Turbulent shear flow over large Martian ripples

On Mars, large aeolian ripples with wavelengths typically 1-3 m but lacking very coarse sand at crests have been encountered by rovers and observed from orbit.  These bedforms have no terrestrial counterpart and several hypotheses for origins have been proposed.  This work reports results of Computational Fluid Dynamics (CFD) experiments with ANSYS Fluent under terrestrial and Martian boundary layer conditions, using the k - w SST turbulence model to evaluate shear stress along a topographic profile of large Martian ripples at different boundary layer wind speeds.  Results indicate that, compared with Earth conditions: (1) boundary-layer flow along large ripples under Martian conditions is less turbulent due to higher kinematic viscosity; (2) reverse-flow vortex regions from crests at ripple lee flanks are larger; and (3) shear stresses at crests of large ripples are relatively low so ripple flattening is less likely at high wind speeds.  These results indicate Martian ripples formed by the saltation impact splash mechanism should be less constrained by shear stress effects limiting growth of exposed ripple crests, because the low-density Martian atmosphere applies relatively low wind-related shear stress to ripple surfaces.  Other origins for the large Martian ripples are not excluded, however.  On Earth, very large ripples with crests unprotected by very coarse grains do not develop due to higher wind-related shear stresses. 

------------------------------------------ FILES IN THIS ARCHIVE RELATED TO NASA DATA ------------------------------------------ 2020JE006515_DataAvailability-PDSimages.csv 2020JE006515_DataAvailability-PDSimages.xlsx These two files help locate MER and MSL image data sources utilized in the paper, all of which are available for free, worldwide, from the NASA Planetary Data System (PDS) at https://pds.nasa.gov. Original raw and refined image data products in the PDS archive commonly are in .IMG format, which is not native to most image processing/display software. For this reason, we recommend that readers first try the MER Analyst Notebook and MSL Analyst Notebook (http://an.rsl.wustl.edu/mer/ and http://an.rsl.wustl.edu/msl, respectively), where options exist for interacting with image data more efficiently (e.g., stretching, distance measurement, mosaics, downloading image versions in commonly-readable formats). Nevertheless, the two files listed above provide guidance directly into the main PDS archive for all image data displayed in the paper's figures. Figure_2_Csol1173_gobabeb_dune1_stereo_12x1_MR_mcam05311.png Full resolution mosaic acquired by Mastcam100 (sequence mcam05311) on sol 1173 of the MSL Curiosity mission to Gale crater. This product is a mosaic of 12 RGB images. The mosaic was originally compiled and provided to the MSL team by personnel at Malin Space Science Systems. Figure 2 is an excerpt from this mosaic. The 2.6 m wavelength spacing location is the same as shown in Figure 5D (see Figure 5D products above for information on that measurement).

Figure_2_Csol1169_dune_1_stoss_12x1_MR_mcam05302.png Full resolution mosaic acquired by Mastcam100 (sequence mcam05302) on sol 1169 of the MSL Curiosity mission to Gale crater. This product is a mosaic of 12 RGB images. The mosaic was originally compiled and provided to the MSL team by personnel at Malin Space Science Systems. This mosaic is not shown in Yizhaq et al. (2020) J. Geophys. Res. Planets, https://doi.org/10.1029/2020JE006515, but was used with other data to determine the scale information shown in Fig. 2 of that paper. Figure_2_Csol1168ncamRippleWavelengthGobabeb_Scale_(S=RhoTheta).png

Figure_2_Csol1168ncamRippleWavelengthGobabeb_Scale_(S=RhoTheta).txt These products document the sample wavelength measurement of Figure 2. The .png product is a screenshot record from the MSLICE software (MSL team planning and data analysis software that provides functions similar to publicly available MSL Analyst Notebook) showing camera ranges to the two endpoints of the line segment joining two adjacent ripple crests, in Navcam data acquired from the MSL rover's sol 1168-1169 position (where the above entry was also obtained). The view is approximately perpendicular to the sampled wavelength span, a circumstance used for advantage here: Because each of the two ranges measured with MSLICE has larger along-range uncertainties than in azimuth (which is defined by camera optical design) in this instance, the possibility exists of range uncertainties leading to an overestimate of the true three-dimensional span distance. Accordingly, the two ranges were averaged to provide a representative distance "rho" and azimuth was measured to provide "theta" to obtain wavelength span "S" = rho x theta, as indicated in the .txt file. The exact location of this span was also captured from a somewhat different viewing angle within

Figure_2_Csol1173_gobabeb_dune1_stereo_12x1_MR_mcam05311.png, an excerpt of which is Figure 2, and provides scale for that figure. Figure_3_Asol706-708_Pancam_P2267_P2268_L257F.png This mosaic was acquired by the Pancam instrument on sols 706 and 708 during the Mars Exploration Rover "Spirit" during its mission to Gusev crater. The mosaic was produced from Pancam sequences P2267 and P2268 by the MER Pancam team. As of 4/20, the mosaic can be obtained at http://pancam.sese.asu.edu/eldorado_new.html. Figure_3_2P189219098FFLAL00P2267L2M1.IMG

Figure_3_2P189219098XYLAL00P2267L2M1.IMG These are FFL and XYL versions of a single image (part of the mosaic of the above entry) that together allowed extraction of the ripple profile in Figure 7 of Sullivan et al. (2008) J. Geophys. Res. Planets, https://doi.org/10.1029/2008JE003101, which was adapted for use in the numerical experiments of Yizhaq et al. (2020) J. Geophys. Res. Planets, https://doi.org/10.1029/2020JE006515, and is shown in its leveled, adapted form in Figure 3 of the paper. These .IMG files are in PDS format. More easily displayable image formats (same 2P189219098 designation, which is based on the SCLK or mission clock time when the image was acquired on sol 708) are available within the MER Analyst Notebook by navigating to Pancam products acquired on sol 708 of Spirit's mission. The FFL version is the processed image; the XYL version is the underlying stereo terrain (x,y,z) points.

Figure_3_Asol708_2P189219098.101.ripple.jpg Image product based on 2P189219098 showing surface location of Figure 3 profile. See also Figure 7 of Sullivan et al. (2008) J. Geophys. Res. Planets, https://doi.org/10.1029/2008JE003101.

Figure_3_Asol708_2P189219098XYLAL00P2267L2M1.101.ripple.csv Figure_3_Asol708_2P189219098XYLAL00P2267L2M1.101.ripple.csv.xlsx The distance vs. height data of the ripple profile from Fig. 7 of Sullivan et al. (2008) J. Geophys. Res. Planets, https://doi.org/10.1029/2008JE003101 that was adapted for use in the numerical experiments of Yizhaq et al. (2020) J. Geophys. Res. Planets, https://doi.org/10.1029/2020JE006515.

For the other figures in the manuscript use the following instructions: 1.Download the folder YizhaqJGRfigures from https://drive.google.com/drive/folders/1TdfXg5iOoYRhfkRL5hfJ_nYq2r3SG_JA 2.Unzip the folder; it contains 5 folders: FigureA1, Figure3, Figure5, Figure6, Figure7. with .m (MATLAB script), .mat (data). 3.Open the desirable *.m file using MATLAB. 4.Each .m file creates a corresponding Figure.