Plain Language Summary
Strong winds mobilize sand grains that can abrade rock surfaces to form lineated bedrock features. These wind abrasion textures record the direction of the sand-driving winds that created them. Wind abrasion textures were observed on rocks on the floor of Jezero crater traversed during the early part of the Mars 2020 mission. Orientations of these abrasion textures were measured using rover-acquired three-dimensional stereo image data. Results indicate that the sand-driving wind directions that abraded these rocks are very different—nearly opposite—from the current strongest wind directions indicated by orientations of sandy wind tails extending from behind obstacles, measured wind velocities, and climate model predictions for the area. Collectively, these results provide a record of changing atmospheric circulation patterns in the Jezero region.
1 Introduction
Ventifacts are rocks that have been abraded and shaped by wind-blown, sand-size particles. The abrasion process forms flutes, pits, and grooves that typically align along the flow direction of strong, sand-driving winds on both Earth and Mars (Bridges et al., 2004; Laity & Bridges, 2009). There is abundant evidence that aeolian abrasion modifies exposed rock surfaces on Mars, based on observations of ventifacts at previous landing sites (Bridges et al., 1999, 2014; Greeley et al., 2006, 2008; Thomson et al., 2008). Ventifacts also are present on the floor of Jezero crater, as observed by thePerseverance rover during the first 400 sols (martian days) of the Mars 2020 mission (Farley et al., 2020) and are thought to record the direction of the winds that formed them (Herkenhoff et al., 2021). This study reports orientation measurements of ventifact textures on the floor of Jezero and compares these with orientation measurements of recent wind direction features (regolith wind tails, i.e., sandy drifts behind obstacles). Similar comparisons of wind direction inferences at previous landing sites have shown significant differences in recent and paleo-wind indicators, suggesting significant changes in wind directions. The implications of our observations in Jezero crater for paleoclimatic variations are discussed along with similar results at other Martian landing sites. Atmospheric circulation modeling results are then presented and compared with the measured orientations.
2 Methods
Ventifacts and other wind-formed features were identified and measured in stereo data derived from images acquired by the Navcam (Maki et al., 2020; Maki, 2021) and Mastcam-Z (Bell et al., 2021; Hayes et al., 2021; Bell & Maki, 2021) cameras on the Perseverance rover. Navcam has a focal length of 19.1 mm and instantaneous field of view (IFOV) of 330 µrad/pixel, while Mastcam-Z’s focal length ranges from 26 mm (IFOV of 283 µrad/pixel) to 110 mm (IFOV of 67.4 µrad/pixel) (Maki et al., 2020; Bell et al., 2021). At a typical range from the cameras of 3 m, Navcam and Mastcam-Z 110 mm focal length images have pixel scales of 1 and 0.2 mm, respectively. Mastcam-Z stereo mosaics acquired at 110 mm focal length allowed more detailed views supporting more accurate assessments of ventifact texture and orientations compared with shorter focal lengths and other onboard camera systems, so these were used heavily for ventifact orientation measurements in this study. All measured features were within 30 meters of the rover cameras when images were acquired. Care was taken to measure only aeolian abrasion textures with clearly expressed orientations; features that appeared to be aligned along bedding or other primary structures were not analyzed further.
Once linear abrasion features of interest were identified, their orientations were measured in digital ordered point cloud (OPC) datasets using the PRo3D software package (Barnes et al., 2018). The OPCs were produced by 3D vision processing of Mastcam-Z stereo pairs (Paar et al., 2022), georeferenced by the 20 to 50-cm-level-accurate rover localization (relative to HiRISE DTMs) provided by the Mapping Specialists on the Mars 2020 science team. At the maximum distance of measured features (30 m), the Mastcam-Z stereo range error is 7 cm. Each end of linear features, such as flutes, was selected manually and the bearing and slope of the line between them were recorded; an example is shown in Figure 1. Even at the maximum range of 30 m, feature measurements span multiple OPC grid points, so that the features are well resolved in the OPCs. Upwind points were picked first, followed by downwind points, to maintain a consistent dataset. Typically, the upwind ends of flutes are at a lower elevation than the downwind ends (Laity & Bridges, 2009), but an exception was found on the rock named “Rochette.” As shown in Figure 2, Rochette does not appear to be in-place bedrock, and therefore may have tilted since the aeolian abrasion features were formed. In this one case, the downwind direction was reversed for comparison with other measurements, assuming that after most ventifact textures were formed on Rochette, the block tilted (counterclockwise about a ~horizontal axis in the mosaic view) slightly to place the downwind ends of the aeolian flutes at a lower elevation than the upwind ends. This assumption is supported by observations of linear features on larger rocks in the same Mastcam-Z mosaic, which show more typical flute orientations (e.g., Figure 1). Measurements of features that were questionable or poorly resolved in the OPC were not included in statistical analyses. Flutes on nearly vertical rock faces showed higher variability in azimuth, as expected because wind-driven particles are deflected laterally when impacting such rock faces (Laity & Bridges, 2009).
Figure 1. Example measurements of flute orientations on ventifact. View toward south, approximate true color. (left) Part of Mastcam-Z image acquired on Sol 180 by sequence ZCAM08195. Three-dimensional (3D) downwind orientations of flutes indicated by yellow arrows were measured. (right) PRo3D visualization of ordered point cloud data derived from Mastcam-Z 110-mm stereo images acquired by same sequence. Endpoints of flutes were selected manually and are shown connected by blue lines. The highlighted line with red endpoints is 2 cm long. Context is shown in Movie S1.
Figure 2. Rochette ventifact. (top) Measurements of linear abrasion features on Rochette rock using PRo3D, shown as blue lines. Data at lower right for line highlighted with red dots at endpoints (length is 9.6 cm). (bottom) Part of enhanced-color Mastcam-Z mosaic ZCAM08195 acquired on Sol 180, showing Rochette rock and nearby terrain. View toward south-southeast.
The orientations of regolith wind tails (drifts or sand shadows) extending from behind rocks were measured using Navcam and Mastcam-Z stereo data acquired through Sol 400. The three-dimensional (3D) locations of the upwind and downwind ends of each wind tail were measured using Mars 2020 flight operations ASTTRO software (Advanced Science Targeting Toolkit for Robotic Operations; Abercrombie et al., 2019), allowing slope and azimuth (clockwise from north) of each feature to be derived. An example downwind endpoint is shown in Figure 3 . Similar to ventifact measurements, azimuths of regolith wind tails were calculated in the inferred downwind direction. An important qualitative criterion for feature selection was relative isolation in open ground away from adjacent large obstacles like outcrops, boulder clusters, and other features that might have blocked winds and/or wind-blown material from some azimuth ranges, or locally altered the wind direction.
Figure 3. Screen capture of Mars 2020 ASTTRO visualization of part of Navcam image acquired on Sol 355, showing location of end of wind tail (length = 17.3 cm) at white cross. Other wind tails are visible near right side of frame and 3-D (XYZ) and other data shown at far right.
3 Ventifact Orientation Results
Measurements of flutes using Mastcam-Z mosaics of the rock “Rochette” acquired on Sols 180 and 197 (after sample acquisition) differ by 7 degrees, reflecting the uncertainties inherent in our approach including manual point selection and variations in viewing geometry. Standard deviations of each sol’s measurements are similar to this value (Table 1 ). Early in the mission, OPCs were generated in the rover coordinate frame, so that the rover heading had to be subtracted from measurements of features (corrections for tilt were not performed due to our focus on the azimuth of ventifact features). More recently, OPCs have been generated in the Mars local level coordinate frame. Measurements made using the new OPCs agree with measurements of the same features made using the old OPCs to within 4 degrees, reflecting errors in coordinate transformations and uncertainties in picking endpoints of linear features. The orientations of linear abrasion features seen in Navcam and Mastcam-Z stereo data acquired through Sol 57 were measured independently, and agree very well with the OPC measurements: the average downwind azimuth of features measured in terrain meshes generated at the Jet Propulsion Laboratory (JPL) using Navcam and Mastcam-Z images is 97° ± 6° while the average downwind azimuth of features measured in Mastcam-Z OPCs is 95° ± 4° (in both cases, the standard deviation in the measured azimuths is given rather than the uncertainty in the individual measurements). Similarly, the average downwind azimuth of all features measured in OPCs derived from Mastcam-Z stereo images acquired through Sol 400 is 94° ± 7°. Based on the comparisons summarized above, true variations in the orientations of linear features exceed the uncertainties in the measurements.
Table 1 . Sol number of image acquisitions for each set of aeolian abrasion features, number of orientation measurements (N), average downwind feature azimuth (clockwise from north, in degrees) and standard deviation for each sol.