Irregular dust devil pressure drops on Earth and Mars: Effect of cycloidal tracks

In a survey of dust devil activity at a desert playa using continuous monitoring by a pressure logger, we have detected a number of pressure drops with complex structures: simple and symmetric drops make up only 25–30% of the total. In contrast to the simple, symmetric single-dip profiles expected for single-cell vertical vortices gliding past the pressure sensor, many profiles have an asymmetric shape, double dips, or ‘shoulders’ where a broad shallow dip is superposed on a narrow deeper one. A double dip in Mars Phoenix data was attributed in prior work to a near-simultaneous encounter with two dust devils, while laboratory experiments with two-cell vortices find a local peak in pressure at the center, also yielding a double dip in a transect profile. However, we suggest instead that a likely explanation for many complex pressure profiles measured in the field and on Mars is in fact the trochoidal path of a dust devil across the terrain, rather than the straight-line constant-speed path usually assumed. Images of the Martian surface show that many dust devil tracks have such a trochoidal or cycloidal path, which can be parametrically described. A model of the pressure profile driven by this parametric path description can reproduce observations.