Observation of CO2++ Dication in the Dayside Martian Upper Atmosphere

Earth and Planetary Physics RESEARCH ARTICLE 4: 396–402, 2020 PLANETARY SCIENCES doi: 10.26464/epp2020036

++ Observation of CO2 dication in the dayside Martian upper atmosphere

Hao Gu1, Jun Cui2,3,4*, DanDan Niu1, LongKang Dai2, JianPing Huang2, XiaoShu Wu3,4, YongQiang Hao5, and Yong Wei6,7 1State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau 999078, China; 2School of Atmospheric Sciences, Sun Yat-Sen University, Zhuhai Guangdong 519082, China; 3Key Laboratory of Lunar and Deep Space Exploration, Chinese Academy of Sciences, Beijing 100101, China; 4Chinese Academy of Sciences Center for Excellence in Comparative Planetology, Hefei 230026, China; 5School of Earth and Space Sciences, Beijing University, Beijing 100871, China; 6Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 7School of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China Key Points: ● ++ CO2 dication is detected in the Martian ionosphere for the first time ● Simplified photochemical model calculations are carried out and potential reasons for the data-model disagreement are discussed ● ++ The CO2 abundance in the dayside Martian upper atmosphere is more sensitive to solar irradiance at short wavelengths than the + CO2 abundance

++ Citation: Gu, H., Cui, J., Niu, D. D., Dai, L. K., Huang, J. P., Wu, X. S., Hao, Y. Q., and Wei, Y. (2020). Observation of CO2 dication in the dayside Martian upper atmosphere. Earth Planet. Phys., 4(4), 396–402. http://doi.org/10.26464/epp2020036

Abstract: Doubly charged positive ions (dications) are an important component of planetary ionospheres because of the large energy required for their formation. Observations of these ions are exceptionally difficult due to their low abundances; until now, only atomic dications have been detected. The Neutral Gas and Ion Mass Spectrometer (NGIMS) measurements made on board the recent Mars ++ Atmosphere and Volatile Evolution mission provide the first opportunity for decisive detection of molecular dications, CO2 in this case, ++ −3 in a planetary upper atmosphere. The NGIMS data reveal a dayside averaged CO2 distribution declining steadily from 5.6 cm at 160 −3 ++ km to below 1 cm above 200 km. The dominant CO2 production mechanisms are double photoionization of CO2 below 190 km and + ++ single photoionization of CO2 at higher altitudes; CO2 destruction is dominated by natural dissociation, but reactions with atmospheric CO2 and O become important below 160 km. Simplified photochemical model calculations are carried out and reasonably reproduce the data at low altitudes within a factor of 2 but underestimate the data at high altitudes by a factor of 4. Finally, we report a much stronger ++ + solar control of the CO2 density than of the CO2 density . Keywords: MAVEN; Martian ionosphere; dication

1. Introduction on singly charged positive ions (monocations). It has also been proposed that the dissociative recombination (DR) of molecular Despite their low abundances, the study of doubly charged posi- dications makes a non-negligible contribution to atmospheric es- tive ions (dications) has attracted considerable interest over the cape (Lilensten et al., 2013). past few decades, as science has sought a more complete under- standing of ionospheric composition on various Solar System The detection of dications in planetary upper atmospheres is diffi- bodies (Thissen et al., 2011). The large amounts of input energy cult (Thissen et al., 2011). O++ and S++ ions were the first to be required for their formation makes these ions particularly interes- measured, O++ being detected first in the topside terrestrial iono- ting, as does the observation that field-aligned transport of sphere by the mass spectrometer on board Explorer 31 (Hoffman, doubly charged ions is exceptionally efficient because they feel an 1967); since then, numerous satellite experiments have been ++ ambipolar electric field force twice as great as the force exerted made, providing mass spectrometric measurements of O in the ionospheres of the Earth (Taylor, 1973; Hoffman et al., 1974; Breig et al., 1977, 1982), Venus (Taylor et al., 1980; Ghosh et al., 1995), Correspondence to: J. Cui, [email protected] Received 28 MAR 2020; Accepted 15 APR 2020. and Mars (Benna et al., 2015; Gu H et al., 2020). The Galileo plasma Accepted article online 04 JUN 2020. analyzers identified O++ and S++ in Io’s ionosphere (Frank et al.,

Earth and Planetary Physics doi: 10.26464/epp2020036 397 al theoretical models predicted O++ densities in the upper atmo- 240 spheres of the Earth (e.g., Breig et al., 1977, 1982) and Venus (e.g., Fox and Victor, 1981). 220 Despite these efforts, ionospheric molecular dications, such as ++ ++ ++ 200 N2 , O2 and CO2 , have remained undetected until recently, though their presence in the ionosphere of Earth has been predicted for many decades (e.g., Simon et al., 2005) and more re- 180

cently in that of Mars (e.g., Witasse et al., 2002, 2003), of Titan (e.g., Altitude (km) Lilensten et al., 2005), and of Venus (e.g., Gronoff et al., 2007). 160 Witasse M6 Witasse V1 + In recent years, significant progress has been made to improve Without CO2 PI 140 + the resolution and sensitivity of mass spectrometers. Since the With CO2 PI MAVEN observations successful launch of the Mars Atmosphere and Volatile Evolution 120 10-3 10-2 10-1 100 101 (MAVEN) spacecraft in October 2014 (Jakosky et al., 2015), its Density (cm-3) Neutral Gas and Ion Mass Spectrometer (NGIMS) has been providing extensive measurements of the Martian ionospheric composi- ++ tion at altitudes from 120 km to 500 km (Mahaffy et al., 2015). We Figure 1. The dayside averaged CO2 density profile in the Martian show here that the NGIMS data offer the first opportunity of ob- upper atmosphere over the altitude range of 160−220 km based on ++ the MAVEN NGIMS measurements, along with standard deviations serving dications, CO2 in this case, in a planetary ionosphere within the predefined altitude bins. Model profiles from our (Section 2). Simplified model calculations are made to identify its calculations are superimposed, either with or without CO ++ dominant production and destruction processes (Section 3). Po- 2 production from single photoionization of CO +. For comparison, the tential reasons for disagreement between the data and our mod- 2 ++ model predictions of Witasse et al. (2003) appropriate for Viking 1 (V1) el are discussed (Section 4). We also compare CO2 observations and Mariner 6 (M6) conditions are also indicated. under low and high solar activities (Section 5). and 69 SFU (solar flux unit, 10−22 m−2·Hz−1); for M6 the respective ++ 2. Observations of CO2 Dication values are 1.43 AU, 57°, and 166 SFU. Witasse et al.’s calculations This study relies mainly on ion density data accumulated by the predicted a distinctive layer structure peaking at 145 km with dif- MAVEN NGIMS over three years and a half, up to May 2018. The ferent peak densities of 1.4 cm−3 (V1) and 4.5 cm−3 (M6). In prin- ++ NGIMS is a high-resolution mass spectrometer with a mass range ciple, the predicted CO2 peak could be compared with NGIMS of 2–150 Da and a mass resolution of 1 Da (Mahaffy et al., 2015). data from two dayside Deep Dip campaigns on 17−22 April 2015 The background level is channel dependent and is typically be- and on 16−23 October 2017, during which the periapsis was low 0.01 cm−3 (Benna et al., 2015). In this study, the mean count lowered to 120−130 km (Stone et al., 2018), but the DD sample is rate in channel M/Z = 22 above 300 km, which becomes inde- too limited to allow an unambiguous identification of the peak. pendent of altitude, is treated as the background level, and is sub- For this reason, throughout the rest of this paper we focus on the ++ ++ tracted to obtain two CO2 density profiles during each orbit, one CO2 distribution above 160 km. for inbound and the other one for outbound. The contribution from other ion species such as 22Ne++ is too tiny to be relevant Above the peak, Figure 1 indicates that the observed trend for M6 (Thissen et al., 2011). The NGIMS resolution does not allow reli- is consistent with the model prediction, despite that the meas- ++ ured density is on average 50% higher than the predicted density. able detection of some other important dications, such as O2 ++ ++ + The V1 model profile is clearly different from observations in both and N2 /CO , which have the same mass-to-charge ratios as O and N+, respectively. magnitude and trend; at 160 km the measured density is higher by a factor of four than the predicted density and the data-model During the nominal mission phase, the typical MAVEN periapsis is disagreement is exacerbated with increasing altitude. at ~160 km. To ensure near identical samplings at different alti- ++ A comparison of V6 and M1 model results suggests that the CO ++ tudes, we show in Figure 1 the dayside averaged CO2 distribu- 2 tion from 160 km up to 220 km obtained from a total of 831 orbits distribution in the Martian ionosphere depends critically on the with periapsis solar zenith angle (SZA) below 85°. The horizontal solar illumination condition and the background atmosphere. For ++ a more rigorous interpretation of the NGIMS CO ++ data, it is ne- error bars represent the standard deviations of the CO2 density 2 measurements within predefined altitude bins with a common cessary to repeat the calculations using appropriate model inputs, ++ width of 5 km. The NGIMS-derived CO2 density declines steadily which is the subject of the next section. with increasing altitude, from 5.6±1.5 cm−3 at 160 km to 0.7± 0.3 cm−3 at 220 km. ++ 3. A Simplified Photochemical Model of CO2 For comparison, the model predictions of Witasse et al. (2003) are According to laboratory measurements, the natural lifetime of the ++ 3 − superimposed in Figure 1, with the dashed and dash-dotted lines ground state of CO2 , denoted as X Σg, is ~ 4 s before it dissoci- ++ representing the CO2 profiles appropriate for Viking 1 (V1) and ates to two singly charged fragments (Mathur et al., 1995), while 1 for Mariner 6 (M6) conditions: for V1, the heliocentric distance, the natural lifetimes of the two excited states, denoted as A Δg 1 +

SZA, and 10.7 cm solar radio index at the Earth are 1.61 AU, 44°, and B Σg, are several microseconds (Slattery et al., 2005). Hence

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398 Earth and Planetary Physics doi: 10.26464/epp2020036

++ we consider only the ground state CO2 in our calculations. the dayside averaged NGIMS data. The electron density profile represents the best-fit to the dayside averaged LPW data with the Following Witasse et al. (2002, 2003), we assume that CO ++ is pro- 2 revised Chapman model of Cui J et al. (2015). The neutral temper- duced by the double photoionization (PI) and electron impact ature profiles from individual orbits are constructed by down- ionization (EI) of CO , and destroyed via three chemical channels 2 ward integrating the CO density profiles, assuming hydrostatic — reactions with CO and DR, and natural dissociation. The reac- 2 2 balance (Cui J et al., 2018), from which the dayside averaged tem- tion of CO ++ with atomic O is also taken into account following 2 perature profile is computed and fitted polynomially, as dis- Gronoff et al. (2007). Under the condition of photochemical equi- played. The MAVEN-based averaged ion and electron temperat- ++ + librium (PCE), the CO2 density, denoted as nCO2 , is favorably 2 ures, given by the dashed lines in Figure 2b, are empirically computed from Equation (1) of Witasse et al. (2003), written as modeled using the functional form of Ergun et al. (2015). It is well PI + EI PCO2 PCO2 known that the LPW electron temperatures at relatively low alti- + = nCO2 − , 2 + + + 1 tudes are seriously overestimated due to surface resistance or ca- nekdr nCO2 kCO2 nOkO τn pacitance on the LPW instrument sensor (Peterson et al., 2018). To PI is the ion production rate via double PI; EI is the ion pro- PCO2 PCO2 correct for this effect, we derive a modified electron temperature duction rate via double EI; is the thermal electron density; ne nCO2 profile using the same functional form but with the lower bound- is the CO2 number density; nO is the O number density; kdr is the ary temperature manually fixed to be close to the neutral temper- DR coefficient; is the rate coefficient for the reaction between kCO2 ature. To maintain the expected strong coupling between all con- ++ CO2 and CO2; kO is the rate coefficient for the reaction between stituents (Matta et al., 2014), a similar modification is applied also ++ ++ CO2 and O; τn is the CO2 natural lifetime. The relevant rate to the ion temperatures. Our calculations displayed in Figure 2 are coefficients are provided in Table 1 for reference. based on the modified electron and ion temperatures, as given by the solid lines in panel (b). Table 1. The chemical destruction channels of CO ++ and the 2 ++ The dayside averaged CO2 production rates via the double PI respective rate coefficients. (Here Te and Tn are the electron and neutral temperatures). and EI of CO2 are provided in Figure 2c, showing that the latter is only 10% of the former. These production rates are computed Reaction Rate coefficient (cm3·s–1) Reference with the aid of cross section data compiled by Masuoka (1994) ++ −7 1/2 CO2 +e kdr=6.2×10 (300/Te) Seiersen et al. (2003) and Itikawa (2002). The constancy of the EI–to–PI ratio is consistent with the optically thin (at EUV/X-ray) nature of the regions −10 1/2 ++ k =2.1×10 (Tn/300) CO2 +CO2 CO2 Franceschi et al. (2003) considered here. In Figure 2d, we show further the chemical de- ++ ++ −9 struction timescales of CO2 for different channels, revealing that CO2 +O kO=2.0×10 Gronoff et al. (2007) natural dissociation dominates at all altitudes except near the To improve data-model comparison, the model inputs are adap- lower boundary, where the reaction with CO2 becomes important. ted from realistic measurements made by several instruments on Also, according to our calculations, the reaction with O, which was board MAVEN: (1) The solar Extreme Ultraviolet (EUV) and X-ray not considered by Witasse et al. (2002, 2003), cannot be ignored flux is based on the level 3 solar spectral model at Mars, construc- at low altitudes. In addition, DR makes a minor contribution to ++ ted from the Flare Irradiance Spectral Model — Mars (Thiemann et CO2 destruction at all altitudes. The last fact implies that any un- al., 2017) and calibrated with Extreme Ultraviolet Monitor (EUVM) certainty in correcting for the low altitude LPW temperatures band irradiance data (Eparvier et al., 2015); (2) The energetic elec- should not be crucial. ++ tron flux from 3 eV to 4.6 keV is taken from the omnidirectional in- The dayside averaged CO2 density profile predicted by assum- tensity data accumulated by the Solar Wind Electron Analyzer ing PCE is given by the black solid line in Figure 1. When com- (Mitchell et al., 2016); (3) The neutral densities are obtained from pared to NGIMS observations, the model underestimates the ++ NGIMS level 2 data in the Closed Source Neutral mode (Mahaffy et CO2 densities by a factor of 2 at 160 km and by a factor of 35 at al., 2015); (4) The electron densities and temperatures are avail- 220 km. In the following section we address a number of considerable from the current-voltage characteristics measured by the ations that may explain disagreement between the model and the Langmuir Probe and Waves (LPW) (Andersson et al., 2015); (5) The empirical data. ion temperatures are based on the moment computation of the O + energy distribution from Suprathermal and Thermal Ion Com- 2 4. Potential Sources of Data-Model Disagreement position (STATIC) measurements (McFadden et al., 2015). Later in Various rate coefficients of CO ++ destruction have channel-specif- this section the ion temperatures are used to parameterize ion dif- 2 ic uncertainties of 20%–60% (Franceschi et al., 2003; Seiersen et fusion. al., 2003; Gronoff et al., 2007), and the PI and EI cross sections have

In Figures 2a and 2b, we show the altitude profiles of various para- uncertainties of 6%–30% (Masuoka, 1994). Above the CO2 double ++ meters used for calculating the CO2 production and destruction ionization threshold, the uncertainty in the solar EUV/X-ray flux is rates, based on MAVEN data accumulated during the same period 20% (Thiemann et al., 2017). In addition, Mahaffy et al. (2015) re- ++ of CO2 observations (see Figure 1). To minimize possible con- ported systematic uncertainties in neutral and ion densities to be tamination by heterogeneous chemistry on the NGIMS antecham- 20% and 25%, respectively, which cannot be removed by aver- ber walls (Stone et al., 2018), only inbound data are used. The dis- aging over many orbits. The above uncertainties can be com- ++ played neutral density profiles represent logarithmic fittings to bined to estimate the uncertainty in the predicted CO2 density,

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Earth and Planetary Physics doi: 10.26464/epp2020036 399

220 220

210 CO2 210 Tn O Ti + 5 200 CO2 ×10 200 Te e×105 190 190

180 180 Altitude (km) 170 Altitude (km) 170

160 160

150 150 (a) (b) 140 140 106 107 108 109 1010 1011 102 103 Density (cm-3) Temperature (K) 220 220 ++ CO2 PI CO2 +CO2 ++ 210 CO2 EI 210 CO2 +O + ++ CO2 PI CO2 +e 200 200 Dissociation Ion-neutral diff 190 190 Ion-ion diff

180 180 Altitude (km) 170 Altitude (km) 170

160 160

150 150 (c) (d) 140 140 10-4 10-3 10-2 10-1 100 101 100 101 102 103 104 105 ++ -3. -1 ++ CO2 production rate (cm s ) CO2 destruction timescale (s)

+ Figure 2. MAVEN-based altitude profiles of various parameters in the dayside averaged sense, including CO2, O, CO2 , and electron densities in ++ panel (a), neutral, ion, and electron temperatures in panel (b), CO2 production rates via the double PI and EI of CO2, as well as the single PI of + ++ ++ CO2 in panel (c), and in panel (d) various CO2 destruction timescales as indicated in the figure legend, along with the ion–neutral (CO2 –CO2) ++ + and ion–ion (CO2 –O2 ) diffusion timescales. In panel (b), the solid and dashed lines represent the empirical fittings to direct MAVEN measurements and the modified temperature profiles, respectively (see text for details). depending on the dominant production and destruction chan- between model results and observations. nels at any given altitude. Specifically, the uncertainty near the The above discussions imply that extra sources of CO ++ are re- lower boundary is ~40%, indicating that the predicted density de- 2 quired. One candidate is the single photoionization of CO ++, viates from the observed density by no more than 3σ. At high alti- 2 tudes, however, it is clear that the combined experimental uncer- which was not considered by Witasse et al. (2002, 2003). Laborat- tainty is insufficient to explain the model’s serious underestim- ory measurements of the relevant cross section cover the energy ates. range of 23–26 eV (Bizau et al., 2012), which is too narrow to be directly applicable to our calculations. Here for illustrative pur- The model calculations presented so far are made assuming PCE, + poses, we assume that the ratio of the single PI frequency of CO2 but at high altitudes diffusion is likely more important than pho- + ++ to the double PI frequency of CO2 is equal to the respective O to tochemistry (e.g. Fox, 2009). The CO2 diffusion timescales are O ratio. The altitude profiles of the CO + PI rate and CO ++ density presented in Figure 2d. The figure indicates that diffusion be- 2 2 ++ thus obtained are displayed in Figures 2c and 1, respectively. Our comes important only above 200 km. To evaluate CO2 diffusion properly, ion–neutral and ion–ion collisions should both be con- calculations indicate that this extra channel is important at high sidered (Matta et al., 2013). Note that the respective collision fre- altitudes. Near the upper boundary, it increases the predicted √ ++ quencies should be enhanced by a factor of 2 (ion–neutral) or 2 CO2 density under PCE by a factor of 9 and hence reduces the ++ + (ion–ion) for CO2 dication relative to CO2 monocation (Schunk model difference from the observed density to a factor of 4. Al- and Nagy, 2009). In addition, the effect of diffusion is manifest as a though the effect is expected to be small, inclusion of the single ++ + bulk CO2 outflow (e.g. Wu XS et al., 2019), which reduces top- ionization of CO2 by photoelectrons should improve further the ++ side CO2 densities and causes an even larger discrepancy data-model agreement.

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++ ++ 5. Solar Control of CO2 Dication This is to be distinguished from CO2 , which is destroyed mainly NGIMS measurements exhibit a clear solar cycle variation in the via natural dissociation at a constant frequency, independent of ++ CO2 distribution. This is displayed in Figure 3, comparing the solar inputs (Mathur et al., 1995). The above distinction is respons- ++ ++ + dayside averaged CO2 profiles under low and high solar activity ible for the stronger solar control of CO2 than of CO2 . conditions (defined as an integrated solar EUV/X-ray energy flux −2 −1 of less than or greater than 1.2 erg·cm ·s , based on the EUVM 6. Concluding Remarks solar spectral model up to 90 nm). These two cases include 464 The study of dications in planetary upper atmospheres has attrac- and 367 orbits, respectively. Figure 3 reveals a remarkable differ- ted extensive research interest over the past few decades. Histor- ence between the two cases, indicating a strong influence of sol- ically, observations have been made only of atomic dications such ++ ar EUV/X-ray radiation on CO2 abundance in the dayside Mar- as O++ and S++ (Simon et al., 2005; Thissen et al., 2011, and refer- tian upper atmosphere. According to Section 3, the density differences therein). The NGIMS instrument on board MAVEN has been ence is a function of the differences in both the solar irradiance providing extensive measurements of the ionospheric composi- and background atmosphere, except near the lower boundary tion on Mars (Benna et al., 2015), allowing the first ever detection where the difference in the background atmosphere drops out ++ of molecular dications, CO2 in this case, in a planetary upper at- ++ because there the rates of production and of destruction of CO2 mosphere. are both proportional to neutral density. ++ The NGIMS-measured CO2 density decreases steadily with altitude from 5.6 cm−3 at 160 km to below 1 cm−3 above 200 km. Us- 220 ++ CO high SA ++ 2 ing MAVEN-based model inputs, we calculate the CO2 density ++ 210 CO2 low SA profile under PCE. Our calculations suggest that the dominant CO+ high SA 2 ++ + CO2 production channel is the double PI of CO2 below 190 km 200 CO2 low SA + ++ and the single PI of CO2 at higher altitudes; CO2 is destroyed 190 primarily via natural dissociation over the entire altitude range of interest but the contributions from its reactions with CO2 and O

Altitude (km) 180 are important near the lower boundary. Meanwhile, ion diffusion ++ 170 becomes non-negligible above 200 km. The CO2 density predicted by our nominal model is lower than the NGIMS-measured 160 10-2 10-1 100 101 102 103 104 105 density by a factor of 2 at 160 km and by a factor of 4 at 220 km. Density (cm-3) Considering the combined experimental uncertainty of at least 40%, we conclude that our model reasonably reproduces the data ++ + Figure 3. A comparison of the dayside averaged CO2 and CO2 at low altitudes but modestly underestimates the data at high alti- density profiles between high and low solar activity conditions, tudes. A better data-model agreement could possibly be achieved defined as integrated solar EUV/X-ray energy flux at Mars up to 90 nm once improved knowledge of the relevant cross sections and reac- of less than or greater than 1.2 erg·cm-2·s-1. The two cases contain 464 tion coefficients becomes available (e.g. Bizau et al., 2012). and 367 orbits, respectively. Finally, our analysis reveals the expected strong solar control of ++ It is also instructive to compare monocations and dications in the CO2 distribution in the dayside Martian upper atmosphere, terms of their different responses to solar inputs. Figure 3 presents manifest as a factor of 25 density enhancement under high solar ++ activity relative to low solar activity. Such a density enhancement CO2 density profiles under high and low solar activity condi- + + is much greater (by a factor of 4 four) than that observed for CO2 , tions, indicating that from low to high solar activities, the CO2 monocation density increases on average by a factor of 4, but the indicating that the abundances of dications are more sensitive to ++ the solar irradiance at short wavelengths than are the abund- CO2 dication density increases by a factor of 25. Without show- ances of monocations. This is partly due to the larger energy re- ing the details, we mention that a similar difference is also seen quired for CO ++ formation and partly due to enhanced CO + de- between O++ and O+ (see also Gu H et al., 2020). These observa- 2 2 struction in response to enhanced thermospheric O abundances tions clearly demonstrate that the abundances of dications are at higher levels of solar activitiy. more sensitive to solar irradiance at short wavelengths than are the abundances of monocations. Acknowledgments ++ A more pronounced difference in CO2 density between differ- This work is supported by the B-type Strategic Priority Program ent solar inputs is related to an enhanced variability in the solar ir- No. XDB41000000 funded by the Chinese Academy of Sciences, radiance at short wavelengths combined with a larger energy re- and the pre-research projects on Civil Aerospace Technologies No. ++ + quired for CO2 formation (above 37.6 eV) as compared to CO2 D020105 and D020103 funded by China's National Space Admin- formation (above 13.8 eV) (Masuoka, 1994). Meanwhile, the dom- istration. The authors also acknowledge support from the Nation- + inant CO2 destruction channel is its reaction with atmospheric O, al Science Foundation of China through grants 41525015 and whose abundance is enhanced under high solar activities be- 41774186 to JC, 41904154 to WXS, and 41525016 to YW. The cause of enhanced photolysis of CO2 and CO (e.g., Withers et al., multi-instrument MAVEN data set used in this work is publicly + 2015). Hence enhanced CO2 production under high solar activit- available at the MAVEN Science Data Center (http://lasp. + ies is partially counterbalanced by enhanced CO2 destruction. colorado.edu/maven/sdc/public/).

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