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 (O
3
) 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 (L
s
= 163–193°). During this fast transition phase between summer and winter ozone distribution, the O
3
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 (CH
4
)
ν
3
vibration-rotation band, and it, along with a newly discovered CO
2
band in the same region, may interfere with measurements of methane abundance.
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.
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 1
σ
upper limits of around 10 pptv (20 pptv at 2
σ
), 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 1
σ
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.
The atmosphere of Mars is dominated by CO2, making it a natural laboratory for studying CO2 spectroscopy. 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 methane ν3 absorption 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 methane Q-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 the P-, Q-, and R-branches of the magnetic dipole and electric quadrupole 01111-00001 (ν2 + ν3) absorption bands of the main CO2 isotopologue; 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 CO2 absorption.
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 (H 216 $_2^{16}$ 2 16 O, H 218 $_2^{18}$ 2 18 O, and H 217 $_2^{17}$ 2 17 O) 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.
•Equation for the transition electric quadrupole moment squared of carbon dioxide.•Selection rules for the magnetic dipole and electric quadrupole absorption spectra of carbon dioxide.•Observed value ...of the vibrational transition magnetic dipole moment for the 01111–00001 band of 12C16O2.
Magnetic dipole and electric quadrupole absorption in carbon dioxide are addressed in details. The selection rules for both processes are presented. The equations for the line intensities are given. In the case of the quadrupole absorption the Herman-Wallis functions are derived. The results of the present paper were used in the analysis of the carbon dioxide absorption band at 3.3 µm in the atmosphere of Mars (Trokhimovskiy A, Perevalov V, Korablev O, Fedorova A, Olsen KS, Bertaux JL, Patrakeev A, Shakun A, Montmessin F, Lefèvre F, Lukashevskaya A. First observation of the magnetic dipole CO2 absorption band at 3.3 µm in the atmosphere of Mars by ExoMars Trace Gas Orbiter ACS instrument. A&A 639, A142 (2020)). The retrieved from the Martian atmosphere spectra vibrational transition magnetic dipole moment for the 01111–00001 (ν2+ν3) band of 12C16O2M01111←000001|Δℓ2|=1=0.96μN (where μN is nuclear magneton) is one order of magnitude larger than the gyromagnetic ratio in the case of the rotation-induced magnetic dipole moment.
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The 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.
Plain Language Summary
Suspended in the air, mineral dust and water ice particles play a key role in thermal balance and circulation of the atmosphere of Mars and its climate. In that context, global dust storms are rare but powerful events that significantly alter spatial and vertical distributions of dust and water ice particles and their properties. The most recent event, which occurred in the summer of 2018, was monitored by the Atmospheric Chemistry Suite instrument onboard the ESA‐Roscosmos Trace Gas Orbiter mission. Solar occultation observations carried out in the visible and the middle infrared spectral ranges enabled monitoring of dust and water ice particles properties such as size, number of particles and their mass per unit volume, and the width of their size distribution during the evolution of the storm. Dust was lifted to 85 km, while water ice clouds were observed at even higher altitudes, up to 100 km.
Key Points
Atmospheric Chemistry Suite solar occultations in the 0.7–6 μm spectral range separate microphysical properties of dust and water ice aerosols in Martian atmosphere
The observed particle radius ranges from 0.1 to 3.5 μm for dust and from 0.1 to 5.5 μm for water ice clouds, with the largest particles found below 10 km
Water ice clouds with 0.1–1 μm particles reached altitudes up to 100 km during the peak of the 2018 global dust storm
The 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‐
μ 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
μ m and applied it to observations gathered between
Ls=165∘ and
Ls=243∘. 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 (
reff≤0.3μ 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 (
reff≥1.5μ 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.
Plain Language Summary
In this article, we use data from the Atmospheric Chemistry Suite infrared spectrometer onboard the European Space Agency‐Roscosmos ExoMars Trace Gas Orbiter mission to study water ice clouds in the Martian atmosphere. More specifically, we aim to characterize the evolution of their altitude, geographic distribution, and microphysical properties before and during the planet‐wide dust storm that occurred during the summer of 2018. In particular, we developed a method to simultaneously detect the water ice clouds and constrain their particle size using simulated spectra of water ice. We observe that the maximal altitude of the clouds increased from 60 km to above 90 km during the storm. Most high‐altitude clouds have small particle sizes (lower than 0.3
μ m) as expected from the low pressure at such altitude. However, we also observe for the first time large (larger than 1.5
μ m) water ice particles at unusually high altitude (higher than 60 km), uniquely during the storm. This suggests that the increased atmospheric activity associated with global dust storm significantly impacts water ice cloud formation.
Key Points
Monitoring of Martian water ice clouds and derivation of vertical profiles of particle size using the 3‐
μ m spectral band
Observation of mesospheric water ice clouds at altitudes greater than 90 km during the MY 34 GDS
Evidence of water ice particles larger than 1.5
μ m between 50 and 70 km during the GDS
Phosphine (PH
3
) 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 (CH
4
), 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 PH
3
with line strengths comparable to previously observed CH
4
lines. The signature of PH
3
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.
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