Dust Lifting Through Surface Albedo Changes at Jezero Crater, Mars

Abstract We have identified temporal variations in surface albedo at Jezero Crater using first-of-their-kind high-cadence in-situ measurements of reflected shortwave radiation during the first 350 sols (Martian days) of the Mars 2020 mission. Simultaneous MEDA measurements of pressure, radiative fluxes, winds and sky brightness indicate that these albedo changes are caused by encounters with dust devils (dusty convective vortices) under typical conditions and by a dust storm that affected Jezero (sols 313-318). Albedo changes caused by dust devils are more frequent, but their magnitude is smaller compared to that caused by the storm. Spectral reflectance measurements from Mastcam-Z images indicate that the decrease in albedo is mainly caused by dust removal. The intensity, size and trajectory of the convective vortex, as well as the availability and mobility of small particles at the surface, play a role in the ability of detecting albedo changes from MEDA measurements. These results show synergies between multiple instruments (MEDA, Mastcam-Z, Navcam and Supercam microphone) to improve our understanding of aeolian processes on Mars. 1. Introduction The Martian atmosphere interacts with the surface, redistributing dust and sand particles (Kahre et al., 2017). Dust particles are lifted by convective vortices and by strong wind gusts, and are subsequently transported before settling again (Newman et al., 2002a; Newman et al., 2002b; Basu et al., 2004; Kahre et al., 2006; Vicente-Retortillo et al., 2018). The redistribution of small particles modifies the surface albedo (Wells et al., 1984; Szwast et al., 2006; Reiss et al., 2010). Since February 2021, the environmental conditions at Jezero Crater have been monitored by the Mars Environmental Dynamics Analyzer (MEDA), the suite of meteorological sensors onboard the Mars 2020 Perseverance rover (Rodriguez-Manfredi et al., 2021; Farley et al., 2020). Simultaneous measurements from the Thermal and Infrared Sensor (TIRS), the Radiation and Dust Sensor (RDS), the Pressure Sensor and the Wind Sensor allow studying dust lifting at the Martian surface by analyzing an unprecedented set of magnitudes, including high frequency measurements (1 – 2 Hz) of surface broadband albedo and temperature, wind speed and direction, downwelling radiation, sky radiance and pressure. In addition, Mastcam-Z (Bell et al., 2021) and Navcam (Maki et al., 2020) images allow quantitative estimations of the spectral reflectance of the surface and provide additional visual context of the rover surroundings. Finally, recordings of the Supercam microphone (Maurice et al., 2021) provide additional information on winds during some periods for which MEDA measurements are not available. Surface albedo changes have been typically analyzed using satellite measurements (Cantor et al., 2006; Whelley and Greeley, 2008; Greeley et al., 2010; Reiss et al., 2016; Geissler et al., 2016; Wellington and Bell, 2020). These studies provide relevant information about the temporal and spatial variability of dust lifting processes. However, satellite observations generally lack of contemporaneous near-surface environmental measurements and have a low temporal coverage at a given location. Albedo has also been studied using images acquired by landed missions (Greeley et al., 2005; Bell III et al., 2008; Rice et al., 2018). Images provide a good context of the terrain, but due to their nature, they require more energy and storage availability than a single albedo value. Due to their limited temporal coverage and to the rover motion, these studies are typically focused on spatial variations in albedo. Here we use MEDA measurements to analyze for the first time environmental variables coincident (temporally and spatially) with the dust lifting event that leads to a change in surface albedo, providing insights into dust lifting mechanisms and effects on the surface, as well as ground-truth for orbital observations. Jezero Crater was affected by a dust storm between sols 313 and 318 of the mission (Ls ~155º), which caused a significant change in environmental conditions. The rover remained at the same location between sols 287 and 328, providing a unique opportunity to study the surface albedo and temperature before, during and after the storm. Section 2 describes the Mars 2020 instruments that provided the measurements for this work and the methodology to analyze the albedo changes. Sections 3 and 4 are dovoted to the surface albedo changes under typical conditions and during a dust storm, respectively. In Section 5, we provide an overall discussion of the variables involved in surface albedo changes. Finally, we highlight the main results of this work in Section 6.摘要 本研究借助火星2020任务前350个火星日（sols）期间同类首创的高时间分辨率原位反射短波辐射测量数据，识别出杰泽罗撞击坑地表反照率的时间变化特征。同步开展的火星环境动力学分析仪（Mars Environmental Dynamics Analyzer, MEDA）对气压、辐射通量、风速风向以及天空亮度的测量结果表明，这些反照率变化一方面源于典型工况下遭遇尘卷风（含尘对流涡旋），另一方面则由影响杰泽罗撞击坑的一场沙尘暴（第313至318个火星日）引发。尘卷风引发的反照率变化更为频繁，但相较于沙尘暴引发的变化，其幅度更小。桅杆相机-Z（Mastcam-Z）影像的光谱反射率测量结果显示，反照率降低主要源于沙尘被移除。对流涡旋的强度、尺度与运动轨迹，以及地表细小颗粒的可获取性与移动性，均会影响通过MEDA测量数据识别反照率变化的能力。本研究结果彰显了多仪器（MEDA、Mastcam-Z、导航相机Navcam与超级相机麦克风）协同观测的优势，有助于加深我们对火星风成过程的认知。1. 引言 火星大气与地表相互作用，对沙尘颗粒进行再分布（Kahre et al., 2017）。对流涡旋与强阵风会扬起沙尘颗粒，随后这些颗粒会被输送并再次沉降（Newman et al., 2002a; Newman et al., 2002b; Basu et al., 2004; Kahre et al., 2006; Vicente-Retortillo et al., 2018）。细小颗粒的再分布会改变地表反照率（Wells et al., 1984; Szwast et al., 2006; Reiss et al., 2010）。自2021年2月起，火星2020“毅力号”漫游车搭载的气象传感器套件——火星环境动力学分析仪（MEDA），便对杰泽罗撞击坑的环境条件开展了持续监测（Rodriguez-Manfredi et al., 2021; Farley et al., 2020）。热红外传感器（Thermal and Infrared Sensor, TIRS）、辐射与粉尘传感器（Radiation and Dust Sensor, RDS）、气压传感器以及风速风向传感器可同步获取测量数据，通过分析一系列前所未有的参数集——包括地表宽带反照率与温度、风速风向、下行辐射、天空辐射以及气压的1~2 Hz高频测量数据——来研究火星地表的沙尘扬起过程。此外，桅杆相机-Z（Mastcam-Z, Bell et al., 2021）与导航相机（Navcam, Maki et al., 2020）可获取地表影像，用于定量估算地表光谱反射率，并为漫游车周边环境提供可视化背景信息。最后，超级相机麦克风（Supercam microphone, Maurice et al., 2021）的录音可在部分MEDA测量缺失的时段，补充提供风速相关的辅助信息。此前，地表反照率变化的研究通常依赖卫星测量数据（Cantor et al., 2006; Whelley and Greeley, 2008; Greeley et al., 2010; Reiss et al., 2016; Geissler et al., 2016; Wellington and Bell, 2020）。这类研究可为沙尘扬起过程的时空变化特征提供重要参考。然而，卫星观测通常缺乏同步的近地表环境测量数据，且对特定地点的时间覆盖度较低。着陆巡视任务获取的影像也曾被用于反照率相关研究（Greeley et al., 2005; Bell III et al., 2008; Rice et al., 2018）。影像可提供良好的地形背景信息，但受限于其自身特性，相较于单一反照率数值，影像采集需要消耗更多的能量与存储资源。同时受限于时间覆盖度以及漫游车的移动特性，这类研究通常聚焦于反照率的空间变化。本研究首次借助MEDA测量数据，分析了与引发地表反照率变化的沙尘扬起事件在时间与空间上均匹配的环境变量，可为沙尘扬起机制及其对地表的影响提供新的认知视角，并为轨道观测提供地面验证数据。杰泽罗撞击坑在任务第313至318个火星日期间受沙尘暴影响（Ls≈155°），该事件导致当地环境条件发生显著变化。漫游车在第287至328个火星日期间停留于同一位置，为研究沙尘暴前后及期间的地表反照率与温度提供了独一无二的契机。第2节介绍了为本研究提供测量数据的火星2020任务仪器，以及分析反照率变化的研究方法。第3节与第4节分别阐述典型条件下与沙尘暴期间的地表反照率变化。第5节将综合讨论影响地表反照率变化的各类变量。最后，第6节总结本研究的主要成果。