The wind tail orientations indicate that they were formed by winds
blowing toward the west-northwest (WNW) while the orientations of
ventifact features suggest formative, sand-driving winds blowing toward
the east, almost in the opposite direction (Figure 7 ). Note
that the inferred direction of recent winds that formed the wind tails
is consistent with the orientation of the dunes visible inFigure 6 . These and other aeolian features were used by Day &
Dorn (2019) to infer that they were formed by winds trending toward 263°
± 8°, very similar to the trend derived from the wind tail azimuths
summarized in Table 2 .
Figure 7.Orientations of linear ventifact features (orange) and regolith wind
tails (green) averaged over 15-degree azimuth bins. In both cases
downwind azimuths are shown.
The surface wind patterns in the tropical region of Mars reflect complex
interactions between the large-scale circulation, which is dominated by
Hadley cell flows, thermal tides and other planetary waves, and slope
flows at regional and local scales (Rafkin et al., 2016; Newman et al.,
2017; Viúdez-Moreiras et al., 2019). These flows can be affected by
smaller-scale, complex topography. Jezero crater is located on the
northwestern slopes of Isidis basin, a region that presents a steep
slope, thus strong near-surface regional flows are expected in the
region. Pre-landing mesoscale simulations using nine different models
showed control by regional and local slope flows (Newman et al., 2021),
which is consistent with Mars 2020 observations (Newman et al., 2022;
Viúdez-Moreiras et al., 2022a). Chojnacki et al. (2018) found that sand
ripples with 3-5 m wavelengths within the Mars 2020 landing ellipse in
Jezero crater had moved ~0.2 m/yr toward the
west-northwest based on HiRISE orbital observations, also consistent
with the Mars 2020 observations.
The diurnal cycle of winds observed during the first 315 sols of the
mission by Perseverance ’s Mars Environmental Dynamics Analyzer
(MEDA, Rodriguez-Manfredi et al., 2021; Rodriguez-Manfredi & de la
Torre Juarez, 2021) instrument suite included part of northern spring
and summer and presented two regimes (Viúdez-Moreiras et al., 2022a):
(i) a daytime regime, from dawn to sunset, with average easterly (i.e.,
from the east) to southeasterly winds during which maximum wind speeds
were measured, and (ii) a nighttime regime with a period of westerly and
northwesterly downslope winds followed by a relatively calm period until
sunrise. Maximum average wind speeds of ~7
ms-1 were measured during the afternoon, when winds
were easterlies. Weibull models using high-frequency wind data show that
winds exceed 8 ms-1 for about 20% of the afternoon
period but for less than 0.2% of the nighttime period (Viúdez-Moreiras
et al., 2022b), highlighting the strength and the convective activity
involved in the easterly and southeasterly winds observed during the
daytime. Although wind patterns have not been observed in northern
autumn and winter, the strong effect of regional flows observed during
spring and summer suggests that the aforementioned near-surface wind
regimes at Jezero will be roughly maintained throughout the rest of the
year, probably disturbed to a greater extent around the winter solstice
due to the strength of the zonal-mean meridional circulation
(cross-equatorial Hadley cell) in that season. In a pre-landing,
multi-model intercomparison (Newman et al., 2021), most models predict
in general the highest maximum and mean wind speeds in northern summer.
This is also the period of generally highest wind stress and estimated
sand flux for most models (see Figures 5 and 11 of Newman et al. 2021,
respectively), despite atmospheric density reaching its annual minimum
in late summer. Peak sand flux is predicted in the first half of the
year by all models that resolve the crater. Hence the seasons already
observed by Mars 2020 are expected to have included the period of
maximum aeolian transport in Jezero, at least in the absence of major
dust storm activity (the effects of which were not modeled).
The moderately strong wind speeds observed during the first part of the
evening (the wind speed drops after midnight) are aligned with ventifact
orientations but are rarely strong enough to drive sand and form the
ventifacts observed by Mars 2020. In addition, ventifacts indicate a
roughly opposite direction to the strong daytime winds observed by Mars
2020 under nominal conditions, which suggests that ventifacts were not
produced under nominal wind patterns in the present-day Mars climate.
Ventifact formation in the current era might occur only during anomalous
weather conditions (e.g., rare strong, sand-driving wind events from
atypical directions, associated with dust storms or other unusual
weather phenomena). Alternatively, ventifact orientations might indicate
a paleowind regime different from the current prevailing conditions
(Newman et al., 2022; Bell et al., 2022), as discussed further in the
next section.
4 Interpretation using atmospheric circulation models
Perseverance has yet to observe the circulation for northern
autumn and winter, and damage to the MEDA wind sensors, likely due to
flying debris on Sol 315 (Viúdez-Moreiras et al., 2022b), may preclude
future observations of those seasons. However, winds measured to date
show good agreement with the predictions of some atmospheric models that
resolve the crater topography. Figure S1 of Newman et al. (2022) shows a
mostly very good match between ~1.5 m altitude winds
modeled by MarsWRF and those measured by MEDA for Sols 180-188,
corresponding to the period around the northern summer solstice
(areocentric solar longitude Ls ~90°).
Figure 8 herein shows similarly good agreement for Sols 47-53,
corresponding to the period around the northern fall equinox
(Ls ~30°). Figure S1 also shows good
agreement between the MarsWRF model and data acquired near
Ls 150º. The agreement between model-predicted and
1-minute average observed wind speed is excellent at all times of sol,
while the agreement with wind direction mainly diverges in the late
afternoon when the model predicts southeasterly winds while observed
winds are roughly easterly. These results lend greater confidence to
MarsWRF predictions of how the circulation may differ at other seasons,
as shown in Figures 2 and 8 (for Ls ~0°
and 270°, respectively) of Newman et al. (2021) and in the
high-resolution modeling results of Pla-Garcia et al. (2020). While
there are subtle differences, those predictions show a lack of
significant seasonal variation in diurnal wind patterns, except for a
reduction in wind speeds at Ls ~270°
compared to other seasons. Of course, the model may be incorrect, but it
appears unlikely that seasonal changes can explain the difference
between ventifact and wind tail orientations.
Figure 8. Comparison of
minute-averaged MEDA wind data with WRF model results at every minute
(black symbols) for Ls = 30°. Key for colors of various
data shown in top panel.
As noted above, nighttime wind orientations (from the west-northwest)
are quite consistent with ventifact orientations; the problem is that
the nighttime wind speeds are typically very low compared to those
during the day and thus unlikely to dominate sand transport and
abrasion. However, the key variable here is the force on the surface,
which is related to wind stress, rather than wind speed directly. The
wind stress is given by air density times friction velocity (u*)
squared, where we may estimate u* based upon the wind speed at 1.5 m
above the surface by assuming a logarithmic wind profile and value of
surface roughness, z0, with u(z) = u*
ln(z/z0) / kappa, where kappa is the Von Kármán constant
(~0.4) and z=1.5 m. Air density is proportional to
pressure and 1/temperature, hence increases at night when air
temperature on Mars decreases significantly. This density effect means
that if the daytime wind speed only slightly exceeds that at night, the
nighttime wind stress may be greater. In the case of the rover’s recent
locations in Jezero crater, however, the daytime wind
speed far exceeds that at night, hence the estimated nighttime
wind stresses are also low compared to those during the daytime. Newman
et al. (2022) used MarsWRF climate modeling to show that nighttime winds
in the northwest quadrant of Jezero are likely dominated by the effect
of the local crater rim to the northwest and west, on which strong,
thermally-driven downslope flows are predicted to develop and intensify
after sunset. For the first half of the night, these winds, from the
west through the northwest, are predicted to penetrate well into the
crater. Although strongest on the rim, they remain moderately strong by
the time they reach the location of the rover. However, after
~02:30 local true solar time (LTST), modeled wind speeds
drop rapidly at the rover’s position, although they continue to
intensify on the crater rim. The reason for this drop in nighttime wind
speeds is not yet confirmed, but one possibility is that the mountain to
the southeast of the crater generates strong downslope flows that induce
an intra-crater circulation, which restricts the strong crater rim
downslope flows closer to the rim as the night progresses.
Other than seasonal changes, there are two other possibilities for
producing the observed ventifact orientations. One is that they are
formed predominantly during dust storms, when winds could potentially be
stronger and thus reflect the altered wind patterns during such events.
Unfortunately, the MEDA wind sensor was damaged during the early part of
the January 2022 regional dust storm, and although data from right
beforehand does indicate a shift in the wind, it is difficult to
determine whether storm winds are consistent with the observed ventifact
orientations (Lemmon et al., 2022; Viudez-Moreiras et al., 2022a).
A second possibility is that they formed during a different orbital
epoch, such as one with a different obliquity compared to the present
day (~25°) or a different seasonal timing of perihelion.
Newman et al. (2022) offer a hypothesis that involves strengthening of
the winter-solstice Hadley circulation–in which the zonal-mean flow
near the surface blows from north to south–at higher obliquities, with
this increase due to the latitudinal shift in solar forcing (e.g.,
Haberle et al., 2003; Newman et al., 2005). The idea is then that
stronger background northerlies might enhance and/or push nighttime
downslope winds from the northwest/west-northwest deeper into the
crater, which might result in stronger wind speeds from this direction
at the rover’s location. While a thorough exploration of a range of
orbital changes is beyond the scope of this paper, Figure 9 shows the
predicted change in sand flux and sand transport direction predicted by
MarsWRF if the orbital obliquity is increased to 45°, which may last
have been the case ~5 Myr ago (Laskar et al., 2004).
Here the present-day simulations are the same nested mesoscale
(crater-resolving, grid spacing ~1.4 km) MarsWRF
simulations shown in Newman et al. (2021), while the 45° obliquity
simulations use the same model setup but with an obliquity of 45°. While
it is possible that higher obliquity periods resulted in stronger global
circulations and greater dust lifting (e.g., Newman et al., 2005), it is
also possible that dust was rapidly depleted from most source regions
and dust loading was reduced. In the absence of a clear answer, these
simulations use the same prescribed atmospheric dust distribution, based
on observed years with no major dust storms, as described by Newman et
al. (2021), for both the present day and the past climate epoch. Note
that MarsWRF includes a CO2 cycle that parameterizes
surface-atmosphere exchange of CO2 and redistributes the
CO2 ice cover with the seasons. However, while the
seasonal variation of CO2 ice cover changes for the
higher obliquity simulation, this produces only a small change in the
surface pressure cycle compared to the present-day simulation, and we do
not explore more significant changes that might have resulted from the
proposed buried CO2 release. All simulations were
conducted for 9 sols at Ls ~0° and
Ls ~270°, with the first two sols (in
which the model may still be adjusting to its initialization) not used,
and the global domain in which they were embedded had first been run for
at least one Mars year before the nested simulation start time, again to
allow the model to adjust and reach a quasi-seasonally-repeatable state.
The main result shown in Figure 9 is that, at both seasons examined, the
high-obliquity simulations generally predict a balance between daytime
and nighttime sand flux magnitudes similar to that found for the present
day. The exception is at Ls ~270° in the
high obliquity simulation, in which the peak flux across all sols at
~23:00 LTST is comparable to the peak mid-sol. However,
the sand transport direction in that particular sol (at
~23:00) is from the northeast. Further, peak fluxes
during this simulation are lower than those predicted during the daytime
for the present-day Ls ~270° simulation,
and are much lower than those predicted in the late afternoon at
Ls ~0° for both the present-day and 45°
obliquity simulations, during all of which times the predicted sand
transport is from between the east and southeast. These preliminary
results do not support the idea that higher obliquities could explain
the observed ventifact orientations. However, far more study is needed
of other seasons, including the idea that at Ls~270° at higher obliquity the atmosphere might have much
greater dust content due to feedback between winds and dust lifting,
driving a stronger zonal mean circulation, although this may not occur
if the dust supply is limited. This preliminary investigation barely
scratches the surface of exploring past orbital configurations; further
modeling could be beneficial.
Figure 9. Predicted sand
flux (left) and direction from which the sand blows (right) assuming the
Lettau and Lettau (1978) sand flux equation and a threshold of 0, using
output from the MarsWRF model every five minutes, for (a) present-day
orbital settings at Ls ~0°, (b) same
season as (a) but for an orbital obliquity of 45°, (c) present-day
orbital settings at Ls ~270°, (d) same
season as (c) but for an orbital obliquity of 45°. As in Figure 8,
colors indicate different sols, with 7 sols shown for each seasonal
period.
We conclude this section by noting that Newman et al. (2021) used the
output from eight Mars atmospheric model simulations (involving six
different models) to estimate the net sand transport direction as a
function of season at the center of the landing ellipse in Jezero crater
(77.43°E, 18.47°N), which is close to the rover’s true location during
this period. Fig. 11 of that paper presents the results, which show the
expected sand transport direction to be toward the west or north-west in
all seasons for most simulations and choices of sand motion threshold,
consistent with observations of regolith wind tails in the crater and
with the estimated net sand transport direction based on MEDA winds and
air densities measured over the first 216 sols (see above). The
exception is at zero sand motion threshold, where there is greater
spread (sand transport also toward the south-west, south, or even
south-east) in some simulations and seasons. Even then, however, the
season of peak sand flux still coincides with transport toward the west
or north-west, again consistent with wind tail observations in Jezero.
4 Discussion
The distribution of measured ventifacts (Figure 6) does not suggest
correlations of ventifact susceptibility with various rock units. Rocks
with measurable ventifact orientations were found locally among rocks
that displayed less or no obvious ventifact textures. This suggests that
the susceptibility to aeolian abrasion of source bedrock now weathering
at the surface varies across relatively short length scales, and/or that
the current population of rocks exposed along the rover traverse also
includes an important fraction of “erratics” different from local
source bedrock, contributed either as impact ejecta from elsewhere
and/or as resistant remnants lowered into place by erosion of
stratigraphically younger/higher materials (e.g., mixed delta deposits
that once covered the rover traverse) to the current level of the crater
floor. Figure 6 indicates that ventifact orientations do not vary
significantly along the rover traverse through Sol 379, so most likely
they record strong sand transport from a narrow range of directions in a
different past climate regime.
Evidence for changes in strong wind directions has been noted at other
locations on Mars. Bridges et al. (1999) concluded that ventifacts at
the Mars Pathfinder landing site have orientations that are
significantly different from recent wind directions indicated by the
orientations of wind tails and local wind streaks, strongly suggesting a
change in local circulation patterns since the ventifacts were formed.
At the MER Opportunity site at Meridiani Planum, orientation
differences between aeolian features of different relative ages indicate
that strong wind events have changed direction by about 40° over time
(Sullivan et al., 2005). Fenton et al. (2018) modeled changes in
atmospheric circulation over the past 400,000 years to infer that the
ripples on Meridiani Planum observed by Opportunity were likely
formed by strong winds during the most recent obliquity maximum (between
26° and 27°) about 100,000 years ago when the atmospheric pressure was
~25% higher than today. Evidence for abrasion from
periodically sustained paleo-winds was observed in the sculpted surfaces
of iron-nickel meteorites by Opportunity at Meridiani Planum
(Ashley et al., 2011; 2022). Microscopic Imager mosaics and their
respective 0.09 mm/post DTMs show oriented hollows and fluted oxide
coatings on Meridiani meteorites, as well as patterns in coating
occurrence with respect to topography (e.g., wind shadows), and possibly
even gross meteorite morphology in at least one instance. However, these
patterns do not appear to be consistent with either recent wind streak
orientations (Sullivan et al., 2005) or the most recent aeolian bedform
migration directions (Golombek et al., 2010). Similarly, orientations
differ between recent and paleo-wind direction indicators at Gale
crater, based on observations of ventifacts near the Mars Science
Laboratory landing site (Bridges et al., 2014). Measured orientations of
thousands of aeolian bedforms and Periodic Bedrock Ridges (PBRs) at Oxia
Planum, the intended landing site for the ExoMars Rosalind Franklin
rover, indicate changes in the direction of formative winds with time
(Favaro et al., 2021). In addition, Favaro et al. (2021) found that
modeled contemporary wind directions and strengths are not consistent
with the orientations of either the aeolian bedforms or PBRs. Hence, the
differences between Jezero floor ventifact feature orientations and
models or other indicators of current wind directions in Jezero crater
are not atypical for Mars, because such differences have been observed
at other landing sites (e.g., Greeley et al., 2000). Collectively, all
of these examples indicate the potential of aeolian abrasion textures to
preserve records of past wind conditions for relatively long surface
exposure times under arid martian conditions.
5 Conclusions
The capabilities of Perseverance ’s Mastcam-Z and Navcam stereo
instruments and their data products and visualization assets allow
statistically significant 3D measurement and analysis of ventifacts
observed on the floor of Jezero crater during the first 400 sols of the
landed mission. These measurements of features located within
~1 km of the landing site show that strong winds forming
the ventifacts blew from the west (toward azimuth 94 ± 7°), nearly the
opposite direction of the winds that formed more recent aeolian features
(toward azimuth 285 ± 15°). Therefore, a major change in the direction
of strong winds in Jezero crater is recorded by surface features of
different ages. In the past, sand-driving winds consistently from the
WNW prevailed long enough to establish ventifact textures on rock
exposures throughout the study area, but the strongest winds of the
current wind regime have not prevailed long enough, with whatever sand
supply exists upwind, to leave a comparable ventifact record. Current
nighttime wind directions are similar to the paleowind direction
inferred from the ventifact orientations, so a past wind regime that
causes nighttime wind speeds to increase and dominate over daytime winds
may explain the formation of the ventifacts. Further work is needed to
determine what changes in global orbital/axial parameters might cause
such a wind regime.