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Interactions between boulders and aeolian ridges on Mars

Here we present new observations of two different interactions between aeolian ridges and boulder fields on Mars that provide insight into past wind conditions. First, an analysis of ridge and boulder interactions at two test sites in Proctor Crater and an additional site ~430 km to the northeast shows that ridge geometry can be affected by changes in surface roughness elements generated by boulder fields. Second, a detailed examination of some of the boulder fields found that individual boulders can generate multi‐armed ‘wakes’ that have no clear proxy on Earth. The ridge/boulder dynamics suggest that transverse aeolian ridges acted as roughness during their development, indicating that they formed at a length scale greater than wind ripples. The boulder wakes seem to represent an unusual interaction between flow separation and pre‐exiting ridges; why this pattern is not observed on Earth remains uncertain.

TARs can be morphometrically similar to both ripples and dunes.
Most TARs are nearly symmetrical in profile, comparable to large ripples or very small reversing dunes (Shockey & Zimbelman, 2013;Zimbelman & Williams, 2007). TARs similarly occupy a middle ground between ripples and dunes in terms of wavelength. They are typically on the order of tens of metres; larger than impact ripples on Mars or Earth (typically decimetres) but smaller than dunes (high decametres to hundreds of metres when fully developed) (Bourke et al., 2006;Lapotre et al., 2016;Leeder, 1982;Milana, 2009;Williams, 2002;Wilson, 2003). Further, TARs can be observed in locations prohibitive to reversing wind patterns (e.g. TARs within large fields of asymmetrical dunes as in this study). Many studies have identified TAR proxies on Earth: gravel megaripples in Argentina (de Silva et al., 2013;Hugenholtz et al., 2015), megaripples in Iran and Libya (Foroutan & Zimbelman, 2016;Foroutan et al., 2019;Gough et al., 2020;, and reversing dunes in Idaho (Zimbelman & Scheidt, 2014), but a definitive terrestrial analogue has not been determined.
Martian aeolian features lie on a continuum that does not lend itself to easy discrimination between classes. There is strong and growing evidence of overlapping amplitude and wavelength characteristics between various types of aeolian features on Mars (Lapotre et al., 2016). For example, such overlap occurs between active ripples and inactive course-grained ripples (Silvestro et al., 2010;Sullivan et al., 2008), between large ripples and TARs (Geissler & Wilgus, 2017;, and between active megaripples and TARs (Chojnacki et al., 2019;Fenton et al., 2021;Silvestro et al., 2020). For this study, we draw a distinction between apparently inactive bright-toned ridges with wavelengths of 10 m (referred to here as TARs) and smaller inactive dark-toned ridges with wavelengths of <10 m (inferred to be ripples based on this study).
Crater counting is currently the best method for calculating surface ages on planetary bodies (Benedix et al., 2020;Hartmann, 2005;Hartmann & Daubar, 2017;Hartmann & Neukum, 2001;Shoemaker, 1961;Shoemaker & Hackman, 1962). A study of TARs using Mars Orbiter Camera data found sparse cratering on the large TARs in Nirgal Vallis, concluding that the TARs were 300 ka-1.4 Ma in age (Reiss et al., 2004). A survey of uncratered TAR fields (those with no craters 5-10 m in diameter) versus cratered TAR fields estimated that the uncratered fields were 100 ka old, while the cratered fields were 1-3 Ma old (Berman et al., 2011). Similarly, in-situ studies by the Opportunity Rover of cratering in large ripples found that even relatively small aeolian bedforms could be preserved for tens or even a few hundred thousand years (Golombek et al., 2010). No craters have been identified in well-documented dune fields, but crater-retention estimates and superpositioning over Amazonian surface puts the age of most Martian dune fields at 100-8 Ma in the southern highlands and 3 Ga-8 Ma at higher latitudes (Fenton & Hayward, 2010).
The lengthy duration of these aeolian features on the Martian surface offers an opportunity to examine the atmospheric past to different degrees (Fenton et al., 2003(Fenton et al., , 2005. Given a much slower resurfacing rate as compared to Earth, even small Martian bedforms offer insight that extends one to two orders of magnitude further back into the past than can be found on Earth, where our observations are limited to dune fields dating approximately to the Last Glacial Maximum (31-16 kya) (Beveridge et al., 2006;Kocurek, 1991;Lancaster et al., 2003;Wolfe et al., 2004).
Here we document previously undescribed patterns in small aeolian features in three boulder fields in and around the Proctor Crater region on Mars and use comparative analyses to analyse similar areas on Mars. While absolute dating is unavailable for the fea- In the remainder of this Introduction, we give a brief overview of surface roughness dynamics and lee-side topographic features. In our Methods section we describe how we extracted boulder and ridge characteristics. In our Results section, we (1) summarize the ridge characteristics of the study sites, (2) compare the boulder distributions between the sites, (3) compare the ridge orientations and spacings interior and exterior to the boulder fields, and (4) describe and illustrate unusual fan-like downflow features on some of the ridges. In our discussion, we explore inferences about the formative conditions for each of the study sites, outline the novelty of the fan features, and finally confirm that the ridges were inactive in our study sites.

| Roughness density
We observe the effects of locally varying roughness density (λ) on aeolian ridges by comparing ridges in boulder fields to adjacent ridges without boulders. We can thus infer the flow regime that may have dominated when the ridges were still active in the past, and hypothesize some of the formative mechanisms at work. A change in roughness density affects aerodynamic roughness length (z 0 ): the length scale at which a wind speed profile can be accurately approximated by a logarithmic function (Charru et al., 2013;Raupach, 1992;Raupach et al., 1993). We examine these effects through the lens of three possible flow regimes depending on the roughness density (Lee & Soliman, 1977;Wolfe & Nickling, 1993) causing the wind to seemingly pass smoothly above the roughness elements (Lee & Soliman, 1977;Raupach, 1992;Raupach et al., 1993).
The three flow regimes can be characterized by the ratio of z 0 to roughness element height (z 0 /h) (Raupach, 1992;Raupach et al., 1993), hereafter referred to as Λ. Isolated roughness flow F I G U R E 1 Simplified diagrams of well-documented lee-side features (red) downflow of obstacles (yellow). Left: An isolated obstacle such as a shrub or rock. Right: A linear feature such as a dune. Note that the lee-side features have a single central crest and come to one point [Color figure can be viewed at wileyonlinelibrary.com] occurs when Λ is relatively low. As roughness element density increases, Λ increases to higher values distinctive of wake interference flow. As roughness elements become even more tightly packed, initiating the formation of a new surface, Λ decreases to a stable intermediate value typical of the new skimming flow regime (Charru et al., 2013;Raupach, 1992;Raupach et al., 1993).

| METHODS
We documented surface features with observations made at three sites: two fields in Proctor Crater and a field in an unnamed area to the northeast of Proctor Crater ( Figure 2, Table 1). Georeferenced High-Resolution Imaging Science Experiment (HiRISE) images (McEwen et al., 2007) were analysed in ArcGIS Pro (ESRI, 2020). A sub-area from each image was selected for closer examination ( Figure 2). Each study area contains large dark dunes that have been well documented and described in the literature (Fenton & Mellon, 2006;Fenton et al., 2003Fenton et al., , 2005Jackson et al., 2015;Taniguchi & Endo, 2007). These dune fields contain a range of TAR morphologies, and extensive boulder fields ( Figure 3). The boulder fields are hypothesized to have been generated via impacts to the lithified sedimentary in-fill in the crater (Fenton et al., 2003).
Morphological assessments were derived for the TARs in the three study areas (Figure 3). TAR wavelength, ridge-crest length, and width were extracted using the distance measurement tool in ArcGIS Pro (ESRI, 2020). Using standards defined in Balme et al. (2008), width is measured as the total linear distance of a ridge; measured end-toend along the average orientation of the ridge, length is the maximum distance perpendicular from toe to toe across a given ridge crest; wavelength is the spacing between ridge crests as measured perpendicular from the centre of the ridge (Figure 4).

| Extracting boulder sizes
Measurements were also taken of the boulder fields in the study areas. Clasts larger than 0.5 m 2 in diameter were extracted using an

| Characteristics of the three study areas
Site 1 exhibits the smallest and most regular ridges, likely impact ripples based on their size and spacing (Table 2, Figure 6). TAR lengths at Site 2 were highly irregular, likely the result of sand in-fill of the area (Table 2, Figure 7) (Fenton et al., 2003). Sites 2 and 3 exhibit similar TAR wavelengths and lengths, but different widths (Table 2). However, the TARs at Site 3 are highly sinuous (Figure 8), with bends approximately every 3-6 m, and have notably greater amplitudes than those at Sites 1 or 2 and are usually taller and larger than the boulders based on shadow length comparisons.
Ridges in all three study areas had prominent and well-developed secondary ridges; smaller crests that form near perpendicular to main ridges (Lapotre et al., 2016). Secondary ridges extend crest to crest throughout Sites 1 and 2 (Figures 6 and 7), and in most locations at Site 3 (Figure 8).

| Boulder distributions
The three study areas required different thresholds for effective extraction of boulder pixels given the slightly different lighting conditions in each scene. Rare cases of false-positive detections of bright ridge crests occurred and were especially evident at Site 2 ( Figure 9).
However, these false positives are greatly outnumbered by the falsenegative detections of boulders, and the overall undercount of boulder pixels throughout the images (Figure 9). The boulder areas were likely systematically underestimated in each area, but this bias should not affect relative comparisons across the regions. Further, the boulder count frequency plots shown in Figure 10 should thus be considered minimum values.
Sites 1 and 2 contained similar boulder morphologies in terms of their typical widths, lengths, and areas, while Site 3 generally contained smaller and less elongate boulders (Table 3, Figure 10).
It is useful to compare Site 2 to the other two study sites: Site 2 contains boulders that are similar to those found at Site 1, and TARs that are similar in size and wavelength to those found at Site 3 ( Figure 10,

| Ridge geometry
The orientations of the TARs at Sites 2 and 3 were consistent Site 1 appeared to be affected by the presence of roughness elements (boulders). The wavelengths within the boulder field at Site 1 were 1.3 m shorter than those to the west where no boulders were present (Table 4, Figure 6). This is a difference of 30%, or more than six standard deviations different from the exterior field. The wavelengths interior to the boulder field were also less regular, with a standard deviation seven times larger than those exterior to the boulder field. The shortened wavelengths suggest that drag from the boulders decreased the local wind velocity, and the greater variability in wavelengths inside the boulder field is indicative of increased turbulence. In contrast, the TARs at Site 3 were not affected by the roughness of the boulder field: wavelengths differed by less than 3% between the boulder-free area and the boulder field (Table 4).

| Unusual 'wakes' behind boulders
At Site 1, within the boulder field, the ridges formed multi-armed fan-shaped wakes to the east-northeast of large boulders, opposite the inferred wind direction (Figure 11). Outside the boulder field the ridges form long, continuous, slightly arcing crests. The divergent wakes are typically 10 m long and deviate from the prevailing local ridge orientation by up to 90 ( Figure 12).
T A B L E 2 The wavelengths, lengths, and widths for n samples in each study area. Mean and median statistics are presented for each characteristic. Note that the ridges in Site 1 are much smaller by every metric

| Morphological responses to changes in roughness density
The boulders at Site 1 likely disrupted the local airflow by changing the roughness density within the boulder field, causing the wakes and more broadly, less uniform bedform orientations. Based on their size and observed changes, the boulders themselves are likely responsible for altering the aerodynamic roughness length. Ridges within the boulder field at Site 1 likely formed under a wake interference flow regime.
z 0 peaks when roughness elements (in this case, boulders) are spaced approximately twice their diameter apart (Greeley & Iversen, 1987), and the boulders at Site 1 are separated by two to three times the average boulder length of 3 m (Table 3). Further, many of the boulder wakes can be observed overlapping and interfering, a characteristic of wake interference flow (Wolfe & Nickling, 1993).
Although a combination of conditions likely plays a role, based on our observations, flow separation and deceleration caused by the boulder field likely caused the decrease in wavelength at Site 1. This is F I G U R E 1 0 Normalized boulder size distributions. Note the log y-axis scale. There are similarities between the distributions at Sites 1 (red) and 2 (green); Site 3 (blue) has substantially more boulders, but they were generally smaller [Color figure can be viewed at wileyonlinelibrary.com] T A B L E 3 Mean and median boulder characteristics for each of the three study areas. Note the similarity in boulder density and size between Sites 1 and 2; Site 3 had many more boulders that were smaller than the other two locations F I G U R E 9 A detailed view of some thresholding challenges. Extracted boulder pixels (light green) with corresponding polygons shown with dark green lines. Non-outlined pixels were those below the area threshold set above. Two TAR crests were falsely detected as boulders, producing four false-positive (FP) 'boulders'. Conversely, three boulders were too dark and were not detected, producing falsenegative (FN) detections (PSP_004077_1325) [Color figure can be viewed at wileyonlinelibrary.com] consistent with the longstanding observation that wind speed and ripple spacing are proportional (Anderson, 1987;Goossens, 1991;Nishimori & Ouchi, 1993;Sharp, 1963). Further, the spatially rapid change in wavelength over short spatial scales at Site 1 is characteristic of saltation, the principle formative mechanism of ripples. Other sediment transport mechanisms such as suspension, which operate over longer distances, are unlikely to alter bedform wavelengths by 30% over a span of just a few wavelengths. We thus F I G U R E 1 2 Manually digitized boulder wakes (dashed yellow lines in red insets) at Site 1 (centre image). The inferred wind direction is from the west-southwest to east-northeast (see Figure 11). Note the range of ridge orientations behind each obstacle and the distinct fan-like shape of the wake (PSP_006806_1,365) [Color figure can be viewed at wileyonlinelibrary.com] F I G U R E 1 1 Example boulder wake initiation points at Site 1 marked by orange arrows. Note how ridge orientations east of the boulders (right side of the image) differ from those in the boulderless area in the west of the image (left side of the image). The local wind direction is inferred from the orientation of the wakes, and from scouring and excavation observed on the west of the boulders (inferred as being upwind) (PSP_006806_1,365) [Color figure can be viewed at wileyonlinelibrary.com] interpret this deceleration as evidence that the ridges are large ripples formed via saltation within the lower boundary layer.
Unlike Site 1, the wavelengths at Site 3 did not meaningfully change between the barren area and the boulder field. The Site 3 TARs likely developed under a skimming flow regime where the more tightly packed boulders and the TARs themselves contributed to an already high roughness density. Given that the TARs at Site 3 do not appear to respond to changes in the roughness density caused by the boulder field, the TARs could be (1) too large relative to the boulders to be affected by changes in boulder distribution and/or (2) unaffected by the processes that decreased the saltation length that generated shorter ripple wavelengths in Site 1.

| Implications of the 'wakes'
At present, Site 1 is the only area where wakes have been well documented. However, more comprehensive surveys looking for this type of feature may be fruitful, particularly in areas with ongoing sediment transport (i.e. proximal to sand dunes) and with boulders or other obstacles. Several similar morphologies from other locations are shown in Figure 13.
The multi-armed wakes downwind of boulders are extremely unusual surficial features with no analogue in the literature to the authors' knowledge. As described above, an obstacle in steady flow near universally results in single-pointed features as the flow converges or reattaches behind the obstacle (Cooper, 1958;Hesp, 1981;Hesp & Hyde, 1978;McKenna Neuman & Bédard, 2015;McKenna Neuman et al., 2013;Melton, 1940;Pidgeon, 1940;Salisbury, 1952). The wakes suggest that there is an interruption to this convergence of the flow. We hypothesize that an interaction between pre-existing ridges and the flow separation around the obstacles interrupted the convergence of the flow, and generated the boulder wakes ( Figure 14). Why this process would be undocumented on Earth or in numerical simulations remains unclear and warrants further study.
We interpret the wakes at Site 1 as further evidence that the features are ripples. The small length scale of these interactions, just a metre or two, is highly suggestive of ripple-like dynamics. Further, given our proposed formation process, the rapid temporal response of ripples to changes in flow conditions is more plausible than a dunelike formation.

| No detected TAR activation
Recent research has suggested that some TARs may still be active on Mars, with migration rates of 0.13 m/year as measured over 8-10 Earth years (Silvestro et al., 2020). Active TARs are likely to be found in areas with moving dunes, where dune-sourced saltating grains have been hypothesized as enabling TARTAR activation and migration via impact creep (Anderson, 1987;Berman et al., 2011;Kok et al., 2012;Mladenoff et al., 1997;Swet et al., 2019 Previous work has concluded that Proctor Crater experienced significant aeolian erosion in the past, and continues to be shaped by aeolian processes (Fenton et al., 2003). The visibility of the fractured boulders in the TAR fields at Sites 2 and 3 indicates that several hundred metres of sediment were likely eroded from these sites, and ongoing dune and ripple movement illustrates the ongoing evolution in these areas (Fenton et al., 2003(Fenton et al., , 2005Jackson et al., 2015). Both Sites 2 and 3 are excellent candidates for more rigorous changedetection studies as they have already been imaged repeatedly. Site 2 has eight images taken over 11 years (2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018), and Site 3 has 22 images taken over 13 years (2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020). Sites 2 and 3 would thus be ideal for further study because ongoing saltation and sediment transport are likely a requirement for TAR activation (Berman et al., 2011;Silvestro et al., 2020), and the TARs in these two sites are surrounded by active dunes.

| CONCLUSION
Here we observed TARs and ripples in three locations in and near Proctor Crater with boulder fields to infer some properties of their formative flow regimes. Characteristics of the boulder fields and aeolian forms at each of the sites were documented and analysed.
Site 1 contained ripples, while Sites 2 and 3 contained TARs. Sites