Comparing the Dust Devil Vortex Populations on Mars and Earth

Abstract

  Convective vortices, and their particle-laden counterparts dust devils, are an important feature of the meteorology of both Mars and terrestrial desert areas. In addition to being interesting phenomena in their own right, they can cause occasional damage and even death.   The last 12 years has seen the generation of statistically-robust catalogs of vortex encounters from long-lived (>1000 Sol) landers and rovers equipped with meteorological instrumentation, namely MSL, InSight and Mars 2020.   Although previous landers (Phoenix and Pathfinder, lasting ~100 Sols) yielded catalogs that were enough to indicate useful analytic function descriptions of vortex population functions (e.g. power laws or exponentials of number versus measured pressure drop), the new generation of missions provide much more robust statistics, and also have had more extensive instrumentation, permitting the documentation of wind speed, dust loading and even seismic characteristics of vortex encounters. In the 2000s, the Mars statistics were in fact rather better than those available for the Earth, but the advent of inexpensive and low-power data logging systems with flash memory permitted the long-duration (months) acquisition of high-cadence (>1/second) pressure and other data required to detect small vortices in unattended field measurement campaigns. Inexpensive timelapse cameras have also permitted optical surveys of dust devils that are comparable with those from Mars landed and orbital missions. In many respects the populations of vortex events are surprisingly similar on the two worlds, when expressed as a normalized peak pressure drop (pressure drop divided by ambient pressure : this quantity is proportional to the peak wind speed at the wall of the vortex).  The cumulative rate of encounters typically varies as a power law with an exponent of about -2, and Martian rates are a factor of several higher than those on Earth.  Although the convective heating rates of the respective surfaces are somewhat different, a key difference is that the Martian Planetary Boundary Layer, which sets the upper limit on vortex size, is much deeper, and Martian dust devils are typically larger than Earth’s as a result.   I will review these population functions and their implications for meteorology, dust lifting and safety.