Convective vortices and dust devils detected and characterized by Mars 2020

Abstract 33 We characterize the vortex and dust devil activity at Jezero from pressure and winds ob- 34 tained with the MEDA instrument on Mars 2020 over the first 415 sols of the mission 35 (Ls=6-213◦, Martian Year 36, MY36). Vortices are abundant (4.9 vortices per sol with 36 pressure drops >0.5 Pa when correcting from gaps in coverage) and peak at noon. One 37 in every 5 vortices carries dust detected with the RDS on MEDA, and more intense vor- 38 tices are more likely to be dust devils. Seasonal variability was small but dust devils were 39 abundant during a dust storm at Jezero (Ls=152-156◦). Vortices are more frequent and 40 intense over terrains with lower thermal inertia favoring a higher day-time thermal gra- 41 dient. We model pressure and winds to find the physical characteristics of dust devils. 42 Diameters range 5-135 m with a mean of 20 m. Three 100-m size events approached to 43 Perseverance closer than 30 m. From the close encounters we estimate a mean dust devil 44 activity of 2.0-4.0 dust devils km−2 sol−1. A comparison of MEDA observations with 45 a Large Eddy Simulation of Jezero at Ls=90◦ produces a similar estimation. We esti- 46 mate that large dust devils with diameters > 100 m have a density of 0.1 dust devils 47 km−2sol−1, which implies that dust lifting is dominated by the largest vortices in the 48 region. From our models we estimate that a few vortices in Jezero can have central pres- 49 sure drops of 9.0 Pa and horizontal wind speeds of 25 ms−1. Two dust devils partially 50 damaged the MEDA wind sensors and we detail their characteristics.1 Introduction 66 Dust Devils (DDs) are common on Mars and Earth (?). They constitute one of the 67 various phenomena developing in the Planetary Boundary Layer (PBL) of both plan- 68 ets, but they are much more common on Mars due to the more extended PBL created 69 by the lower atmospheric density. DDs constitute an important element of the Martian 70 atmospheric dust cycle (???). DDs can change the local albedo creating Dust Devil Tracks 71 (DDTs) (??). They can also produce cleaning events over solar panels (?) and produce 72 short intense bursts of wind as they pass over a specific location (?). Dust grains car- 73 ried by intense winds can represent a hazard to hardware in missions at the surface (?). 74 Thus, characterizing the dust devil activity at Jezero, the location of the Mars 2020 mis- 75 sion which is collecting the first samples of Mars to be brought to Earth (?), and under- 76 standing risks associated to DDs, is an important step towards the design of the Mars 77 Sample Return Mission. 78 Introduce MERS 79 Field data obtained by meteorological sensors can characterize the physical prop- 80 erties of convective vortices (?). Vortices can be identified in dips in the pressure record –2– manuscript submitted to JGR: Planets 81 and in sharp changes in wind intensity and direction (?). Vortices can also produce warmer 82 temperatures at their core, and the presence of dust can be investigated with photodi- 83 odes observing changes in the scattering of light caused by dust (??). The MEDA in- 84 strument on the Mars 2020 Perseverance rover carries sensors capable to simultaneously 85 obtain those measurements (?). MEDA measures air pressure and horizontal winds that 86 can be used to determine the physical properties of the vortices. MEDA counts with pho- 87 todiodes pointing at different directions in the Radiation and Dust Sensors (RDS), which 88 includes a panchromatic sensor pointing to the vertical (RDS Top 7) with a 90◦ Field 89 of View (?). On Earth and Mars the near surface temperature lapse rate is a key ele- 90 ment in determining the frequency, intensity, and the horizontal size and vertical exten- 91 sion of the vortices (???). MEDA measures the ground temperature and air tempera- 92 tures at different altitudes using a combination of thermocouples and infrared sensors 93 in the ATS and TIRS packages (???). Thus, multi-sensorial investigations of convective 94 vortices and DDs and the properties of the environment where they develop are possi- 95 ble with MEDA. 96 The Mars 2020 Perseverance rover landed in Mars in Jezero crater at 18.4◦N at Ls=6.2◦ 97 (North hemisphere Spring). Predictions before landing based on Mars atmospheric mod- 98 els suggested that Jezero is a location where intense vortices and frequent DDs form reg- 99 ularly peaking in activity at around Ls∼ 90 (?). MEDA data, as well as DD surveys and 100 movies obtained by the cameras on Perseverance resulted in abundant observations of 101 vortices and DDs in the first 215 sols of the mission (Ls=6.2-105◦) (?). The frequency 102 of vortices carrying dust, and the associated number of images of DDs was high and the 103 copiousness of DDs in Jezero, was one of the main conclusions of an early analysis of pres- 104 sure and RDS data from MEDA data covering the first 89 sols of the mission (?). 105 Images of DDs at Jezero have a stark contrast with the lack of DDs observed in 106 Elysium Planitia, where the Insight mission has observed thousands of vortices (???) with 107 no evident dust activity ?. Following arguments presented by ? of vortex abundance and 108 their detections by Insight being influenced by environment winds on Elysium Planitia, 109 ? concluded that the intrinsic activity of vortices at Elysium Planitia and Jezero are very 110 similar, but strongest winds at Elysium make vortices drift faster resulting in more de- 111 tections. Differences in the DD abundance must be related with surface properties, pos- 112 sibly including the availability at the surface of fine dust particles. 113 Here we extend over the vortex and DD results presented in ? and we analyze the 114 vortex and DD activity from MEDA data over the first 415 sols of the mission up to to 115 Ls=213◦ (North hemisphere Autumn). This allows to explore the effects on the vortex 116 and DD activity caused by seasonal variations and changes in the properties of the ter- 117 rain traversed by Perseverance. In addition, a regional dust storm covered Jezero on Ls=152- 118 156◦ (Mars 2020 sols 310-318), significantly affecting the local environment (?). 119 The multi-sensor capabilities of MEDA allow a characterization of the physical prop- 120 erties of vortices and dust devils on Jezero. Further insights can be gained through com- 121 parisons with models of vortices and simulations of the convective activity at Jezero. Here 122 we present such a characterization of vortices and DDs observed with MEDA. The struc- 123 ture of this paper is the following. We describe MEDA data in Section 2. We present 124 the global vortex and DD activity in Section 3. Section 4 describes the variability of this 125 activity, including seasonal evolution, variations associated to the dust storm, and ef- 126 fects caused over different terrains. Section 5 analyzes a subset DDs from fitting pres- 127 sure and winds to a model of a drifting vortex. In section 6 we explore a comparison of 128 our in situ data with a Large-Eddy Simulation of vortex activity at Jezero. We discuss 129 our findings on section 7, where we also incorporate a hazard analysis of dust devils at 130 Jezero. We present a summary of our conclusions in Section 8.