The longevity and aspect ratio of dust devils: Effects on detection efficiencies and comparison of landed and orbital imaging at Mars

We summarize data from Earth and Mars on the duration of dust devils as a function of their diameter: a strong positive correlation is seen (duration in minutes appears to vary as ∼0.66d0.66, where d is the diameter in meters). This duration information allows orbital measurements at Mars (where only one observation per day is typical) to be compared with landed or terrestrial surveys with continuous monitoring. Counts from orbit appear higher than those from the surface, possibly due to higher detection efficiency from near-vertical orbital imaging where the dust loading integrated along the line-of-sight may be higher than for near-horizontal field/landed imaging due to the slender aspect ratio (∼5) of dust devils, indicated by both Earth and Mars data. The diameter dependence of duration results in an enhancement in the effective detection area for large devils for small-area surveys which do not discriminate between devils that form within their defined nominal survey areas, and those that are advected into it by winds.