Figure 4 evaluates the potential effects of varying
illumination angle on the azimuth measurements, comparing average
azimuth for each sol (through Sol 238) and local true solar time of
image acquisition. The Mastcam-Z mosaics that were analyzed typically
required a few minutes to acquire, so the local true solar time of the
midpoint of each mosaic acquisition was used for this analysis. While
there is more scatter in the measurements when the sun was lower, there
is no apparent systematic effect of illumination differences on the
results. Potential systematic errors from viewing azimuth (measured
clockwise from N, centered on Mastcam-Z) were also evaluated in
ventifact orientations measured through Sol 238. Figure 5 shows
no apparent dependence of feature azimuth on viewing geometry either. We
therefore conclude that neither illumination nor viewing geometry
significantly affects our results.
Figure 4. Bearing
(azimuth clockwise from north) of ventifact features on the floor of
Jezero crater, averaged over measurements made in each Mastcam-Z
panorama (acquisition sol indicated in boxes) through Sol 238. Vertical
bars show the standard deviation of each range of measurements. Three
panoramas were acquired between 12:04 and 12:41 on Sol 180; the midpoint
time of the second one is plotted here. The two bearing measurements of
features seen in the Sol 112 panorama differ by only 0.1° so standard
deviation is not visible for the Sol 112 data point. There is no
significant correlation between time of day and ventifact orientation,
indicating that illumination conditions do not affect these
three-dimensional measurements.
Figure 5 . Bearing
(azimuth clockwise from north) of ventifact features on the floor of
Jezero crater vs. viewing azimuth for data acquired through Sol 238.
Overall, variations in ventifact orientations along the first 400 sols
of the rover traverse are minor, as shown in Figure 6 , perhaps
because the area explored by the Mars 2020 rover over that period is
only about 1 km across. Some of the variability in linear feature
orientations is due to variations in the 3D geometry of the rock faces
relative to the wind azimuth, as expected. It is also expected that
local topography will deflect sand-transporting winds and that some of
the observed variability may be caused by nearby topographic obstacles.
To determine the significance of such effects on our measurements,
individual ventifact azimuth measurements that differ from the global
average by more than two standard deviations were examined.
Many of the >2-sigma variations in azimuth appear to be
caused by the deflection of winds on more steeply inclined rock faces
(e.g., on Sols 53, 254, 353, and 362). In these cases, differences
between the normal to the abraded rock face and the direction of strong
winds may cause the winds to be deflected horizontally. Such deflections
have been observed on terrestrial ventifacts. To understand this,
consider a hemispherical rock that is subjected to abrasion by saltating
sand blowing from a single direction. The orientation of aeolian flutes
will depend on the local surface normal at each grain impact site,
resulting in variable flute orientations that radiate away from the
upstream point on the rock. Lower-resolution topographic data on more
distant features also contributed to uncertainty and more scatter in
measured azimuths on some features. Evidence for large-scale wind
deflections by upstream topography is limited, with possible cases
observed on Sols 53 and 180. As described above, the Rochette float
block may have moved slightly after the ventifact features were formed,
contributing to differences in their orientations relative to other
ventifacts.
Figure 6 . Locations of
measurements of aeolian feature orientations (arrows) plotted on HiRISE
color image (north up, illumination from left). White dots show rover
locations, connected by grey traverse path with white arrows showing
drive direction. Sols and average orientations of ventifact features
shown by yellow arrows, average orientations of wind tails shown as
green arrows. Wind tails were measured at multiple locations on Sols
340, 341, and 360; suffixes “a” and “b” were added to the sol
numbers annotating these locations. Wind tail orientations measured
later in the mission are not included, as the traverse extended to the
north after Sol 379, beyond the area mapped.
4 Comparison with indicators of recent aeolian transport
Wind tail orientations, averaged over observations made at each rover
location, are summarized in Table 2. These 101 downwind orientations
were measured using Navcam stereo data in all cases except on Sols 34
and 52, when Mastcam-Z stereo images were analyzed. Sometimes images
obtained at mid-drive pauses showed measurable features (i.e., in
addition to features at the final, end-of-drive position). These
instances are distinguished in Table 2 with letters appended to the sol
number. Downwind azimuths that differ from the mean by more than two
standard deviations were measured in images acquired on Sols 130, 137,
173, 199, 202, and 355. In all cases, topographic features near the
measured wind tails are too small to have affected wind flow
significantly, so the cause of the orientation differences is unclear.
There is no evidence that differences in grain or feature size
influenced susceptibility to recent wind modification.
Table 2. Sol number of
image acquisition, number of measurements (N), average wind tail azimuth
(clockwise from north, in degrees) and standard deviation. Measurements
were made at multiple rover locations on Sols 32, 47, 340, 341, 355, and
360.