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The set of instruments carried by Mars 2020, described in Sect. 2 and in other papers in this special issue, are able to monitor a wide range of variables that are relevant to meteorological and aeolian studies. They will measure both meteorological variables (pressure, temperature, winds, and water vapor) and the forcing that drives their behavior over time (e.g. aerosol abundances and properties, and radiation fluxes at the surface). These measurements will enable investigations of processes operating on timescales from seconds to years, ranging from understanding the statistics of convective vortices and atmospheric turbulence to determining the impact of local topography or atmospheric dust opacity on near-surface wind stress, sand motion, and dust lifting. Images of the surface, combined with measurements of the local circulation, will provide insight into aeolian features such as ripples, dunes, ventifacts, and dusty convective vortex (dust devil) tracks. Changes detected on the surface or rover deck will be correlated with winds and vortex activity to shed light on aeolian processes and the threshold conditions that must be exceeded for dust lifting or active saltation.
Download Dune (2021) Lat mp4
In this study, we therefore make aeolian predictions for a range of plausible wind stress thresholds, from 0 Pa (which is clearly unphysical but provides an upper limit on sand motion) to 0.01 Pa (which was found to be a good effective threshold for matching observed seasonal dune migration using wind stresses from a low-resolution simulation; see Ayoub et al. 2014). Predicted sand fluxes and net sand transport directions over this range of thresholds, as a function of season, are predicted at the Mars 2020 landing site in Sect. 8.1 and at several points along the expected rover traverse in Sect. 8.3.
A reasonable assumption in aeolian theory is that bedforms that migrate or elongate do so in the direction of the net sand flux vector when observed over some moderately long time period (Wilson 1971). For small and large ripples, this may be hours and weeks, respectively; for large, active dunes, this may be seasons to years. The dune migration direction may therefore be estimated by calculating the net sand flux vector multiple times per Mars sol over a full Mars year (or by sampling the diurnal cycle at regular intervals over a Mars year), as described in Sect. 4.1, then performing a vector sum over the year. These results are shown for the landing site in Sect. 8.2.
The development of the Gross Bedform-Normal Transport (GBNT) theory of Rubin and Hunter (1987) provided the first means of predicting dune orientations given a known wind field. This approach has been verified in laboratory experiments (Rubin and Hunter 1987; Rubin and Ikeda 1990; Reffet et al. 2008), numerical modeling (Werner and Kocurek 1997; Kocurek and Ewing 2005; Reffet et al. 2008), and field studies (Lancaster et al. 2002; Rubin et al. 2008), and is widely accepted as producing the most accurate prediction of dune orientations in regions with loose, free-moving sediment for all dune types. The basic idea is that dunes will align so as to maximize the GBNT, which is simply the magnitude of wind-driven sand transport normal to the bedform, summed over all orientations between 0 and \(180^\circ\) and over all times. The key here is to consider gross rather than net sand transport; transport to and fro across a dune crest cancels out in a net sense, but in reality both directions of transport move sand across the bedform and can thus build it over time. The optimal dune orientation is found by determining the orientation that results in the largest GBNT given the long-term (e.g. annual) wind field at a given location.
Predicted net dune migration direction over a Mars year (black arrows) as well as predicted orientation of bedform crests (red lines) assuming the (a) GBNT or (b) Fingering Mode (bottom) theory of dune formation, for the region surrounding the landing site in NW Jezero crater using mesoscale MarsWRF output. The shaded background shows topography. Note that the net dune migration is independent of the chosen dune formation theory hence the black arrows are identical between plots. Also shown is topography (colored shading) and the location of the Mars 2020 landing site and Midway (white and pink circles, respectively). Note that arrows show only net sand transport direction and are not scaled to show predicted sand flux
In terms of predicting bedform orientations, the GBNT theory (Fig. 13a) predicts these will largely be from SW to NE over most of the area shown. Again, however, near the crater rim a greater range of orientations is predicted, including nearly S to N (just inside the western part of the crater) and W to E (on the area of high topography outside it). Interestingly, some of the bedforms along or just inside the western crater rim are predicted to be nearly aligned with the net transport direction, suggesting nearly equal and opposite sand fluxes from either side of the bedform over a year. By contrast, the Fingering Mode theory (Fig. 13b), which should apply in the case of a limited sand supply, predicts dunes virtually parallel to the predicted sand transport directions in all locations. Under this assumption, bedforms would be oriented from NW to SE, again except along the crater rim where bedforms oriented SW to NE or S to N would occur, and to the east of a topographic ridge (top left) where S to N bedforms are also predicted.
Easterly (or slightly more northerly) wind directions are predicted at the LTST of peak wind stress by most simulations at \(\textLs\sim 180^\circ\) and \(270^\circ\), which appears consistent with the orientation of wind streaks observed in these periods, although as most of these streaks are bright (hence likely depositional) they may have been emplaced when atmospheric stability was reduced, rather than when wind stress peaked, thus more investigation is needed here. The prediction of net annual sand transport to the WNW at the landing site by most simulations appears consistent with general sand transport directions across the region, as inferred from aeolian features inside Jezero crater and with the observed motion of dunes just to its north. The mesoscale MarsWRF simulation is used to expand this prediction over the region containing the likely Mars 2020 rover traverse, and predicts sand transport to the NW over most regions except close to the crater rim and topographic ridges. This shows that its more southerly daytime wind directions than other simulations are not restricted to the landing site region, and the mismatch with aeolian features in the region suggests that this prediction may be incorrect. 041b061a72