Widespread Megaripple Activity Across the North Polar Ergs of Mars

Abstract

The most expansive dune fields on Mars surround the northern polar cap where various aeolian bedform classes are modified by wind and ice. The morphology and dynamics of these ripples, intermediate-scale bedforms (termed megaripples and Transverse Aeolian Ridges [TARs]), and sand dunes reflect information regarding regional boundary conditions. We found that populations of polar megaripples and larger TARs are distinct in terms of their morphology, spatial distribution, and mobility. Whereas regionally restricted TARs appeared degraded and static in long-baseline observations, polar megaripples were not only widespread but migrating at relatively high rates (0.13 ± 0.03 m/Earth year) and possibly more active than other regions on Mars. This high level of activity is somewhat surprising since there is limited seasonality for aeolian transport due to surficial frost and ice during the latter half of the martian year. A comprehensive analysis of an Olympia Cavi dune field estimated that the advancement of megaripples, ripples, and dunes avalanches accounted for ~1%, ~10%, and ~100%, respectively, of the total aeolian system’s sand fluxes. This included dark-toned ripples that migrated the average equivalent of 9.6 ± 6 m/yr over just 22 days in northern summer—unprecedented rates for Mars. While bedform transport rates are some of the highest yet reported on Mars, the sand flux contribution between the different bedforms does not substantially vary from equatorial sites with lower rates. Seasonal off-cap sublimation winds and summer-time polar storms are attributed as the cause for the elevated activity, rather than cryospheric processes.

Plain Language Summary

“Megaripples” are distinct wind-driven bedforms that occur on the surface of Earth and Mars, often with sizes between that of smaller ripples and larger dunes. Recent work has found the thin martian atmosphere can mobilize some coarse-grained megaripples, overturning prior notions that these were static relic landforms from a past climate. We mapped megaripples and adjacent bedforms across the north polar sand seas, the most expansive collection of dune fields on Mars. Megaripples were found to be widespread across the region and migrating at relatively high rates relative to other sites on Mars that are at lower latitudes. This enhanced activity is likely related to the greater sand fluxes found for neighboring dunes which are driven by summer-time seasonal winds when polar ice is sublimating. In contrast, other megaripples appear to be stabilized, a likely result of intergranular ice within low wind areas.

1. Introduction and Motivation

Dune fields across Earth and Mars host a variety of aeolian bedform classes (e.g., ripples, megaripples, dunes) that vary in terms of size and particle size distribution (Bagnold, 1941; Greeley & Iversen, 1985; Wilson, 1973). Planetary bedform types include sand dunes, decimeter-wavelength impact ripples, and the generally larger ripple class of coarse-grained “megaripples” (Greeley et al., 1992; Lancaster, 2009; Malin & Edgett, 2001; Sullivan et al., 2005, 2008). Martian dark-toned, decameter-wavelength ripples are an exception, with no counterpart in terrestrial aeolian environments (Vaz et al., 2017). The last several decades of Mars exploration and the arrival of high resolution orbital imaging and surface rovers have also revealed some of these bedform classes are migrating under the current climate (Chojnacki et al., 2015; Silvestro et al., 2010, 2020; Sullivan et al., 2005). Ultimately the presence and activity of a given bedform class reflects differences in their boundary conditions (e.g., grain size, wind energy, sediment supply; Chojnacki et al., 2019; Kocurek & Ewing, 2012). For example, terrestrial megaripples that are often partially sourced by an abundant coarse sand population may rarely migrate except for very strong storm events (Isenberg et al., 2011; Milana, 2009; Sakamoto-Arnold, 1981).

The smallest bedform class observed from orbit on Mars (1–5-m spacing and ~40-cm tall) are dark-toned ripples (DTRs) found migrating atop dunes or within isolated patches (Bridges et al., 2011; Lapotre et al., 2016, 2018; Sullivan & Kok, 2017). The larger (10–100-m spacing and 1–14-m tall), light-toned Transverse Aeolian Ridges (TARs) can occur in association with dunes or as large TAR fields, but often lack unambiguous signs of activity (Balme et al., 2008; Berman et al., 2018; Geissler & Wilgus, 2017). The size range in between DTR and TAR populations has been largely unexplored and generally assumed to be inactive like TARs (Chojnacki et al., 2018). We term these intermediate-scale (5–40-m spacing, ~1–2-m tall) bedforms as “megaripples” based on their greater dimensions and brighter crests than DTRs (Figure 1), where we infer the higher albedo areas to be due to a coarser grain size component (Greeley & Iversen, 1985; Zimbelman, 2019). It is noted that granulometrical analysis, which is required to properly distinguish between unimodal and bimodal sand of a given bedform (Greeley & Iversen, 1985; Sullivan et al., 2008; Yizhaq et al., 2012), is unavailable for most locations on Mars.

Figure 1.

Polar bedform sites with active megaripples, as viewed in High Resolution Imaging Science Experiment (HiRISE) at the same scale. Approximate transport directions (dashed arrows) are shown. Insets are 100-m wide. All images are oriented north up unless otherwise indicated. (a) High flux bedforms in the Olympia Cavi dune field termed “Buzzel.” (b) Polar bedforms that resemble Transverse Aeolian Ridges (TARs), alongside DTRs and megaripples. (c) Loath crater megaripples, some partially restricted by late season frost and possible intergranular ice. (d) Bright, thin megaripples found to be active in Scandi Cavi. (e) Modern bedforms that migrate over aeolian stratigraphy described by Brothers and Kocurek (2018). (f) Megaripples arranged upwind and flanking intererg megadunes.

Recent analysis using images acquired by the High Resolution Imaging Science Experiment (HiRISE) camera (0.25–1 m/pix) (McEwen et al., 2007) have shown certain locations host migrating megaripples, which are most apparent among high flux dunes (Chojnacki et al., 2019; Silvestro et al., 2020). However, it is not clear how frequent this mobility is or even their broader occurrence. In particular, the northern polar latitudes of Mars has been found to have extensive migration of DTRs and dunes (Bridges et al., 2011; Chojnacki et al., 2019; Fenton et al., 2021; Hansen et al., 2011), but also has been cited to lack intermediate-scale bedforms of megaripples or TARs (Balme et al., 2008; Wilson & Zimbelman, 2004). Additionally, this effort seeks to constrain the sand flux contributions of megaripples relative to other polar bedforms.

The goal of this project is to better understand the intriguing bedform class of megaripples in one of the most complex planetary aeolian systems, namely the north circumpolar ergs of Mars. Objectives here are to (a) survey aeolian sites across the study region for the presence of intermediate-scale bedforms, (b) assess megaripple and TAR dynamics and evaluate if any activity is restricted to certain areas or is widespread across polar sites, and (c) quantify relative sand flux contributions of polar megaripples related to other bedforms. All the objective results will be viewed in the context of the polar environment and how regional boundary conditions impact bedform mobility across the erg. In this way, we seek to better understand polar aeolian processes, identify any seasonal effects, and quantify landscape evolution in one of the most active regions on Mars.

2. Study Region

The north polar region of Mars displays a range of seasonal and annual atmospheric and surface processes that continually reshape the local landscape (Smith et al., 2018). These processes are linked to the volatile and dust exchange between polar and nonpolar reservoirs, where the north polar cap is composed of seasonal CO₂ ice, residual H₂O ice, and dust (Khayat et al., 2019; Langevin, 2005). This surface-atmospheric exchange is known to drive various aeolian phenomena, such as wind streaks (Howard, 2000), seasonal and interannual albedo variations (Calvin et al., 2015), spiral trough evolution (Smith & Holt, 2010), dust storms (Wang & Fisher, 2009), and bedform migration (Bourke et al., 2008; Bridges et al., 2011). Expansive dune systems nearest the north polar layered deposits (NPLD) and residual cap (Fenton, 2020; Hayward, 2011) are primarily driven by Coriolis force deflected katabatic (downslope) winds from the northeast descending into a series of reentrant chasms. Indeed, sand pathways sourced from the NPLD’s Planum Boreum Cavi and Rupes units are most evident in Chasma Boreale, Olympia Cavi, and other reentrants that spiral southward to merge with the main erg (Figure 2; Fishbaugh & Head, 2005; Tanaka et al., 2008). The high level of bedform migration occurs despite the limited sediment availability caused by autumn/winter CO₂/H₂O ice accumulation that restricts saltation for most of the year (Chojnacki et al., 2019). Dune sand becomes ice-cemented while winter-time CO₂ ice buries dunes and then slowly sublimes through the northern spring/summer until bedforms are “frost free” and mobile by summer (Ewing et al., 2010; Hansen et al., 2011). Some ice-cemented bedforms do not appear to regain mobility and were deposited into the geologic record as evidenced by the exposed aeolian cross-strata (Brothers & Kocurek, 2018). The elements of these putative remnant dune fields are present throughout the study region and are noted in relevant figures.

Figure 2.

Polar bedform occurrence and activity results. Dune field distribution are shown in red (Fenton, 2020; Hayward et al., 2014). Base maps are MOLA shaded relief with gray-scale or colourized elevation. (a) Survey results showing High Resolution Imaging Science Experiment (HiRISE) locations of dune fields with megaripples, Transverse Aeolian Ridge (TAR) candidates, or sites lacking either intermediate-scale bedform class. See Table S1 in Supporting Information S1. (b) Results showing megaripple fluxes (colored circles) from manual mapping along with two sites lacking megaripples (white circles; Figure S1 in Supporting Information S1). (c) Prior results showing sand dune fluxes at the same locations (colored circles; Chojnacki et al., 2019) and simplified transport directions (dashed arrows).

3. Overview of Approach Data Sets and Methods

For the objective 1 survey, we assessed bedform morphology using HiRISE images (0.25–1 m/pix) acquired in northern summer and criteria described in Section 4.1 (Table S1 in Supporting Information S1). Objective 2 and 3’s assessment of bedform activity relied on long-baseline (4–7 Mars years) orthoimages of sites with prior HiRISE Digital Terrain Models (DTMs) (1 m/post) (Chojnacki et al., 2019). Orthorectification was carried out using SOCET SET^® BAE system photogrammetry software (Kirk et al., 2008), where image pairs for change detection were typically acquired within 20° of solar longitude (L_s), but in different Mars years (see Section S1 in Supporting Information S1 for more details). Activity was quantified by mapping 3 or more consecutive megaripple crests (per area) in both the Time 1 (T1) and T2 images (Table S2 in Supporting Information S1) using ArcGIS^® or QGIS. Wavelength (wr) and migration rate (m/Earth year) were calculated using QGIS software and in-house code which ingests manually mapped crest lines from different images. Wavelength measurements correspond to the average spacing computed along transects orthogonal to the bedform traces, while migration rates were quantified assuming a local biorthogonal migration trend between mapped crest lines (Silvestro et al., 2020). Bedform half-heights (h_r), and ultimately megaripple flux estimates, were derived using Equation 1

$$h_r=wr/20$$ (1)

The wavelength-height relationship, uncertainty (Bridges et al., 2012; Runyon et al., 2017b), and application of Equation 1 to megaripples (Silvestro et al., 2020) were developed over several investigations.

Objective 3’s quantification of whole dune field fluxes required multiple approaches applied to an Olympia Cavi reentrant aeolian site (232.9°E; 84.0°N) termed here as “Buzzel” (see Diniega et al., 2017). This site was chosen due to the abundance of adequate data and known activity (Chojnacki et al., 2019; Diniega et al., 2017). Dune front advancements were recorded with the tracing of lee fronts in ArcMap^® on the T1, T2, and T3 images (Table S3 in Supporting Information S1), allowing migration rates and directions to be semiautomatically computed. This process integrates data derived from HiRISE orthoimages and DTMs, generating continuous measurements of migration and heights along the slip faces allowing volumetric sand fluxes (q) (m³ m⁻¹ yr⁻¹) to be estimated (Urso et al., 2018). Migration rates and sand fluxes are reported with temporal units of Earth years, while the elapsed time between images is generally reported in Mars years. Instead of reporting peak fluxes (multiplying the maximum height by the average migration, like in Urso et al. (2018)) we multiply slip face heights and migration rates along the slip faces in order to compute mean and median fluxes. This generates lower average fluxes, yet it is a more accurate representation of the overall fluxes (the same approach was followed in the flux comparison presented by Silvestro et al. (2020)). Ripple and megaripple displacements were quantified for the Buzzel site using “Co-registration of Optically Sensed Images and Correlation” (COSI-Corr) software (Leprince et al., 2007) which produces a dense vectorial map of ripple migration (Bridges et al., 2012; Vaz et al., 2017). The rapid migration rates of DTRs required early summer images (Mars Year (MY) 35, L_s 95°–105°), whereas MY 30 and 35 images were used to assess slower megaripples. Fluxes were derived using the method of Silvestro et al. (2020). For flux comparison purposes, all three bedform classes were characterized in the northeast ~1–2 km (upwind) section of the dune field based on image pair constraints (see SM Section S2 for more details).

Statistics were collected using various techniques given the constraints of the available data and bedform class. Reported fluxes are deemed representative of the whole bedform movement and are intended for comparison, but these various measurements have different accuracies and nuances. For example, “Dune crest flux” measurements embody the vast majority of the Buzzel sand movement in that zone of the field, but no doubt some sand is passing through the area unrecorded via lateral deflection of saltons (Hersen, 2004; Momiji & Warren, 2000). The relative amounts, in terms of order of magnitude, are more important to note rather than absolute values and reported percentages are approximate. While median values are more robust statistics to compare fluxes for different areas by reducing possible bias caused by outliers (Table 1), mean flux metrics are also useful for comparison with other investigations.

Table 1.

Polar Megaripple Activity Results (Objective 2)

Site ID and namea	Elapsed time (EY/MY)b	Displacement (m)	Migration rate (m/yr)	Wavelength (m)	Half-average height (m)	Flux (m3 m−1 yr−1)	N c
3374 + 216 McLaughlin	7.6/4.0	0.97 ± 0.5	0.13 ± 0.06	5.8 ± 2	0.29 ± 0.08	0.035 ± 0.02	6,459
0742 + 214 NiliFossae	9.4/5.0	1.5 ± 0.9	0.16 ± 0.09	6.9 ± 2	0.35 ± 0.09	0.05 ± 0.03	2,953
2329 + 840 Buzzel dunesd	9.5/5.0	0.92 ± 0.4	0.097 ± 0.05	6.5 ± 2	0.33 ± 0.08	0.03 ± 0.02	4,151
0953 + 761 Palma dunes	9.3/5.0	1.4 ± 0.5	0.15 ± 0.06	8.7 ± 2	0.43 ± 0.1	0.062 ± 0.04	797
2121 + 790 Gypsum erg	7.6/4.0	1 ± 0.2	0.13 ± 0.03	11 ± 3	0.55 ± 0.1	0.069 ± 0.02	678
1788 + 816 Olympia Undae	7.5/4.0	1.1 ± 0.3	0.14 ± 0.04	9.7 ± 2	0.49 ± 0.08	0.069 ± 0.02	359
1035 + 703 Louth Crater	12.9/6.9	2.2 ± 0.5	0.17 ± 0.04	11 ± 2	0.55 ± 0.09	0.086 ± 0.02	553
1186 + 835 Tleilax dunes	11.0/5.9	1.8 ± 0.4	0.16 ± 0.03	11 ± 3	0.56 ± 0.1	0.087 ± 0.03	611
2798 + 809 Abalos Scopuli	9.5/5.0	2.6 ± 0.6	0.27 ± 0.07	9.4 ± 2	0.47 ± 0.1	0.13 ± 0.04	713
2705 + 761 Abalos dunes	11.3/6.0	1.7 ± 0.3	0.15 ± 0.03	7.2 ± 1	0.36 ± 0.07	0.057 ± 0.01	517
3393 + 850 Chasma Boreale	11.2/6.0	0.95 ± 0.3	0.084 ± 0.03	11 ± 3	0.53 ± 0.2	0.044 ± 0.02	1,329
2095 + 780 Scandia Cavi	11.2/6.0	1.1 ± 0.2	0.1 ± 0.02	9.3 ± 0.8	0.47 ± 0.04	0.046 ± 0.01	397
3154 + 827 Chasma Boreale-Megadunes	7.6/4.0	1.5 ± 0.5	0.2 ± 0.07	11 ± 4	0.56 ± 0.2	0.1 ± 0.04	417
All areas		1.2 ± 0.5	0.13 ± 0.06	7.2 ± 2	0.36 ± 0.1	0.046 ± 0.03	19,934

Note. Reported values are medianmedian absolute deviations. The first two sites of McLaughlin crater and Nili Fossae are the lower latitude fields discussed by Silvestro et al. (2020). Also see Figures 2 and 5.

^aDune field site IDs, where the first four digits are the monitoring site’s centroid east longitude, the last three digits are the site’s latitude (no decimals), and the “+” indicates the northern hemisphere. Informal site names are also provided where some correspond with those investigated by Diniega et al. (2017).

^bSee Table S2 in Supporting Information S1 for relevant HiRISE data information.

^cThe number of measurements.

^dThe values for the Buzzel dunes correspond to the mapping of the full area (a total of 247 slip faces), while the measurements discussed in Section 4.3 correspond to a buffer area (148 slip faces) shown in Figures 6 and 7.

4. Results

4.1. Survey of Polar Megaripple Occurrence

We surveyed dune fields imaged by HiRISE across all latitudes including those above 65°N to assess the presence or absence of intermediate-scaled bedforms (Figures 1 and 2a). TARs and megaripples were classified separately based on their different size, albedo, and stratigraphic relationship (Figure 1). TAR candidates were designated for light-toned, transverse bedforms, which were interpreted as being stratigraphically below dark dunes and meter-scale ripples (Figure 1b). In contrast, megaripples were noted to be present for typically smaller, variable-albedo bedforms which were in most cases stratigraphically above or in continuity with neighboring bedforms (Figure 1a). Of the 67 north polar locations surveyed, 88.1% had megaripples, 9.0% had TARs, 9.0% had both, and 11.9% had neither class of intermediate-sized bedforms (Table S1 in Supporting Information S1). DTRs were found at all erg locations. Megaripples were commonly found upwind of erg areas, climbing dune slopes, or in small interdune fields. Bright-toned TAR fields were identified in the region, but notable below polar scarps nearest the NPLD-erg margins (Figures 2a, 3 and 4). Finally, the greatest proportion of HiRISE images lacking either intermediate-sized class were in polar regions (Figures 2a and Supporting Information S1). Prior global surveys had described in passing the lack of TARs in polar regions compared with lower-latitude regions (Balme et al., 2008; Wilson & Zimbelman, 2004). Consistent with these findings, our global HiRISE survey of dune fields identified close to half of all sites (52.5%) had bedforms identified as TARs (Figure 4) (Chojnacki et al., 2021).

Figure 3.

Examples of polar megaripples and Transverse Aeolian Ridges (TARs) near the north polar layered deposits (NPLD) and basal unit sand sources at (a) Chasma Boreale and (b) west Olympia Cavi. Along with being underneath some dark dunes or ripples, polar TARs can be found superposed with boulders or with crests in opposing directions as nearby dunes transport. Approximate transport directions (dashed arrows) are shown. Inset maps show site locations (star).

Figure 4.

Global trends of intermediate-sized aeolian bedforms (black circles) using High Resolution Imaging Science Experiment (HiRISE) images of dune fields. Megaripples (MR) and Transverse Aeolian Ridges (TARs) were classified separately based on their different size, albedo, and stratigraphic relationship. Also see Table S1 in Supporting Information S1 and Chojnacki et al. (2021). Compare with earlier survey results that also found a lack of polar TARs (Wilson & Zimbelman, 2004). Base maps are MOLA shaded relief with dunes field in red (Fenton, 2020; Hayward et al., 2014).

4.2. Assessment of Polar Megaripple Activity

To qualitatively and quantitatively assess the activity of polar megaripples we examined HiRISE long-baseline orthoimages (Table S2 in Supporting Information S1). Of the 13 monitoring sites in the north polar ergs, 85% (11) showed unambiguous migration of megaripples in (downwind) directions that are broadly aligned with that of nearby DTRs and dunes (Figure 2b; Animation S1–S5). The remaining two sites had no megaripples present to observe, although DTRs and dunes were migrating at those locations (Figure S1 in Supporting Information S1). Megaripple activity is most evident on the upwind edges of dune fields and in some cases within intererg areas or below arcuate scarps. Clusters of contiguous megaripple fields often flanked by ripples and dunes were most common, while occasionally occurrences of mobile megaripple trains atop of bedrock were observed (Figure 1a, Animation S3). Crestlines may bifurcate, split, or merge with other megaripples moving at slower rates resulting in changes in crestline patterns. In many cases, unambiguous megaripple migration was observed in shorter-term annual pairs as well (2–3 Mars years). Some of these swifter examples migrating several wavelengths made them difficult to track in longer baseline image pairs, whereas certain slower ones were overtaken and buried by dunes (Animation S6 and S7). In contrast, all occurrences of polar TARs remained static at the time scale and spatial resolution of this survey.

The median wavelength for active megaripple sites ranged between 5.8 and 11 m (average 7.2 ± 2 m (Figure S2 in Supporting Information S1); all reported uncertainties correspond to 1σ) and rates between 0.08 and 0.27 m/yr (0.13 ± 0.06 m/yr for all sites) (Figure 5; Table 1; Table S2 in Supporting Information S1). For comparison, global average dune rates were ~0.5 m/yr (Chojnacki et al., 2019), average southern latitude ripple rates were 0.35 m/yr (Banks et al., 2018), and tropical latitude megaripples migrated at 0.12–0.13 m/yr (Silvestro et al., 2020). Average wavelength-derived heights for all sites were between 0.7 and 0.9 m (Table 1), but topographic profiles show some individual megaripples 1–2-m tall (Figure S3 in Supporting Information S1). The manually derived median megaripple sand fluxes ranged between 0.034 and 0.099 m³ m⁻¹ yr⁻¹ (average q = 0.046 ± 0.02 m³ m⁻¹ yr⁻¹) (Figures 2b and 5; Table 1). Average sand dune crest fluxes for all but one of these sites (Louth crater) were moderate to high (7.4–18.6 m³ m⁻¹ yr⁻¹) based on earlier measurements (Figure 2c; Chojnacki et al., 2019). A comparison between the megaripple and sand dune flux distributions (Figures 2b and 2c) shows a moderate correlation for monitoring sites implying a relation. That is, areas possessing dunes with moderate to high fluxes often host very active megaripples. However, a more holistic approach is required to better understand the spatial and temporal aspects of megaripples within a given aeolian system (see Section 4.3). Overall, we found that polar megaripple activity is widespread in various contexts (e.g., reentrant troughs, interergs, polar craters), whereas static TAR candidates displayed rounded, broad, or pitted crests were found within otherwise active sand corridors adjacent to the NPLD (Figures 2b and 2c, Animation S8; see Section 5.2).

Figure 5.

Manually derived megaripple sand flux results for 11 polar sites as compared with those in McLaughlin/Nili Fossae (top left two plots, Silvestro et al., 2020). The bottom right plot integrates the measurements from all areas. Migration rates (y-axis) were derived from crestline mapping whereas heights (x-axis) were computed from wavelength-height relationships. Median sand fluxes (±median absolute deviation) are reported in red. Corresponding wavelength are provided in Figure S2 in Supporting Information S1 and detailed statistics are reported in Table 1.

4.3. Polar Megaripple Fluxes and Comparisons to Other Bedforms

The Buzzel site represents a typical polar basin dune field, which is located just downwind of its basal unit sand source at the erg margin (Figure 6) (Fishbaugh & Head, 2005; Nerozzi & Holt, 2019). Protodunes and sand sheets lead southwestward to more developed barchans and barchanoids as sediment supply increases (Ewing et al., 2015). In order to best constrain whole dune field fluxes, we used a buffer area located on the upwind edge of the site (Figure 6a). This area critically included all three bedform classes and was covered with the appropriate data for their characterization (Table S3 in Supporting Information S1). It is noted, this upwind area is arguably one of the most dynamic sections of the dune field with little flux decay as it does not include downwind sections where the internal boundary layer increases with greater dune topography (Runyon et al., 2017a).

Figure 6.

Context for the Buzzel study site. (a) View of the Buzzel site in Context Camera (CTX) images. Dune crest flux vectors (colored arrows) and the buffer area (white polygon) for Objective 3’s whole dune field flux analysis are shown—see Figure 7 for details. Dunes are migrating south to southwest and downwind of steep NPLD scarps and the regional sand source to the northeast. (b) Regional view showing the field-of-view for (a) (white box) with Buzzel at the head of the Olympia Cavi reentrant. CTX mosaic colorized with MOLA elevation. (c) Examples of dune lee face positions during 3 Mars years (MY) and nearby megaripples (green polygons). (d) Oblique view looking downwind (white arrow in (a)) from a projected orthoimage. (inset) Closer view of dunes and megaripples in HiRISE color.

Sand dune migration rates (0.2–5.4 m/yr) and fluxes (1.1–35.7 m³ m⁻¹ yr⁻¹) are variable in the cross-field directions (NW-SE) (Figure 7a, Figure S4 in Supporting Information S1; Table 2). Dune measurements were collected for two consecutive time periods (spanning 3.8 and 5.7 EY), resulting in similar average fluxes, respectively 10.5 ± 8 and 7.3 ± 6 m³ m⁻¹ yr⁻¹ (Figure S4 in Supporting Information S1, Animation S9). The highest flux measurements tended to be either small, extremely swift upwind dunes (Figure 6c) and, in some cases, associated with trains of barchans that get progressively taller downwind.

Figure 7.

Comparison of the sand fluxes at the edge of the “Buzzel” dune field. Also see Figure 6 for context, Figure S4 in Supporting Information S1 for mapping examples, Figure S5 in Supporting Information S1 for sand rose diagrams, Figure S6 in Supporting Information S1 for COSI-Corr details, and Table 2 for summary statistics. (a) Sand dune crest flux results. (a) Fluxes were evaluated at two time steps, spaced in time 3.8 and 5.7 EY. (b) Megaripple fluxes, which were estimated using two approaches: (b) automatic tracking of the bedforms with COSI-Corr, using a mask to select the intermediate-scale bedforms and a constant half-height of 38 cm (corresponding to the average half-height estimated from the manual approach); and (bʺ) manual mapping of bedform crest traces, which allowed migration rates and wavelength-derived bedform half-heights to be estimated at multiple locations (time interval of 9.5 EY). (c, c) Sand fluxes of meter-scale dark ripples that were quantified using COSI-Corr (for dark-toned ripple (DTR) displacements) and a constant half-height of 12.5 cm is assumed (time interval of 22.6 days). (d) Circular plots showing the fluxes mean vectors (m³ m⁻¹ yr⁻¹) and circular standard deviation intervals (dashed lines) for the bedforms and measurement techniques (in the case of the megaripples). (e) Flux comparisons for the Buzzel site bedforms that highlight the different modes of fluxes (fluxes distributions on the right), with megaripple’s fluxes one and 2 orders of magnitude lower than DTR and dune crest fluxes, respectively.

Table 2.

Summary Statistics for the Compared Bedforms’ Fluxes^a

	Mean flux azimuth (°)	Mean flux magnitude	Circular variance	Circular STD (°)	N b	Median flux	Median STD	Mean flux	Flux STD
Dark-toned ripples	246.7	1.01	0.16	33.5	373,172	1.05	0.42	1.2	0.77
Megaripples-COSI-Corr	215.6	0.04	0.31	49.8	27,562	0.04	0.02	0.06	0.05
Megaripples-manual	222.3	0.05	0.06	19.7	2,758	0.03	0.02	0.05	0.05
Dune crest flux	226.4	7.73	0.13	30.7	9,706	6.97	3.47	8.92	7.4

Note. The reported values correspond to a common area, located up to 1 km from the dune field edge at the Buzzel site.

^aStatistics were collected using various techniques given the constraints of the available data and bedform class details. See Section 2 for details. All flux metrics are given in units of m³ m⁻¹ yr⁻¹.

^bThe number of measurements.

Dark-toned ripples migrated at high rates throughout the site but are greatest along higher dune slopes and crests (Figure 7c). Indeed, DTR migration rates ranged from 1 to 84 m/yr and averaged a high value of 9.6 ± 6 m/yr in the brief period between images (L_s 94.96°–105.08° or 22.6 days in MY35/2019). A longer baseline pair (L_s 105.08°–128.4°) was investigated, but ripples had displaced too much for the COSI-Corr correlator to track them preventing a more precise computation of the migration rates. Associated DTRs fluxes were 0.2–10 m³ m⁻¹ yr⁻¹ (average q = 1.2 ± 0.8 m³ m⁻¹ yr⁻¹) (Figure 7e).

Megaripples are distributed across the study area but more often in the upwind locations (Figure 7b). Megaripple sand fluxes here were 0.05–0.5 m³ m⁻¹ yr⁻¹, which are similar to earlier analysis (see below). The very similar COSI-Corr (Figure 7b) and manually derived (Figure 7b) megaripple rates (Figure 7e; Table 2) illustrate the robustness of our analysis. Although megaripple crests that are armored by coarse grains are probably impervious to direct aeolian mobilization, copious amounts of DTR saltation events are available for creep transport. Note, megaripple rates do not exceed that of DTRs or dunes, indicating the slower megaripples do not contribute to total crest fluxes since they never or infrequently approach dune brinks. Instead megaripple populations are overtaken and occasionally buried by swifter dunes or DTR groups (Animation S6 and S7). Active megaripples migration trends (216 ± 50°) are closely aligned with those of dunes (226 ± 31°), whereas migrating DTRs (247 ± 34°) show a more westward trend (Figure 7d, S5 in Supporting Information S1). Overall, it is estimated that the advancement of megaripples, reptation of DTRs, and dune slip face avalanches account for ~1%, ~10%, and ~100%, respectively, of the sand fluxes at the Buzzel dune field (Figure 7e).

It is worthwhile to compare these results with nonpolar megaripple sites. For example, dune crest fluxes at tropical latitudes in Nili Fossae and McLaughlin are ~3 m³ m⁻¹ yr⁻¹, while fluxes derived for the megaripples in the same regions are 2 orders of magnitude lower (0.03–0.04 m³ m⁻¹ yr⁻¹) (Silvestro et al., 2020). Therefore, despite the higher magnitude of fluxes (more than double) and differing boundary conditions at the polar site, we observe a similar relation between the megaripples reptation and slip face advancement fluxes (~1%).

Dark-toned ripple reptation fluxes in Nili Patera were found to correspond to 20% of the slip face fluxes (6.9 m³ m⁻¹ yr⁻¹; Bridges et al., 2012). Slower ripples at the Herschel crater dune field were estimated to have lower reptation fluxes (0.06 m³ m⁻¹ yr⁻¹, see Supporting Information S1; Cardinale et al., 2016), which would equate to ~5% of the bulk flux there (1.2 m³ m⁻¹ yr⁻¹; Vaz et al., 2017). A similar relationship is found for the Bagnold dune field located in Gale crater, where a reptation/bulk flux partition of 4% is estimated (Silvestro et al., 2016, 2020). Overall, the mentioned DTRs reptation fluxes represent 4–20% of the bulk sedimentary flux inferred from the slip face advancements, in line with the 10% estimate for the Buzzel polar site. In addition, there appears to be a positive correlation between bulk crest fluxes and the relative weight of DTR reptation fluxes, which will be tested in the future.

5. Discussion

5.1. Spatial Heterogeneity of Polar Intermediate-Scale Bedforms

North polar megaripples and TARs show spatial heterogeneity in their distribution motivating the question—why are these bedforms relatively abundant at some north polar sites, but absent at others? Survey results indicate aeolian megaripples are widespread in the north polar region particularly for areas of higher dune density or sand volume (Figures 2a and 4). If the identified bedforms are truly composed of bimodal sand this indicates an abundant coarse-grained sand population is present for the ergs—an interesting revelation considering the regional sand source. The consensus view holds most regional sand is sourced from basal Cavi or platy units underlying the NPLD (Figure S1 in Supporting Information S1) (Byrne & Murray, 2002; Fishbaugh & Head, 2005; Tanaka et al., 2008). Based on cross-bedding exposures, internal radar reflections, compositional links, and tendency to produce sand, the Cavi units are widely agreed to be elements of a massive erg that was buried by an expanding ice cap following environmental or climatic change (Brothers et al., 2018; Massé et al., 2010; Nerozzi & Holt, 2019). The presence of an ample coarse sand population, as inferred for megaripples based on their greater size and bright crests, suggests the paleo-erg source was not mature enough to be dominated by fine, well sorted sand. That is, a recycled sand source that has gone through repeated periods of sedimentation (i.e., aeolian sandstone units) (Edgett et al., 2020) is more likely to be rich in fine sand, as compared with a primary sand source (i.e., volcaniclastic units) (Chojnacki et al., 2014; Kocurek & Lancaster, 1999). This evidence of a variable particle size distribution of paleo-erg sand would be consistent with shallow radar interpretations of the basal unit that found alternating layers of water ice and sand dunes (Nerozzi & Holt, 2019). The deposition of sand with variable water content was likely accompanied by sublimation lags (Carr & Head, 2010). Thus, the ancient ergs that are sourcing modern polar dunes were far from being spatially or compositional uniform and may have contained variable bedform classes that were deposited under shifting environmental conditions.

In contrast, there are certain sand transport corridors which lack both megaripples or TARs. For example, the low sand density barchans and ripples of the west Olympia Cavi reentrant (Figure 2a; 110°E, 81°N) are largely without intermediate-bedforms. Other intererg areas mapped with low to moderate sand coverage by Hayward et al. (2014) are similar (Figure 2a). In these locations, scattered barchans or dome dunes migrate in low sediment supply conditions (Figure S1 in Supporting Information S1), and are apparently deficient in a coarse sand population conducive to megaripple formation. These aeolian systems maybe ultimately sourced by a more mature (finer-grained) sand supply or have migrated far downwind of any accompanying coarser-sand megaripple population.

Interestingly, bedforms with characteristics commonly attributed to TARs (e.g., light-toned, transverse, >20 m in spacing) are absent in most polar regions except at the higher latitude NPLD-erg contact areas (Figures 2a, 4, and 8). The identified north polar TAR candidates generally appeared weathered, cracked, with rounded or “boxy” crests, or partially buried (as do some lower latitude examples (Chojnacki et al., 2018; Sullivan et al., 2008)), consistent with long-term inactivity (Figures 1a and 8). Indeed, a detailed study in Scandi Cavi by Fenton et al. (2021) estimated a lower limit age of TAR-like bedforms there to be ~270 kyrs. Additionally, we suggest the stability and appearance for some polar TARs is most readily explained by an intergranular ice component (see Section 5.2). Their close proximity to scarps (the likely regional sand source) and lack of mobility may suggest regional polar TARs did not migrate far after formation.

Figure 8.

Polar erg sites with progressively more degraded Transverse Aeolian Ridges (TARs) or megaripples (top left and working clockwise). All annotated megaripples lacked motion in our analysis. These bedforms appeared weathered, cracked, with rounded or “boxy” crests, or partially buried. Approximate transport directions (dashed arrows) are shown. Insets are 100-m wide. These static bedforms often show nearby mobile megaripples and dark-toned ripples (DTRs) without similar morphologic characteristics. (a) Chasma Boreale, (b) west Olympia Cavi, (c) Chasma Boreale, and (d) Louth crater. Inset maps show site locations (star) and High Resolution Imaging Science Experiment (HiRISE) insets are 100-m wide.

5.2. Sand Fluxes of Polar Megaripples and the Seasonal Cycle

Prior work demonstrated that north polar dune systems show 50% greater crest fluxes than elsewhere on Mars (11.4 versus 7.8 m³ m⁻¹ yr⁻¹) (Chojnacki et al., 2019). The high level of megaripple dynamics, both broadly around the erg and at the Buzzel study site (Figure 7), support this notion of elevated polar bedform activity. This high level of bedform activity is somewhat surprising due to the short period (northern summer and autumn) for frost-free sediment availability (Chojnacki et al., 2019; Hansen et al., 2013, 2015). An important relevant question pertains to whether polar seasonal processes promote or retard megaripple activity. At a broadscale, the unique surface-atmospheric volatile interactions found at the martian north polar region and resulting wind regime is likely a governing factor for the observed enhanced megaripple migration. The north polar wind regime is dominated by off-cap katabatic “sublimation winds,” which occur regularly during the northern spring-summer. These are modeled to be high in magnitude, consistent in direction, and persistent throughout the polar day during the late spring to early summer (Massé et al., 2012; Smith & Spiga, 2018). Winds are driven by seasonal and thermal effects of the retreating spring/summer CO₂ ice and strong contrast between polar cap and erg surfaces in terms of elevation (~2-km high), temperature (23 K), and albedo (15–25%) (Chojnacki et al., 2019; Howard, 2000; Smith & Spiga, 2018). These seasonally forced winds and occasional storm events (Calvin et al., 2015; Wang & Fisher, 2009) appear to drive the high frequency megaripple activity in the region (Figures 2 and 7).

What about more direct evidence of polar processes impacting bedform movement at a finer scale?

For example, following summer-time oversteepening by aeolian processes (Horgan & Bell, 2012) dune slip face alcove formation is seasonally constrained to the autumn/winter, further expanded during springtime frost sublimation, and estimated to contribute to 2–20% of dune movement (Diniega et al., 2017; Hansen et al., 2015). A similar process of seasonal fracturing and wasting of steeper megaripple lee-ward faces may lead to movement, even under frost veneers (Figure 9). However, it is unclear that bulk megaripple displacements and their direction(s), which is well-correlated with that of the nearby bedforms (Figure 7d), are caused by or are significantly influenced by cryospheric processes. Buzzel’s relative flux partitions between bedform classes are comparable to other equatorial dune fields, suggesting that primary aeolian transport modes (impact-driven creep and reptation + saltation) are driving most of the activity in the polar sites, instead of ice-related seasonal processes. Additionally, most megaripple lee and stoss areas appear to be relatively symmetric and not substantially steeper on the lee-side, which might cause oversteepening and mini-alcoves. Although these features would be challenging to track, lee-side slumps, or alcoves on megaripples are not clearly evident even when fully illuminated.

Figure 9.

Seasonal changes between northern spring and summer for polar megaripples and dunes. All sites are for active megaripples. Approximate transport directions (dashed arrows) are shown in summer images. Insets are 100-m wide. (a) Buzzel dunes in east Olympia Cavi reentrant, (b) Loath crater, and (c) west Olympia Cavi reentrant. Inset maps show site locations (star) and HiRISE insets are 100-m wide.

Instead, the cyclical deposition of CO₂/H₂O frost and ice has an important role in regional bedform stabilization over different time frames (Brothers et al., 2018; Schatz et al., 2006). Dark material with icy foresets, isolated dunes, cross-bedded strata, and bounding surfaces have been identified in either Cavi scarp units or interdune areas, which are interpreted as various components of an ancient aeolian sand sea related to past climate change (Brothers et al., 2018; Ewing et al., 2010; Nerozzi & Holt, 2019). Certain modern duneforms also show evidence for cross-stratified ice, partial burial by residual frost (Figures 1e and 8), and thermal properties consistent with a shallow ice table (<1 m and as low as ~3-cm deep) (Brothers & Kocurek, 2018; Putzig et al., 2014). We suggest these cryospheric processes impact intermediate-scale bedforms as well and help explain the degraded morphology of some TAR-like bedforms. Polar TARs that are often characterized by weathered, occasionally pitted, or rounded crests that remain static despite being located in active sand pathways (Figures 1b and 8; Animation S8). More extreme examples can be found of TARs buried in late season frost or perennial water ice (Figures 1c and 8b). In these cases, volatile-related processes and any accompanying cohesion may have outpaced bedform mobility. An analogous process has been described for lower-latitude immobile bedforms which display evidence for dry-condition induration (e.g., cohesion, chemical weathering) and are thought to occur over long periods of inactivity (Sullivan et al., 2008, 2020).

In contrast to the static, possibly ice-cemented polar TARs, megaripples in the same environment remain mobile over many polar winters even when temporarily buried then exhumed by swifter duneforms (Animation S6 and S7). Whereas spring ice does not fill in megaripple troughs (estimated to be ~1 m; e.g., Figure S3 in Supporting Information S1), DTR areas appear to be smoothed over, suggesting decimeter-thick winter frost accumulation (Figure 9). CO₂ frost is typically fully sublimated off sandy surfaces by late spring (L_s ~ 80°–90°) (Hansen et al., 2013; Portyankina et al., 2013), possibly slightly earlier for sites like Buzzel where slip faces are orientated southward (Pommerol et al., 2013). DTRs and megaripple surfaces may respond to wind and regain mobility promptly around the northern summer solstice. While many of the boundary conditions are nearly identical for the adjacent populations of polar megaripples and TARs (e.g., seasonal cycle, wind regime, topography), sand availability for saltation/creep and intergrain ice-content may be large factors in determining mobile versus immobile.

6. Conclusions

This effort identified the presence, activity, and sand flux contribution of intermediate-scale aeolian bedforms across the north polar erg. Megaripples may grade into smaller DTRs with subtle changes in migration and tone, suggesting these populations exist on a continuum (Zimbelman, 2019). The megaripple populations found at these locations were found to be migrating with dunes and dark-toned ripples when adequate data was inspected. Other key findings include the following:

• While megaripples are relatively minor components of terrestrial aeolian systems they are abundant on Mars and the north polar erg. Bedforms identified as TARs display characteristics consistent with inactivity (e.g., rounded or pitted crests) and are primary concentrated at the base of the polar layered deposit scarps and nearby erg areas. These (static) bedforms are adjacent to the regional sand source of the basal unit, which suggests polar TARs did not migrate far after formation

• A lesser amount of polar aeolian systems, often on erg margins, lack megaripples but show widespread and mobile ripples and dunes (e.g., west Olympia Cavi reentrant). These areas are under low sediment supply conditions where widely separated low sand volume barchan/dome dunes migrate. These areas may lack a particle size distribution conducive to megaripple formation (i.e., no coarse sand size fraction)

• Megaripples are highly active in the north polar region including NPLD reentrants (e.g., Chasma Boreale), interior-erg areas, and polar craters (Figure 2). The high level of observed activity seems to be associated with high sand fluxes of dunes, despite the limited sediment availability when sandy areas are under variable autumn, winter, and spring CO₂ frost and ice (Chojnacki et al., 2019; Hansen et al., 2013)

• Polar megaripples yield fluxes that are 2 orders of magnitude lower than neighboring dunes, consistent with earlier work (Silvestro et al., 2020). While these bedforms do not show significantly greater migration rates or fluxes, activity does occur with a higher frequency across the polar ergs than lower latitudes, possibly due to the greater occurrence of high seasonal winds

• A focused analysis of an Olympia Cavi reentrant aeolian system estimated that the advancement of megaripples, saltation and reptation of DTRs, and dune slip face avalanches account for ~1%, ~10%, and ~100%, respectively, of the sand fluxes (Figure 7). To our knowledge DTR and dune rates are some of the highest yet documented on Mars, yet the flux partition between the various bedforms does not seem to differ from equatorial sites with lower sand fluxes

• Whereas seasonal ice contributes to some bedforms movement, such as dune slip face alcoves (Diniega et al., 2017), no evidence was found that cryospheric processes directly promoted megaripple migration. However, late spring-summer off-cap katabatic “sublimation winds” along with polar storm induced winds are deemed major factors for the high levels of observed bedform activity

Data Availability Statement

All data used for this investigation can be found at the HiRISE Planetary Data System (PDS) website (https://hirise.lpl.arizona.edu/PDS/). More specifically, McEwen (2007) for Reduced Data Records and McEwen (2009) for Digital Terrain Models. Additional data access is available via the Planetary Data System Imaging and Geosciences nodes (https://pds-imaging.jpl.nasa.gov/volumes/mro.html) and the Geosciences node (https://pds-geosciences.wustl.edu/missions/mep/index.htm). All map-projected/orthorectified HiRISE images in figures are courtesy NASA/JPL/University of Arizona, and map-projected CTX images (Malin, 2007) are courtesy NASA/JPL/MSSS/University of Arizona.

Key Points:

• Abundant megaripple populations were identified across the north polar ergs of Mars and found to be migrating with dunes and ripples

• Polar megaripple dynamics and sand fluxes are enhanced relative to lower-latitude sites, despite the shorter migration season due to ice

• Seasonal sublimation winds and polar storms were attributed as the cause for the elevated activity rather than cryospheric processes