Strong increase in dust devil activity at Gale crater on the third year of the MSL mission and suppression during the 2018 Global Dust Storm

The Rover Environmental Monitoring Station (REMS) instrument in the Mars Science Laboratory (MSL) mission has collected atmospheric data from Gale crater for more than three Martian years. These data correspond to a path travelled by the Curiosity rover of 18 km in length with differences in altitude of 400 m through a variety of terrains in terms of the thermal inertia and nearby topography in the complex environment formed by Gale crater and Aeolis Mons. Pressure measurements obtained by REMS contain sudden drops on day-time hours caused by convective vortices passing nearby, which in some cases raise dust and become dust devils. Pressure measurements also contain pressure drops at night-time possibly caused by atmospheric turbulence and showing a strong seasonality. We present an analysis of these pressure drops for 2224 sols of the MSL mission extending previous results. The new data obtained over the third Martian year of the mission show a large increase in the frequency and intensity of vortices accompanied by a dramatic increase of dust devils that are simultaneously detected by lower values of UV radiation measured with REMS photodiodes. These dust devil events increased from one to two per Martian year in the two first Martian years of the MSL mission to 36 events in the third Martian year of the mission. We argue that this increase in dust devil activity is related to two different factors: (i) more intense vortices produced by a combination of the higher elevation of the terrain and lower thermal inertia of the environment favouring larger thermal differences between the ground and the air close to the terrain; (ii) a higher dust availability in the terrains traversed during the third Martian year of the MSL mission. An alternative explanation is that dust devils may need a minimum central pressure drop threshold to raise dust, and that a slightly enhanced convective activity after the second Martian year of the mission largely increased the dust devil events. Our observations suggest a minimum pressure drop of 3.5 Pa value for high dust loads in the terrain studied with the MSL mission. The new measurements also cover the period of activity of the Global Dust Storm of 2018. During the roughly 100 sols that the storm lasted, all dust devil activity ceased. Nocturnal pressure drops similar to daytime convective vortices continued to be a strongly seasonal effect and were not affected by the Global Dust Storm.