Diurnal Variability in Aeolian Sediment Transport at Gale Crater, Mars

Abstract

A suite of high resolution cameras onboard the Mars Science Laboratory (MSL) Curiosity rover have provided an unparalleled look at active aeolian processes on Mars, including within the first active dune field explored on another planet, the Bagnold Dunes. Here we present results from a subset of MSL's repeat imaging (“change detection”) experiments with temporal resolutions sufficient to probe the diurnal variability in winds within Gale crater. Images reveal that saltation is a near‐daily phenomenon during southern summer, with repeatable diurnal circulation patterns producing steady impact ripple migration toward the west/southwest. Nighttime fluxes are inferred to be ∼four times larger than daytime fluxes, consistent with predictions from the MarsWRF model of multiple periods of enhanced wind between sunset and sunrise. Multiple factors are likely facilitating saltation at this time: (a) time‐averaged nighttime winds have a higher degree of variance (i.e., higher peak friction speeds) than daytime winds, (b) interactions between regional Hadley flows and local, thermally driven slope winds cause increased turbulence at night, and (c) relatively higher atmospheric density produces correspondingly higher shear stresses and decreases critical thresholds. Observations of sand transport at a range of spatiotemporal scales (down to scale of individual particles moving on the timescale of seconds) support the idea that bedform migration is driven by intermittent, low‐flux saltation events when winds fluctuate between canonical impact and fluid thresholds. Yet, whereas gustiness may play a role in initiating transport, saltation is found to be highly predictable on diurnal timescales and is only stochastic on the shortest timescales characteristic of turbulent fluctuations in wind.

The Aeolian Environment in Glen Torridon, Gale Crater, Mars

“…These migration rates and fluxes indicate that, even during the windy season near planet perihelion, ripple migration at Glen Torridon is slower and more sporadic than at the Bagnold dunes reported by . MARDI images of sand and bedrock acquired near dawn and dusk at the Mary Anning drill site indicate that sand fluxes were significantly higher at night (i.e., between ∼18:00 and 06:00) than during the day, consistent with observations from earlier in the traverse (Baker et al, 2022). These MARDI images captured ripples migrating toward azimuths spanning S to NW (Movie S6 in Supporting Information S1).…”

“…The same effect should make a small sandy wind tail on Mars less vulnerable to erosion from winds approaching it obliquely. Furthermore, only short ripple migrations <1 wavelength per wind event were observed at Glen Torridon, even during the windy season (Section 5.4, Movie 5, Text S1, Text S2 in Supporting Information S1), similar to active ripple migrations observed elsewhere along the traverse (Baker et al, 2022). Wind tails such as those in Figure 10 could require many such wind events to reach observed lengths that in some cases exceed 1 m. In this scenario, (a) brief, low-flux sand-driving events from off-axis directions contributing to wind tail accumulation are unlikely to erase or deflect the growing wind tail, and (b) sand-driving wind directions of multiple contributing events probably would not be perfectly aligned, so would be expected over time to apply ripples obliquely to the average orientation of the hosting wind tail.…”

“…The second possibility is suggested by MarsWRF experiments that do not predict strong wind events consistently blowing WSW at Glen Torridon, even during the windy season near planet perihelion. As discussed in Section 5.6, strong nighttime regional winds and katabatic winds are predicted as separate incursions into the MSL operating area at different times of night blowing in different directions (S, then later W) (see also Baker et al, 2022) (Figure 19). Tentative support for this scenario affecting active sand deposits at Glen Torridon is provided by morphologic details of some sandy wind tails.…”

Orbital and In‐Situ Investigation of Periodic Bedrock Ridges in Glen Torridon, Gale Crater, Mars

“…The orientation of Glen Torridon ridges was compared against outputs from the Mars Weather Research and Forecasting (MarsWRF) model (Newman, 2022). The MarsWRF model has been used to simulate the atmospheric circulation inside Gale crater to compare with MSL Rover Environmental Monitoring Station (REMS) wind measurements (Newman et al, 2017) and to assist in interpreting observations of dust devils (Newman et al, 2019) and aeolian-driven surface changes (Baker, Lapotre, Minitti et al, 2018;Baker et al, 2022). The output used in this work comes from the "vertical grid B" simulations described in Newman et al (2017), which provide the best match to observed winds and aeolian features.…”

The diurnal cycle and temporal trends of surface winds

“…Interestingly, it was recently discovered that in the Gale Crater on Mars, the winds are stronger at night (Baker et al, 2021), possibly due to some local effects. There are also some locations on earth in which the winds are stronger at night, so it may be that in other places on Mars outside the Gale Crater, the winds are stronger during the day.…”