambertian profile. Plans are in place to compare Lambertian profiles to the more accurate relief obtained from stereo HiRISE images (e.g., Kirk et al., 2007), but results from such a comparison are not available at present. In the meantime, slopes on profiles presented here should be considered to represent upper limits. Remarkably symmetric profiles were obtained for TARs from the five locations examined within the Ius Chasma image. The profile for location C (Fig. 3) is illustrative of TAR profiles from all five locations Fig. 3. Profile across TAR at location C in Fig. 2. The profile shape is remarkably symmetric, similar to symmetric reversing transverse dunes on Earth (see Fig. 4). Inset (data for the C profile appear in Fig. 6b of Balme et al., 2008). When shows profile location. ‘v’s indicate breaks in slope at the bases of both sides of the ridge, fi the two sides of the TAR pro le are folded back on itself at the crest, which is defined here to represent the width of the feature. Vertical exaggeration of the the upper slopes on either side of the crest are essentially identical. profile is 6.1×. Data from HiRISE image TRA_000823_1720 (Fig. 2). J.R. Zimbelman / Geomorphology 121 (2010) 22–29 25 active transverse dunes, covering more than three orders of Table 1 magnitude variation in feature size (Zimbelman and Williams, Dimensions of individual TARs in Ius Chasma and Terra Sirenum. 2005). The one terrestrial feature that can approach the symmetry Loca Wavelengthb (m) Widthc (m) Heightd (m) Ave. slopee (°) fi displayed by the TAR pro les is a reversing sand dune, where bimodal A7 7.01.016 pile up the sand along a ridge that is perpendicular to both wind B 24 18.0 1.7 11 directions (e.g., Burkinshaw et al., 1993; Frank and Kocurek, 1996; C 38 45.3 5.8 14 McKenna Neuman et al., 1997; Walker, 1999; Pye and Tsoar, 2009, pp. D 70 29.0 5.8 22 – E 21 13.5 1.7 14 270 272). While active reversing dunes may not often achieve the F 41 20.5 4.7 25 striking profile symmetry of the TARs, one reversing dune at Great G41 11.51.616 Sand Dunes National Park and Preserve (GSDNPP), in south-central H 41 10.5 1.5 16 Colorado, produced a measured profile similar to that of TARs like the J 40 34.0 8.0 25 K 40 22.5 5.7 27 one shown in Fig. 3 (Zimbelman, 2008). L 40 24.5 5.4 24 A tool that holds promise for assessing the similarity of dune and M 40 16.5 2.3 16 ripple shapes to that of TARs is the comparison of similarly scaled N 40 25.5 6.7 28 fi terrestrial and Martian pro les. We experimented with scaling a TAR a See Figs. 2, 9, and 10 for locations. profile from Ius Chasma and surveyed profiles of several terrestrial b A–E, average of crest distance to either side of measured TAR crest; F–H and J–N, aeolian landforms, with mixed results. Use of wavelength as the scaling average crest-to-crest distance across Figs. 9 and 10, respectively. c factor suffered from the great degree of variability in wavelength present A–E, distance between basal breaks in slope on either side of crest, from TAR profile; F–N, twice the length of the sunlit face of TAR, perpendicular to crest direction. in aeolian environments on Earth and Mars; an ‘average’ wavelength d A–E, height difference from crest to basal break in slope, from TAR profile; F–N, fi showed such variability, even within the same dune or ripple eld, that height from shadow length. use of this parameter as a scaling factor proved to not be very satisfactory e Average slope is the arc-tangent of height divided by half the width. (Zimbelman and Williams, 2007a). Next, scaling of the profiles by the height of the feature was explored (Zimbelman and Williams, 2007b), but height is the most difficult dimension to measure remotely (Bourke Table 1 provides dimensions obtained from profiles across et al., 2006), so that uncertainty and variability was not improved over individual TARs at the five locations indicated in Fig. 2, where scaling by wavelength. Scaling of both the horizontal and vertical wavelength is the average of the distance between the crests to either dimension of profiles by the width of the feature, defined to be the side of the measured crest (following the profile orientation), width is distance between the basal break in slope on either side of the crest (‘v’s the horizontal distance between the basal breaks in slope on both in Fig. 3), has proven to be the best scaling approach examined to date, sides of the crest, and height is the elevation difference between the which improved the reproducibility of profiles over both the variability basal breaks in slope and the crest. If height is divided by half of the of the wavelength scaling and the uncertainty of the height scaling feature width, the average slope values are near 15°, with one outlier (Zimbelman and Williams, 2007c, 2008). To illustrate this technique, the (D in Table 1). Even as upper limits, these average slopes are indicative TAR profile from Fig. 3 was scaled by the feature width (determined of landforms that are intermediate between that of granule ripples from the breaks in slope in the profile) and compared to similarly scaled (~8°; Zimbelman et al., 2009) and sand at the angle of repose tape-inclinometer profiles of reversing transverse dunes at Coral Pink (~33°). Sand Dunes State Park in southern Utah (Fig. 4); the profile similarities become readily apparent in the scaled plots. The advantage of using 4. Gamboa crater scaled profiles is that details of the profile shape are preserved (compare TAR profiles in Figs. 3 and 4) while eliminating the size differences Gamboa impact crater, at 40.5°N, 315.5°E (THEMIS, Thermal between the features being compared. Emission Imaging Spectrometer; Christensen et al., 2003, image V12571006), was the target of repeated imaging by MOC to monitor gullies present on the interior crater walls. Some of the MOC images also crossed the central peak on the crater floor (e.g., MOC image R16- 00011), around which are located several dune patches with a lower albedo than the surroundings, similar to the “Large Dark Dunes" discussed by Balme et al. (2008). The extensive image archive for this crater from earlier missions caused it to be targeted early by HiRISE; image PSP_002721_2210 (25 cm/pixel map-projected resolution) provides new insight into dunes and ripples present around the central peak. The solar incidence angle in this HiRISE image is 24° above the horizon, which makes even subtle features clearly visible. The dark dune patches seen in the MOC images are now resolved into rounded to elongate sand masses surrounded by a complex array of TAR-like ripples (Fig. 5). No slip faces are evident on any of the sand patches near the center of Gamboa crater, but the patches are all thoroughly covered by ripples of wavelength ~1 m. The overall appearance of the sand patches (Fig. 5) is quite different from that of TARs (inset in Fig. 3), where a sharp crest is a dominant attribute, and the sand patches do not fit into any of the TAR classification types of Balme et al. (2008). The sand patch dimensions are greater than Fig. 4. Comparison of profiles across reversing transverse dunes at Coral Pink Sand ~40 m, which is consistent with the concept of Bagnold (1941, p. 183) Dunes State Park in southern Utah (Earth1 and Earth2) and the Ius Chasma TAR shown regarding the minimum size of a sand patch that can be considered a in Fig. 3 (Mars). Both height and distance have been scaled by the width of each feature, dune, where saltation across the patch alters the wind sufficiently to which is shown in parentheses (in meters) in the symbol key. The scaled heights of both induce sand deposition on (rather than erosion of) the sand patch. The the terrestrial transverse dunes and the TAR are nearly twice the scaled height of granule ripples and Gamboa crater ripples (see Fig. 8). Vertical exaggeration of the ripples surrounding the sand patches display crests that are oriented profiles is 5.0×. radial to the sand mass (Fig. 5), which suggests that the local wind 26 J.R. Zimbelman / Geomorphology 121 (2010) 22–29

Fig. 5. Broad sand patches, surrounded by ripples, on the floor of Gamboa crater south of the central peak. The sand patches lack any evidence of a slip face, and are covered by ripples with wavelength ~1 m. Lines A–A′ and B–B′ show the location of profiles in Figs. 6 Fig. 7. Profile across two TAR-like ripples adjacent to a broad sand patch (B–B′ in Fig. 5). and 7, respectively. C is a boulder whose shadow length indicates it is 0.8 m high. Portion Inset shows the location of the profile. The ripples have dimensions and shape very of HiRISE image PSP_002721_2210, 40.8° N, 315.7° E, 25 m/pixel map-projected similar to that of granule ripples (see Fig. 8). Vertical exaggeration of the profile is 21.9×. resolution, taken 2/24/07, NASA/JPL/U of A. Data from HiRISE image PSP_002721_2210 (Fig. 5).

flow was influenced by the presence of the sand patches; some of the radially oriented ripples may be superposed on the lower margins of sand patches may be granule ripples whereas those on the sand the dune, although it is also possible that the sand patches may bury patches are more likely to be normal sand ripples (Zimbelman and some preexisting ripples. Williams, 2005, 2006). The granule ripple hypothesis can be evaluated The photoclinometry profiling technique described above was by considering some of the large ripples well removed from the sand applied to a broad sand patch and nearby ripples along a single line, patch (Fig. 7). When the two Martian ripple profiles are individually taking care to restrict the profile line to a region that appears to have a scaled by the feature width, as described earlier, the Martian profiles relatively uniform albedo. The sand patch has limited relief, b2min are seen to be very comparable to similarly scaled profiles of granule height, and shallow slopes across the entire feature (Fig. 6); similar ripples at GSDNPP (Zimbelman and Williams, 2006) and in the heights are expected anywhere along the sand patch. The sand patch Coachella Valley of southern California (Sharp, 1963; Zimbelman and relief (Fig. 6) is less than half the relief of a TAR of comparable width Williams, 2006)(Fig. 8). The scaled height of the terrestrial granule (Fig. 3), and the sand patch also lacks even a hint of the sharp crest ripples and the Gamboa ripples (Fig. 8) are much smaller than the observed on all TARs. Ripples immediately adjacent to the sand patch scaled heights of either the Ius Chasma TAR or the terrestrial rever- have relief of 0.2 to 0.3 m, while ripples on the surface of the sand sing dunes (Fig. 4), providing a good method to distinguish between patch have relief ≤0.05 m; these heights suggest that ripples off the dunes and granule ripples in HiRISE images. The Gamboa image

Fig. 6. Profile across a broad sand patch on the floor of Gamboa crater (A–A′ in Fig. 5). Fig. 8. Comparison of profiles across granule ripples at Great Sand Dunes National Park Inset shows the location of the profile. The sand patch is b2 m tall and shows no and Preserve in south-central Colorado (Earth1), in the Coachella Valley of southern evidence of a slip face, but it is still sufficiently broad to be considered a minimal dune California (Earth2), and two Martian TAR-like ripples (Mars1 and Mars2, left and right (see text). Small ripples are superposed on the dune; they are most discernable along in Fig. 7, respectively). Both height and distance have been scaled by the width of each the profile on the sunlit side of the dune. Ripples immediately adjacent to the margin of feature, which is shown in parentheses (in meters) in the symbol key. The scaled the dune are 4 times the height of ripples on the sand patch surface, and are likely small heights of both the terrestrial and Martian ripples are about half the scaled height of examples of ripples like those in Fig. 7. Vertical exaggeration of the profile is 16.9×. Data terrestrial transverse dunes and a TAR (see Fig. 4). Vertical exaggeration of the profiles from HiRISE image PSP_002721_2210 (Fig. 5). is 10.0×. J.R. Zimbelman / Geomorphology 121 (2010) 22–29 27 provides examples of features that can be confidently categorized as small dunes (the broad sand patches lacking slip faces), normal sand ripples (the small ripples on the sand patches), and granule ripples (the large TAR-like ripples surrounding the sand patches), all of which are very distinct from the Ius Chasma TARs (Fig. 3).

5. Terra Sirenum

A HiRISE image from the cratered highlands in the southern mid- latitudes was examined to compare attributes of mid-southern- latitude TARs to those seen in the HiRISE image of equatorial Ius Chasma. Crater walls in the Terra Sirenum region of Mars were imaged by HiRISE to gain insight into the numerous gullies present in this area. HiRISE image PSP_001684_1410, centered at 38.9°S, 196.0°E, has 25 cm/pixel map-projected resolution, and while gullies are common throughout the image, so are many interesting TARs. The solar incidence angle in this image is only 16° above the horizon, an angle comparable to the average slope of the TARs in Ius Chasma, so that distinct shadows are cast by individual TARs (Figs. 9 and 10). Shadow lengths were measured along the solar incidence azimuth (clearly indicated by the shadows of individual boulders; see Fig. 8), and then converted into the height of the TAR crests. The numbers in Figs. 9 and 10 give the TAR crest height in meters, next to the line where the shadow length was measured. The strong shadows preclude applying the photoclinometric technique to generate a Fig. 10. Forked terminations of Terra Sirenum TAR crests, along with evidence of eroded layers exposed both in the interdune regions and even on the slopes of some TARs. The low fi complete pro le across the features, but obtaining a height from a solar incidence angle (16°) casts long shadows that provide heights (in meters) for several shadow length is generally less dependent on assumptions than is TARs (white lines show where shadow length was measured) and one boulder. Letters the photoclinometry procedure. However, the shadow lengths can indicate measurement locations for Table 1. Portion of HiRISE image PSP_001684_1410 be affected by subtle irregularities on the surface over which the (as in Fig. 9). shadow is cast, which might cause some shadows to appear longer than they would on a flat surface. TAR heights (obtained from the comparable to the majority of the average slopes for the Terra shadow lengths) and widths (obtained from doubling the sunlit Sirenum TARs, suggesting a possible bimodal distribution of average width perpendicular to the crest at the point of the shadow slope may exist for TARs in general. measurement) at Terra Sirenum are generally consistent with Many TARs in Terra Sirenum show a pronounced ‘tail’ from the those obtained from profilesacrosstheIusChasmaTARs(Table 1). northeast end of their crest, some of which can be followed down to The average slopes of the Terra Sirenum TARs fall into two distinct widths of only 1 m, while others break into meter-wide linear groups, much like the Ius Chasma TARs did. It is instructive to note features aligned with the tapered end of a nearby TAR (small arrows that the one average slope outlier from Ius Chasma (D in Table 1)is in Fig. 9). For a tapered end 1 m wide, if two of the four pixels are considered to be in shadow, then the crest at this location is b14 cm high. Such narrow tails are exceedingly difficult to explain as original depositional features associated with the emplacement of the main TAR. Predictions that sand saltation path lengths might be one or more meters under current conditions on Mars (White, 1979; Almedia et al., 2008)makeitdifficult to envision how blowing sand could be deposited while confined onto such narrow features, particularly for winds transverse to the crest orientation. If the last winds were parallel to the crest orientation, perhaps the feature could have extended along the crest direction. Terrestrial long- itudinal dunes are thought to involve winds at slight angles to either side of the crest orientation (Tsoar, 1983; Tsoar et al., 2004), which might be expected to preserve some visible sinuosity of the crest line or regular variation in the crest height, neither of which are apparent in the HiRISE image. The discreet linear segments aligned with the tapered end of one TAR (upper left of Fig. 9) represent an even tighter constraint against the dominant wind being along the crestline orientation. If sand was being shed from the tapered tip, sand accumulations downwind should develop into barchanoid shapes pointing toward the tip rather than the observed linear axis parallel to the crest at the tip. All of the Fig. 9. TARs in Terra Sirenum. TARs taper to very thin linear ridges extending from their northern ends (three small arrows, upper right), some of which separate into isolated above considerations lead to a favored interpretation that the tapered linear segments following the trend of the nearby TAR (four small arrows, upper left). extensions of the TARs represent erosional remnants of once more The narrow extensions suggest erosion of inactive TARs rather than extension of active extensive TARs, rather than depositional features resulting from winds TARs along such narrow bands. Subtle aeolian bedforms are visible both between and on parallel to the TAR crest. the TARs (large arrows). Shadow lengths provide TAR heights (in meters) at three The textures on and around the Terra Sirenum TARs, revealed by the locations (lines indicate where shadow length was measured). Letters indicate measurement locations for Table 1. Portion of HiRISE image PSP_001684_1410, 38.9° S, shallow illumination conditions, suggest the presence of second-order 196.0° E, 25 cm/pixel map-projected resolution, taken 12/5/06, NASA/JPL/U of A. ripples (e.g., Bridges et al., 2007). In particular, features between the 28 J.R. Zimbelman / Geomorphology 121 (2010) 22–29

TARs (large arrows in Fig. 9) display linear trends traceable for tens of 3) TARs in a HiRISE image from Terra Sirenum are comparable to meters, but without the gentle crenulations prevalent on the sand those in Ius Chasma, but with average slopes grouped around ~16° ripples superposed on the Gamboa sand patches (Fig. 5). Some of the and ~25°. Many of the Terra Sirenum TARs have tapered extensions textural patterns continue from the inter-TAR plains onto the sunlit sides to the northeast that are more consistent with being erosional of some TARs (Fig.10). The Terra Sirenum textures seem more consistent remnants of formerly larger TARs rather than the result of with second-order ripples (Bridges et al., 2007) than to possible extension of the feature by deposition from the tapered end. exposures of eroded layers. Terra Sirenum is within the latitudinal 4) The new observations generally support a reversing transverse band of a ‘mid-latitude mantle’ identified in MOC images, interpreted to dune origin for TARs with heights ≥1 m, and a granule ripple incorporate volatiles within the mantling unit (Mustard et al., 2001; origin for TAR-like features with heights ≤0.5 m. Kreslavsky and Head, 2002). Perhaps the Terra Sirenum image may reveal layering that is part of the composite mantling layer observed Acknowledgements in MOC images. However, it is important to note that no layer-like texture is evident in the Gamboa crater image, which is from a similar Comments by two anonymous reviewers were very helpful during latitude in the northern hemisphere. Study of many HiRISE images the revision of the manuscript. This work was supported by grants from both hemispheres is needed to investigate further the possible NNG04GN88G and NNX08AK90G to JRZ from the Mars Data Analysis causes of the subtle textures seen in Figs. 9 and 10. Whatever their Program of NASA. cause, the textures are clearly superposed on the TARs and thus do not appear to be intimately involved with the original emplacement References of the TAR materials. Almedia, M.P., Parteli, E.J.R., Andrade, J.S., Hermann, H.J., 2008. Giant saltation on Mars. Proc. Nat. Acad. Sci. 105 (17), 6222–6226. doi:10.1073/pnas.0800202105. 6. Discussion Anderson, R.S., 1987. A theoretical model for aeolian impact ripples. Sed. 34, 943–956. Anderson, R.S., Haff, P.K., 1988. Simulation of eolian saltation. Science 241, 820–823. Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. 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