Atmospheric Chemistry Suite Research Articles

Thermal Tides in the Martian Atmosphere Near Northern Summer Solstice Observed by ACS/TIRVIM Onboard TGO

AbstractThermal tides in the Martian atmosphere are analyzed using temperature profiles retrieved from nadir observations obtained by the TIRVIM Fourier‐spectrometer, part of the Atmospheric Chemistry Suite onboard the ExoMars Trace Gas Orbiter. The data is selected near the northern summer solstice at solar longitude (LS) 75°–105° of Martian Year 35. The observations have a full local time coverage, which enables analyses of daily temperature anomalies. The observed zonal mean temperature is lower by 4–6 K at ∼100 Pa, but higher toward the summer pole, compared to the Laboratoire de Météorologie Dynamique (LMD) Mars General Circulation Model (GCM). Wave mode decomposition shows dominant diurnal tide and important semi‐diurnal tide and diurnal Kelvin wave, with maximal amplitudes of 5, 3, and 2.5 K, respectively, from tens to hundreds of Pa. The results generally agree well with the LMD Mars GCM, but with noticeable earlier phases of diurnal (∼1 hr) and semi‐diurnal (∼3 hr) tides.

Thermal Structure and Aerosols in Mars’ Atmosphere From TIRVIM/ACS Onboard the ExoMars Trace Gas Orbiter: Validation of the Retrieval Algorithm

AbstractThe Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter monitors the Martian atmosphere through different spectral intervals in the infrared light. We present a retrieval algorithm tailored to the analysis of spectra acquired in nadir geometry by Thermal InfraRed channel in honor of professor Vassilii Ivanovich Moroz (TIRVIM), the thermal infrared channel of ACS. Our algorithm simultaneously retrieves vertical profile of atmospheric temperature up to 50 km, surface temperature, and integrated optical depth of dust and water ice clouds. The specificity of the TIRVIM data set lies in its capacity to resolve the diurnal cycle over a 54 sol period. However, it is uncertain to what extent can the desired atmospheric quantities be accurately estimated at different times of day. Here we first present an Observing System Simulation Experiment (OSSE). We produce synthetic observations at various latitudes, seasons and local times and run our retrieval algorithm on these synthetic data, to evaluate its robustness. Different sources of biases are documented, in particular regarding aerosol retrievals. Atmospheric temperature retrievals are found robust even when dust and/or water ice cloud opacities are not well estimated in our OSSE. We then apply our algorithm to TIRVIM observations in April–May 2018 and perform a cross‐validation of retrieved atmospheric temperature and dust integrated opacity by comparisons with thousands of colocated Mars Climate Sounder (MCS) retrievals. Most differences between TIRVIM and MCS atmospheric temperatures can be attributed to differences in vertical sensitivity. Daytime dust opacities agree well with each other, while biases are found in nighttime dust opacity retrieved from TIRVIM at this season.

No detection of SO2, H2S, or OCS in the atmosphere of Mars from the first two Martian years of observations from TGO/ACS

Context. The detection of sulphur species in the Martian atmosphere would be a strong indicator of volcanic outgassing from the surface of Mars. Aims. We wish to establish the presence of SO2, H2S, or OCS in the Martian atmosphere or determine upper limits on their concentration in the absence of a detection. Methods. We perform a comprehensive analysis of solar occultation data from the mid-infrared channel of the Atmospheric Chemistry Suite instrument, on board the ExoMars Trace Gas Orbiter, obtained during Martian years 34 and 35. Results. For the most optimal sensitivity conditions, we determine 1σ upper limits of SO2 at 20 ppbv, H2S at 15 ppbv, and OCS at 0.4 ppbv; the last value is lower than any previous upper limits imposed on OCS in the literature. We find no evidence of any of these species above a 3σ confidence threshold. We therefore infer that passive volcanic outgassing of SO2 must be below 2 ktons day−1.

Isotopic Composition of CO2 in the Atmosphere of Mars: Fractionation by Diffusive Separation Observed by the ExoMars Trace Gas Orbiter

AbstractIsotopic ratios in atmospheric CO2 are shaped by various processes throughout Mars' history, and can help understand what the atmosphere of early Mars was like to sustain liquid water on its surface. In this study, we monitor the O and C isotopic composition of CO2 between 70 and 130 km for more than half a Martian year using solar occultation observations by the Atmospheric Chemistry Suite onboard the ExoMars Trace Gas Orbiter. We find the vertical trends of the isotopic ratios to be consistent with the expectations from diffusive separation above the homopause, with average values below this altitude being consistent with Earth‐like fractionation (δ13C = −3 ± 37‰; δ18O = −29 ± 38‰; and δ17O = −11 ± 41‰). Using these measurements, we estimate that at least 20%–40% of primordial C on Mars has escaped to space throughout history. The total amount of C lost from the atmosphere is likely to be well in excess of this lower limit, due to carbonate formation and further sink processes. In addition, we propose a photochemical transfer of light O from H2O to CO2 to explain the larger enrichment in the ratio in H2O than in CO2.

Gravity Wave Activity in the Martian Atmosphere at Altitudes 20–160 km From ACS/TGO Occultation Measurements

AbstractThe paper presents observations of gravity wave‐induced temperature disturbances in the Martian atmosphere obtained with the mid‐infrared (MIR) spectrometer, a channel of the Atmospheric Chemistry Suite instrument on board the Trace Gas Orbiter (ACS/TGO). Solar occultation measurements of aabsorption band at 2.7m were used for retrieving density and temperature profiles between heights of 20 and 160 km with vertical resolution sufficient for deriving small‐scale structures associated with gravity waves. Several techniques for distinguishing disturbances from the background temperature have been explored and compared. Instantaneous temperature profiles, amplitudes of wave packets and potential energy have been determined. Horizontal momentum fluxes and associated wave drag have been estimated. The analyzed data set of 144 profiles encompasses the measurements made over the second half of Martian Year 34, from the Solar longitudethrough. We observe enhanced gravity wave dissipation/breaking in the mesopause region of 100–130 km. Our analysis shows no direct correlation between the wave amplitude and Brunt‐Väisälä frequency. It may indicate that convective instability may not be the main mechanism limiting gravity wave growth in the middle atmosphere of Mars.

A new laboratory emissivity and reflectance spectral library for the interpretation of Mars thermal infrared spectral data

New spectral orbital thermal infrared data of Mars are being acquired by the thermal infrared channel TIRVIM (in honor of Vassily Ivanovich Moroz) of the Atmospheric Chemistry Suite (ACS) of spectrometers on board of ExoMars2016 mission. TIRVIM encompasses the spectral range of 1.7–17 μm. A major challenge brought by the analysis of these data of planetary bodies with atmospheres is the ability to extract from the data the relevant information about the surface. Thus, laboratory work plays an essential role, providing end-member and mixture spectral data of planetary analogs to fit the orbital data by means of deconvolution techniques. At the Planetary Spectroscopy Laboratory (PSL) of the German Aerospace Center (DLR), we are performing new laboratory experiments on Martian analogs in order to provide a new and updated library of spectra optimized for the interpretation of TIRVIM data. Emissivity measurements, recorded at increasing temperatures, are coupled with reflectance measurements on fresh and thermally processed samples acquired between 1.7 and 17 μm. Building on measurements previously collected on Martian analogues, we have paid particular attention to the study of the spectral behaviour of mixtures of carbonates and phyllosilicates. The main goal of this analysis is to study the variation of the main carbonate spectral features in mixtures with a phyllosilicate component, an important factor for understanding the story of carbonates detections on planetary surfaces and to provide insights for new detections. The results obtained in this work show that the presence of a phyllosilicate component affect the appearance of the carbonate spectral features in the spectral range studied, with a stronger effect in the range between 1.7 and 5 μm. Effects of mineral type and particle size are also investigated and shown to strongly affect the spectral behaviour of laboratory samples. Finally, deconvolution techniques of laboratory emissivity spectra are studied in preparation for the interpretation of atmospherically corrected TIRVIM spectral data, showing that modelled mixtures spectra represent an acceptable reproduction of laboratory spectra of mixtures.

Isotopes of chlorine from HCl in the Martian atmosphere

Hydrogen chloride gas was recently discovered in the atmosphere of Mars during southern summer seasons. Its connection with potential chlorine reservoirs and the related atmospheric chemistry is now of particular interest and actively studied. Measurements by the Atmospheric Chemistry Suite mid-infrared channel (ACS MIR) on the ExoMars Trace Gas Orbiter allow us to measure the ratio of hydrogen chloride two stable isotopologues, H35Cl and H37Cl. This work describes the observation, processing technique, and derived values for the chloride isotope ratio. Unlike other volatiles in the Martian atmosphere, because it is enriched with heavier isotopes, the δ37Cl is measured to be − 7 ± 20°, which is almost indistinguishable from the terrestrial ratio for chlorine. This value agrees with available measurements of the surface materials on Mars. We conclude that chlorine in observed HCl likely originates from dust and is not involved in any long-term, surface-atmosphere cycle.

Isotopic fractionation of water and its photolytic products in the atmosphere of Mars

The current Martian atmosphere is about five times more enriched in deuterium than Earth’s, providing direct testimony that Mars hosted vastly more water in its early youth than nowadays. Estimates of the total amount of water lost to space from the current mean D/H value depend on a rigorous appraisal of the relative escape between deuterated and non-deuterated water. Isotopic fractionation of D/H between the lower and the upper atmospheres of Mars has been assumed to be controlled by water condensation and photolysis, although their respective roles in influencing the proportions of atomic D and H populations have remained speculative. Here we report HDO and H2O profiles observed by the Atmospheric Chemistry Suite (ExoMars Trace Gas Orbiter) in orbit around Mars that, once combined with expected photolysis rates, reveal the prevalence of the perihelion season for the formation of atomic H and D at altitudes relevant for escape. In addition, while condensation-induced fractionation is the main driver of variations of D/H in water vapour, the differential photolysis of HDO and H2O is a more important factor in determining the isotopic composition of the dissociation products. Three years of observations with the ExoMars TGO Atmospheric Chemistry Suite have clarified the mechanism of atmospheric water loss from Mars, and particularly the fraction of deuterium compared with hydrogen. Analysis of several isotopologues of water in the mid-atmosphere of Mars shows that atomic H and D are produced at perihelion in relative amounts controlled by photolysis. These atoms are able to escape from the atmosphere.

Revealing a High Water Abundance in the Upper Mesosphere of Mars With ACS Onboard TGO

AbstractWe present the first water vapor profiles encompassing the upper mesosphere of Mars, 100–120 km, far exceeding the maximum altitudes where remote sensing has been able to observe water to date. Our results are based on solar occultation measurements by Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO). The observed wavelength range around 2.7 μm possesses strong CO2 and H2O absorption lines allowing sensitive temperature and density retrievals. We report a maximum H2O mixing ratio varying from 10 to 50 ppmv at 100–120 km during the global dust storm (GDS) of Martian Year (MY) 34 and around southern summer solstice of MY 34 and 35. During other seasons water remains persistently below ∼2 ppmv. We claim that contributions of the MY34 GDS and perihelion periods into the projected hydrogen escape from Mars are nearly equivalent.

A stringent upper limit of 20 pptv for methane on Mars and constraints on its dispersion outside Gale crater

Context. Reports on the detection of methane in the Martian atmosphere have motivated numerous studies aiming to confirm or explain its presence on a planet where it might imply a biogenic or more likely a geophysical origin. Aims. Our intent is to complement and improve on the previously reported detection attempts by the Atmospheric Chemistry Suite (ACS) on board the ExoMars Trace Gas Orbiter (TGO). This latter study reported the results of a campaign that was a few months in length, and was significantly hindered by a dusty period that impaired detection performances. Methods. We unveil 640 solar occultation measurements gathering 1.44 Martian years worth of data produced by the ACS. Results. No methane was detected. Probing the clear northern summer season allowed us to reach 1σ upper limits of around 10 pptv (20 pptv at 2σ), with an annual mean of the smallest upper limits of 20 pptv. Upper limits are controlled by the amount of dust in the atmosphere, which impairs detection performance around the equator and during the southern spring and summer seasons. Observations performed near Gale crater yielded 1σ upper limits of up to four times less than the background values measured by the Curiosity rover during the corresponding seasons. Conclusions. Reconciliation of the absence of methane in the TGO spectra with the positive detections by Curiosity is even more difficult in light of this annual survey performed by ACS. Stronger constraints are placed on the physical and chemical mechanism capable of explaining why the mean of the best overall upper limits of ACS is ten times below the smallest methane abundances measured by Curiosity.

Asymmetric Impacts on Mars’ Polar Vortices From an Equinoctial Global Dust Storm

AbstractMars possesses dynamical features called polar vortices: regions of cold, isolated air over the poles circumscribed by powerful westerly jets which can act as barriers to transport to dust, water, and chemical species. The 2018 Global Dust Storm was observed by multiple orbiters and offered a valuable opportunity to study the effects of such a storm on polar dynamics. To this end, we assimilate data from the Mars Climate Sounder and Atmospheric Chemistry Suite into a Mars Global Climate Model. We find that the storm had asymmetrical hemispherical impacts, with the northern vortex remaining relatively robust while the southern vortex was substantially diminished in its intensity. We propose that this asymmetry was due both to the storm’s latitudinal extent, as it extended further south than north, and to its equinoctial timing, occurring as the southern vortex was already decaying. We show that both polar vortices, in particular the northern, were reduced in ellipticity by the storm. There was a well‐correlated reduction in stationary topographic wave activity at high latitudes in both hemispheres. We demonstrate that the characteristic elliptical Martian polar vortex shape is the pattern of the stationary waves, which was suppressed by the shifting of the polar jet away from regions of high mechanical forcing (north) or reduction of polar jet intensity by a reduced meridional temperature gradient (south). These asymmetric effects suggest increased transport into the southern (but not northern) polar region during global dust storms at northern autumn equinox, and more longitudinally symmetric transport around both poles.

Upper limits for phosphine (PH3) in the atmosphere of Mars

Phosphine (PH3) is proposed to be a possible biomarker in planetary atmospheres and has been claimed to have been observed in the atmosphere of Venus, sparking interest in the habitability of Venus’s atmosphere. Observations of another biomarker, methane (CH4), have been reported several times in the atmosphere of Mars, hinting at the possibility of a past or present biosphere. The Atmospheric Chemistry Suite on the ExoMars Trace Gas Orbiter has a spectral range that includes several absorption lines of PH3 with line strengths comparable to previously observed CH4 lines. The signature of PH3 was not observed in the 192 observations made over a full Martian year of observations, and here we report upper limits of 0.1–0.6 ppbv.

Seasonal reappearance of HCl in the atmosphere of Mars during the Mars year 35 dusty season

Hydrogen chloride was discovered in the atmosphere of Mars for the first time during the global dust storm in Mars year (MY) 34 (July 2018) using the Atmospheric Chemistry Suite mid-infrared channel (ACS MIR) on the ExoMars Trace Gas Orbiter. The simultaneity of variations in dust and HCl, and a correlation between water vapour and HCl, led to the proposal of a novel surface-atmosphere coupling analogous to terrestrial HCl production in the troposphere from salt aerosols. After seasonal dust activity restarted in MY 35 (August 2020), we have been monitoring HCl activity to determine whether such a coupling was validated. Here we present a new technique for analysing the absorption features of trace gases close to the ACS MIR noise level and report that HCl mixing ratios are observed to rapidly increase in both hemispheres coincidentally with the onset of the MY 35 perihelion dust season. We present the temporal evolution of the vertical distribution of HCl (0.1–6 ppbv) and of dust activity in both hemispheres. We also report two observations of >2 ppbv HCl below 10 km in the northern hemisphere during the aphelion period.

Transient HCl in the atmosphere of Mars.

A major quest in Mars' exploration has been the hunt for atmospheric gases, potentially unveiling ongoing activity of geophysical or biological origin. Here, we report the first detection of a halogen gas, HCl, which could, in theory, originate from contemporary volcanic degassing or chlorine released from gas-solid reactions. Our detections made at ~3.2 to 3.8 μm with the Atmospheric Chemistry Suite and confirmed with Nadir and Occultation for Mars Discovery instruments onboard the ExoMars Trace Gas Orbiter, reveal widely distributed HCl in the 1- to 4-ppbv range, 20 times greater than previously reported upper limits. HCl increased during the 2018 global dust storm and declined soon after its end, pointing to the exchange between the dust and the atmosphere. Understanding the origin and variability of HCl shall constitute a major advance in our appraisal of martian geo- and photochemistry.

The vertical structure of CO in the Martian atmosphere from the ExoMars Trace Gas Orbiter

Carbon monoxide (CO) is the main product of CO2 photolysis in the Martian atmosphere. Production of CO is balanced by its loss reaction with OH, which recycles CO into CO2. CO is therefore a sensitive tracer of the OH-catalysed chemistry that contributes to the stability of CO2 in the atmosphere of Mars. To date, CO has been measured only in terms of vertically integrated column abundances, and the upper atmosphere, where CO is produced, is largely unconstrained by observations. Here we report vertical profiles of CO from 10 to 120 km, and from a broad range of latitudes, inferred from the Atmospheric Chemistry Suite on board the ExoMars Trace Gas Orbiter. At solar longitudes 164–190°, we observe an equatorial CO mixing ratio of ~1,000 ppmv (10–80 km), increasing towards the polar regions to more than 3,000 ppmv under the influence of downward transport of CO from the upper atmosphere, providing a view of the Hadley cell circulation at Mars’s equinox. Observations also cover the 2018 global dust storm, during which we observe a prominent depletion in the CO mixing ratio up to 100 km. This is indicative of increased CO oxidation in a context of unusually large high-altitude water vapour, boosting OH abundance.

Properties of Water Ice and Dust Particles in the Atmosphere of Mars During the 2018 Global Dust Storm as Inferred From the Atmospheric Chemistry Suite

AbstractThe properties of Martian aerosols are an integral part of the planetary climatology. Global dust storms (GDS) significantly alter spatial and vertical distributions of dust and water ice aerosols and their microphysical properties. We explored the 2018/Martian year 34 GDS with the Atmospheric Chemistry Suite instrument onboard the ESA‐Roscosmos Trace Gas Orbiter mission. Solar occultation observations of thermal infrared and near infrared channels in the 0.7–6 μm spectral range with >103 signal‐to‐noise ratio are used to constrain the vertical dependence and the temporal evolution of the particle properties of water ice and dust (effective radius, effective variance, number density, and mass loading) before the 2018 GDS and during its onset and decay phases. In most of the observations, the particle size of dust and water ice decreases with altitude. The effective radius of dust and water ice particles ranges in 0.1−3.5 μm and 0.1–5.5 μm, respectively. The largest aerosol particles (> 2.5 μm for dust and > 3.5 μm for water ice) are present below 10 km before the onset and during the GDS decay phase. During the peak of the GDS, dust reached altitudes of 85 km; the most frequently observed effective radius is 1–2 μm with 0.1–1 cm−3 number density and 0.1 effective variance. Detached layers of water ice composed of 0.1–1 μm particles are systematically observed at 50–100 km during this period. Below, at 0–50 km, we see the dust mixed with the main water ice layer comprising 1–4 μm particles.

First observation of the magnetic dipole CO2absorption band at 3.3μm in the atmosphere of Mars by the ExoMars Trace Gas Orbiter ACS instrument

The atmosphere of Mars is dominated by CO2, making it a natural laboratory for studying CO2spectroscopy. The Atmospheric Chemistry Suite (ACS) on board the ExoMars Trace Gas Orbiter uses solar occultation geometry to search for minor atmospheric species. During the first year of ACS observations, the attention was focused on the spectral range covering the methaneν3absorption band, 2900–3300 cm−1, which has previously been observed on Mars. No methane was detected by ACS; instead, an improvement of the data processing has led to the identification of 30 weak absorption lines that were missing from spectroscopic databases. Periodic series of absorptions up to ~1.6% deep are observed systematically around the position of the methaneQ-branch when the line of sight penetrates below 20 km (creating an optical path length of 300–400 km, with an effective pressure of a few millibar). The observed frequencies of the discovered lines match theoretically computed positions of theP-,Q-, andR-branches of the magnetic dipole and electric quadrupole 01111-00001 (ν2+ν3) absorption bands of the main CO2isotopologue; neither band has been measured or computed before. The relative depths of the observed spectral features support the magnetic dipole origin of the band. The contribution of the electric quadrupole absorption is several times smaller. Here we report the first observational evidence of a magnetic dipole CO2absorption.

First detection of ozone in the mid-infrared at Mars: implications for methane detection

Aims. The ExoMars Trace Gas Orbiter was sent to Mars in March 2016 to search for trace gases diagnostic of active geological or biogenic processes. Methods. We report the first observation of the spectral features of Martian ozone (O3) in the mid-infrared range using the Atmospheric Chemistry Suite Mid-InfaRed (MIR) channel, a cross-dispersion spectrometer operating in solar occultation mode with the finest spectral resolution of any remote sensing mission to Mars. Results. Observations of ozone were made at high northern latitudes (>65°N) prior to the onset of the 2018 global dust storm (Ls = 163–193°). During this fast transition phase between summer and winter ozone distribution, the O3 volume mixing ratio observed is 100–200 ppbv near 20 km. These amounts are consistent with past observations made at the edge of the southern polar vortex in the ultraviolet range. The observed spectral signature of ozone at 3000–3060 cm−1 directly overlaps with the spectral range of the methane (CH4) ν3 vibration-rotation band, and it, along with a newly discovered CO2 band in the same region, may interfere with measurements of methane abundance.

Martian Water Ice Clouds During the 2018 Global Dust Storm as Observed by the ACS‐MIR Channel Onboard the Trace Gas Orbiter

AbstractThe Atmospheric Chemistry Suite (ACS) instrument onboard the ExoMars Trace Gas Orbiter (TGO) European Space Agency‐Roscosmos mission began science operations in March 2018. ACS Mid‐InfraRed (MIR) channel notably provides solar occultation observations of the Martian atmosphere in the 2.3‐ to 4.2‐ m spectral range. Here, we use these observations to characterize water ice clouds before and during the MY 34 Global Dust Storm (GDS). We developed a method to detect water ice clouds with mean particle size 2 m and applied it to observations gathered between and . We observe a shift in water ice cloud maximum altitudes from about 60 km before the GDS to above 90 km during the storm. These very high altitude, small‐sized ( m) water ice clouds are more frequent during MY 34 compared to non‐GDS years at the same season. Particle size frequently decreases with altitude, both locally within a given profile and globally in the whole data set. We observe that the maximum altitude at which a given size is observed can increase during the GDS by several tens of kilometers for certain sizes. We notably notice some large water ice particles ( m) at surprisingly high altitudes during the GDS (50–70 km). These results suggest that GDS can significantly impact the formation and properties of high‐altitude water ice clouds as compared to the usual perihelion dust activity.

Oxygen isotopic ratios in Martian water vapour observed by ACS MIR on board the ExoMars Trace Gas Orbiter

Oxygen isotope ratios provide important constraints on the history of the Martian volatile system, revealing the impact of several processes that might fractionate them, such as atmospheric loss into space or interaction with the surface. We report infrared measurements of the Martian atmosphere obtained with the mid-infrared channel (MIR) of the Atmospheric Chemistry Suite (ACS), onboard the ExoMars Trace Gas Orbiter. Absorption lines of the three main oxygen isotopologues of water vapour (H216O, H218O, and H217O) observed in the transmission spectra allow, for the first time, the measurement of vertical profiles of the 18O/16O and 17O/16O ratios in atmospheric water vapour. The observed ratios are enriched with respect to Earth-like values (δ18O = 200 ± 80‰ and δ17O = 230 ± 110‰ corresponding to the Vienna Standard Mean Ocean Water). The vertical structure of these ratios does not appear to show significant evidence of altitudinal variations.