Seasonal variation of vortex and dust devil activity on Jezero and physical characterization of selected events

Abstract

<p>The Mars 2020 Perseverance rover landed in Mars in February 2021 in Jezero crater at 18.4ºN. One of its instruments is MEDA, the Mars Environmental Dynamics Analyzer, which measures among other properties air pressure, air temperature at different levels, surface temperature from its infrared emission, and the presence of dust. The latter is provided by a set of photodiodes pointing in different directions that constitute the Remote Dust Sensor or RDS. MEDA data are acquired with a frequency of 1 or 2 Hz in data sessions that cover about 50% of a full sol allowing a full characterization of daily and seasonal cycles.</p><p>Predictions before landing indicated that Jezero should be a location favoring the formation of intense vortices and dust devils in Spring to Summer. These expectations were fulfilled with frequent observations of vortices and dust devils observed with MEDA and the rover cameras. A systematic analysis of MEDA’s pressure sensor shows the close passage of convective vortices. These are detected as events that range from short and sharp pressure drops to long and deep pressure drops. Wind measurements during the vortex passage, combined with their duration, give information about the size and distance of the vortex. Many of the most intense events in terms of the pressure drop and peak winds detected have simultaneous drops of light measured with the RDS and are dust devils equivalent to those observed at much higher distances with Perseverance cameras. The combination of pressure, wind and RDS measurements largely constrain the geometry effects associated to these close passing dust devils. Some of them also have additional clear counterparts in other MEDA sensors including temperatures, which allows for an in-depth investigation of the physical properties of selected dust devils. Some events might also be captured by the SuperCam microphone, that records pressure fluctuations in the audible domain. The acoustic signal can provide insights into the short term behavior of vortices, and can contribute to the determination of the vortex physical properties. Statistics of vortices allow us to determine the probability of finding these events with the SuperCam microphone.</p><p>We present results for over one Earth year (Ls=6; Feb. 2021, Northern Hemisphere Spring – Ls=180; Feb. 2022; Northern Autumn Equinox). We show the daily cycle of vortex and dust devil activity and how this has evolved from early Spring until the start of the dust storms season. We present results of the distribution of sizes of vortices and dust devils and a selection of some remarkable events. These include direct hits of dust devils passing right through Perseverance, tangential passes in which one wall of the vortex passes over Perseverance, and more distant passages of very dusty events whose diameter in some cases largely exceed 100 m. A comparison of the vortex convective activity observed at Jezero with results from a Large-Eddy-Simulations (LES) using the MarsWRF model helps us to gain insight into how the detected vortices and their properties can constrain other general properties of the atmospheric dynamics at Jezero crater.</p>