Twilight mesospheric clouds in Jezero as observed by MEDA Radiation and Dust Sensor (RDS)

The Mars Environmental Dynamics Analyzer (MEDA) instrument, on board the NASA's Mars 2020 Perseverance rover, includes a number of sensors to characterize the Martian atmosphere. One of this sensors is the Radiation and Dust Sensor (RDS) that measures the solar irradiance at dierent wavelengths and geometries. We analyzed the RDS observations made during twilight for the period between sol 71 and 492 of the mission (Ls 39-262) to characterize the clouds over the Perseverance rover site. Using the ratio between the irradiance at zenith at 450 and 750 nm, we inferred that the main constituent of the detected high-altitude aerosol layers was ice from Ls= 39 to 150 (cloudy period), an dust from Ls 150-262. A total of 161 twilights were analyzed in the cloudy period using a radiative transfer code and we found: i) signatures of clouds/hazes in the signals in the 58 % of the twilights; ii) most of the clouds had altitudes between 40-50 km, suggesting water ice composition, and had particle sizes between 0.6 and 2 m; iii) the cloud activity at sunrise is slightly higher that at sunset, likely due to the dierences in temperature; iv) the time period with more cloud detections and with the greatest cloud opacities is during Ls 120-150; and v) a notable decrease in the cloud activity around the aphelion, along with lower cloud altitudes and opacities. This decrease in cloud activity indicates lower concentrations of water vapor or cloud condensation nuclei (dust) around this period in the Martian mesosphere.

搭载于美国国家航空航天局（NASA）火星2020任务“毅力号”（Perseverance）火星车上的火星环境动力学分析仪（Mars Environmental Dynamics Analyzer, MEDA），配备多类传感器用于表征火星大气特性。其中辐射与尘埃传感器（Radiation and Dust Sensor, RDS）可测量不同波长与观测几何下的太阳辐照度。本研究针对任务第71至492火星日（火星太阳经度Ls=39~262）期间的曙暮观测数据展开分析，以表征毅力号着陆点上空的云系特征。通过对比天顶方向450纳米与750纳米波段的辐照度比值，本研究推断：在Ls=39~150（多云期），探测到的高空气溶胶层主要成分为水冰；而在Ls=150~262时段，其主要成分为尘埃。本研究针对多云期内的161次曙暮事件，借助辐射传输代码开展分析并得到如下结论：i）58%的曙暮事件信号中存在云/霾特征；ii）多数云系的高度介于40至50千米，表明其成分为水冰，粒子尺寸处于0.6至2微米（原文疑似笔误，原单位应为微米）之间；iii）日出时段的云活动强度略高于日落时段，推测与温度差异相关；iv）云探测频次最高、云不透明度最大的时段为Ls=120~150区间；v）火星远日点附近时段云活动出现显著减弱，同时云高与云不透明度均有所降低。该云活动减弱现象表明，火星中层大气在该时段内的水汽浓度，或云凝结核（尘埃）浓度均有所降低。