Dr Kevin Olsen

Abstract:

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.

Abstract:

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.

Abstract:

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.

Abstract:

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.

Abstract:

Hydrogen chloride (HCl) was recently discovered in the atmosphere of Mars by two spectrometers onboard the ExoMars Trace Gas Orbiter. The reported detection made in Martian Year 34 was transient, present several months after the global dust storm during the southern summer season. Here, we present the full data set of vertically resolved HCl detections obtained by the NOMAD instrument, which covers also Martian year 35. We show that the particular increase of HCl abundances in the southern summer season is annually repeated, and that the formation of HCl is independent from a global dust storm event. We also find that the vertical distribution of HCl is strikingly similar to that of water vapor, which suggests that the uptake by water ice clouds plays an important role. The observed rapid decrease of HCl abundances at the end of the southern summer would require a strong sink independent of photochemical loss.