Dr Kevin Olsen

Abstract:

Context. Hydrogen chloride (HCl) was independently detected in the Martian atmosphere by the Nadir and Occultation for MArs Discovery (NOMAD) and Atmospheric Chemistry Suite (ACS) spectrometers aboard the ExoMars Trace Gas Orbiter (TGO). Photochemical models show that using gas-phase chemistry alone is insufficient to reproduce these data. Recent work has developed a heterogeneous chemical network within a 1D photochemistry model, guided by the seasonal variability in HCl. This variability includes detection almost exclusively during the dust season, a positive correlation with water vapour, and an anticorrelation with water ice. Aims. The aim of this work is to show that incorporating heterogeneous chlorine chemistry into a global 3D model of Martian photochemistry with conventional gas-phase chemistry can reproduce spatial and temporal changes in hydrogen chloride on Mars, as observed by instruments aboard the TGO. Methods. We incorporated this heterogeneous chlorine scheme into the Mars Planetary Climate Model (MPCM). After some refinements to the scheme, mainly associated with it being employed in a 3D model, we used it to model chlorine photochemistry during Mars Years (MYs) 34 and 35. These two years provide contrasting dust scenarios, with MY 34 featuring a global dust storm. We also examined correlations in the model results between HCl and other key atmospheric quantities, as well as production and loss processes, to understand the impact of different factors driving changes in HCl. Results. We find that the 3D model of Martian photochemistry using the proposed heterogeneous chemistry is consistent with the changes in HCl observed by ACS in MY 34 and MY 35, including detections and 70% of non-detections. For the remaining 30% of non-detections, model HCl is higher than the ACS detection limit due to biases associated with water vapour, dust, or water ice content at these locations. As with previous 1D model calculations, we find that heterogeneous chemistry is required to describe the loss of HCl, resulting in a lifetime of a few sols that is consistent with the observed seasonal variation in HCl. As a result of this proposed chemistry, modelled HCl is correlated with water vapour, airborne dust, and temperature, and anticorrelated with water ice. Our work shows that this chemical scheme enables the reproduction of aphelion detections in MY 35.

Abstract:

In this presentation, we will show efforts made to include accurate photochemical modelling of hydrogen chloride (HCl) and ozone (O3) in the Mars Planetary Climate Model in order to reconcile recent observations.The ExoMars Trace Gas Orbiter (TGO) has detected and characterised trace gases in the Martian atmosphere over several Mars years. With its data, upper limits of potential constituents have been constrained, the accuracy of species’ concentration measurements has been improved, and seasonal and spatial variations in the atmosphere have been observed. The wealth of data obtained has addressed several open questions about the nature of Mars’ atmosphere, while other measurements have revealed much that remains poorly understood. For example, models continue to struggle to reproduce ozone distributions, both spatially and temporally, as well as seasonal variations in atmospheric oxygen (O2), suggesting that some key photochemical interactions may be being overlooked. As another example, despite seven years of dedicated observations producing very low upper limits on atmospheric methane levels, there remains no unifying hypothesis that simultaneously explains the detections reported by other Mars assets at Gale Crater [e.g., 1-4].Hydrogen chloride—the first new gas detected by TGO [5,6]—has been investigated recently using the mid-infrared channel on TGO’s Atmospheric Chemistry Suite (ACS MIR) [7,8]. Observations show a strong seasonal dependence of HCl in the atmosphere, with almost all detections occurring during the latter half of the year between the start of dust activity and the southern hemisphere autumnal equinox. There are also unusual measurements of HCl, localised in both time and space, during the aphelion season. Chlorine-bearing species such as HCl are important to understand in the Mars atmosphere because on Earth they are involved in numerous processes throughout the planetary system, including volcanism, from which HCl on Earth ultimately originates. Further, chlorine species play a key role in atmospheric chemistry: they influence oxidative chemistry and variations in the aforementioned O2 and O3 concentrations (e.g., by catalysing the destruction of ozone), and by extension, potential CH4 in the Martian atmosphere [9]. However, much remains unknown about original source and sinks of HCl, as well as the factors controlling its distribution and variation.Here, we use the Mars Planetary Climate Model—a 3-D global climate model that includes a photochemical network—to investigate potential mechanisms accounting for patterns in ozone and HCl detections and interactions between them. We begin with the role of heterogeneous chemistry involving ice and dust aerosols, by implementing modelling developed for the Open University Mars Global Climate Model [10] and building on existing chlorine photochemical model networks [11,12,13]. Heterogeneous chemistry affects the abundances of oxidative species such as OH and HO2, and by extension, O and O3. In addition, we investigate how such processes can potentially serve as a mechanism for direct release and sequestration of HCl from the atmosphere. We also explore potential mechanisms behind the annual occurrence of spatially-constrained aphelion HCl, including volcanic sources, and we investigate the interplay between chlorine-bearing species and OH, HO2,O, and O3. Figure 1 shows the way that HCl appears during spring and summer in the southern hemisphere (solar longitudes 180-360°) when water vapour is present in the Martian atmosphere. Ozone behaves in the opposite manner and is present when water vapour abundances are low. As shown, these species are anti-correlated; we explore the important chemical pathways connecting them.Understanding the role of oxidative chemistry on HCl and other trace gases is key to achieving a more complete picture of processes occurring in the present-day Mars atmosphere, as well as processes that have shaped its evolution and habitability.Figure 1: Observations of CO, O2, O3 and HCl seasonally and across multiple Mars Years. Upper panel: CO and O2 observations from Curiosity’s Sample Analysis at Mars (SAM) instrument (stars; [14]) and the Mars Climate Database (lines; [15]). Lower panel: O3 and HCl observations from TGO’s ACS instrument [8]. MY=Mars Year; NH/SH=northern/southern hemisphere. Figure from Kevin Olsen.References:[1] Giuranna, M., et al. (2019). Nat. Geosci. 12, 326–332. [2] Korablev, O. et al. (2019). Nature 568, 517–520. [3] Montmessin, F. et al. (2021). Astron. Astrophys. 650, A140. [4] Webster, C. R. et al. (2015). Science 347, 415-417. [5] Korablev O. I. et al. (2021). Sci. Adv., 7, eabe4386. [6] Olsen K. S. et al. (2021). Astron. Astrophys., 647, A161. [7] Olsen K. S. et al. (2024a). JGR, 129, e2024JE008350. [8] Olsen K. S. et al. (2024b). JGR, 129, e2024JE008351. [9] Taysum, B. M. et al. (2024). Astron. Astrophys., 687, A191. [10] Brown M. A. J. et al. (2022). JGR, 127, e2022JE007346. [11] Rajendran, K. et al. (2025). JGR: Planets 130(3), p.e2024JE008537. [12] Streeter, P. M. et al. (2025). GRL 52(6), p.e2024GL111059. [13] Benne, B. et al. (2024). EPSC, pEPSC2024-1037. [14] Trainer, M. G. et al. (2019). JGR 124, 3000. [15] Millour, E. et al. (2022). Mars Atmosphere: Modelling and Observations, p. 1103.

Abstract:

Mars has an axial tilt of 25.2°, comparable to that on Earth of 23.4°. This gives rise to very similar seasons, and even leads to our definition of Martian time, aligning the solar longitudes (Ls) such that Ls 0° and 180° occur at the equinoxes. In the northern hemisphere, between the equinoxes, the north polar region experiences polar days without darkness in spring and summer, and days of total darkness in the fall and winter. The dark polar winters give rise to a polar vortex that encircles the polar region and encircles an atmosphere of very cold and dry air bound within (1-3).The Atmospheric Chemistry Suite (ACS) mid-infrared channel (MIR) on the ExoMars Trace Gas Orbiter (TGO; 4) operates in solar occultation mode in which the Sun is used as a light source when the atmosphere lies between the Sun and TGO. The tangent point locations of ACS MIR observation necessarily lie on the solar terminator on Mars. At the poles when either polar night or polar day are experienced, there is no terminator, and solar occultations are restricted to outside such a region. The latitudinal distribution of ACS MIR solar occultations during the north polar fall and winter over four Mars years (MYs) is shown in Fig. 1. The furthest northern extent of observations occurs at the equinoxes, and falling northern boundary is seen between, as the north pole points further away from the Sun (similarly in the south, where it is polar day).While direct observations of the north polar vortex are forbidden with solar occultations, the polar vortex is not perfectly circular (1-3) and occasionally, descends into the illuminated region where we are making observations. The characteristic signs that we are sampling the polar vortex are a sudden drop in temperature below 20 km, the almost complete reduction in water vapour volume mixing ratio (VMR) and an enhancement in ozone VMR, the latter of which is extremely rare (5).To measure the extent of the polar vortex, we use temperature measurements from the Mars Climate Sounder (MCS; 6, 7) on Mars Reconnaissance Orbiter (MRO). We define the polar vortex as the average temperature over 10-20 km being within a boundary of 170 K (30). We introduce a novel technique to determine this boundary during a 1° Ls period using an alpha hull. We show that we can accurately measure the area of the polar vortex and achieve similar results to (3). The impact of the southern summer and dust activity is clearly visible in the time series of the northern polar vortex extent, leading to maxima occurring at the equinoxes, and shrinking toward perihelion. The impact of global dust storms and the late season dust storms are also pronounced.We will show the vertical structure of water vapour and ozone VMRs inside and outside the north polar vortex, the results of a search for polar vortex temperatures from the near-infrared channel (NIR) of ACS (along the dark blue dots in Fig. 1), and show whether these results agree with the polar vortex extent measurements using MCS.       Figure 1: The latitudes of ACS MIR solar occultation as a function of time (solar longitude Ls) during northern fall (Ls 180-270°) and winter (Ls 270-360°). Data from Mars years (MYs) 34-37 are indicated with colours. The region of interest in searching for polar vortex excursions is highlighted in blue.References:(1) Streeter, P. M. et al. J. Geophys. Res. 126, e2020JE006774 (2021).(2) Streeter, P. M., Lewis, S. R., Patel, M. R., Holmes, J. A., & Rajendran, K. Icarus 409, 115864 (2024).(3) Alsaeed, N.R., Hayne, P. O. & Concepcion, V. J. Geophys. Res. 129, e2024JE008397 (2024).(4) Korablev, O. et al. Space Sci. Rev. 214, 7 (2018).(5) Olsen, K. S., et al. J. Geophys. Res. 127, e2022JE007213 (2022).(6) Kleinböhl, A., et al. J. Geophys. Res., 114, E10006 (2009).(7) Kleinböhl, A., Friedson, A. J., & Schofield, J. T. J. Quant. Spectrosc. Radiat. Transfer. 187, 511-522 (2017).

Abstract:

Recent observations from the ExoMars Trace Gas Orbiter (TGO) have revealed the presence of hydrogen chloride (HCl) in the martian atmosphere. HCl shows strong seasonality, primarily appearing during Mars' perihelion period before decreasing faster than projected from photolysis and gas‐phase chemistry. HCl profiles also display local anti‐correlation with water ice aerosol. One candidate explanation is heterogeneous chemistry. We present the first results from a heterogeneous chlorine chemistry scheme incorporated into a Mars global climate model (GCM), with atmospheric dust/water ice parameterized as an HCl source/sink respectively. Results were compared against a Mars GCM with gas‐phase only chlorine chemistry and observations from TGO's Atmospheric Chemistry Suite. We found that the heterogeneous scheme significantly improved the modeled HCl seasonal, latitudinal, and vertical distribution, supporting a crucial role for heterogeneous chemistry in Mars' chlorine cycle. Remaining discrepancies show that further work is needed to characterize the exact aerosol reactions involved.

Abstract:

Recent observations by instruments aboard the ExoMars Trace Gas Orbiter (TGO) have revealed the seasonal presence of hydrogen chloride ( HCl $\text{HCl}$ ) in the Martian atmosphere. This discovery may have important implications for Martian photochemistry as chlorine species are chemically active, and it may provide a link between the atmosphere and known surface reservoirs of chlorine. However, the global distribution of atmospheric HCl $\text{HCl}$ is unknown beyond the very sparse TGO observations, and the source and sink processes driving the observed variability of HCl $\text{HCl}$ are not currently understood. We used a Martian global climate model to investigate, for the first time, the spatial distribution of chlorine species in the Martian atmosphere, and the resulting distribution of surface perchlorates formed via adsorption of atmospheric chlorine species. We adapted an existing Martian photochemical scheme to include gas‐phase chlorine chemistry with HCl as the source species, and the resulting atmospheric perchloric acid was allowed to deposit onto the Martian surface via a heterogeneous adsorption scheme. We found that odd‐oxygen ( O , O 3 $\mathrm{O},{\mathrm{O}}_{3}$ ) and odd‐hydrogen ( H , OH , HO 2 $\mathrm{H},\text{OH},{\text{HO}}_{2}$ ) species play a major role in controlling the distribution of atmospheric chorine species. Surface perchlorate deposition was found to occur preferentially at high latitudes; in the tropics, the perchlorate distribution was anti‐correlated with surface thermal inertia and agreed qualitatively with observations of surface chlorine. Our model predicted a relative enhancement of HCl in polar regions, but it did not reproduce the observed strong seasonality of HCl, suggesting that heterogeneous chemistry may be required to explain the observed chlorine cycle.