Effect of Incoming Solar Particle Radiations on The Exosphere of Mars
Kamsali Nagaraja, Praveen Kumar Basuvaraj, S. C. Chakravarty, Praveen Kumar Kuttanpillai
EEffect of Incoming Solar Particle Radiations on The
Exosphere of Mars
Kamsali Nagaraja a(cid:63) , Praveen Kumar Basuvaraj a , S. C. Chakravarty a , and PraveenKumar Kuttanpillai ba Department of Physics, Bangalore University, Bengaluru, India b Indian Space Research Organisation Headquarters, Bengaluru, India (cid:63)
Correspondence to : [email protected]
Abstract
Mars Exospheric Neutral Composition Analyzer (MENCA) of Mars Orbiter Mission (MOM)measures the in-situ neutral upper atmospheric constituents of Mars. Martian lower atmospherepredominated by the presence of CO which photo-dissociates into atomic oxygen ( O ) in higheraltitudes much near the exobase. Atomic O plays a significant role in invoking stronger presenceof O +2 in the Martian ionosphere. Primary photo-dissociative species CO , crossover its neutralabundance with atomic O in the collisonless hetergenous atmosphere with varying local solar con-ditions. Initial measurements from Neutral Gas and Ion Mass Spectrometer (NGIMS) instrumenton Mars Atmosphere and Volatile Evolution (MAVEN) estimated these crossover/transition al-titude wavering between ≈
225 km to 240 km during solar maximum conditions with peak solarilluminations. MENCA sampled the neutral atmospheric species, below the exobase upto peri-areion of ≈
160 km, under low solar active conditions during June 2018. Observations of partialpressures of CO and O in subsequent orbits reveals that solar inputs are crucial in quantifyingthese crossing points, where [ O ] / [ CO ] remain unity, alongside the influences from temperature.The multi-spacecraft measurements of the direct influences of solar wind charged particle fluxesand velocity on the daily variation of neutral thermospheric/exospheric compositions were ob-served on the local evening hours of Mars and presented. It marks the first-ever direct in-situ observation of interaction between the energetic solar particle radiations on Martian exosphericcompositions, potentially contibuting for the steady escape and differing population of atomic [ O ] in the exosphere. Keywords:
Mars, MOM, MAVEN, Solar Activity
The seasonal cycles of summer, autumn, winter and spring on Mars are similar to that of Earthdue to their near similar axial tilts (25 ◦ for Mars and 23.5 ◦ for Earth), but each season lasts foralmost double the time on Mars as compared to Earth because of the difference in their periodsof revolution around the Sun. Along with seasons, parameters such as solar extreme ultravioletfluxes, radiative and collisonal cooling, gravity waves, heliocentric distance, laitute, etc., would haveinfluenced the upper atmospheric processes on Mars (Bougher et al., 2015a). For several decades,the results on upper atmospheric composition and density of Mars were limited to the entry, descentand landing (EDL) observations carried out by the Viking Lander 1/2 that reached Martian surface1 a r X i v : . [ phy s i c s . s p ace - ph ] A ug n 20 July 1976 at 22.5 ◦ N, 48 ◦ W and 03 September 1976 at 48 ◦ N, 22 ◦ W respectively. These in-situ measurement of atmospheric profile below 200 km confirmed the presence of abundant CO supporting earlier Ultra-Violet Spectroscopic (UVS) observation using Mariner 6, 7 and 9 missions(Barth et al., 1971, 1972; Stewart et al., 1972). Alongside CO , the presence of N , Ar , and tracemeasurements of atomic and molecular oxygen ( O and O ), CO , N O , Kr , N e and Xe were reported(Nier and McElroy, 1976, 1977; Owen and Biemann, 1976; Owen et al., 1977). The complete set ofnear surface meteorological data obtained from Viking Landers (1976) to the Curiosity Rover (2012)has been analysed and results have been consolidated in terms of diurnal, seasonal and inter-annualvariations of meteorological parameters including dust storms over a span of more than 20 Martianyears (Martínez et al., 2017). Though, the Viking missions succesfully identified the major and minorspecies of Martian atmosphere to surface, it is to be noted that this observational data providespaucity on the study pertaining to seasonal and solar activity related variations of the Martianthermosphere and exosphere. A similar analysis to characterise the thermosphere/exosphere systemhas not been possible due to the lack of observational data on the atmospheric neutral/ionised gasconstituents covering the exospheric region (Bougher et al., 2015a).Indian Space Research Organisation (ISRO) has launched its first interplanetary mission to Mars viz. Mars Orbiter Mission (MOM) on 05 November 2013, reached Mars on 24 September 2014 duringMartian nighttime. MOM arrived at Mars with its initial primary objective concerning studies onplanet’s morphological features, detection of Methane ( CH ), estimating the ratio between Deuteriumand atomic Hydrogen ( D/H ), coverage of spatio-temporal profiles of various neutral atmosphericconstituents and cartographic events including dust storms, atmospheric clouds and so-on (Kumarand Chauhan, 2014). As of August 2020, the MOM spacecraft remains healthy in performing theextended mission objectives.Utilizing the opportunity that Mars approaches Earth closely, for every ≈ ≈
150 km), also investigated the role of solarwind plasma and habitability aspects of Mars (Jakosky et al., 2015). The ExoMars Trace Gas Orbiter(ExoMars TGO) mission launched on 14 March 2016, as an joint venture between the European SpaceAgency (ESA) and Roscosmos (Russian Space Agency), inserted intially into a periareion of ≈
400 kmexospheric orbit at an inclination of 74 ◦ , much similar to MAVEN’s orbit inclination, for observingbiologically relevant trace constituents such as Methane ( CH ) and potential organic gases (Olsenet al., 2017).Based on the observations using MENCA payload between December 2014 and May 2015, analysisto extract spatial and temporal distribution of the atmospheric composition of the thermosphere-exosphere region of Mars have been carried out by Nagaraja et al. (2020). The results obtainedfrom NGIMS-MAVEN data have provided valuable information about the spatial variation of thethermospheric neutral/ion constituents delineating their vertical and latitudinal distribution, andthe effect of solar zenith angle (Mahaffy et al., 2015a). MAVEN Deep Dip campaign to samplethe sub-solar collisional homosphere and magnetic field structure in April 2015 demonstrated theorbit-to-orbit variation of thermosphere and ionosphere of Mars (Bougher et al., 2015b). Directmeasurements of atmospheric neutrals from MENCA-MOM and NGIM-MAVEN has been used inour study to examine the influence of solar activity driven changes on Martian upper atmosphere.The results from both the spacecrafts agrees that the energetic particle fluxes deposited over theexosphere plays a major role in steering the daily variations on the exospheric compositions of Mars.2 Data Analysis and Methodology
Mars Orbiter Mission spacecraft inserted into Martian orbit on 24 September 2014, initially with aperiareion of ≈
400 km. MOM had a orbital period of ≈
72 hours due to its highly eccentric orbitinclined at 150 ◦ focusing observations over the Martian equator. Comet Siding Spring (C/2013 A1)had a closest approach by Mars on 19 October 2014, within a month of arrival of MOM and MAVENspacecrafts at Mars. Operations of all spacecrafts were ceased to protective mode to prevent fromthe meteor shower caused during the passage of the comet (Schneider et al., 2015). MOM occultedbehind Mars through orbital manoeuvres, later brought down its periareion altitudes to around262 km during December 2014, more feasible region to study the Martian exosphere.Figure 1: Mars Orbiter Mission’s periareion altitudes between October 2014 and October 2020, asprojected by the MOM team at Jet Propulsion Laboratory. The figure forecasts the periareionaltitudes reached by MOM immediate after orbital maneuvers due to the closest encounter of CometSiding Spring (C/2013 A1) by Mars on 19 October 2014 (Helfrich et al., 2015).Mars Exospheric Neutral Composition Analyser (MENCA), a dedicated atmospheric suite ofMOM, sampled exospheric neutrals near the exobase from December 2014 through May 2015 com-prising of 88 orbits. MENCA measures the total atmospheric pressure and partial pressures of variousatmospheric constituents covering the mass range from 1-300 amu , which is further programmableto limit its operation under mass-sweep and trend modes. MENCA consists of a built-in electronimpact ionizer operated at ≈
70 eV to ionize the atmospheric neutrals, a set of four quadrupole rodsand a detector assembly. The detector comprises of Faraday Cup and Channel Electron Multipliermeasures the partial pressures of gas constituents with a mass resolution of 1 amu . Part of postivelyionized atmospheric neutrals inside the ionization chamber known as the source grid region drags theions inside the quadrupole mass filter system. Another part of neutrals ionized outside the sourcegrid were collected by the Bayard-Alpert gauge, which is calibrated to measure the total pressure.The detailed description of the instrumentation, calibration factors, complete working mechanism,sensitivity and measurement limitations were given by Bhardwaj et al. (2016).3ndian Space Science Data Centre (ISSDC) at Bengaluru, India archives the MOM observa-tional data and disseminate to scientific users. This archived data consists of total pressure andpartial pressure values in the units of Torr with variable time resolution of ≈
12 to 30 s. The datais used for scientific studies after incorporating calibration, correction and normalization factors,and time tagging the processed data with ancillary information such as latitude, longitude, altitudeand solar zenith angle (SZA) corresponding to the orbital phase of MENCA-MOM using relevantSPICE/Ephemeris kernels (Nagaraja et al., 2020). The data sets are identified and arranged withrespect to different orbit numbers of MOM. However due to the oscillatory nature of periareion heightof MOM, good coverage of lower and crucial exospheric altitudes has been obtained only for a smallnumber of orbits (in spite of some orbit lowering excercises) during 5-13, June 2018.Figure 1 shows the periareion altitude forecast of MOM spacecraft from October 2014 throughOctober 2020. It is clear that after 2014-2015 there has been only a very short period during 2018-2019 when useful data could be collected closer to the exobase and above. The predicted minimumperiareion altitude reached by MOM will be 112.9 km on 08 July 2029 and can traverse above 100 km(Helfrich et al., 2015). The analysis was carried out for the additional MENCA data available duringJune 2018 and results obtaiend are discussed in detail.
The Mars Atmosphere and Volatile EvolutioN (MAVEN) mission inserted into a highly ellipticalorbit, focuses on the search for past history of Martian atmosphere and to understand its present cli-mate. The Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument of the MAVEN spacecraftstudies the structure and composition of the upper neutral atmosphere of planet Mars, measures iso-tope ratios, and measures thermal and supra-thermal ions (Mahaffy et al., 2015b). NGIMS measuresin a mass range of 2-150 amu . NGIMS science operation starts below 500 km to periapsis (inbound)and periapsis to 500 km (outbound) during each orbit lasting for ≈
600 s for each leg, with spatialresolution of 1 km. These observation covers both exosphere as well as thermosphere of Martianatmosphere that peers through the exobase at ≈
200 km, falls within NGIMS limits. The level-2(version 08 and revision 01) data-sets of NGIMS-MAVEN has been retrieved from MAVEN ScienceData Center at Laboratory for Atmosphere and Space Physics (LASP), during the period June 2018were used for this study. NGIMS-MAVEN observation has been been chosen from 03 June 2018through 15 June 2018 in parallel to MENCA-MOM observations.
Figure 2 shows the analysis of atomic H ion current data on 5 June 2018 covering the time period 10to 13 h UTC having both temporal and spatial variations. The time period of closest approach i.e. near periareion, of MOM towards Mars has been shown along with altitude coverage. The top panelshows altitude profile derived from SPICE kernels provided along with the observational data and ioncurrent of raw data. The middle panel shows the ion current of atomic H with background correctedat ≈
500 km. The bottom panel shows the calibrated partial pressure of atomic H for the actual timesampled values with background correction. The pressure values have been converted to Torr unitsby using the calibration procedure prescribed in Bhardwaj et al. (2016). The termainal panel plotclearly shows that the maximum pressure values occur much near to the periareion altitudes thoughit can vary from orbit-to-orbit. The figure also shows a small height interval near pariareion withincreasing pressure much above the noise levels of the instrument. This is the region of useful datato be used for further analysis.From the ion currents differentiated with amu values, the parial pressures of atmospheric con-stituents can be estimated for obtaining the time/altitude profiles. Since the traverse of spacecraftthrough the periareion is relaively of shorter time to collect useful data within same orbit, the effect4igure 2: Spatio-temporal variation of raw ion current and calibrated partial pressure of atomicHydrogen ( H ) observed from MENCA-MOM on 05 June 2018.of change in solar zenith angle may be insignificant. However, for long-term variabilty, the seasonalchanges in solar zenith angles need to be corrected for studying any effect of solar activity relatedvariations in exospheric partial pressures. -14 -13 -12 -11 -10 -9 -8 -14 -13 -12 -11 -10 -9 -8 -14 -13 -12 -11 -10 -9 -8 -14 -13 -12 -11 -10 -9 -8 -14 -13 -12 -11 -10 -9 -8 -14 -13 -12 -11 -10 -9 -8 Figure 3: Variation of atmospheric CO and O abundances observed on 05, 08 and 13 June 2018 byMENCA-MOM. Corresponding variation in local solar time (LST) in hours and solar zenith angle(SZA) in degrees has been shown. 5igure 3 shows details of the variation of atmospheric CO and O partial pressures near theperiareion region between ≈
160 km to 500 km, subtracting the background noise at 500 km. Thiscovers the Martian upper thermosphere, its exobase and lower exosphere regions for ascertainingrelative effects of gaseous transition and escape. While the pattern of variation of partial pressureis similar on 05 June 2018 and 08 June 2018, there is a shift of about few minutes in the peakdensity observed on 13 June 2018, where maximum of pressure has not occurred at the lowest heightof observation in both CO and atomic O . This anomaly needs to be examined by studying thecondition of atmospheric dynamics. It is also observed that the rate of change of pressure values forinbound and outbound trajectories are generally symmetric. It can be seen that the SZA variationis from the nighttime and moving towards the evening hours of Mars. This transition does not affectthe density profiles appreciably for short duration of observations. Time Period (UTC) -15 -14 -13 -12 -11 -10 -9 -8 P a r t i a l P r e ss u r e ( T o rr) A l t i t ude ( k m ) MENCA-MOM observations on 05 June 2018 (Periareion at 165.4 km) HH OH ON +COO ArCO Altitude
Figure 4: Relative abundance of major atmospheric species measured from MENCA-MOM on05 June 2018. Increased N + CO/amu has contributions from CO /amu . Variation of watervapour ( H O ) is mainly due to the outgassing of MOM spacecraft and eliminated during backgroundcorrection.Figure 4 shows the relative partial pressure of major atmospheric constituents. It is noted thatthe CO shows the maximum density values. According to the known pattern of variations, the otherconstituents have relatively lower pressures compared to CO . In addition, the water vapor density,product of outgassing, in MOM has reduced considerably compared to the values observed duringDecember 2014. However, the temporal profile of H O follows similar pattern of other species dueto reduction in further degassing. The escaping remanant water vapour may be maintained throughthe hydrostatic equilibrium.To undersand the day-to-day variations of the partial pressures of the exospheric constituents,the profiles of CO , Ar , O and N + CO are plotted for the three orbits of observations in Figure5. It is seen that during a short period of nine days, there is considerable reduction in pressureon 13 June 2018 compared to 05 June 2018 and 08 June 2018. Such short period changes of gasconcentrations in the thermosphere/exosphere need to be first confirmed before extending any pos-sible explanation. To strengthen MENCA observations and analysis, the simultaneous observationsfrom MAVEN on Martian thermosphere/exosphere were used. NGIMS-MAVEN atmospheric payloadmeasures the neutral/ions of upper atmosphere has been utilised to validate the observed variations.A detailed analysis was carried out using NGIMS-MAVEN data during 03-14, June 2018 to matchperiod of MENCA-MOM. There are about five orbits of MAVEN per Earth day with NGIMS datacovering the Martian atmospheric altitude between ≈
150 km and 500 km taking about 11-min for6 -14 -12 -10 -8 -14 -12 -10 -8 -14 -12 -10 -8 -14 -12 -10 -8 -14 -12 -10 -8 -14 -12 -10 -8 -14 -12 -10 -8 -14 -12 -10 -8 Figure 5: Inbound (top) and Outbound (bottom) altitude profiles of CO , O , CO + N and Ar on05, 08 and 13 June 2018 from MENCA of Mars Orbiter Mission.Figure 6: Vertical profiles of upper atmospheric neutral gas constituents for four days viz.,
03, 04, 08and 13 June 2018. The corresponding variations of SZA during the period of each profiles are alsoshown.the track/traverse. The daily average values of parameters are computed from the available dataduring the 24 h of each Earth day. Figure 6 shows the results of the altitude profiles of a few selectedMartian atmospheric constituents in the units of number density per cubic centimeter for four daysof June 2018.On 13 June 2018, differences in the density profiles can be seen compared to the profiles on 03,04 and 08 June 2018 that are similar. Particularly, the [ CO ] and [ O ] crossover point has shifted toa higher altitude of ≈
240 km on 13 June 2018 compared to ≈
190 km on other days. This indicatesa reduction in photodissociation driven concentration of atomic oxygen ( O ) on 13 June 2018. Such7igure 7: Vertical profiles of daily mean conentrations of CO and O densities on 03, 04, 08 and13 June 2018. [ CO ] steadily increase with decline in [ O ] during this period as observed fromNGIMS-MAVEN.Figure 8: Ion and electron fluxes signifying the solar-wind variations as measured from the SEPinstrument (forward and rear-view) of MAVEN during 03-13, June 2018. Steady reduction in theincoming solar flux, immediate after 05 June 2018, shows significant contribution to the decrease ofthe photodissociative product, atomic oxygen ( O ).reduction of atomic oxygen concentrations has been observed by MENCA also (see Figure 5). Theindependent results from two separate spacecrafts confirms the anomalies which could have resulted8rom the solar wind particle velocity and flux variations. Figure 7 shows the mean daily profiles of CO and O . There exists a striking relationship between the decrease in O and increase in CO densities during the period from 03 June 2018 to 13 June 2018.The Solar Energetic Particle (SEP) instrument onboard MAVEN measured the solar-wind electronand ion fluxes over various energy levels during the same period. The SEP sensors are positionedon two corners to ensure that the field of view (FOV) adequately cover the canonical Parker spiraldirection around which solar energetic particle distributions are normally centered. SEP providesmeasurements with ≈ CO . Further, the dissociation of CO under this condition has limited theprodution of O , and the same is reflected. The upper atmosphere of Mars has been observed from MENCA-MOM and NGIMS-MAVEN instru-ments during June 2018. Interesting, the solar wind particle fluxes show a steady decrease between09 June 2018 and 13 June 2018, providing the possible clues of charged particle interactions withMartian atmospheric constituents playing a major role in the day-to-day variation of O and CO concentrations. This study shows the increase in concentration of O and decrease in CO during03-08, June 2018 indicates an enhanced production of O from the dissociation of CO correspondingto higher energy and fluxes of solar wind particles. Hence, it marks the first-ever observation thatin absence of a global magnetic field of Mars, the direct interaction of solar wind charged parti-cles affect the daily variation of thermosphere/exosphere gaseous concentrations, contributing to thesteady escape of O with enhanced solar activity. Acknowledgement
This work was funded by the Indian Space Research Organisation (ISRO) under Mars Orbiter Mis-sion’s Announcement of Opportunity Program through the research project, Observation and Model-ing Studies of the Atmospheric Composition of Mars (OMAC), vide reference number ISRO:SPL:01.01.33/16.We greatly acknowledge the use of MENCA data from MOM, archived at Payload Operation Cen-ter, Space Physics Laboratory, Vikram Sarabhai Space Centre, Thiruvananthapuram, India. TheNGIMS and SEP datasets of MAVEN used for this study were publicly available on MAVEN Sci-ence Data Center at LASP (https://lasp.colorado.edu/maven/sdc/public/) as well as the PlanetaryData System (http://pds.nasa.gov). The MAVEN mission is supported by NASA through the MarsExploration Program. We thank AMDA Science Analysis System (http://amda.irap.omp.eu/) pro-vided by the Centre de Données de la Physique des Plasmas (CDPP) supported by CNRS, CNES,Observatoire de Paris and Université Paul Sabatier, Toulouse for performing initial MAVEN dataanalysis on NGIMS and SEP instruments. This research work was carried at Atmospheric and SpaceScience Research Lab, Department of Physics, Bangalore University, Bengaluru, India.
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