Dust devil speeds, directions of motion and general characteristics observed by the Mars Express High Resolution Stereo Camera

a b s t r a c tDust devil diameters, like many features in nature, have skewed distributions. I summarize, and present in a unified manner, literature values of dust devil optical diameters from seven terrestrial surveys, three rover-based surveys on Mars and two orbital surveys on Mars. The problems of appropriately treating these data are analogous to those for impact craters, and similar display and binning approaches are suggested. Remarkably, the Mars dust devil population remains better-known than Earth's. The theoretical justifications for possible log-normal, and (truncated) exponential and power-law descriptions of dust devil properties are discussed, and the challenges of discriminating between these candidate distributions with finite (and often, coarsely-binned) observation sets are noted: the best-sampled datasets so far appear well-described by power laws. Data required for advances in model discrimination are discussed: data binned in four or fewer ranges are useless for this purpose. Caution must be exercised in applying the notion of an 'average' dust devil and in calculating population-integral properties such as dust flux.

Section: Dust Devil Datasets and Model Fitsmentioning

confidence: 99%

On the statistical distribution of dust devil diameters

Lorenz

2011

“…The largest ones might easily lift dust. However, Stanzel et al (2008) and Reiss et al (2014) find wind speeds (tangential and transversal) which are not always large enough. Also, Reiss et al (2009) observed dust devil activity on Arsia Mons.…”

Section: Introductionmentioning

confidence: 96%

An insolation activated dust layer on Mars

Beule

Wurm

Kelling

et al. 2015

The illuminated dusty surface of Mars acts like a gas pump. It is driven by thermal creep at low pressure within the soil. In the top soil layer this gas flow has to be sustained by a pressure gradient. This is equivalent to a lifting force on the dust grains. The top layer is therefore under tension which reduces the threshold wind speed for saltation. We carried out laboratory experiments to quantify the thickness of this activated layer. We use basalt with an average particle size of 67 µm. We find a depth of the active layer of 100 to 200 µm. Scaled to Mars the activation will reduce threshold wind speeds for saltation by about 10%.

“…They are powered by solar insolation and form most commonly in hot, arid environments where there are strong vertical temperature gradients (Renno et al, 1998). Dust devils are not limited to the Earth, and have been identified on Mars from orbit in both high and low resolution imaging data (e.g., Thomas and Gierasch, 1985;Malin and Edgett, 2001;Fisher et al, 2005;Cantor et al, 2006;Stanzel et al, 2008;Towner, 2009) and from the surface in both images and meteorology data (e.g., Ryan and Lucich, 1983;Metzger et al, 1999;Ferri et al, 2003;Greeley et al, 2006;Ellehoj et al, 2010). Martian dust devils might be responsible for the persistent dustiness of the martian atmosphere especially in the northern hemisphere summer when weather systems are generally weaker (Newman et al, 2002), as their ability to lift dust could be enhanced both by the local wind shear and their low-pressure cores which could act to 'suck-up' material as they move across the surface Balme and Hagermann, 2006).…”

Section: Introductionmentioning

confidence: 99%

Field measurements of horizontal forward motion velocities of terrestrial dust devils: Towards a proxy for ambient winds on Mars and Earth

Balme

Pathare

Metzger

et al. 2012

a b s t r a c tDust devils -convective vortices made visible by the dust and debris they entrain -are common in arid environments and have been observed on Earth and Mars. Martian dust devils have been identified both in images taken at the surface and in remote sensing observations from orbiting spacecraft. Observations from landing craft and orbiting instruments have allowed the dust devil translational forward motion (ground velocity) to be calculated, but it is unclear how these velocities relate to the local ambient wind conditions, for (i) only model wind speeds are generally available for Mars, and (ii) on Earth only anecdotal evidence exists that compares dust devil ground velocity with ambient wind velocity. If dust devil ground velocity can be reliably correlated to the ambient wind regime, observations of dust devils could provide a proxy for wind speed and direction measurements on Mars. Hence, dust devil ground velocities could be used to probe the circulation of the martian boundary layer and help constrain climate models or assess the safety of future landing sites.We present results from a field study of terrestrial dust devils performed in the southwest USA in which we measured dust devil horizontal velocity as a function of ambient wind velocity. We acquired stereo images of more than a 100 active dust devils and recorded multiple size and position measurements for each dust devil. We used these data to calculate dust devil translational velocity. The dust devils were within a study area bounded by 10 m high meteorology towers such that dust devil speed and direction could be correlated with the local ambient wind speed and direction measurements.Daily (10:00-16:00 local time) and 2-h averaged dust devil ground speeds correlate well with ambient wind speeds averaged over the same period. Unsurprisingly, individual measurements of dust devil ground speed match instantaneous measurements of ambient wind speed more poorly; a 20-min smoothing window applied to the ambient wind speed data improves the correlation. In general, dust devils travel 10-20% faster than ambient wind speed measured at 10 m height, suggesting that their ground speeds are representative of the boundary layer winds a few tens of meters above ground level. Dust devil ground motion direction closely matches the measured ambient wind direction.The link between ambient winds and dust devil ground velocity demonstrated here suggests that a similar one should apply on Mars. Determining the details of the martian relationship between dust devil ground velocity and ambient wind velocity might require new in situ or modelling studies but, if completed successfully, would provide a quantitative means of measuring wind velocities on Mars that would otherwise be impossible to obtain.