DOI: 10.48577/jpl.2lwkl8
title: Initial Results of the relative humidity observations by MEDA instrument onboard the Mars 2020 peseverance Rover

description: Abstract 26 Mars 2020 mission rover “Perseverance”, launched 30th Jul. 2020 by NASA, landed suc- 27 cessfully 18th Feb. 2021 at Jezero Crater, Mars (Lon. E 77.4509° Lat. N 18.4446°). Land- 28 ing season was at Mars solar longitude Ls = 5.2°, close to start of the northern spring. 29 Perseverance’s payload includes the relative humidity sensor MEDA-HS, which opera- 30 tions, performance, and the first observations from sol 80 to sol 410 (Ls 44°- 210°) of Per- 31 severance’s operations we describe. MEDA-HS output is reliable above 2% relative hu- 32 midity readings (referenced to sensor temperature), thus the sensor output is useful at 33 local night from late night hours to few tens of minutes after sunrise. Data delivered to 34 the Planetary Data System (PDS) includes also absolute humidity in volume mixing ra- 35 tio (VMR), VMR is calculated using the MEDA-PS pressure sensor values. According 36 to observations, nighttime absolute humidity follows a seasonal curve in which release 37 of water vapor from the northern cap with advancing northern spring and summer is vis- 38 ible. The sensor relative humidity output is transferred to air relative humidity values. 39 At ground level, frost conditions may have been reached a few times. Nocturnal volume 40 mixing ratio values show a declining trend suggesting adsorption of humidity into the 41 ground. Observations are compared with an adsorptive single column model, which com- 42 plies with observations. The model allows estimating daytime VMR levels. Short term 43 sub-hour time scales show large temporal fluctuations in humidity, which suggest ver- 44 tical and spatial advection. 45 Plain Language Summary 46 Mars 2020 mission rover ”Perseverance” landed successfully 18th Feb. 2021 at Jezero 47 Crater, Mars. The rover’s payload includes a versatile instrument suite called MEDA. 48 One of the MEDA instruments is a relative humidity sensor, which observations for the 49 first 410 martian days are described here. Sensor operations and accuracy estimates are 50 presented. Relative humidity together with MEDA pressure and air temperature obser- 51 vations allow calculating absolute water vapor content of air at the sensor level at night- 52 time. Humidity observations are also compared with models describing water vapor ad- 53 sorption and desorption into and out from soil. The results show how atmospheric hu- 54 midity at the rover’s site experience large subhour variability. Humidity observations help 55 to understand interchange of humidity between the soil and the atmosphere. Water is 56 mandatory for life such as in earth, thus understanding these water cycle processes bet- 57 ter are important for evaluating possibilities of past and current habitability of Mars. 58 Perseverance is also collecting samples which maybe returned to Earth one day. Knowl- 59 edge of the conditions at the times when samples were collected maybe useful. 60 1 Introduction 61 The Mars 2020 mission rover “Perseverance,” launched 30th July 2020 by NASA, 62 landed successfully 18th Feb. 2021 at Jezero Crater, Mars (Lon. E 77.4509°, Lat. N 18.4446°) 63 at solar longitude Ls = 5.2°. Jezero is a 45-km wide impact crater located in the Nili Fos- 64 sae region close to western edge of Isidis Planitia. It is thought that an ancient river flowed 65 into Jezero, forming a delta, flooding the crater and forming a lake, both of which have 66 dried out long ago (Mangold et al., 2021). 67 Perseverance’s science objectives include studying signatures of past habitability. 68 It will also collect and store a set of samples for possible recovery by a later mission. To 69 prepare for human exploration, environmental conditions are recorded by Perseverance’s 70 Mars Environmental Dynamics Analyzer (MEDA) instrument package, which is one of 71 the rover’s seven primary instruments. MEDA has a set of six sensors: Air Temperature 72 Sensor (ATS), Pressure Sensor (PS), Radiation and Dust Sensor (RDS), Relative Hu- 73 midity Sensor (HS), Thermal Infrared Sensor (TIRS) and Wind Sensor (WS) (Rodriguez- 74 Manfredi et al., 2021). In addition to MEDA’s importance to future human exploration, –2– manuscript submitted to JGR: Planets 75 MEDA can be used to address environmental scientific goals, including understanding 76 the near-surface atmosphere and its relationship to the surface over which the rover is 77 driving. In this paper, we focus on the first results of the MEDA Relative Humidity Sen- 78 sor. 79 The Relative Humidity Sensor (MEDA-HS) is based on capacitive polymer sensors 80 developed by Vaisala Oyj. During the daytime, the relative humidity drops close to 0%, 81 below the accuracy of the sensor. 82 This paper describes the observations by MEDA-HS of the first 410 Sols of oper- 83 ations of the Perseverance Rover, MEDA-HS operational cycles, and the limitations of 84 the sensor as well as some initial interpretations of those results. Section 2 describes the 85 background of the MEDA-HS and water vapor in Martian atmosphere. Section 3 gives 86 a description of the sensor, section 4 describes how the MEDA-HS has been operated 87 onboard Perseverance, section 5 presents an overview of the observations, and section 88 6 presents comparisons between a column water model and the observations. Conclu- 89 sions and discussion are in Section 7.