Seasonal Variations in Atmospheric Composition as Measured in Gale Crater, Mars
The Sample Analysis at Mars (SAM) instrument onboard the Mars Science Laboratory Curiosity rover measures the chemical composition of major atmospheric species (CO2, N2, 40Ar, O2, and CO) through a dedicated atmospheric inlet. We report here measurements of volume mixing ratios in Gale Crater using the SAM quadrupole mass spectrometer, obtained over a period of nearly 5 years (3 Mars years) from landing. The observation period spans the northern summer of MY 31 and solar longitude (LS) of 175° through spring of MY 34, LS = 12°. This work expands upon prior reports of the mixing ratios measured by SAM QMS in the first 105 sols of the mission. The SAM QMS atmospheric measurements were taken periodically, with a cumulative coverage of four or five experiments per season on Mars. Major observations include the seasonal cycle of CO2, N2, and Ar, which lags approximately 20–40° of LS behind the pressure cycle driven by CO2 condensation and sublimation from the winter poles. This seasonal cycle indicates that transport occurs on faster timescales than mixing. The mixing ratio of O2 shows significant seasonal and interannual variability, suggesting an unknown atmospheric or surface process at work. The O2 measurements are compared to several parameters, including dust optical depth and trace CH4 measurements by Curiosity. We derive annual mean volume mixing ratios for the atmosphere in Gale Crater: CO2 = 0.951 (±0.003), N2 = 0.0259 (±0.0006), 40Ar = 0.0194 (±0.0004), O2 = 1.61 (±0.09) x 10-3, and CO = 5.8 (±0.8) x 10-4.
- First multiyear in situ measurements of the major components of the Mars atmosphere have been obtained by the MSL/SAM investigation
- Seasonal variation of CO2, N2, and Ar reveals differences in atmospheric transport and mixing timescales
- Oxygen varies seasonally and interannually, independently from Ar and N2, on timescales too fast to be explained by known chemistry
Plain Language Summary
The atmosphere of Mars is made up of primarily carbon dioxide, and during the Martian year, the barometric pressure is known to cycle up and down substantially as this carbon dioxide freezes out and then is rereleased from polar caps. The Mars Science Laboratory Curiosity rover has now acquired atmospheric composition measurements at the ground over multiple years, capturing the variations in the major gases over several seasonal cycles for the first time. With the Sample Analysis at Mars instrument, the annual average composition in Gale Crater was measured as 95.1% carbon dioxide, 2.59% nitrogen, 1.94% argon, 0.161% oxygen, and 0.058% carbon monoxide. However, the abundances of some of these gases were observed to vary up to 40% throughout the year due to the seasonal cycle. Nitrogen and argon follow the pressure changes but with a delay, indicating that transport of the atmosphere from pole to pole occurs on faster timescales than mixing of the components. Oxygen has been observed to show significant seasonal and year-to-year variability, suggesting an unknown atmospheric or surface process at work. These data can be used to better understand how the surface and atmosphere interact as we search for signs of habitability.
The present-day 25.2° axial tilt or obliquity of Mars is similar to the Earth's 23.5°, resulting in seasonal changes as on Earth. However, each season on Mars is nearly twice as long as on Earth due to Mars' longer orbital period of 687 days compared to the Earth's 365 days, and the more elliptical orbit of Mars strongly affects the seasonal variation as compared to Earth. The seasonal change in Mars' meteorological parameters has been monitored by a number of spacecraft, beginning with the Viking landers (VL1 and VL2) in 1976 and continuing to this day on the Curiosity rover. As a result, a long time record of the surface pressure and temperature has become available, largely from VL1 and VL2 from MY 12 to MY 15 at their respective landing sites of 22.4°N and 47.9°N, MER-A (Spirit) and MER-B (Opportunity) from MY 26 to MY 33 at 14.6°S and 1.9°S, and now Curiosity at Gale Crater from MY 31 to present at 4.6°S. Other environmental properties, including aerosol opacity, UV flux, relative humidity, and water vapor content, are also being measured on the Curiosity rover (Martínez et al., 2017). These parameters are valuable for interpreting the observations of seasonal, temporal, or sporadic changes in other atmospheric properties such as the abundances of atmospheric volatiles, which are the focus of this paper.
Prior to Curiosity, little information was available on long-term trends in the behavior of the atmospheric constituents. While the GCMS on the Viking landers measured the bulk volume mixing ratios (VMR) of the main atmospheric constituents CO2, N2, and 40Ar (95.3%, 2.7%, and 1.6%, respectively), O2 and CO had large uncertainties (Owen, 1992; Owen et al., 1977; Oyama & Berdahl, 1977). No useful information about their temporal or seasonal change could be derived from those data; however, on the other hand, the seasonal change in surface pressure measured by Viking was instrumental in understanding the unique annual cycle of condensation and sublimation of atmospheric carbon dioxide to and from the poles of Mars (Hess et al., 1980; James et al., 1992). As CO2 is the principal component of the atmosphere, and the other two gases N2 and Ar are not condensable at Martian temperatures and pressures, the observed change in the surface pressure can be attributed to seasonal change in the atmospheric CO2 content. Those data also revealed a time lag between the onset of CO2 deposition/sublimation and the resulting surface pressure change, which is related to the dynamics of CO2 migration to and from the poles.
Seasonal variation of the second most abundant gas on Mars, N2, could not be studied in situ or by remote sensing before Curiosity. However, the next most abundant constituent, radiogenic argon (40Ar, referred to generally as Ar), was measured over several years in situ by the APXS instrument on MER (VanBommel et al., 2018) and by remote sensing using the gamma subsystem of the gamma ray spectrometer on Mars Odyssey orbiter (Sprague et al., 2012). The MER data correspond to the equatorial region, where the VMR of Ar were found to have a relatively small seasonal variation of 10%. The Mars Odyssey data lacked the precision for such small changes in Ar in the equatorial region but revealed a dramatic change in Ar over the poles. The Ar mixing ratio was found to increase by a factor of 6 over the southern pole in winter and by a factor of 3 over the northern pole in winter. As argon is a noncondensable gas, its total atmospheric content remains unchanged through the Martian seasons. The observed seasonal variation in its mixing ratio is therefore due to the dynamical processes induced by the deposition and sublimation of the principal atmospheric constituent, CO2, at the poles.
Like N2 and Ar, O2 and CO are long-lived constituents on Mars, with lifetimes greater than the year on Mars (Atreya & Gu, 1995; Krasnopolsky, 1993; Wong et al., 2003). Besides aforementioned Viking measurements, only a few sporadic ground-based measurements of O2 were available pre-Curiosity. However, an extensive set of data on CO has been collected between MY 28 and MY 33 by CRISM on MRO (Smith et al., 2017). The CO VMRs from those orbital observations are found to be generally slightly larger than the values obtained from the Earth or from Mars Express observations for corresponding regions (e.g., Billebaud et al., 2009; Encrenaz et al., 2006; Krasnopolsky, 2015).
The SAM results discussed in this paper provide the first simultaneous measurements in the equatorial region of all key constituents. Including CO2, N2, Ar, O2, and CO, these measurements were made over nearly 6 years using the same mass spectrometer of the SAM instrument suite. Additionally, methane has been measured over the same period with the tunable laser spectrometer (TLS) of the SAM suite. Mars is found to have a persistent low background level of methane (CH4) with a mean value of 0.41 ± 0.16 ppbv, but it undergoes an unexpected seasonal variation of a factor of ~3 from 0.24 to 0.65 ppbv. The observed variation is unrelated to any known environmental factors, which can account for only about ±20% seasonal change in CH4. The magnitude of the seasonal change in the CH4 background is much greater than in other long-lived atmospheric volatiles (N2, Ar, O2, and CO) discussed in this paper. The TLS results on the methane background and occasional spikes have been published elsewhere (Webster et al., 2015; Webster et al., 2018).
2.1 SAM QMS Measurements
We report here the measurements of atmospheric volume mixing ratios on Mars using the SAM QMS onboard the Mars Science Laboratory Curiosity rover, taken in Gale Crater (4.5°S, 137°E) over a period of almost 3 Mars years (>5 Earth years) from landing (Vasavada et al., 2014). Although Gale Crater is located just south of the equator, in this paper, the seasons will be referenced with respect to the northern hemisphere. The observation period spans the northern summer of MY 31, solar longitude (LS) of 175°, and through northern spring of MY 34, LS = 85°. During the measurement timeframe, Curiosity traversed a distance of over 16 km in Gale Crater, spanning elevations from –20 to +329 m relative to the landing site (–4501 m). The SAM QMS atmospheric measurements were taken periodically during this time, interspersed between solid sample measurements and other rover activities, with a cumulative coverage of four or five experiments per season (Figure 1).
Atmospheric mixing ratios measured by the SAM QMS were reported for the first 105 sols of the mission (Mahaffy et al., 2013). These were recently updated to account for newly developed calibration factors following in situ calibration experiments using SAM's onboard calibration cell (Franz et al., 2017). Periodic sampling has continued throughout the mission to explore variations related to the seasonal CO2 cycle, as indicated by the annual pressure curves (Figure 1). The SAM atmospheric measurements taken through MY 34 LS 85° overlap seasonally with the first reported measurements in MY 31, testing whether there is a repeatable annual cycle in the atmospheric composition. With the exception of three daytime runs, atmospheric ingestions were taken near local midnight (Table 1).
|Mars year||Test ID (TID)||MSL sol||Ls (°)||Ingest start LMST||REMS air temp (°C)||REMS ground temp (°C)||REMS pressure (mbar)|
- a Daytime ingest.
Following the acquisition of the atmospheric data set presented in this manuscript, the SAM instrument completed its first full derivatization experiment on the Ogunquit Beach (OG) dune sample (Malespin et al., 2018). High signal levels in the wet chemistry experiment on sol 1909 (December, 2017) caused a shift in the sensitivity of the instrument detectors, requiring a change in the QMS electron multiplier gain setting. The gain state change restored QMS count rates to their previous levels, but detailed analysis is ongoing to ensure that deadtime corrections and calibration constants for all species are adjusted. Thus, this report is restricted in scope to atmospheric measurements prior to sol 1909.
2.1.1 Experimental Details and Calibration
The SAM instrument suite consists of three instruments supported by a gas separation and processing subsystem and a solid sample manipulation system (Mahaffy et al., 2012). Results presented here were obtained with the QMS through a dedicated heated (50 °C) atmospheric inlet. The QMS employs hyperbolic rods, redundant 70-eV electron beam energy ion sources, and redundant pulse counting Channeltron detectors. Separate miniaturized turbomolecular pumps (compression ratio ~5x108) evacuate the small QMS sensor volume and the much larger inlet manifold volume prior to Martian atmosphere ingestion. Experiments include two background scans: One with the QMS sensor volume isolated from the manifold and one with the QMS open to the evacuated manifold. The QMS is operated in a dynamic sampling mode with continuous pumping by one of the turbomolecular pumps. An atmospheric sample is acquired by opening a valve on the sample inlet tube for ~30 sec to introduce gas to a portion of the manifold. A small fraction of this gas is then leaked into the QMS and scanned over a specified mass range (1.5–149.9 Da) for several minutes. This process is repeated for most runs to provide two ingestions per experiment (Table 1). Detailed descriptions of the experimental design and flight instrument calibration for the atmospheric investigation can be found in Franz et al. (2014).
Direct atmospheric QMS data were acquired in both a fractional scan mode with 0.1-Da step size and a unit scan mode with 1.0-Da step size. A typical fractional scan spectrum acquired during an atmospheric experiment is shown in Figure 2. Fractional scan mode reduces uncertainties related to the tuning and shape of spectral peaks, while unit scan mode reduces uncertainties due to time variation in the signal (as both background and sample are pumped out). Repeated experiments have shown that sample and background signals are well characterized by exponential functions of time, so best results are determined through processing of data acquired in fractional scan mode (Franz et al., 2014; Franz et al., 2015). The method involves correcting mass spectra for detecting deadtime effects at high count rates and integrating peak areas (±0.4 Da) for the m/z values of the major atmospheric components, CO2, Ar, N2, O2, and CO.
Overall sources of error in the mixing ratio calculation include (i) measurement noise (detector noise following Poisson counting statistics), (ii) errors in the data corrections (detector deadtime, background subtractions, and isobaric interferences), and (iii) uncertainties in the calibration constants. Some of these errors can be reduced by averaging data points within individual experiments and by averaging multiple experiments (i.e., measurement noise and background subtractions), while other sources are systematic and are not reduced by averaging multiple experiments together (i.e., calibration constants and corrections for deadtime and isobaric interference). Discussions of these sources of error and their estimation are included in Franz et al. (2014) and in the supplemental material of Wong, Atreya, et al. (2013). For the major atmospheric gases with multiple ion fragments measured by the QMS, the m/z values used for calculation of mixing ratios were selected to have large enough count rates to minimize detector noise but low enough count rates to minimize saturation and deadtime effects. Although contributions of residual gas molecules to the background are minimal for most species, the background signal for the primary peak for O2 (m/z 32) is high, thus a large proportion of the total signal. This is due to O2 permeation from the high conductance valve seat after opening, and the background signal decreases exponentially once the QMS and manifold are actively evacuated. This background correction is therefore challenging and is a major source of uncertainty in the O2 mixing ratios measured by the SAM QMS (see Figure S3 of the supporting information).
Figure 3 shows a typical experiment timeline with backgrounds and atmospheric samples for the primary atmospheric species of interest. The approach used for the background correction is to fit an exponential signal as a function of time to the data within the background interval (specifically, the background scan with the QMS exposed to the evacuated manifold) and extrapolating to the sample interval to determine the time-dependent background signal. This correction is a small percentage of the total signal for four of the species under study and thus introduces a minor uncertainty to the final mixing ratio. For the O2 peak at m/z 32, the correction is a large proportion of the signal (≥50%) and varies between different experiments and different ingested samples within each experiment. To accurately capture the inherent uncertainty in this correction in the O2 mixing ratio, the 1-σ confidence intervals for the exponential fit at ta are included as part of the error estimation. Our uncertainty estimates include variations in the functional form of the background fit, in specific cases where the form of the fit significantly influenced the results.
As seen in Figure 3, the magnitude of the QMS signal for the atmospheric sample is also a time variable, as the sample trapped in the manifold is gradually pumped through the QMS. Ratios for the major ions of each gas are taken at each time point (i.e., from each mass spectrum) and then averaged to remove the time variability from the mixing ratio determinations. Resulting ratios are constant across a sampling interval. Mixing ratios are then computed at each time point and averaged for each ingestion. For experiments with two atmospheric ingests, the resulting mixing ratios from each ingestion are combined into a weighted mean for the sol, reducing systematic errors due to the background corrections. The values reported in Table S1 are these weighted means.
Finally, for CO2, the calculated uncertainties in the reported mixing ratios in this manuscript differ from those in the previous publications (Franz et al., 2014; Franz et al., 2015; Franz et al., 2017; Mahaffy et al., 2013). For this work, we are most interested in identifying seasonal trends and therefore relative behavior of the measured mixing ratios throughout the mission. For CO2 in particular, the errors previously reported are largely introduced by the uncertainty in the calibration constant derived during the prelaunch calibration. This constant introduces an uncertainty on the CO2 VMR on the order of 3%. This uncertainty would apply to the absolute amount of CO2 (such as a calculated partial pressure or number density) but is not optimal for characterizing the VMR. The uncertainty on the relative abundance of CO2 is best modeled by assuming that the total atmospheric composition must be equal to 1 and thus the uncertainty on the CO2 is equal to the total uncertainties on the trace gases (Ar, N2, O2, and CO, added in quadrature) that comprise the balance of the atmosphere.
2.2 Supporting Curiosity Measurements
2.2.1 Rover Environmental Monitoring Station
Throughout the mission, meteorological conditions along the traverse in Gale Crater have been measured by the Rover Environmental Monitoring Station (REMS) sensor suite (Gómez-Elvira et al., 2012). The REMS suite performs measurements of atmospheric pressure, ground and atmospheric temperatures, atmospheric relative humidity, UV radiation fluxes, and horizontal wind speeds (e.g., Martínez et al., 2017). To provide context for the SAM VMR measurements and the seasonal behavior of the major gases, we focus primarily on the analysis of REMS pressure (P) and atmospheric temperature (T) measurements. Comparisons to other environmental conditions, including those measured by REMS, are discussed below in section 4.2, with more detail in the supporting information Figure S7.
The pressure and temperature conditions at the time of sample ingestion for each QMS experiment are provided in Table 1. Conditions were calculated as described in Wong, LeFavor, et al. (2013). Uncertainties include estimated extrapolation uncertainties at times when simultaneous REMS measurements are not available, variation over the duration of the ingest event, and instrumental uncertainties. Updated instrumental uncertainty values correspond to the 9th PDS data release. Ingest start times are rounded to the nearest minute, in local mean solar time (LMST). As indicated in the table, the majority of the atmospheric samples was acquired in near midnight. Three experiments were conducted in the midafternoon to late afternoon, two of these in close proximity to nighttime experiments (within ten sols). Figure 4 shows the daily temperature and pressure curves for two of the sols in which the QMS sampled, 292 and 278, acquired during northern winter/southern summer. The vertical gray bars indicate the times of the QMS sample ingestions, showing a typical nighttime experiment and one of the daytime experiment. The search for possible diurnal trends was of interest in part because of the large diurnal pressure variations observed in Gale Crater, driven by a combination of thermal tides and topographical effects. Haberle et al. (2014) discussed this in their interpretation of the pressure cycles over the first 100 sols on Mars, concluding that in addition to the global thermal tides in the atmosphere from solar heating, the diurnal cycle of upslope/downslope flows driving crater circulation likely have a significant effect on the pressure amplitudes, with implications for the mixing of air in the bottom of the crater with air on the surrounding plateau [see also Rafkin et al., 2014; Tyler & Barnes, 2013]. The planetary boundary layer (PBL) in Gale Crater may be particularly suppressed in comparison with other locations on Mars, with significant impact on the observations of trace gases made by MSL/SAM (Moores et al., 2019; Newman et al., 2017; Rafkin et al., 2016).
2.2.2 ChemCam Passive Sky Observations
Although the MSL ChemCam spectrometer was designed primarily for laser-induced breakdown spectroscopy (LIBS) of Martian surface materials (Maurice et al., 2012; Wiens et al., 2012), it can also operate in “passive” mode to observe solar radiation scattered by the surface and atmosphere. The ChemCam passive mode was initially used only for reflectance spectroscopy of surface materials (Johnson et al., 2015). However, routine ChemCam passive sky spectroscopy started on sol 230 and McConnochie et al. (2017a) has used these observations to derive aerosol properties and water vapor column abundances using the instrument's visible and near-infrared (VNIR) spectral band. ChemCam also observes O2 absorption near 762 nm (McConnochie et al., 2017a), and preliminary results (McConnochie et al., 2017b) indicate that this can be used to derive quantitative O2 column abundances, pending further work to better characterize measurement uncertainties. Because the ChemCam passive sky data are taken at a higher frequency and lower precision than the SAM QMS mixing ratios, they may be able to provide a complementary measurement to track the seasonal behavior of molecular oxygen in and above Gale Crater. Future reports from the ChemCam passive sky measurements may prove valuable for the interpretation of the SAM O2 mixing ratios.
3 Results: In situ Volume Mixing Ratios
3.1 Carbon Dioxide (CO2)
Carbon dioxide is the primary component of the Martian atmosphere, with large seasonal variations in the global surface density due to the condensation and sublimation of CO2 in the polar regions during winter and spring, respectively. This was first observed by the Viking landers through global fluctuations of about 30% in surface pressure (e.g., Tillman et al., 1993). Curiosity pressure measurements to date have shown this cycle as well (Haberle et al., 2014; Harri et al., 2014). The Curiosity landing and science investigations commenced shortly after the annual pressure minimum, which corresponds with the formation of the larger southern polar cap (Figure 1). During the first 200 sols of the mission, the average surface pressure rose steadily to the annual maximum during northern fall, shortly after perihelion. The pressure then decreased to a local minimum near MSL sol 470 during the formation of the north polar cap, which begins to sublime again during the northern spring. The pressure was observed to decrease again as the southern seasonal polar cap was formed, repeating the annual cycle as observed in Gale Crater. SAM QMS measurements after MSL sol 680 have been timed to check for interannual repeatability and to achieve reasonable coverage over the pressure curve.
The SAM measurements are the first comprehensive compositional measurements of the atmosphere taken at intervals throughout Mars' CO2 cycle. Figure 5 shows the local VMR values determined through MSL sol 1869 (MY 34, LS 85°) as a function of solar longitude, with the daily average pressure included for comparison to the global cycle. Tabulated data are given in Table S1 and are publicly available (Trainer, 2019a, 2019b). Although there are large fluctuations in CO2 VMR driven by seasonal and diurnal cycles, CO2 is so dominant that the volume mixing ratio only varies by 1% about a computed average mixing ratio of 0.951 ± 0.003.
The small seasonal perturbation (~1%) in the measured VMR is related to, but does not strictly follow, the surface pressure cycle. The decrease of the relative amount of CO2 in the atmosphere shows a lag behind the decrease of the average surface pressure by roughly 20–40° of LS during the northern summer as the pressure approaches the annual minimum. The data are sparser during the northern winter, but a similar lag is also indicated as the pressure approaches the seasonal minimum. The hemisphere-to-hemisphere redistribution of mass that occurs during these periods of cap sublimation and condensation pulls atmospheric components toward the winter pole. Transport acts very rapidly to maintain pressure equilibrium, but changes in VMRs lag behind because physical mixing of air masses is a slower process.
The details of the pressure fit, this calculation, and the derived Fp values are provided in Figure S2.
Similarly, in the (unrealistic) fast-mixing case, PCO2(t) and Patm(t) should be linearly related. This scenario is plotted as VMR versus Patm in Figure 6, where the expected relationship is represented by the black curve, and measured values are shown with symbols. The expected value for the CO2 mixing ratio was calculated by subtracting ∑Pi = 0.41 mbar from the daily mean pressure at the same LS of the measurement and dividing the Patm(t) to get the mixing ratio (equation (4)). The SAM CO2 VMR data show a deviation from this relation, in particular the seasons containing the pressure minima. (N. summer and winter) show consistent enhancements above the fast-mixing relation. In the periods during which the seasonal caps are subliming, the measured CO2 mixing ratio is more likely to match or run below the fast-mixing relation, as can also be seen in Figure 5. By comparing the actual data with the fast-mixing relation in Figure 6, we calculate a correlation coefficient of R2 = 0.36. This poor correlation indicates that the fast-mixing model does not adequately match the observations.
The seasonal fluctuations of the atmospheric pressure, and the influences of dynamics on the atmospheric composition at this location on Mars, are more easily identified and understood by studying the behavior of the trace, noncondensable components, which are more sensitive to the changes in global pressure. These are discussed in the next section.
3.2 Noncondensable Atmospheric Components
The mixing ratios of argon (40Ar), nitrogen (N2), and oxygen (O2) are shown in Figure 7, and carbon monoxide (CO) is shown in Figure 8. These next four most abundant species are not condensable at Mars surface and atmospheric temperatures and pressures and thus are not expected to deposit or sublime from the polar caps as does CO2. However, there are seasonal trends in the VMR of these molecules that are a response to mixing of air masses during the seasonal CO2 cycle and unexplained additional processes affecting O2 and CO.
For reference, Table 2 provides instantaneous volume mixing ratios for these gases measured by MSL/SAM at approximately the same time of the Mars year as the Viking landers. The previous in situ measurements were not operational for the full Mars seasonal cycle.
|Viking GCMSa||MSL SAM||Viking GEXb||MSL SAM|
|Ls (°)||100||108||121 – 142||134,141c|
|CO2||95.32%||95.2% ± 0.1%||96.2% ± 5%||95.0% ± 0.2%|
|N2||2.7% ± 0.5%||2.55% ± 0.06%||2.3% ± 0.3%||2.70% ± 0.11%|
|40Ar||1.6% ± 0.3%||1.95% ± 0.03%||1.5% ± 0.3%||2.03% ± 0.06%|
|O2||1300 ± 260 ppmv||1940 ± 40 ppmv||<1500 ppmv||1950 ± 290 ppmvd|
- a Owen et al., 1977 (±20% uncertainty on values were prefinal calibration estimates, as stated in their text, and O2 might be based on earlier ground-based observations (see discussion in section 4.2 here)).
- b Oyama & Berdahl, 1977.
- c Average of two measurements taken at these Ls values. Local “instantaneous” mixing ratio is increasing during this time.
- d Oxygen exhibits large interannual variability in the MSL/SAM data set, reflected in the uncertainty on this average.
3.2.1 Argon and Nitrogen
Argon and nitrogen serve as excellent tracers of global transport, since they are chemically inert in addition to remain solely in the gas phase. Figure 7b shows that, as expected, these two species track each other consistently through the Mars year, with good year-to-year repeatability. Instantaneous composition is plotted (i.e., VMR as opposed to VMR'). Pressure maxima occur near LS 60° and 250°, after the seasonal northern and southern polar caps (respectively) have sublimated. Minima in Ar and N2 VMRs lag behind these pressure maxima by ~30° LS, suggesting a slower mixing timescale (although the timing of the northern winter minima in Ar and N2 VMRs are poorly constrained by the SAM QMS data, we assume the minimum is at LS 280°). The lag between pressure minima and VMR maxima (~15° LS) is much smaller than the lag between pressure maxima and VMR minima (~30° LS), but note that VMR' is better suited to compare global transport and mixing timescales. The VMR data are provided in Figure 11 with a detailed discussion in section 4.1.
The four right-most points in Figure 7b for both Ar and N2 show the time period in MY 31 during which daytime and nighttime measurements were taken within close proximity. From left to right, this group of points represents: day–night–night–day (Table 1). Although there appears to be a change in the mixing ratio among these points, there does not appear an obvious correlation with time of day.
N2 and 40Ar have mean mixing ratios of approximately 2.6% and 1.9%, respectively, with a seasonal variation of ±10% of these values throughout the year (Table 3). Note that the temporal average reported here is affected by limited number of samples (Figure 1); we use a strict numerical average without regard to temporal coverage. The nitrogen mixing ratio has been increased substantially (Franz et al., 2017) from the originally published value in Mahaffy et al. (2013) and is now consistent with the Viking values within the uncertainty of those previous measurements. A detailed discussion of the updated calibration constants for the SAM QMS and the adjusted mixing ratios for the first sols of the SAM QMS measurements is given in Franz et al. (2017), which included corrections based on a calibration cell experiment on Mars.
|Atmospheric component||Annual mean mixing ratio||Uncertainty on meanb||Seasonal variation from mean||Approximate measurement error c|
|O2||0.00161||±9 × 10-5||13%||18%|
|CO d||0.00058||±8 × 10-5||36%||6.1%|
- a Corrected for annual mean pressure (× P/P0; using equation (5)), except for CO2. P is the annual mean pressure in Gale Crater, 8.46 mbar.
- b Uncertainty on the annual mean mixing ratio is reported as 2× the standard deviation of the mean.
- c Typical uncertainty on the individual instantaneous measurements for each atmospheric component.
- d Reflects measurements from the first 830 sol only; see discussion in section 3.2.3. This likely skews the mean value toward northern winter (36% of measurements; Figure 8).
The 40Ar/14N ratio of the Mars atmosphere, in combination with 14N/15N, has been used as a diagnostic tool for verifying the inclusion of trapped atmosphere in Martian meteorites, as it has a unique signature as compared to Earth (Becker & Pepin, 1984). This has been discussed previously in the context of the SAM 14N/15N measurements (Wong, Atreya, et al., 2013), but the value has been updated with the new SAM calibration (Franz et al., 2017). The 40Ar/14N ratio also serves as a useful metric by which to evaluate the robustness of the mixing ratio measurements of these trace gases over time. The relative abundance of these two gases should remain constant, since neither is expected to react or condense, and their similar abundances ensure near-identical transport during the seasonal cycle on Mars. The 40Ar/14N measured by SAM throughout the mission is shown in Figure 9 as a function of MSL sol and indeed shows multiyear consistency within measurement uncertainty, with an average value of 0.376 ± 0.008.
The measured mixing ratio of O2 has varied from approximately 1,300 to 2,200 ppmv during MSL's first 1,900 sols at Gale Crater (Figure 7b). Two unexpected features of the seasonal behavior of O2 are immediately apparent when comparing to the other major inert species (Figure 7b). First, O2 does not follow the same general pattern as the Ar and N2, particularly through the beginning of the year. Second, the O2 mixing ratio shows substantial interannual variability. Both of these features are surprising, because the chemical lifetime of Martian atmospheric O2 is estimated to be ≥10 Earth years (Krasnopolsky, 2017). Like Ar and N2, O2 does not condense under Mars atmospheric conditions, so the O2/40Ar ratio is expected to be constant, as for the Ar and N2. Despite the larger uncertainties on the derived O2 mixing ratios (section 2.1.1), two prominent seasonal features are apparent in the O2 VMR data (Figure 7b) and the O2/40Ar ratio (Figure 10): There is a gradual northern spring/summer increase in O2, followed by a potentially rapid reset to a constant level over much of northern summer and fall (LS 160°–315°).
The spring/summer increase in O2 can only be characterized in a broad sense, due to the large error bars and coarse sampling of the time series. If we make the simplest assumption, a linear change in VMR as a function of LS in the LS 0°–150° period, there is almost a factor of 3 variation in rate of change in MY 32, MY 33, and MY 34 (from about 1.3 to 3.6 ppm/°LS; Figure 7b). The VMR values plotted in the figure include the effects of both seasonal CO2 condensation/sublimation and global mixing. By instead considering the O2/40Ar ratio (Figure 10), we can eliminate all changes due to condensation/sublimation and global mixing, and it becomes more reasonable to apply a constant rate of change to all 3 years of O2 observations. Any remaining changes in the O2/40Ar ratio indicate other factors controlling the local mixing ratio of oxygen in Gale Crater besides the large-scale global dynamics controlling transport during the seasonal cycle. We will discuss this result in more detail below in section 4.2. A significant change is clear over the LS 0°–150° period, with a consistent rate of 0.012%–0.015% /°LS. The exact onset and end of the O2 increase season are not well defined, and the data are not finely sampled enough to determine whether the increase is truly linear (or whether there is variation year to year).
The second O2 seasonal feature is that O2/40Ar mixing ratios (Figure 10) are more or less constant and identical in all Mars years, over the period of LS 160°–315°. During this part of northern summer and fall, the O2/40Ar mixing ratio is identical to the annual mean of 0.083 (within uncertainties). It is important to note that the sparse temporal sampling of the data is not sufficient to precisely characterize the bounds of this constant period. However, MY 33 data on LS 141° and 161° show a rapid decrease of 20%–25% in O2/40Ar. This is a remarkably rapid change, potentially giving insight into the processes modifying atmospheric O2 abundances, though we note that the rapid decrease was only observed during MY 33.
Finally, interannual variation is clearly apparent from offsets between measurements in the spring/summer season of increasing O2/40Ar mixing ratio. Full characterization of the interannual variation is not possible given the sparse temporal sampling, but the onset of the variable period seems to be sometime later than LS 270° and sometime earlier than LS 30°.
We investigated the possibility of instrument effects or other measurement artifacts affecting the retrieval of the O2 mixing ratio and thus contributing to the apparent variability. As noted in section 2.1.1, the O2 mixing ratio is computed based on the signal at m/z 32, which has both a high background signal and a contribution from the fragmentation of CO2+. The backgrounds and corrections have been tracked through the course of the mission, and we have found no correlation with the increases in both absolute and relative O2 mixing ratios. Further, the O2 measurements were checked against the solid sample analyses to determine whether there could be contamination from a pyrolysis experiment. There was no unique correspondence between solid samples with O2 release and large increases in the atmospheric O2 mixing ratio or background measurement (Figure S4).
3.2.3 Carbon Monoxide
Carbon monoxide (CO) has been detected and is quantified as reported in Franz et al. (2017, 2015). As discussed in those publications and in Mahaffy et al. (2013), the quantification of CO relies on a marginal detection above the dominant CO2 component, which generates CO+ and other interfering fragments in the QMS. Even when the CO abundance was measured at a relatively high value (i.e., near LS 180°, TID 25012), the signal at m/z 12 is estimated to be comprised of 15% directly ionized CO and 85% from CO2 fragmenting into CO+. The derived CO mixing ratios for the first part of the mission are given in Figure 8. It can be seen that these species approximately track the seasonal trend in argon, behaving much like a passive tracer species through the atmosphere. This is generally what is assumed in photochemical models, although there has been some observational variability that has challenged this assumption [see discussion in Krasnopolsky, 2015]. Smith et al. (2017) recently published CRISM column observations of CO for MY 28 through MY 33, therefore overlapping in the time period of the reported MSL measurements. The CO derived from CRISM data for the latitude band at 0°–20°S shows a similar trend, with a minimum at LS 90° and a subsequent rise to a peak value just before LS 180°. The absolute value of the mixing ratio measured by SAM ranges from 400 ppm to 800 ppm during this period, which is slightly lower than the 700–1000 ppm reported by CRISM for this region on Mars. Other orbital and ground-based measurements also report greater CO mixing ratios than the in situ values (Billebaud et al., 2009; Encrenaz et al., 2006; Hartogh, Błęcka, et al., 2010; Krasnopolsky, 2003, 2015; Sindoni et al., 2011; Smith et al., 2009). There are several possible explanations for any discrepancy between the surface and the orbital and ground-based observations, including the difference in spatial sampling, the effect of any local depletions in Gale Crater, errors in the SAM CO measurement, and the distinction between a point and column-integrated measurement.
Although we have measured the CO in each atmospheric experiment through MSL sol 1,711, measurements after sol 1,000 show significantly elevated signal at m/z 12 and therefore very high CO mixing ratios. The VMR for CO obtained in MY 32, near LS 250° (MSL sol 830, TID 25232, Tables 1 and S1) shows the onset of this trend, in which the CO begins to diverge from the repeated seasonal trend in Ar (Figure 8). In addition to the derived CO mixing ratio, more than doubling from MY 32 to MY 33, the elevated measurements have not decreased or shown any seasonal modulation in MY 33 and MY 34, in contrast to the O2 measurement. This behavior is suspect, and at this time, a possible contamination or instrument effect cannot be ruled out. Especially because we know the m/z 12 signal to be highly sensitive to such effects (Franz et al., 2015), we are cautiously omitting the questionable observations from this paper. Those CO measurements thus require further investigation and will be reported at a later time.
4.1 Seasonal Transport
Argon and nitrogen in Mars atmosphere have extremely long lifetimes (~Gyr) against losses from photochemistry, sputtering, and escape, and they do not condense under any of the conditions reached during the current seasonal cycle. Condensation points at Mars ambient pressures are ~63 and 53 K for Ar and N2, respectively. Thus, these gases serve as excellent tracers of the complicated dynamics induced by the cycling of CO2 into and out of the polar caps on yearly timescales. Measurements of Ar from orbit using gamma subsystem (GS) of the gamma ray spectrometer (GRS) on the Mars Odyssey spacecraft first identified a “freeze distillation” effect, in which Ar (and other noncondensables) are carried to the polar regions by advection of the bulk atmosphere during autumn and winter when CO2 freezes out on the polar cap (Sprague et al., 2007). The GRS measurements provide a column-averaged mean mixing ratio (mmr) of Ar relative to the total atmospheric gas and show an enrichment of up to 6 times at the southern winter pole and 3 times at the northern winter pole (Lian et al., 2012; Sprague et al., 2012). However, the orbital measurements of Ar at the lower latitudes have too much scatter to detect what could be seasonal variations of a smaller magnitude (Sprague et al., 2012).
Recently published work from the alpha particle X-ray spectrometer (APXS) on the Mars Exploration Rover Opportunity (MER-B) has reported a relative atmospheric argon density over 6 years of observation from ~2°S, a similar latitude as MSL (VanBommel et al., 2018). The MER-B measurements, from Mars years 28–33, provide normalized argon mixing ratios that show a seasonal variation of up to 15% from LS 0°, which is comparable in magnitude to the observed variation in absolute mixing ratios measured by SAM, with broadly similar seasonal trends. More detailed analysis, beyond the scope of this paper, will be needed to assess the statistical and physical significance of the modest differences between SAM and APXS. Note however that the spatial separation of MER-B and MSL and the much larger volume of atmosphere sampled by the APXS detection method could contribute to such differences due to the significance of atmospheric dynamics in mixing ratio trends.
With SAM QMS measurements, we have the first data set of compositional measurements near the equator of all the major species, thus painting a more complete picture of the behavior of Mars' atmosphere throughout the seasonal cycle. The instantaneous mixing ratio data presented in Figure 7 and Table S1 were subsequently adjusted for the annual global pressure cycle, using the method described in section 3.1. We used equation (5) to correct the in situ values to produce global mean annual mixing ratios (VMR', Figure S5). If the atmospheric gases were perfectly globally mixed and transport effects on trace species were negligible at this low latitude, the amount of Ar and N2 would show a constant value once the pressure variation was removed. In other words, the Ar and N2 VMR' values in Gale Crater would remain constant with time, if atmospheric transport and mixing timescales were identical. The corrected mixing values (Figure 11) show that there are clear deviations to such behavior, because the transport (traced by adjustment of atmospheric pressure due to polar CO2 condensation/sublimation) is faster than mixing (traced by adjustment of composition). Global mean values were calculated from the Ar and N2 VMR' (0.0194 and 0.0259, respectively), about which the individual mixing ratios are plotted. In this manner, the two atmospheric components can be plotted on the same axis, highlighting deviations from the average value as a function of LS. The data indicate that the minimum in Ar and N2 VMR' in N. spring/summer lags behind the pressure maximum by approximately 78° of LS, and similarly the maximum in Ar and N2 in N. summer/fall follows the pressure minimum by 67° of LS, due to delay in mixing.
The corrected mixing ratios indicate mixing of air masses of distinct composition, moving through the equator during the seasonal cycle. At the south pole, maximum enrichment of Ar observed at the southern pole by the GRS (Sprague et al., 2012) occurs near LS 120°, an effect of “freeze distillation” of trace gases occurring at the pole during the local winter (Lian et al., 2012). Although pressure at all latitudes adjusts relatively and rapidly to polar sublimation/condensation, compositional differences can be maintained longer by the southern polar vortex until it mixes with higher latitude air in southern spring, pushed by the large amounts of CO2 subliming off the cap. Repeating cycles of enrichment and depletion of Ar and N2 are observed at Gale Crater as the composition mixes between low latitude air masses and seasonally varying polar air masses. It should be noted that the orbital GRS measurements of Ar at low latitudes (where both Opportunity and Curiosity sit), and even in north polar regions, are dominated by scatter and the variations described here are not resolvable.
Differences in the timescales for atmospheric transport and mixing are emphasized by plotting global mean values (VMR'; Figure 11) using the pressure correction described above, since VMR' should not change if transport and mixing timescales are identical. We use pressure-corrected VMR' data to compute mean annual mixing ratios (Table 3), but as indicated by the residuals plotted in Figure S5, uncertainties in the global average are affected more by seasonal variation than by instrumental uncertainties. In Figure 12, we plot the instantaneous VMR of Ar and N2 on top of a modeled Ar mixing ratio in Gale Crater as predicted by the Laboratoire de Météorologie Dynamique (LMD) Mars GCM (MGCM) with coupled photochemistry (Lefevre et al., 2004). All are normalized to LS 345°, selected as one of the SAM measurement points closest to LS 0° (360°). The MGCM prediction shows generally good agreement with the measured mixing ratios for Ar and N2, with some discrepancy between LS 90° and180° when a lower mixing ratio is observed by SAM. The discrepancy between the model and the data may reflect an inaccurate estimation of the local isolation in Gale Crater that limits mixing between the MSL measurement sites and the surrounding atmosphere (Pla-Garcia et al., 2016; Rafkin et al., 2016). Thus, the SAM data set provides unprecedented ground truth measurements of these tracers to aid in improving the representation of various processes within the Martian climate system in global circulation models. This has significant implications for proper understanding of the water cycle, CH4 abundance and distribution (Webster et al., 2018), and possibly also the dust cycle on Mars (Lian et al., 2012).
MER-B used APXS data to measure seasonal trends in Ar mixing ratio at Meridiani Planum. General trends are very consistent with SAM results at Gale Crater (Figure 7), but the cadence of the SAM data preclude a definitive observation of the argon “pulse” near LS 150° observed in the APXS data [see Figure 9 in VanBommel et al., 2018]. The SAM data are not inconsistent with a pulse; qualitatively, in particular with the pressure-corrected data (Figure 11), there appears to be a small increase in Ar around LS 150° that deviates from an apparent seasonal curve. Interestingly, LS 150° also corresponds to the timing of the transition between seasonal periods of increasing O2 and constant O2 in the SAM data set (section 3.2.2).
4.1.1 Nitrogen Cycle
Recent detections of nitrate in Martian sediments have indicated the presence of nitrogen fixation cycles on Mars at least at one time in its history (Navarro-Gonzalez et al., 2019; Stern et al., 2015; Stern et al., 2017). Because nitrogen is an essential element for life, nitrogen fixation is a critical process required to support habitable environments as we know them. If currently active, a large nitrogen flux into the regolith could potentially affect the abundance of atmospheric N2 over seasonal or long-term timescales. We explored whether the available SAM atmospheric data set could shed any light on whether there is a currently active nitrogen cycle. As on Earth, the primary reservoir of nitrogen is in the form of N2 in the atmosphere. On Earth, nitrogen fixation proceeds abiotically and biologically, with the biological rate (1014 g N yr-1) occurring about 100 times higher than the abiotic rate triggered by lightning (Menge et al., 2013; Schumann & Huntrieser, 2007; Zehr et al., 2001). Nitrogen-containing end products of biological processes are rapidly recycled back into the atmosphere at a rate such that the nitrogen budget on Earth appears to be in an approximate equilibrium. On Mars, nitrogen fixation could potentially occur both biologically (Klingler et al., 1989) and abiotically (Navarro-Gonzalez et al., 1998; Navarro-Gonzalez et al., 2001; Segura & Navarro-Gonzalez, 2005). Currently there exist no data capable of providing evidence of biological activity on Mars. If reported, CH4 detections (Formisano et al., 2004; Mumma et al., 2009; Webster et al., 2013; Webster et al., 2015; Webster et al., 2018) were assumed to be solely a result of active biological activity; an estimated biomass of ~109 g yr-1 would be required to maintain a CH4 concentration of 10 ppbv (Krasnopolsky et al., 2004). A methanogenic population of this size would be capable of a hypothetical biological nitrogen fixation rate of ~108 g N yr-1 (Frigstad et al., 2011; Leigh, 2000). Conversely, photochemical models predict an abiotic nitrogen fixation rate of ~109 g N yr-1 in the form of nitrates (Yung et al., 1977). Therefore, the abiotic rate in the current atmosphere would be expected to exceed the hypothetical biological rate by at least an order of magnitude. The current annual flow of nitrogen from the atmosphere to the surface is negligible considering a repository in the atmosphere on the order of 1018 g N2.
Further, the abiotic denitrification of nitrates is a viable mechanism to recycle back surface nitrogen to the atmosphere, but the rate of this process is currently unknown. Therefore, it is concluded that current nitrogen cycle on Mars has no measurable impact on the atmospheric N2 mixing ratio and consequently implies nitrogen seasonal stability.
4.2.1 Interannual Variability
The 1300–2200 ppmv abundances of oxygen measured by SAM are generally in the same range as prior measurements that span different sets of in situ and remote sensing observations of O2 in the upper atmosphere and at the surface of Mars. The repeated measurements of SAM provide the most robust measurements to date, as the previously reported values have been made with substantial uncertainty and limited frequency, besides being in different regions of the atmosphere and the surface. From five sets of mass spectral measurements obtained within days of landing of Viking lander 1 (VL1) on 20 July 1976, Owen and Biemann (1976) reported an O2 mixing ratio in the 1000–4000 ppmv range. Owen et al. (1977) subsequently concluded that the oxygen measurements had considerable scatter of a factor of 2, which they attributed to instrumental causes. It seems likely, as England and Hrubes (2004) suggest, that Owen et al. (1977) based the 1300 ppmv value for O2 mixing ratio given in their Table 1 on measurements reported by Barker (1972) and Carleton and Traub (1972), which were obtained with high-resolution 762 nm O2-band spectroscopy from terrestrial observatories (see also Trauger & Lunine, 1983 who report a slightly lower value from ground-based spectroscopy: ~1200 ppm when scaled to 6 mbar surface pressure). The Viking GEx experiment reported an upper limit of 1500 ppmv (Oyama & Berdahl, 1977). England and Hrubes (2004) calculated a seasonal variation in O2 between 2,500 and 3,300 ppmv, by inverse scaling of O2 to atmospheric pressure measured by the Viking. It is important to note that the reference O2 value they used in their scaling calculation (3,000 ppmv) is the value VL1 measured at 125–300 km, in the upper atmosphere above the homopause (Nier & McElroy, 1977), which is not representative of O2 at the surface. Mars Express SPICAM observed 4,000 ppmv O2 averaged over 90–130 km and six observations (Montmessin et al., 2017; Sandel et al., 2015). Any seasonal or temporal variations in O2 at the surface are not expected to propagate to the upper atmosphere. Recent measurements made through disk-averaged observations of Mars with the Herschel Space Observatory's HIFI instrument retrieved a value of 1400 ± 120 ppmv, though they caution the reader the value may not be vertically uniform (Hartogh, Jarchow, et al., 2010). This measurement was taken during MY 30, LS 77°. SAM measurements from surface from similar times of year are slightly higher but with overlapping uncertainty (Table S1).
To quantify the enrichment of the observed O2 compared to what would be predicted, we account for the changes in total number density, which are mainly due to pressure changes (Figure 7a) caused by the global-scale CO2 condensation-sublimation cycle, as well as the expected changes in the mixing ratios of all noncondensable gases, which are caused by the interaction of the global circulation with that condensation-sublimation cycle and are readily visible to us via Ar results. After accounting for both of these, it can be estimated that approximately 1014 O2 molecules cm-3 must be added to the atmosphere sampled by Curiosity in order to explain the observed 1,700 to 2,200 ppmv increase in O2 between LS 60°and 140° in MY 33. In other words, the number density of O2 molecules at LS 140° in MY 33 is ~1014 molecules cm-3 larger than it would have been had the ratio of O2 to Ar remained constant as expected. For reference, given the 1,700 ppm starting value at LS 60°, the O2 mixing ratio would have been ~1830 ppm at LS 140°, had the O2 to Ar ratio remained constant over that time span.
Using the O2/40Ar ratio as an indicator for O2 variability outside of the known and observed seasonal dynamics, a simple linear model can be used to fit a single slope to all the data (Figure 10). This model highlights a consistent seasonal increase in O2/40Ar ratio of 0.014/100° LS during the LS 0–150° period, with an interannual variation in the mean O2/40Ar ratio in this period. For LS >150°, O2/40Ar seems to be more or less constant, with no significant interannual variation. The low values at LS >310° in MY 31 suggest a possibility of additional interannual variation late in the Mars year, and potentially the onset of the increases is observed through the spring of the following years, but future observations would be needed to confirm this possibility.
Within the uncertainties caused by limited sampling and measurement error, this magnitude appears typical of the unexpected seasonal increase, and so going forward, we will adopt ~400 ppm and ~014 molecules cm-3 as the amount that needs to be resolved. Assuming that the unexpected O2 is uniformly mixed in the lower atmosphere, as seems likely for perturbations of this timescale given current assumptions about the eddy diffusion coefficient, the 1014 molecules cm-3 become 1020 molecules cm-2 in the atmospheric column (see, e.g., Krasnopolsky, 2010 who adopts 107 cm2 s-1 for the eddy diffusion coefficient, which gives a ~2-day characteristic timescale for the bottom scale height of the atmosphere).
Given photochemical schemes above, this 400 ppm of extra O2 would require a corresponding destruction of CO2 and H2O molecules in approximately 170 sol. Considering H2O alone, ~800 ppm of H2O would need to be destroyed, which is more than five times larger than the maximum abundance of H2O measured in and around Gale Crater by REMS (Martínez et al., 2016) and ChemCam (Fig. 111 in McConnochie et al., 2018). Thus, it appears unlikely that the needed O2 could be produced from the available atmospheric water for any plausible H2O photolysis or dissociation mechanism. Furthermore, H2O abundance shows an increase during this time period and no strong correlation with O2 (Figure S6). Estimates for the production of O from CO2, using CO2 photolysis rates for the lower atmosphere of Mars (Table 1 in Wong et al., 2003), indicate that this process is much too slow to generate the observed rise over the short (~1/2 yr) time period. For completeness, note that photolysis or other dissociation of CO is negligible, and in any case a seasonal removal of ~800 ppm of CO is clearly ruled out by observations (e.g., Smith et al., 2009).
The primary destruction pathways for O2 are through direct photolysis in the upper atmosphere and reaction with photolysis products of H2O, HO2, and CO2 deeper (Atreya et al., 2006; Lefèvre & Krasnopolsky, 2017; Wong et al., 2003). Even factoring in the effects of dust devils and large dust storms (though no major dust events occurred during the measurement period) (Atreya et al., 2006), the lifetime of O2 against photochemical destruction in the Mars atmosphere is expected to be at least 10 years, possibly longer (Krasnopolsky, 2010; Lefèvre & Krasnopolsky, 2017). Again using the O2/40Ar ratio as an indicator for O2 variability (Figure 10), the SAM measurements in MY 33 show a relative decrease of 23% in a period of 39 days (38 sol). (This is a ~500 ppm absolute O2 decrease from what would be expected from a constant O2/40Ar ratio given the starting O2 abundance at LS 140°.) There is a similar decrease observed from fall to winter of MY 31, although the measurement frequency is such that the period of change appears longer (–20% in 201 sol). In particular, the rapid drop in MY 33 corresponds to a lifetime of 150 days, several orders of magnitude less than the photochemical equilibrium lifetime of ≥10 years. The drop in MY 31 corresponds to a lifetime of ~1000 days, which is still relatively short.
The lack of a known atmospheric source or sink that could explain the apparent behavior of the O2 in Gale Crater suggests the possibility of a temporary surface reservoir. Previously, Herschel WIFI observations found that the O2 vertical profile above the surface is not constant with altitude (Hartogh, Jarchow, et al., 2010) and preliminary analysis of the data shows that a surface flux of O2 may be required to explain the observations (Paul Hartogh, personal communication, 2016). A surface sink has been previously invoked to balance the current redox budget (Zahnle et al., 2008), and the surface is known to harbor a variety of oxidant species (e.g., Lasne et al., 2016 review). In fact, the Viking Gas Exchange experiments found that a significant quantity of O2 was released whenever soil samples were humidified (Klein, 1978; Oyama & Berdahl, 1977) although these experiments were all done at ~10 °C rather than Mars ambient temperatures.
Deposition of oxygen could occur in the form of more reactive oxidized species, superoxides, hydrogen peroxide (H2O2), ozone (O3), or perchlorates, all of which are assumed to have a higher surface reaction probability (γ) with surface materials than molecular oxygen. It is possible to conceive of an oxygen cycle with the appropriate seasonal and interannual variability if oxygen were effectively converted to these species, deposited into the regolith, and then rereleased due to thermal, chemical, or radiation perturbations.
Perchlorates, found to be prevalent in the surface materials in Gale Crater at 0.03–1 wt% level (Sutter et al., 2017), are very stable. They have also been detected at 0.4–0.6 wt% level in the polar landing site of the Phoenix Lander (Hecht et al., 2009). To put perchlorates in context, a 1-cm depth of soil containing 1% by weight of calcium perchlorate has slightly more than enough oxygen to contribute the apparent ~1020 molecules cm-2 of unexpected column O2 variation, and O2 has been shown to be a high yield product of radiolysis of surface perchlorate salts (Quinn et al., 2013). However these results point to a long-term accumulation of O2 in the Martian soil; the production rate of O2 from perchlorate radiolysis is insufficient to produce the observed recurring unexplained signal. More specifically, based on the Quinn et al. (2013) experimental radiation dose and yield, and on their estimated Martian dose rates and estimated 2-m cosmic ray penetration depth, it would take on order of 1 Myr to accumulate enough trapped O2 for one season worth of 1020 molecules cm-2 of column O2 variation. Similarly, the proposed “superoxide” O2- ions, suggested to explain the results of the Viking soil reactivity experiments, could form from ultraviolet radiation on surface minerals and lead to the observed release of O2 with humidification (Yen et al., 2000), but the reported rate of superoxide generation is too small to be consistent with the inferred column O2 signal yet again by a factor of ~106.
The Viking Gas Exchange experiments released up to 770 nanomoles of O2 from a 1 cm3 sample over a period of less than 11 days upon “humidification” at ~10° C (Klein, 1978; Oyama & Berdahl, 1977). If the same abundance of rapidly releasable O2 was present across 2 m of depth (i.e., 200 cm3), this would yield the 1020 molecules cm-2 that would explain the atmospheric measurements. This indicates that sufficient rapidly releasable O2 is present in the Martian soil, although it is not clear that such a rapid release of O2 could have occurred at Mars ambient temperatures. More importantly, this serves to illustrate that explaining a one-time release of O2 is not the main problem. The primary difficulty is that the slow rates of accumulation in the processes considered so far cannot explain the seasonal recurrence of excess O2.
Hydrogen peroxide is worth considering as a solution to this problem, because it is less stable than perchlorates, and is therefore more likely to provide a rapidly exchangeable reservoir of O2. H2O2 has been detected in the atmosphere (Clancy et al., 2004; Encrenaz et al., 2004) and exhibits seasonal and interannual variability (Encrenaz et al., 2019), and it has been suggested that diffusion of atmospheric H2O2 into the regolith and/or mineral/water interactions could supply H2O2 in the near subsurface (Bullock et al., 1994; Lasne et al., 2016).
Current knowledge of H2O2 physics and chemistry in Martian soil is very limited, but based on a coupled soil-atmosphere model by Bullock et al. (1994), it appears that the magnitude and thermal sensitivity of H2O2 soil adsorption is potentially close to the right order of magnitude to supply the unexpected 1020 molecules cm-2 of O2. However, this conclusion only follows from assuming a 107-year chemical lifetime for adsorbed H2O2, which is the longest that Bullock et al. (1994) considered plausible, and it depends on their adopted absorption isotherm, which was based on the Fanale and Cannon (1971) empirically derived expression for H2O since no data for H2O2 were available. Furthermore, once the depth penetration of the annual temperature wave (e.g., Grott et al., 2007) is considered, the amount of H2O2 potentially cycled in and out of the soil is estimated at an order of magnitude less than needed here, at 1019 molecules cm-2. Further, the timescale for diffusion from meter-scale depths may be far too long. Finally, note that although seasonal trends are in fact observed for atmospheric H2O2, this mechanism would require a rapid conversion to O2 immediately at the surface as the observed amount of H2O2 is only on the order of ppb (Encrenaz et al., 2015).
Another potential mechanism involving H2O2 was proposed by Quinn and Zent (1999). They showed that H2O2-TiO2 complexes rapidly released O2 upon humidification at warm (for Mars) temperatures of 10 °C; a more extensive study of regolith analogs and environmental conditions for this type of H2O2 ➔ O2 conversion in the regolith is needed.
Finally, any release of O2 to the atmosphere from surface/subsurface reservoirs of H2O2 and perchlorates (ClO4-) would be associated with concomitant flux of hydrogen and chlorine, respectively, which would impact the chemistry of the atmosphere in ways not seen as well as require their removal from the atmosphere by processes that are poorly understood.
4.2.2 Correlations With CH4 and Environmental Parameters
The in situ CH4 abundances measured by SAM/TLS (Webster et al., 2018) are plotted with the relative instantaneous O2 mixing ratios in Figure 12b. Here, the annual global mean pressure correction is omitted, and the CH4 data are normalized to the value at LS 330°, whereas the O2 data are normalized to LS 344.9° (Figure 12a). Although not displayed on identical vertical scales, it can be seen that both trace gases exhibit seasonal variations with much greater amplitudes than Ar and N2 (Figure 12a). The available CH4 data indicate a smoother seasonal trend than the O2 within the sampling frequency. The observed behavior of either molecule is not currently understood, and a strong relationship between the two might inform the root cause of observed changes in both O2 and CH4, such as the potential seepage or release mechanisms hypothesized for CH4 (Moores et al., 2019). However, it appears that the O2 and CH4 show a similar trend for only part of the year. In particular, the relative decrease during northern fall into winter (as the smaller of the two polar caps forms) is similar for both species. This is a period when O2 also more closely follows the Ar and N2 measurements as well as the modeled seasonal trends. Yet in the northern spring/summer, O2 shows an earlier increase and much more interannual variability than the CH4 measurements (see Figure 13a for O2 and CH4 vs. sol). Thus it seems that at least some component of the variability in the O2 cycle is unique or not directly affected by the processes that regulate CH4. Seemingly, with respect to O2 and CH4 on Mars, the observations to date are inconclusive as to whether there is a definitive correlation between the them.
Finally, we tested correlations between O2 abundance variations with a wide range of environmental parameters in an attempt to explain the compositional variability. For most parameters, no correlations were observed (discussed in more detail in Figure S6). Figure 13 shows the correlations identified between O2/40Ar and the dust opacity and UV absorption in the atmosphere. An overlay of O2/40Ar ratios and dust opacity as a function of MSL sol number (Figure 13a and Figure S6) suggests some type of inverse relationship between dust opacity and oxygen abundance. A similar relationship is apparent in comparisons with atmospheric UV absorption (Figure 13b and Figure S7). Note that dust opacity and atmospheric UV absorption are closely related, because atmospheric UV absorption is more sensitive to variation in dust loading than to insolation.
The observed correlations are not very strong, with reduced χν2 values significantly greater than 1 (Figures S6c and S7c). This suggests that if a real relationship between atmospheric dust loading (and/or atmospheric UV absorption) is present, that relationship is more complex than a simple linear relationship. Agreement between these dust-related environmental parameters and the O2 abundance seems weaker in MY 32, compared to the other Mars years. Within the limitations of this analysis, a real relationship between O2 abundance and atmospheric dust loading (and/or atmospheric UV absorption) could suggest either unknown photochemical or surface chemistry controls on the O2 abundance.
Thus, the observed O2 variability remains a mystery until further measurements, models, or experiments are able to identify likely mechanisms through which the O2 can vary on short timescales. It is hoped that hypotheses that may be testable with further in situ measurements by Curiosity arise, while the mission is still operating in Gale Crater.
The atmospheric compositional data obtained by the SAM instrument in Gale Crater offer unprecedented seasonal and multiyear coverage of the Mars atmosphere at the surface. The abundances of the major atmospheric gases CO2, N2, 40Ar, O2, and CO have been measured from MY 31–MY 34, through three full Martian seasonal cycles. Measurements were sufficiently distributed throughout the mission to provide insight on the behavior of the major atmospheric components in response to the seasonal variations driven by Mars' obliquity and its large orbital eccentricity.
The SAM measurements of volume mixing ratios reveal repeatable cycles in which CO2 is modulated by the formation and sublimation of the polar caps, but with a lag of approximately 20–40° of LS behind the maxima and minima of the total pressure cycle. Similarly, the inert tracer species N2 and Ar show a similar delayed response to the pressure cycle. These data demonstrate that transport acts very rapidly to maintain pressure equilibrium, but changes in VMRs lag behind because physical mixing of air masses is a slower process. Our measurements indicate that for any single location on Mars, the instantaneous composition cannot be simply calculated from the time of year and globally averaged pressure.
Surprisingly, however, we have found that O2 does not demonstrate the predictable seasonal behavior of the other major components. Surface O2 measurements by SAM yield abundances that vary between 1300 and 2200 ppmv; when corrected for the annual global mean pressure, O2 varies from 1300 to 1900 ppmv. Despite large instrument backgrounds, these are the first precise in situ measurements of O2, revealing a surprising seasonal and interannual variation that cannot be accounted for in current chemical models. Though Mars has the potential to generate significant O2 release due to abundances of oxidants in/at its surface, the mechanisms by which O2 could be quickly generated and then quickly destroyed are completely unknown. As with all surprising results, we hope that continued in situ, experimental, and theoretical results may shed light on this intriguing observation.
Observations by MSL and SAM in Gale Crater, such as the atmospheric composition study presented in this paper, have enhanced our understanding of Mars as a complex planetary system. Geophysical and geochemical results have painted a picture of a formerly habitable planet billions of years in Mars' past, and measurements of current processes provide indications that Mars may still potentially harbor habitable environments. The new insights gained on the annual cycle of CO2, N2, and Ar presented here enable a more accurate understanding of the seasonal transport and mixing of the bulk atmosphere, thereby improving our ability to track the sources and sinks of trace species such as water and methane. Further, the significant O2 variability, especially when considered with recently documented seasonal enhancements of CH4 (Webster et al., 2018), hints at an active present-day Mars.
All MSL data used in this manuscript (REMS and SAM) are freely available on NASA's Planetary Data System (PDS) Geosciences Node, from within 6 months after receipt on Earth (http://pds-geosciences.wustl.edu/missions/msl/). The mixing ratios developed and presented in this paper are available at a publicly available archive (dataverse.org: doi.org/10.7910/DVN/CVUOWW) as cited within the manuscript. The successful operation of the Curiosity rover and the SAM instrument on Mars is due to the hard work and dedication of hundreds of scientists, engineers, and managers over more than a decade. Essential contributions to the successful operation of SAM on Mars and the acquisition of SAM data were provided by the SAM development, operations, and test bed teams. The authors gratefully thank the SAM and MSL teams that have contributed in numerous ways to obtain the data that enabled this scientific work. We also thank NASA for the support of the development of SAM, SAM data analysis, and the continued support of the Mars Science Laboratory mission. The contribution of F. Lefèvre was supported by the Programme National de Planétologie (PNP). R. Navarro-Gonzalez acknowledges support from the Universidad Nacional Autónoma de México (PAPIIT IN111619). LPI is operated by USRA under a cooperative agreement with the Science Mission Directorate of the National Aeronautics and Space Administration. We thank members of the SAM and larger MSL team for insightful discussions and support. In particular, we thank R. Becker and R. O. Pepin for careful review of data analysis and interpretation. We thank M. D. Smith for discussion of CRISM CO measurements. We thank A. Brunner, M. Johnson, and M. Lefavor for their development of customized data analysis tools used here and in other SAM publications. The authors acknowledge the work of Daniel Y. Lo (Climate and Space Sciences and Engineering, University of Michigan) for uncovering a data input error affected the plotting of the REMS data associated with sol 45, and for his work to make these corrections in Figures S6c, S7c, and S8 in the Supporting Information included with this publication.
|2019JE006175-sup-0001-Supporting Information SI-S01.docx3.6 MB||Supporting Information S1|
|jgre21250-sup-0002-2019JE006175-ts01.xlsxExcel 2007 spreadsheet , 12.1 KB||Table S1|
|jgre21250-sup-0003-2019JE006175-ts02.xlsxExcel 2007 spreadsheet , 19.1 KB||Table S2|
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In the originally published article, a data input error affected the plotting of the REMS data associated with sol 45 in Figure S8h (correlation between the O2/Ar ratio with the daily maximum relative humidity from REMS). The relative humidity values were determined from REMS values within a ±2°-LS search interval about each SAM measurement time. There was, however, no available REMS data within this interval around sol 45, and a maximum relative humidity of 58.28% from sol 1870 was incorrectly assigned to the sol-45 datapoint. The analysis has been repeated after enlarging the search interval to ±3° for all datapoints, giving a new sol-45 Χν value of 3.06 compared to the previous value of 3.34. This corrected Χν value, while slightly closer to unity, is still too large to indicate a significant linear correlation between O2/Ar and the daily maximum relative humidity, and thus is still consistent with the statement in the original text that "H2O abundance shows...no strong correlation with O2." For consistency, the correlation plots between O2/Ar and other environmental variables (Figures S6c, S7c, and the other plots in S8) have been updated to reflect the larger search interval for all quantities. The larger search interval did not result in significant changes in these other plots. These errors have been corrected in the online version, which may be considered the authoritative version of record, and not the PDF.