Abstract
Abstract The InSight mission has operated on the surface of Mars for nearly two Earth years, returning detections of the first marsquakes. The lander also deployed a meteorological instrument package and cameras to monitor local surface activity. These instruments have detected boundary layer phenomena, including small-scale vortices. These vortices register as short-lived, negative pressure excursions and closely resemble those that could generate dust devils. Although our analysis shows that InSight encountered more than 900 vortices and collected more than 1000 images of the Martian surface, no active dust devils were imaged. In spite of the lack of dust devil detections, we can leverage the vortex detections and InSight’s daily wind speed measurements to learn about the boundary layer processes that create dust devils. We discuss our analysis of InSight’s meteorological data to assess the statistics of vortex and dust devil activity. We also infer encounter distances for the vortices and, therefrom, the maximum vortex wind speeds. Surveying the available imagery, we place upper limits on what fraction of vortices carry dust (i.e., how many are bona fide dust devils) and estimate threshold wind speeds for dust lifting. Comparing our results to detections of dust devil tracks seen in space-based observations of the InSight landing site, we can also infer thresholds and frequency of track formation by vortices. Comparing vortex encounters and parameters with advective wind speeds, we find evidence that high wind speeds at InSight may have suppressed the formation of dust devils, explaining the lack of imaged dust devils.
Highlights
The InSight spacecraft landed on Elysium Planitia (4°.5N, 135°.6E; Golombek et al 2020) on 2018 November 28, carrying suites of both geophysical (Banerdt et al 2020) and meteorological instruments (Banfield et al 2019, 2020)
Wind speeds measured by InSight’s Temperature and Winds for InSight (TWINS) instrument may be combined with measurements of the surface and near-surface temperatures to assess the accuracy and applicability of theoretical predictions of surface layer heat and momentum transport developed for Earth
As we show in Appendix B, fitting both ΔPobs and Vobs and assuming a balance between centrifugal and pressure gradient accelerations allows us to estimate ΔPact and Vact, along with the encounter distance b
Summary
The InSight spacecraft landed on Elysium Planitia (4°.5N, 135°.6E; Golombek et al 2020) on 2018 November 28, carrying suites of both geophysical (Banerdt et al 2020) and meteorological instruments (Banfield et al 2019, 2020). Dust devils may register not just in imagery and in meteorological time series if a vortex passes over or near the lander—the convective cells produce short-lived (few seconds), negative pressure excursions, accompanied by perturbations to the observed wind speed and direction (Lorenz 2016). One significant drawback of such studies, though, is that, without simultaneous imagery or pyranometry, judging whether the encountered convective vortex carries dust is difficult Such dustless vortices are common on both Mars and Earth, and there is no requirement that a vortex lifts dust, even when dust is available. We compared our occurrence rates to analysis of space-based observations of tracks left in the region around InSight (Reiss & Lorenz 2016; Perrin et al 2020), which allows us to assess how frequently vortices leave tracks on the Martian surface. Two appendices follow that provide details on the statistics of our vortex detection scheme and on our model for determining the geometry and uncertainties for each encounter between the InSight lander and a vortex from the observed vortex parameters
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