Coarse‐Grained Ripples Investigated by the Opportunity Rover on Meridiani Planum, Mars

Aeolian coarse‐grained ripples have been found in all regions investigated by Mars rovers: Meridiani Planum, Gusev crater, Gale crater, and Jezero crater. Therefore, it can be assumed that coarse‐grained ripples are one of the most common landforms on Mars. Studying their formation and evolution gives us the opportunity to determine past and current wind patterns. They are also crucial for understanding the formation and evolution of larger aeolian bedforms. Of all locations studied in situ on Mars, coarse‐grained ripples in extensive (∼100 km2) ripple fields were found only on Meridiani Planum. As coarse‐grained ripples on Mars are not well characterized in the literature, in this work, the morphometry, morphology, spatial distribution, and orientation of coarse‐grained ripples investigated along the 45 km long traverse of the Opportunity rover were analyzed. The obtained results allowed for a more precise definition of coarse‐grained ripples and for distinguishing three classes of coarse‐grained ripples on Meridiani Planum: small, medium, and large. The coarse‐grained ripple activity on Meridiani Planum is now limited due to low material supply, and the relatively strong induration of the ripple surfaces. Even though most of the coarse‐grained ripples on Meridiani Planum were formed thousands of years ago, some smaller coarse‐grained ripples were formed by modern winds.


Introduction
Aeolian bedforms are very common on Mars and possibly the most common bedform on the planet are ripples.Knowledge about these bedforms is crucial not only for understanding aeolian processes but also for future Mars missions, especially concerning hazards related to the traversability of wheeled vehicles (see e.g., Arvidson et al., 2017;Balme et al., 2018;Bretzfelder & Day, 2021), and the morphological mapping of the planetary surface.
In planetary science, ripples are mostly divided into classes in terms of their size given by their wavelength (see e.g., Lapotre et al., 2016) as this is the easiest parameter to measure from orbit.Information on bedform particle size distribution (PSD) can only be obtained from in situ data.PSD data in planetary science are mostly available only from the surface layers of bedforms, and in the case of ripples, such layers often are composed of the coarsest fraction within a ripple body (Sharp, 1963).Information on the PSD of ripple surface layers is important as the size of grains on the surface greatly influences ripple dynamics.For example, ripples that are composed only of fine sand (often called fine sand ripples or impact ripples) form faster and are more active than ripples covered by coarser particles (Sharp, 1963).Therefore, the most basic classification of bedforms should include the size of bedforms and the particle size distributions of their surface layers (see e.g., Day & Zimbelman, 2021).
All sand grains are transported by wind, but their transport modes vary depending on their size.Finer grains are easily transported in saltation, a hopping form of movement, while coarser grains (which are larger and hence too heavy) are transported mainly in creep, that is, by rolling or sliding over the surface (Bagnold, 1941).The critical threshold shear velocity necessary to initiate the grain movement has been described in several works (see e.g., Shao & Lu, 2000).Dunes form by saltation, and ripples by saltation and creep (Bagnold, 1941;Sharp, 1963).On Mars, dunes are often composed of fine (0.06-0.25 mm) and medium sands (0.25-0.50 mm; Weitz et al., 2018); therefore, it can be assumed that medium sands are transported in saltation on Mars.Hence, in this work, coarse grains are considered particles larger than 0.5 mm in diameter (i.e., coarse and very coarse sand and gravel), and fine grains particles that have diameters between 0.06 and 0.5 mm (i.e., fine and medium sand; Wentworth, 1922).
Ripples covered by armor composed of coarse grains are often called "megaripples" (see e.g., Ellwood et al., 1975), "coarse-grained ripples" (see e.g., Jerolmack et al., 2006;Sullivan et al., 2008), or "granule ripples" (see e.g., Frydberger et al., 1992;Sharp, 1963).As ripples composed of coarse sand can be very small (about 1 cm tall; see e.g., Jerolmack et al., 2006), therefore, the term "megaripples" does not describe them properly.The term "granule ripples" is also not very adequate as these bedforms are often composed of coarse sands and other fractions which are not granule-sized (see also discussion in Sullivan et al., 2020).As particles that are within the ripple surface determine the entire ripple dynamics and are easily available for measurements, the term "coarsegrained ripples" is used in this work to describe ripples covered by coarse grains (>0.5 mm), even if the interior of these ripples is made up of fine sands (about 0.1-0.2mm in diameter; Sullivan et al., 2005).
Coarse-grained ripples on Mars are observed on various types of terrain, for example, on plains, in craters, on slopes, in trenches, and among dunes.They create ripple fields, but can also be solitary features (Greeley et al., 2006;Sullivan et al., 2008).Several coarse-grained ripples were investigated by the Spirit rover in Gusev crater (e.g., Greeley et al., 2008), and the Curiosity rover in Gale crater (e.g., Lapotre et al., 2018;Weitz et al., 2018Weitz et al., , 2022;;Zimbelman & Foroutan, 2020).Yet, of all the regions investigated by in situ missions, only on Meridiani Planum very large (∼100 km 2 ) fields of coarse-grained ripples are present.They were extensively investigated by the Mars Exploration Rover (MER) Opportunity.
Previous investigations of coarse-grained ripples on Meridiani Planum were mainly based on orbital data.These studies were focused solely on the ripples' orientations and wavelengths (Fenton et al., 2015;Silvestro et al., 2014).Some in situ investigations of coarse-grained ripples were conducted only as a part of other research related to craters (Golombek et al., 2010(Golombek et al., , 2014)), wind streaks (Geissler et al., 2008), and as a part of a more engineering-focused approach to estimate the hazards involved in the rover's traversability (Balme et al., 2018).Some information about coarse-grained ripples on Meridiani Planum was mentioned in the Mars Exploration Rover (MER) mission overviews (Arvidson et al., 2011;Squyres et al., 2006), and the constraints on wind characteristics that are necessary to mobilize particles in ripples were described by Jerolmack et al. (2006).
Coarse-grained ripples on Meridiani Planum are very interesting for investigation as: (a) they have not been studied in detail yet, (b) they represent one of the most common bedforms on Mars, (c) their formation and evolution in the highly uniform region of Meridiani Planum is only slightly influenced by topographic and geologic factors (see e.g., Arvidson et al., 2011;Squyres et al., 2006), and (d), a necessarily large in situ database of ripples and sands on Meridiani Planum is available thanks to the extensive MER mission.
The aim of this work is to characterize coarse-grained ripples on Meridiani Planum.Studying the morphometry, morphology, orientation, and distribution of coarse-grained ripples along the entire traverse of the Opportunity rover allows answering some questions: (a) whether coarse-grained ripples on Meridiani Planum are different or similar to ripples in other locations on Mars, (b) how grain size influences ripple morphometry, (c) what wind patterns were responsible for the formation of coarse-grained ripples on Meridiani Planum.Answering these questions will enable further development of the ripple classification.

Study Area
Meridiani Planum is an equatorial plain located on both sides of the Martian prime meridian that passes through Airy-0 crater (Hynek & Di Achille, 2017;Hynek et al., 2002).The upper layers of Meridiani Planum are made of sedimentary strata of Middle Noachian to Early Hesperian ages and overlay an older heavily cratered terrain (Hynek & Di Achille, 2017).
On 25 January 2004, Opportunity, one of two rovers in the NASA MER mission, landed in the western part of this region (Squyres et al., 2004).Although the MER mission was planned to last only 90 sols (Mars-days), it persisted for almost eight Mars years (MYs).By the time Opportunity had sent its last signal on Sol 5111 (10 June 2018), it had traveled more than 45 km from its landing site in Eagle crater to Perseverance Valley located at the western rim of Endeavour crater (Figure 1).Due to the in situ mission, the western part of Meridiani Planum is densely covered by high-resolution orbital images.The High-Resolution Imaging Science Experiment (HiRISE) camera on board the NASA Mars Reconnaissance Orbiter (MRO) acquired images at a resolution as good as 0.25 m per pixel (McEwen et al., 2007), which enables resolving meter-scale landforms.The MRO has been sending data since 2006, so the first high-resolution orbital images of Merdiani Planum were acquired about 1.5 MY after Opportunity's landing.
From a geological and morphological point of view, the area studied by Opportunity can be divided into two regions: the plains and Endeavour crater's rim (Figure 1).The surface of the plains made of sulfate-rich sandstones, called the Burns formation (Squyres et al., 2004), is populated by impact craters.It is covered by a thin unconsolidated layer, which is a lag deposit consisting of iron-rich spherules, basaltic sands, and gravel of various origins.Iron-rich spherules are derived from Burns formation rocks (see e.g., Squyres et al., 2004) via aeolian erosion that has been taking place since the Hesperian epoch (Hynek & Di Achille, 2017).Endeavour crater is a relatively large impact crater (approx.22 km in diameter) of Noachian age.Its western rim clearly dominates the surrounding plains, and is comprised of Matijevic and Shoemaker formation rocks that are basaltic in composition (Mittlefehldt et al., 2018).The studied area falls away slightly in a south-eastern direction, with the highest point on Opportunity's traverse located in the vicinity of Victoria crater and the lowest points near Endeavour crater.
During the current Amazonian period, Meridiani Planum has been shaped by aeolian processes.An estimation of aeolian erosion rates for Meridiani Planum showed that up to 80 m of the Burns formation has been eroded during the Amazonian (Golombek et al., 2006), and recent erosion rates are estimated to be 1-10 nm/yr (Golombek et al., 2006).
Four types of larger aeolian landforms have been observed on Meridiani Planum: wind streaks, coarse-grained ripples, transverse aeolian ridges (TARs), and dunes (Figure 1; see also Fenton et al., 2015).Dunes on Meridiani Planum, which are large (tens to hundreds of meters in size) dark bedforms, are located solely on the floors of larger craters (>5 km in diameter; Fenton et al., 2015).In the area covered by the Opportunity rover's traverse, dunes were found only in Endeavour crater (Chojnacki et al., 2011(Chojnacki et al., , 2015)).Wind streaks along Opportunity's traverse are not common and only occur at Endurance, Victoria, and Santa Maria craters (Figure 1).It is possible that bedforms that were previously classified as TARs on Meridiani Planum are large coarse-grained ripples (see Sullivan et al., 2020); therefore, in this work, we called them TAR-like bedforms.These TAR-like bedforms are present in some of the craters investigated by the Opportunity rover (e.g., Endurance, Victoria, Santa Maria, and Endeavour crater; Figure S1a in Supporting Information S1; Fenton et al., 2015).Coarsegrained ripples are very common and literally cover the Meridiani Planum (see e.g., Fenton et al., 2015) in contrast to fine sand ripples which are very rare.

Data and Methods
To characterize coarse-grained ripples, it is necessary to study them in situ, as many forms are too small to be visible from orbit even using the highest-resolution images.Also, an in situ approach allows investigating the PSD and the ripple structure.The orbital images in this work were mainly used for mapping and analyzing the morphology of the bedforms and investigating their surroundings.
As there are no meteorological measurements from Meridiani Planum, the only information on wind patterns and wind strength that is not inferred from studying bedforms can be obtained from theoretical models.To determine how theory predicts wind direction, strength, and frequency for this area, we performed numerical simulations using the Mars Regional Atmospheric Modeling System (MRAMS; Rafkin & Michaels, 2019).

In Situ Data Processing and Analysis
To study ripples on Meridiani Planum, the entire Opportunity image database acquired by the onboard cameras: the Navigation Camera (NAVCAM; Maki et al., 2003), the Panoramic Camera (PANCAM; Bell et al., 2003), the Hazard Avoidance Cameras (HAZCAM; Maki et al., 2003), and the Microscopic Imager (MI; Herkenhoff et al., 2003) were employed.The image database was obtained from the Planetary Data System (PDS).
Stereoscopic NAVCAM images allowed us to study the morphometry, morphology, and orientation of ripples.The bedforms were also studied using PANCAM images, which allowed for resolving mm-scale particles on and around ripples.The PANCAM, HAZCAM, and NAVCAM images were employed to determine the exact location of the MI images on the ripples.The MI database was used to measure the size and shape of the sand grains in ripples.The MI was a panchromatic microscope camera located on the rover's arm.Its FOV at best focus was approximately 3.2 × 3.2 cm and its resolution-31 μm/px (Herkenhoff et al., 2003).Therefore, the particles that can be identified from the MI images must be larger than 100 μm.If multiple images were available for a given sand target, a mosaic was created using Adobe Photoshop (Kozakiewicz et al., 2023).To detect individual grains in the MI images, the Particle Detection and Measurement (PADM) algorithm was used-a semiautomated tool that allows fast detection of individual grains (Kozakiewicz, 2018)-and if this was not possible (e.g., due to uneven illumination in images), manual tracking of the inner grain contours was done in Adobe Photoshop.To study the sand samples in a broader context, grains that were detected from the MI images or MI mosaics were overlaid on the reference color PANCAM images of the studied part of the ripple.

Orbital Data Processing and Analysis
For mapping and wavelength measurement of bedforms, georeferenced HiRISE images and a Digital Terrain Model (DTM) were used.The DTM was generated using NASA's Ames Stereo Pipeline (ASP) and the integrated System for Imagers and Spectrometers (ISIS), which is a digital image processing software package developed by the USGS for NASA.ASP is an open-source tool used for the stereo imagery processing of data acquired from satellites around Earth and other planets (Shean et al., 2016).
To create a DTM mosaic that encompassed the entire traverse of the Opportunity rover and its vicinity, five stereo pairs were used: • PSP_001414_1780-PSP_005423_1780.
The HiRISE Experiment Data Record (EDR) files (McEwen, 2007) were downloaded from the PDS and combined into one image.After the DTM was produced, the images were orthorectified using the DTM in order to remove the influence of the terrain height on each image.After this, the five orthoimages were mosaicked into one product, in a similar way to the DTM.The DTM and orthoimage mosaics were horizontally aligned to the data from the High-Resolution Stereo Camera (HRSC) system of the ESA Mars Express mission, and the DTM mosaic was vertically aligned to the NASA Mars Orbiter Laser Altimeter (MOLA) DTM.The final mosaics and the DTM covered an area of approximately 87 km 2 and had a resolution of 27 cm.Next, the individual HiRISE images were georeferenced onto the orthoimage mosaic to provide the best quality maps with a common reference system.

Estimating Wind Patterns and Sediment Flux Using the MRAMS Model
MRAMS is a mesoscale model designed to simulate the Martian atmosphere at various horizontal scales (Rafkin & Michaels, 2019).As it is a regional model, it requires an initial state and time-dependent lateral boundary conditions-items which are interpolated from the output fields of a Mars global climate model (MGCM).Custom simulations with the NASA Ames MGCM code (Haberle et al., 2019) were used to provide these inputs for this work.To study the atmospheric environment at Meridiani Planum, a series of 6 nested grids (each with a successively smaller total area and horizontal grid spacing) was used to bridge the gap between the ∼300 km horizontal grid spacing of the MGCM and our finest horizontal grid spacing of ∼1.6 km.This finest-resolution MRAMS grid encompasses most of the western Meridiani Planum.The HRSC and MOLA Blended Digital Elevation Model at 200 m v2 (Fergason et al., 2018) was used to provide topographic information for the MRAMS runs discussed here.To determine the seasonal dependence of wind speed, direction, and frequency throughout a Mars year, MRAMS simulations were performed at 12 different Ls (planetocentric solar longitude) windows: northern hemisphere spring (10°, 40°, 70°), summer (100°, 130°, 160°), autumn (190°, 220°, 250°), and winter (280°, 310°, 340°).For each Ls window, the instantaneous atmospheric parameter values (recorded at ten-Mars-minute intervals) were analyzed over an entire sol.These atmospheric parameter values are valid at the midpoint of the lowest model layer, in this case at ∼5 m above ground level (AGL).
The potential sediment flux (Q) values were calculated using the MRAMS output and an expression from Kok et al. (2012;Equation 2.34), which employs the fluid threshold shear velocity (u* ft ) for Mars calculated via the technique of Greeley and Iversen (1985), assuming a grain mass density of 2,400 kg/m 3 and a grain diameter of ∼0.1 mm to establish the upper limit of the sediment flux-the sediment flux for particles the most susceptible to aeolian transport.In this work, the impact threshold shear velocity (u* it ) was assumed to be 0.1 u* ft (Kok, 2010).As u* ft is a function of the Martian atmosphere's density and viscosity, both of which do not remain sufficiently constant during each sol or throughout the Martian year, there is no constant critical wind speed that is consistently associated with conditions where saltation is likely.The conditions for saltation of ∼0.1 mm particles within our simulation were expected either when the shear velocity value (u*) was greater than or equal to u* ft or when u* ≥ u* it and sufficient turbulence was present.As MRAMS does not resolve small-scale winds (e.g., turbulent gusts) that are present in reality, the determination of whether sufficient turbulent gusts are present (implicitly initiating saltation) must be parameterized-the method described by Stillman et al. (2021) was used for this.This method involves examining the value of a bulk Richardson number for the surface layer.

Coarse-Grained Ripples on Meridiani Planum
Almost 450 ripples seen up close by the rover were investigated along the entire rover traverse (Figure 2).All these ripples were covered with coarse sand grains.It can be assumed that all ripples in this region similar in terms of morphology, visual expression, and wavelength to the investigated ripples are also coarse-grained ripples, even if they were not directly investigated by the rover, and the HiRISE images did not allow resolving grains in these bedforms.
The surface of the coarse-grained ripples on Meridiani Planum (Figure 3) is armored by particles 1-2 mm in diameter, which cover their crests and slopes.Within their troughs, there is a mix of smaller particles in the form of dust agglomerates and fine sand, and larger particles (>2 mm), such as angular gravel grains and spherules (see e.g., Jerolmack et al., 2006;Kozakiewicz et al., 2023).The coarse grains found on Meridiani Planum are almost free of adhering dust (see Herkenhoff et al., 2004), which could indicate that dust removal episodes take place in this area frequently.Below the thin layer of armoring, fine basaltic sand (<0.2 mm in diameter) is dominant, with lesser amounts of coarser grains similar to or slightly smaller than those in the armoring layer (Figures 4a and 4b).In the case of larger bedforms, stratification in the form of bright and dark layers is clearly visible in places where impacts have produced craters in ripples (Figure 4c).On larger ripples' slopes these layers are visible in the form of bright and dark bands (Figure 3c).These exposed layers of cross-stratification (bright and dark) vary from a  few centimeters to more than 20 cm in width.The cross-strata measured in cross-section within craters indicated that they dip toward the west (see Arvidson et al., 2011).

Ripple Morphometry and Morphology
Coarse-grained ripple classification was based on the in situ measurements of the height, width, wavelength, and morphology of 442 bedforms (Figure 2).Due to the limited image resolution of the in situ data, only ripples that  were taller than 1 cm were studied.The basal width and the height of a ripple were measured from the stereo pairs at the same point, with the profiles being as close to the highest point of the ripple as possible.The base of a ripple was defined by a distinct change in the slope gradient (Figure 5a).
The natural breaks in the wavelength distributions indicate three classes of coarse-grained ripples on Meridiani Planum: small, medium, and large (Figure 6).Even if these three classes of coarse-grained ripples occur on Meridiani Planum, coarse-grained ripples with wavelengths between these classes were also found.Small ripples (Sr) have wavelengths up to 80 cm with the average wavelength 55 cm, medium ripples (Mr) have the average wavelength 200 cm, and large ripples (Lr) have wavelengths above 300 cm with the average wavelength 390 cm (Figure 6).There is a bias in the results for the largest bedforms.Small ripples always occur as ripple groups or fields on sand-gravel covers (layers of sand and gravel available for aeolian transport), but medium and large ripples can also occur on bedrock.The rover mission control avoided regions covered by larger ripples, and also selected areas covered by bedrock and not by loose sediments.Therefore, in the database, there is almost the same number of medium and large ripples on bedrock as in ripple fields and on sand-gravel covers (Figure 2b).On the other hand, orbital investigation indicated that large and medium coarse-grained ripples on Meridiani Planum are more common on sand-gravel covers and in ripple fields than on bedrock (see also Figure 2 in Golombek et al., 2014).The wavelength of coarse-grained ripples is smaller for ripples in ripple fields or ripples on sandgravel covers than for ripples on bedrock.The larger wavelength of coarse-grained ripples on bedrock is responsible for the occurrence of tails on the right side of the peaks in Figure 6, especially for large ripples.
When ripples are located in areas where loose sediments are deflated and ripple fields are eroded ("ripples on bedrock" areas in Figure 2b) or when ripples are solitary bedforms, it is not possible to measure ripple wavelength.Therefore, an additional classification was introduced based on the height to width ratio of the above determined wavelength classes.This ratio can be used for individual ripples as well as for ripples in ripple fields.It was found that small ripples have height mainly up to 5 cm, and width up to 50 cm, and large ripples have height mainly above 18 cm, and width above 280 cm (Table 1).The relationship between height and width for coarsegrained ripples on Meridiani Planum is 0.07, and the ripple index, the ratio between the ripple wavelength and ripple height, is 15 (Figure 7).The obtained ripple index is very close to that observed on Earth, which for many ripple fields is close to 15 (Sharp, 1963;Yizhaq & Katra, 2015).The relationship between height and width for each class of ripples (small, medium, and large) is similar, and for small ripples it is 0.058, for medium 0.060 and for large 0.065 (Figure 7a).
In the most cases, the coarse-grained ripples on Meridiani Planum are characterized by slopes that are almost symmetrical and crests that are straight to slightly sinuous.In the case of medium and large ripples, their western slope angle is 7.5°± 1.5°, and the eastern slope is 8.2°± 1.6°(Figure 5a).The morphology of small ripples cannot be studied precisely due to their size.However, they seem similar to medium and large ripples.Coarse-grained ripples on Meridiani Planum have mainly concave eastern slopes, and slightly convex western slopes (Figures 5a and 5b).Medium and large ripples often have asymmetric crowns (see Figures 5a and 5b).A crown is the uppermost volume of a ripple, distinguished from the slope by a sudden change in the slope angle (Figures 5a and 5b; Figure S2 in Supporting Information S1; Sharp, 1963).These crowns can be susceptible to reshaping (Sharp, 1963), and can sometimes be detached from the main ripple body.Crowns have steeper eastern slopes with angles ∼11°(Figures 5a and 5b).Some medium ripples (16% of cases) have crowns that are detached from the ripple body (Figures 3b and 5c; Figure S2 in Supporting Information S1), and crests that are divided into several fragments (Figure 3b).Such dissected crests are more common for medium ripples (17%) than for large ripples (3%) and small ripples (2%).The crests of medium ripples with detached crowns are armored by well-sorted coarse-grained sand, but the slopes of these forms are often made of fine sand mixed with coarse sand and gravel (Figure S2 in Supporting Information S1).For medium ripples without detached crowns and large ripples, the crests are often not divided and the entire bedform is armored by well-sorted coarse sands (Figures 4a and 4b).
For relatively small features related to local topography a term "rim ripple" was proposed by Sullivan et al. (2007Sullivan et al. ( , 2008)).Rim ripple sizes vary but they are always larger than the ripples in their vicinity (Figure 8a).Rim ripples have undivided crests, and their morphology and orientation are imposed by the geometry of the obstacle.For smaller obstacles, they are isolated forms (Figure 8a), but in the case of larger obstacles such as large craters, they occur in the form of several crests, creating small isolated fields of rim ripples (Figure 8b).The spaces between rim ripple crests are an ideal location for accumulation of fine fractions probably made of very fine sand and dust visible as bright patches (Figure 8b).

Sand Grains in the Ripple Armoring Layer
In order to determine the relationship between the size of ripples and the sand within ripples, sand targets located entirely on the ripple crest or slopes were investigated.Even if fine and coarse particles can be distinguished using  PANCAM images (Figure S3 in Supporting Information S1), in order to measure the exact size of particles, microscopic images must be used.Many of the MI deposit targets were related to the troughs between ripples (Kozakiewicz et al., 2023).Along the traverse, only 16 ripple slopes or crests were investigated using the MI (Table S1 in Supporting Information S1; see also Kozakiewicz et al., 2023).Therefore, targets from 16 ripples: 6 small, 2 medium, 7 large, and 1 rim ripple were analyzed.As only the armoring layer was studied, fractions <0.5 mm in diameter were not included in the analysis.

Class
All 16 ripples were composed of very similar coarse sand grains (Kozakiewicz, et al., 2023).As an example, the results for two neighboring large coarse-grained ripples (named: 498 and 507, Table S1 in Supporting Information S1) are presented here (Figure 9).Several targets on these two ripples were investigated (Table S1 in Supporting Information S1).Two targets: "New Face" and "New Crest" are from ripple 498 (the distance between these two targets is a few centimeters), "Trough Plain" target is from the trough between those two ripples (some 2 m from their crests), "Flank 1" target is on the slope of 507 ripple (tens of centimeters from the crest).On the crests of 498 ripple the grains are smaller (mean diameter 1.35 mm) than grains on the slopes of 507 ripple (mean diameter: 1.85 mm), and grains in the trough between these ripples (mean diameter: 2.11 mm; Kozakiewicz et al., 2023).On the surface of 498 ripples, sands are made of coarser particles (mean diameter: 1.35 mm) than in the subsurface layers (mean diameter: 1.22 mm, Figure 9a).The grains on medium ("Juneau") and small ("Aegean Crest") coarse-grained ripple crests are similar to those on large coarse-grained ripples (Figure 9a).
The exposed cross-strata observed on larger ripples (Figures 2c and 5d) are related to variations in sand fractions.
On the lower parts of slopes, this change is mostly related to the dark layers' enrichment in very coarse sand and gravel.Whereas on the higher parts of slopes the darker layers have similar amount of coarse sand grains as the bright layers (Figures 9b and 9d), yet they are depleted of finer fractions (very fine sand and dust) in comparison to the brighter layers.

Meridiani Planum Ripples as Compared to Other Ripples
The coarse-grained ripples on Meridiani Planum can be compared with other ripples studied in situ on Mars: in Gale and Gusev craters.For this purpose, the correlation between the size of grains in the ripple armoring layer and the wavelength of ripples was used (Day & Zimbelman, 2021;Yizhaq & Katra, 2015).Unfortunately, the number of ripples on Mars for which PSD have been calculated is very limited (see, e.g., Cabrol et al., 2014;Gough et al., 2021;Weitz et al., 2018Weitz et al., , 2022)).Furthermore, ripples composed of coarser sand are often isolated bedforms for which wavelength cannot be estimated.
The results (Figure 10; Table S2 in Supporting Information S1) clearly indicate that coarse-grained ripples on Meridiani Planum have a different characteristic than some ripples studied in other locations as well as fine sand ripples found on Meridiani Planum.Ripples in Gale crater although made of medium sand particles (about 0.5 mm in diameter), are still more similar to fine sand ripples than to coarse-grained ripples on Meridiani Planum.Therefore, it is convenient to introduce two classes of ripples on Mars: fine-grained, which surface layers are composed mainly of very fine to medium sand, and coarse-grained ripples, which surface layers are composed mainly of fractions from coarse sand to gravel.

Ripple Crests
The primary crest alignment of larger coarse-grained ripples on Meridiani Planum is almost N-S; however, some ripples are aligned to NE-SW (Fenton et al., 2018;Sullivan et al., 2005).As mentioned before, many larger ripples have dissected crests, and these fragments are not aligned with the direction of the ripple crests, but orientated NE-SW (Figures 3b and 11a).The dissection of crests led to the creation of smaller crests which are lined up in sequences (Figure 11b).Therefore, two types of ripple orientation can be defined: related either to an individual ripple crest or to ripple crest sequences.
The crestline azimuth orientation of more than 800 coarse-grained ripples as well as about 100 ripple crest sequences were measured (Figure 12).The ends of individual ripple crests were not included in the analysis as in almost all ripples the northern ends of crests deviate toward the east, and southern ends toward the west (Figures 3b,3c,5c,5d and 11b).
The modal crest orientation for all small coarse-grained ripples is 023°, while the typical ripple crest sequence orientation of small coarse-grained ripples is 003°(Figures 12a and 12b).The orientation of small coarse-grained ripple crests changes with location (Figures 12b and 12c).On the plains, the modal orientation is 026°, while at Endeavour crater's western rim, it changes to 017°(Figures 12b and 12c).Seasonal changes in the orientation of small coarse-grained ripples on Meridiani Planum have not been observed.The rover traveled through the plains for more than 4 MYs and along the rim of Endeavour crater for more than 3.5 MYs, taking images in all seasons, and in each season the orientation of small coarse-grained ripples was the same.The orientation of medium coarse-grained ripples also depends on location; their modal crest orientation at the rim of Endeavour crater is 356°, whereas on the plains it is 008°(Figure 12e).As medium ripples are mostly found on the plains, the modal crest orientation for all coarse-grained medium ripple crests is also 008°.The orientation of medium ripple crest sequences was 001°( Figure 12d).The orientation of large coarse-grained ripples was determined only for individual ripple crests, and the modal value is 001°(Figure 12f).This value is similar to the modal orientation of medium ripples, and to the modal orientation of larger ripples obtained from orbital data by Fenton et al. (2015) that equals 002.5°.Rim ripple crests' modal orientation is 020°( Figure 12g), and as they were mostly found at the Endeavour crater rim, spatial changes in their orientation could not be investigated.

Cross-Stratification in Ripples
To establish whether there were changes in the orientation of coarse-grained ripples in the past, the difference between the orientations of ripple crests and the exposed cross-strata (see e.g., Figure 3c) was measured for almost 200 coarse-grained ripples.In most cases the exposed cross-strata were parallel to crests.The average difference was less than 1°, which is on the order of measurement error.Non-parallel orientations were only found in locations where: (a) some local disturbance in the fluid flow was present, such as near craters; (b) when the shape of neighboring ripples changed due to high erosion; (c) in the case of ripples with dissected crests; (d) in bifurcation and termination zones; and (e) on merging ripples.Even in the most extreme cases of typical ripples, the difference was not greater than 6°.

Ripple Spatial Distribution
Although MER's mission control made every effort to avoid hazardous aeolian bedforms, more than 73% of Opportunity's traverse crossed ripples.Coarse-grained ripples were more or less evenly distributed along the traverse (Figure 2).Of these, 50% were in ripple fields, 46% on bedrock, and 4% were solitary ripples on sandgravel covers (Figure 2b).Small ripples were the most numerous of all coarse-grained ripple types due to their small size and a large number of individual ripples for a unit area, even if small ripples fields were not the most frequent (Figure 2b).
On the plains, all the classes of coarse-grained ripples are very common.A zonal structure of ripple distribution is clearly visible (Figure 2b).In the interior of vast ripple fields, large ripples are dominant.On their edges, medium ripples are present: firstly as ripple fields, and then, farther from the center of the large ripple fields, as sparsely distributed medium ripples on sand-gravel covers, which then turn into fields of small ripples (Figure 2b).The plains are also covered by small craters (Golombek et al., 2010(Golombek et al., , 2014)).Small fresh-looking craters (e.g., Eagle crater) are sometimes filled with fine sand, and fine-grained ripples occur on crater floors but not coarse-grained ripples (Figure S4a in Supporting Information S1).On the other hand, small but degraded craters (e.g., Vostok crater; Figure S4b in Supporting Information S1) can be filled with all types of coarse-grained ripples.Sometimes coarse-grained ripples inside these craters are larger than ripples found in their surroundings (Figure S1b in Supporting Information S1).Small craters are often surrounded by all types of coarse-grained ripples (see e.g., Golombek et al., 2010Golombek et al., , 2014)).
Inside large fresh-looking craters, TAR-like bedforms are common but there are usually no ripples similar to those found on the plains (ripples of simple morphology; Figure S1a in Supporting Information S1).Only rarely some highly eroded coarse-grained medium ripples or fresh-looking small coarse-grained ripples are seen around these craters.These medium and small ripples look like the ripples on the plains; however, in both cases, their orientation depends on the crater's topography and is parallel to the craters' rims, indicating that they were formed by local and not regional fluid flow (Figure S5 in Supporting Information S1).
On the other hand, large, degraded craters are filled and surrounded by all types of coarse-grained ripples.Coarsegrained ripples within these craters have a similar orientation to the ripples on the plains, indicating that they formed after the craters were eroded.Often ripples within these craters are smaller than ripples on the plains (Figure S6 in Supporting Information S1).

Secondary Bedforms
Four classes of secondary features were distinguished on and around the primary crests of coarse-grained ripples on Meridiani Planum: small wind streaks, small secondary ripples, hooks, and secondary crests.Small wind streaks are made of dust or very fine sand and occur on the lee side of primary crests (Figures 13a and 13b).Small secondary ripples are found around medium and large ripples.They have wavelengths and sizes similar to small primary ripples (Figure 13c).There are also many ripple-like features; however, in contrast to the secondary ripples, one part of such a feature is still attached to or in line with the primary ripple crest but the other part deviates from the orientation of the primary ripple crest (Figures 13c-13g).These features have a hook-like shape and were named hooks in this work.The distances between individual hooks and between dissected fragments of ripple crests were similar (compare L2 in Figures 11 and 13).Secondary crests join neighboring ripples together and are the largest secondary features (Figure 13d).The secondary bedforms that are formed on the western parts of a primary ripples are oriented toward the west, while on the eastern parts toward the east (Figure 13f).This orientation is similar to the orientation of the ends of primary ripple crests (Figures 3b, 5c, 5d, and 13c).
Secondary crests are only associated with large ripples and are quite rare (3% of ripples have secondary crests).On the other hand, almost all ripples have small wind streaks on their western slopes.Hooks are most common on medium ripples (they are found on 30% of medium ripples) but they are also found on large ripples (14% of large ripples have hooks), and very rarely on small ripples (7% of small ripples have hooks).Hooks are frequently found on medium ripples eastern slopes, especially in the southern part of the rover's traverse.Rim ripples have no secondary bedforms apart from small wind streaks.Wind streaks are made of fine sand and dust not recognizable in the MI images.Hooks are made up of smaller grains than typical grains in ripple crests (Figures 9c and 9e).The orientation of secondary ripples and secondary crests on and around the primary crests is constant along the traverse with the modal value 034°(Figure 12h).

Ripple Streaks
It has been observed that ripples tend to be more prominent when they are placed downwind of intercrater fields of TAR-like bedforms, creating streak-like forms in an area of ripple fields.These extensive bedforms were called (e) Small secondary hooks on the northern end of a ripple crests are oriented toward the east and on the southern end toward the west (Sol 1818, NAVCAM).(f) Secondary features are oriented toward the east if they are on the eastern part and toward the west when they are on the western part of a primary ripple (Sol 2190, NAVCAM).(g) Deviation from the N-S crest orientation: the northern end of the crest deviated to the east and southern to the west leading to the joining of two primary ripples (purple arrow) (Sol 0517, NAVCAM).L2-see Figure 11.Blue arrows -small wind streaks, yellow arrows-hooks oriented toward the west, orange arrows-hooks oriented toward the east, red arrows-small secondary ripples, green arrows -secondary crests.
TARs-streaks by Silvestro et al. (2014) but were renamed as ripple streaks by Fenton et al. (2015).The rover crossed over one of these ripple streaks located downwind from a crater located at the Endeavour crater rim (Figures 2a and 14).The coarse-grained ripples in ripple streaks are taller than those outside the ripple streak (Figure 2) and are characterized by alternating bright and dark regions on their surface (Figure 14); hence, they are clearly visible from orbit.They have relatively bright dusty crests and bright-dark pattern on both sides, which are in contrast to the darker sand-gravel surface in ripple troughs (Figure 14b).Defects in the form of small ellipsoidal depressions are often found on ripple crests inside the ripple streak.

Wind Velocity and Potential Sediment Flux on Meridiani Planum
The MRAMS model has predicted that during the present epoch, wind on Meridiani Planum changes direction during the Martian year and throughout each sol (Chojnacki et al., 2011).New simulations were performed to obtain the output wind velocity & frequency and potential sediment transport (i.e., time-integrated fluxes) for one  grid point located on the rover's traverse (Figure 2a).The analysis location was chosen some distance from larger craters in the vicinity in order to be as representative of the plains of the western Meridiani Planum as possible.The results indicated (Figure 15) that for the northern hemisphere's spring, the strongest winds are WNW winds, and in summer SSE winds, while the most frequent winds for these two seasons blow from the NNW, WNW, and SSE.In the fall, E but also W, ESE, and WNW winds are relatively strong, and during winter, the strongest winds are from the E and ENE, and are quite frequent.During the autumn and winter, a very frequent but moderate wind also blows from N. The MRAMS results also indicated the potential for some sediment transport in western Meridiani Planum every sol throughout the year, although the actual magnitude of any such transport is likely to be quite modest since the available sand supply is clearly limited due to various types of surface armoring.Therefore, the model predicts that the main direction of transport in spring and summer is from the SSE, in autumn and winter from the E, ENE, ESE, N (Figure 15), and in the autumn, additionally from W.

Discussion
Ripple activity depends not only on wind strength and frequency of strong wind events but also on the availability of fine and medium sand.If fine and medium sand is not available, even bedforms composed of fine sand are dormant (Weitz et al., 2018); but when a high flux of such particles is present, even bedforms composed of coarse grains can be activated (Chojnacki et al., 2019;Silvestro et al., 2020).In modern times, the supply of fine and medium sand particles on Meridiani Planum is low, as there are almost no medium sand particles on the Meridiani Planum plains and fine sand particles were immobilized under armoring layers of lag deposits and within coarsegrained ripples (Kozakiewicz et al., 2023).Therefore, both aeolian erosion and accumulation are slow, and erosion dominates.Along the rover's traverse, erosion and deflation indicators can be observed: a lack of fine and medium sand on the surface, scarcity of fine sand bedforms, erosion of old coarse-grained ripples, and a lack of coarse-grained ripples in places where they had been present in the past.The presence of previous generations of ripples is indicated by paleo-troughs and the presence of bands (remnants of eroded ripple cross-stratification) on sand-gravel covers (Figure 16).
Even if aeolian accumulation is not dominant, there are indicators of modern accumulation on the Meridiani Planum.Active dunes occur in large (D > 5 km) craters (Chojnacki et al., 2011;Fenton et al., 2015).Small finegrained ripples form as groups of ripples in small depressions, such as craters or trenches (Figure S4a in Supporting Information S1; see also Sullivan et al., 2005;Sullivan & Kok, 2017).Dust accumulates at the highest points of the terrain on the leeward side of obstacles (Figure S7 in Supporting Information S1).Some secondary bedforms on coarse-grained ripples develop (Figures 13 and 16).The NE-SW orientation of smaller coarsegrained ripple crests indicates that they were formed by NW or SE winds.The NW and SE winds are modern winds: the NW winds are responsible for observed dunes' activity, and the SE winds for wind streaks' activity (Chojnacki et al., 2011).The convex shape of the western slopes of the coarse-grained ripples and the asymmetry of the crowns (Figure 5) indicated that the more formative winds in the cases of coarse-grained ripples were the NW winds.
Based on the distribution and the orientation of different classes of coarse-grained ripples (Figures 2b and 12), a model of ripples' formation and evolution on Meridiani Planum can be inferred.As the last major migration of large ripples on Meridiani Planum was dated to 10 4 -10 5 years ago (Golombek et al., 2010), the model may miss some episodes not preserved in the ripples.
In the first epoch, a high material supply was necessary for the formation of N-S oriented coarse-grained ripples on the Meridiani Planum plains.The cross-stratification in ripples can be explained by the mechanism proposed by Sullivan et al. (2008), which is based on alternating episodes of longer gentle winds with heavy dust fall-out, and stronger wind events.Due to frequent dust events, the coarse-grained ripples became dust-indurated, and light layers were formed.When stronger wind events occurred, huge amounts of saltating particles partially reduced the ripples' induration and led to the transport of coarse sand grains, creating dark layers (Figures 3c and 16d).The orientation and geometry of cross-stratification in the coarse-grained ripples is constant across the entire area, indicating that it was formed by uniform winds perpendicular to the ripple crests.As the cross-stratification inside ripples dip toward the west, it was inferred that the N-S oriented coarse-grained ripples were formed by E winds (Arvidson et al., 2011;Fenton et al., 2015, winds blowing from the right side in Figures 5a and 5b).Higher accumulation of sand in larger fresh-looking craters, which were natural traps for sand, provided enough material for the formation of larger than coarse-grained ripples on the plains TAR-like bedforms (Figure S1a in Supporting Information S1), and dunes (Figure 1).
In the second epoch, the supply of fine sand on Meridiani Planum became limited and deflation took place.Erosion rates on the plains were faster than accumulation, but in craters, accumulation was still more important than erosion.The presence of smaller coarse-grained ripples in the eroded craters (Figure S6 in Supporting Information S1) in comparison to larger ripples on the plains indicated that they were relatively younger bedforms, located in wind-shadow zones, and made up of smaller particles (easier to transport), which are deposited in craters in larger numbers than larger grains.It can be assumed that on the plains, strong E winds were responsible for the formation of the second generation of medium and small N-S oriented coarse-grained ripples, as the material eroded from the first generation of N-S oriented ripples led to short-term periods of increased material supply.The presence of the best-preserved and largest coarse-grained ripples in the interiors of ripple fields (Figure 2b) can be explained by the relatively small amount of free, fine grains in these regions, as the erosion of coarse-grained ripple fields, which led to an increase in fine sand particles, progressed from the ripple fields' edges.Strong erosion by E winds led to individual ripples and entire ripple fields being diminished as well as the removal of sand-gravel covers, which in turn led to the exposure of the bedrock.Due to erosion, the height and width of many older ripples (Figure 5d) were proportionally reduced, although their wavelength was not changed.The erosion of crater rims enabled E winds for partial destruction of TAR-like bedforms located in more eroded craters.Material eroded from these TAR-like bedforms led to downwind formation of ripple streaks on the plains (Figures 2a and 14).
In the third epoch, the E wind became weaker and the NW and SE winds became dominant.On the plains, on the edges of coarse-grained ripples fields, mainly NW winds (from the upper-left side in Figures 5a and 5b) with additional SE winds partially eroded and dissected both the first and second generations of small and medium ripple crests and formed the third generation of NE-SW oriented small ripples (Figures 11 and 12).Near obstacles, these winds formed rim ripples (Figure 8).On the other hand, inside eroded craters, older (mostly better indurated) N-S oriented coarse-grained ripples were protected from aeolian processes and were less influenced by NW and SE winds (Figure S1b in Supporting Information S1).
Although on Meridiani Planum some large ripples seem quite tall, they may only be composed of a very thin veneer of sand (Figure 17).The surface around some partially eroded ripples is much lower than the surface on which these ripples are located.Hence, the amount of material inside a ripple cannot always be measured solely using the ripple's dimensions.This relative higher elevation of bedrock beneath the ripples as compared to bedrock between the ripples could be explained by a model of the development of periodic bedrock ridges (Hugenholtz et al., 2015;Montgomery et al., 2012;Silvestro et al., 2021) proposed by Hugenholtz et al. (2015).In this model, periodic bedrock ridges are formed by the interaction between megaripples and erodible bedrock.It is highly probable that coarse-grained ripples on Meridiani Planum protect bedrock on which they were formed from erosion as coarse-sand armoring, composed mainly of rounded fragments of iron-rich spherules (Kozakiewicz et al., 2023), is more resistant to erosion than highly erodible bedrock composed of Burns formation rocks (see e.g., Fenton et al., 2015).After the loose sediment is deflated from the ripple troughs, the bedrock is exposed and eroded, deepening the areas of the ripple troughs.
Opportunity crossed hundreds of large, medium, and small coarse-grained ripples on its way to Endeavour crater without problems.The hazards associated with the rover's traversability were on Meridiani Planum especially related to modern drifts that formed in the zones where large coarse-grained ripples merged.Such drifts were formed especially on the NW parts of these merging zones, indicating that recently transported sand is more loosely packed than sand that was deposited in the previous epochs by E winds.These drifts were responsible for many unintended breaks in the rover journey and one of them at the Purgatory ripple almost ended the mission (Squyres et al., 2006).
The dust content of coarse-grained ripples on Meridiani Planum is smaller than that of coarse-grained ripples in Gale and Gusev craters (Bretzfelder & Day, 2021;Greeley et al., 2006;Herkenhoff et al., 2004).The rover tracks in coarse-grained ripples as well as on gravel-sand sheets on Meridiani Planum were erased relatively faster (Geissler et al., 2008) than in Gale crater (Bretzfelder & Day, 2021), indicating more intense aeolian transport.However, even here dust was responsible for the cementation of sand grains, increasing the induration of ripple surface layers (Figures 3a and 3b).However, on Meridiani Planum, the lack of finer and medium particles is probably more responsible for the low activity and erosion of coarse-grained ripples than their induration by dust, as happens in Gusev and Gale craters.

Conclusions
The analysis of many hundreds of coarse-grained ripples on Meridiani Planum along the entire Opportunity rover's traverse allowed the development of a classification of ripples.The statistical analysis of the size of sand grains in ripples and the size of ripples indicated that there are two types of ripples on Mars: fine-grained ripples and coarse-grained ripples (Figure 10).Analysis of the orientation and morphology of coarse-grained ripples (Figures 5 and 12) indicated that the last formative wind on Meridiani Planum was from the NW and no seasonal variation in wind direction was observed in coarse-grained ripples' orientation.Some of the secondary bedforms (winds streaks, and hooks) indicated formative winds from the SE, E, or NE (Figure 13).These winds, however, were too gentle or too short to leave evidence in the primary crests of coarse-grained ripples.These findings support those of Chojnacki et al. (2011), who studied dunes in Endeavour crater.
The orbital investigation of Meridiani Planum coarse-grained ripples indicated that coarse-grained ripples did not migrate during 16 years of HiRISE observations (2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021)(2022).This outcome is similar to the findings of the investigation of coarse-grained ripples in Gale crater (Bretzfelder & Day, 2021).Coarse-grained ripples on Meridiani Planum are still active, but their activity is of low intensity and slow.Their activity is often evidenced by: (a) the different orientation of smaller ripples and rim ripples in comparison to the orientation of larger ripples (Figure 12), (b) the difference in the orientation of individual crests and the fragments of crests in comparison to the orientation of crest sequences (Figures 11 and 12), (b) complete filling of the small craters in ripples (Figure S8 in Supporting Information S1), (c) the erosion of ripples (Figure 16), and (d) the formation of secondary bedforms (Figure 13).
Ripples much smaller than topographic obstacles (see Figure S5 in Supporting Information S1) are formed by local (modified by topography) fluid flow.Therefore, the possible presence of modified flow must be considered when inferring wind directions on the basis of bedform orientations in such locations.
The development of coarse-grained ripples is different in different geomorphological settings.The development of ripples inside relatively large craters takes place even if the regional sand supply is low.The interiors of such craters are characterized by higher accumulation rates and lower deflation rates; therefore, they are relatively abundant in material available for the formation of bedforms, such as TAR-like bedforms, and dunes.
Coarse-grained ripples on Meridiani Planum are oriented in two different directions: N-S (due to past winds) and NE-SW (due to relatively modern winds) and these orientations depend directly on the age of bedforms and indirectly on their size and location.At present, on Meridiani Planum, two types of coarse-grained ripples are formed: small ripples on flat surfaces and rim ripples in places where accumulation is enhanced by topography.Small ripples and rim ripples are the youngest bedforms, and do not have dissected crests and rarely have secondary bedforms.
The statistical analysis of the ripple wavelength (Figure 6) allowed us to determine three dominant classes of coarse-grained ripples in terms of size: small, medium, and large.As there is a continuum in the height to width ratio in all studied ripples, a continuous transition in ripple sizes can be assumed (see also Sullivan et al., 2020).The peaks in the wavelength probability density function only indicate that some ripple sizes are more common than others.The measured ripples morphometry slightly varies for different classes (Figures 7a and 8b) due to the fact that on Meridiani Planum coarse-grained ripples at various stages of development co-exist.
Predictions from the MRAMS model are consistent with some of the observations of the bedforms: the presence of two formative wind directions, SE and NW, and the erosion of bedforms by E winds.However, the strength of these winds does not seem to match adequately the observations.The MRAMS model does not predict dominant potential sediment transport from the NW, although it does predict frequent WNW and NW winds and daily potential sediment transport throughout the year.Effective NW winds could also occur sporadically but were not resolved by this particular model configuration.
To characterize coarse-grained ripples across Mars, further extensive investigations based on in situ data are necessary, as well as field studies on terrestrial analogs.On Mars, coarse-grained ripples have been found in all regions studied by rovers, and some of these are similar to the Meridiani Planum ripples.At Gusev crater, coarsegrained ripples are quite common; however, their characteristics are different from those on Meridiani Planum.They are mostly solitary ripples (mainly rim ripples) with much steeper slopes and covered by thicker layers of dust, and therefore are probably more indurated.One ripple, called Serpent, is approximately 20 cm high and 1.5 m wide with asymmetrical slopes of 32°(probably slip-face) and 22°(windward-type slope; Greeley et al., 2006).Its coarse-grained armoring material is very similar to that of Meridiani Planum's ripples (Greeley Journal of Geophysical Research: Planets 10. 1029/2023JE008225 et al., 2006)).At Gale crater, various coarse-grained ripples are present in relatively small (up to 0.1 km 2 ) isolated ripple fields, groups, or as solitary features.They seem to be at various stages of induration.Their distribution is mostly controlled by the topography.They occur in depressions (craters, valleys) and near obstacles (Bretzfelder & Day, 2021;Weitz et al., 2022).Coarse-grained ripples in Gale Crater seem more similar to the ripples at Gusev crater than to those on Meridiani Planum.However, their armoring is similar to both the Meridiani Planum and Gusev crater ripples, such as the armoring of a coarse-grained ripple (rim ripple) at the Dingo Gap (Weitz et al., 2018).The Dingo Gap bedform is also characterized by similar morphometric proportions to ripples on Meridiani Planum.The height to width ratio for this ripple is 0.056 (Zimbelman & Foroutan, 2020), very close to the coefficients obtained for Meridian Planum ripples (Figure 8d).Also, in Jezero crater, coarse-grained ripples with ≤1 mm in diameter grains at their surfaces were found (Bell et al., 2022).In the investigated part of Jezero crater, ripples are found in ripple fields, but the area of these ripple fields is 2 orders smaller than coarse-grained ripple fields on Meridiani Planum.Ripples in Jezero crater are often found near obstacles and in depressions.Even if coarse-grained ripples in Jezero crater have morphology and size similar to coarse-grained ripples observed in other locations on Mars, detailed morphometric and granulometric analyses of these bedforms still need to be performed.

Figure 1 .
Figure 1.The traverse of the Opportunity rover (yellow line) plotted on MRO Context Camera imagery.Dark streaks extend to the NW direction from the Endurance, Victoria and Santa Maria craters.In Endeavour crater, some barchan and barchanoid dunes are visible as darker patches.Green numbers -sol numbers.

Figure 2 .
Figure 2. (a) Distribution of the studied coarse-grained ripples along the traverse (green dots), locations where MI sand images were acquired (red dots), and the location of a point for the MRAMS output (blue square).The orange contour indicates the area of the ripple streak (see Section 4.7).(b) The distribution of coarse-grained ripples along the traverse: redsmall ripples fields on sand-gravel covers, purple-sparsely distributed medium ripples on sand-gravel covers, orangemedium ripples on bedrock, yellow-fields of medium ripples, dark green-large ripples on bedrock, light green -fields of large ripples, gray-no ripples.Rim ripples are not shown as they are solitary bedforms.The regions were classified on the basis of the prevailing ripple classes.

Figure 3 .
Figure 3. Types of coarse-grained ripples on the Meridiani Planum in terms of their size.Scale bars in all in situ images in this work are 0.5 m.(a) A field of small ripples (Sol 0070, NAVCAM); (b) a field of medium ripples (ripples have detached crowns and dissected crests, see text; Sol 0362, NAVCAM); (c) large ripples on bedrock (bedrock is seen as brighter patches), exposed cross-strata in the form of bands are visible on the left-side slope (eastern slope; Sol 0796, NAVCAM).

Figure 4 .
Figure 4. (a) A track mark in a large ripple's western slope (Sol 0502, PANCAM image in false colors).(b) Close-up of the yellow rectangular area (3.2 × 3.2 cm), showing that the armoring layer is very thin (Sol 0510, MI) and under the armor fine sand is present.(c) A crater in a large ripple's crest (yellow dashed line); internal stratification in the ripple is visible in the form of dark and bright layers (white arrow, Sol 0776, NAVCAM).

Figure 5 .
Figure 5. (a) Scheme of a ripple's cross-section (vertical exaggeration 10×).H-height, W-width = a + b, a -basal width of the western slope, b -basal width of the eastern slope.Slope angles are calculated as arctan(H/a) and arctan(H/b).(b) Profiles of a large ripple, and a medium ripple with a detached crown (east slopes on the right).Crowns are characterized by their steeper slopes (see text below).(c) Profile of the medium ripple in a ripple field (Sol 0384, NAVCAM), and (d) large ripple on bedrock (Sol 0765, NAVCAM).

Figure 6 .
Figure 6.Wavelength probability density function of coarse-grained ripples for 318 ripples measured in situ.Three peaks indicate small, medium, and large ripple classes.

Figure 7 .
Figure 7. Coarse-grained ripple morphometry.(a) Height versus width for small (Sr), medium (Mr), and large ripples (Lr).Total number of bedforms: N = 393 (small: 53, medium: 206, large: 134).(b) Wavelength as a function of height for all ripple classes.The relationship between wavelength and height (ripple index) was calculated by averaging the wavelength of bedforms of a given height (every 1 cm in height).
Typical range of height (modal height) Typical range of width (modal width) Typical range of wavelength (average wavelength)

Figure 8 .
Figure 8.(a) A rim ripple (Rr) near the trench (gray elongated depression to the right of the rim ripple indicated by yellow dashed lines) with small ripples (Sr) in the vicinity (Sol 0321, NAVCAM); (b) One of several fields of rim ripples at the rim of Endeavour crater; dust (brighter material) is deposited between individual ripple crests (Sol 3472, NAVCAM).(c) Height versus width for rim ripples.Total number of bedforms: N = 29.

Figure 9 .
Figure 9. (a) PSD of sands from various locations on coarse-grained ripples: beneath a crest, from a crest, slope, and trough.Target names: Aegean Crest (red), Juneau (yellow), New Face (green), New Crest (violet), Flank1 (gray), and Through Plain (black).(b) Isabella and Marchena targets with close-up of three layers visible in a ripple slope: two dark and one bright.The crest of the ripple is outside the image toward the top.The width of the bright band (overlaid by the MI image) is ∼6.2 cm (Sol 0911, PANCAM).(c) Mayberooz and Norooz targets with samples from both sides of the ripple crest.The crest line runs through the image center (Sol 0421, PANCAM).Mayberooz target crossed a hook (blueish area, see Section 4.6) and a small wind streak (yellowish area, see Section 4.6).(b, c) The grains detected by the MI analysis are overlaid on the PANCAM false color image.(d, e) The PSD for the given samples.

Figure 10 .
Figure10.The wavelength of ripples versus the mean diameter of armoring grain samples for various ripples.Clearly visible separation between finegrained and coarse-grained ripples.Information on ripples in Gale crater afterGough et al. (2021) and in Gusev crater afterCabrol et al. (2014).For more detail see TableS2in Supporting Information S1.

Figure 11 .
Figure 11.(a) Dissection of ripple crests and two different orientations of ripple crests (Sol 0070, NAVCAM).The orientation of crest sequences indicated in red, the orientation of individual crests in green.L1-wavelength between ripples, L2-wavelength between the fragments of ripple crests.(b) Two orientations of ripple crests: black lines-ripples crests, blue dashed lines-ripple crest sequence orientation, red dashed lines-individual ripple crest orientation.The northern end of ripple crest deviates toward the east, and the southern toward the west (see also Figures 3b, 3c, 5c, and 5d).North is toward the top.

Figure 12 .
Figure12.The orientation of crests of primary coarse-grained ripples, the coarse-grained ripple crest sequences, and the crests of secondary bedforms in 10°bins.Nthe number of measured bedforms.The color sequence from the smallest to the largest number: brown, yellow, green, and blue.

Figure 13 .
Figure 13.(a) Small wind streaks on a large ripple, clearly visible exposed cross-strata on the eastern slope (Sol 0870, PANCAM).(b) Close-up of small wind streaks on a large ripple's western slope (Sol 0594, PANCAM).(c) Secondary features on a medium ripple.Northern ends of primary crests are deviated toward the east and southern ends toward the west, as seen at the top of the image (Sol 2284, NAVCAM).(d) Secondary crests between primary ripples (Sol 0781, NAVCAM).(e) Small secondary hooks on the northern end of a ripple crests are oriented toward the east and on the southern end toward the west (Sol 1818, NAVCAM).(f) Secondary features are oriented toward the east if they are on the eastern part and toward the west when they are on the western part of a primary ripple (Sol 2190, NAVCAM).(g) Deviation from the N-S crest orientation: the northern end of the crest deviated to the east and southern to the west leading to the joining of two primary ripples (purple arrow) (Sol 0517, NAVCAM).L2-see Figure 11.Blue arrows -small wind streaks, yellow arrows-hooks oriented toward the west, orange arrows-hooks oriented toward the east, red arrows-small secondary ripples, green arrows -secondary crests.

Figure 14 .
Figure 14.Coarse-grained ripples in a ripple streak.Some ellipsoidal defects have been indicated by arrows: (a) from orbit and (b) from the surface (Sol 2624, NAVCAM).These two images are from two different portions of the same ripple streak.

Figure 15 .
Figure 15.Seasonal and annual wind roses for the (a) model point at ∼5 m AGL, where the direction follows the meteorological convention (i.e., the direction the wind is from [degrees from N]), the radial scale is frequency of occurrence [%], and the colors indicate the wind speed distribution for each direction bin ([m/s]; cooler colorsslower, warmer colors-faster; greater wind speeds are at the outside edge of each angular "slice").Seasonal and annual potential sediment transport roses for the northern (b) model point, which show the modeled transport direction [degrees from N] (note that this is 180°from the meteorological wind direction), where the radial scale is the time-integrated potential sediment transport over each seasonal (Ls) range [kg/m].

Figure 16 .
Figure 16.Paleo-troughs oriented N-S (yellow arrows), seen from the surface (a) (Sol 0361, NAVCAM), and from orbit (b) (the crater diameter is ∼12 m, north is toward the top), yellow line-the rover traverse.(c, d) Remnants of ripple crossstratification on the surface in the form of dark bands; the ripples were totally eroded (red arrows).The crest of the ripple is partially eroded and dissected, and hooks and secondary ripples were formed (Sol 0741 [c], Sol 2214 [d], both NAVCAM).

Figure 17 .
Figure 17.The ripple deposit can be very thin, and the undulating bedrock is visible under the thin layer of ripple material on ripple slopes and in troughs (red arrows) (Sol 0734 [a], Sol 1961 [b], Sol 0740 [c], all NAVCAM).