Trace Gas Orbiter Research Articles

On the impact of the vertical structure of Martian water ice clouds on nadir atmospheric retrievals from simultaneous EMM/EXI and TGO/ACS-MIR observations.

Retrieving the optical depth of the Martian clouds (τcld) is a powerful way to monitor their spatial and temporal evolution. However, such retrievals from nadir imagery rely on several assumptions, including the vertical structure of the clouds in the atmosphere. Here we compare the results of cloud optical depth retrievals at 320 nm from the Emirates eXploration Imager (EXI) onboard the Emirates Mars Mission (EMM) “Hope” orbiter performed using a basic uniform cloud profile used in previous studies and using derived cloud profiles obtained from near-simultaneous Solar Occultation observations in the 3.1–3.4 μm spectral range from the Middle-Infrared channel of the Atmospheric Chemistry Suite (ACS) instrument onboard the ESA Trace Gas Orbiter (TGO). We show that the latitudinal dependence of the cloud vertical profiles can have a strong impact on the nadir retrievals; neglecting it can lead to a significant underestimation of τcld in the polar regions (up to 25 % to 50 %, depending on the vertical distribution of the dust in the atmosphere) and to a lesser extent, to an overestimation of τcld around the equator. We also discuss the impact of a vertically-dependent particle size profile, as previous studies have shown the presence of very small water ice particles at the top of the clouds. From this analysis, we provide recommendations for the improvement of water ice cloud parameterization in radiative transfer algorithms in nadir atmospheric retrievals.

Unambiguous detection of mesospheric CO2 clouds on Mars using 2.7 μm absorption band from the ACS/TGO solar occultations

Mesospheric CO2 clouds are one of two types of carbon dioxide clouds known on Mars. We present observations of mesospheric CO2 clouds made by Atmospheric Chemistry Suite (ACS) onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter (TGO). We analyzed 1663 solar occultation sessions of Thermal InfraRed (TIRVIM) and Middle InfraRed (MIR) channels of ACS covering more than two Martian years that contain spectra of 2.7 μm carbon dioxide ice absorption band. That allowed us to unambiguously discriminate carbon dioxide ice aerosols from mineral dust and water ice aerosols, not relying on the information of atmospheric thermal conditions. CO2 clouds were detected in eleven solar occultation observations at altitudes from 39 km to 90 km. In five cases, there were two or three layers of CO2 clouds that were vertically separated by 5–15 km gaps. Effective radius of CO2 aerosol particles is in the range of 0.1–2.2 μm. Spectra produced by the smallest particles indicate a need for a better resolved CO2 ice refractive index. Nadir optical depth of CO2 clouds is in the range 5 × 10−4–4 × 10−2 at both 2.7 μm and 0.8 μm. Asymmetrical diurnal distribution of detections observed by ACS is potentially due to local time variations of temperature induced by thermal tides. Two out of five cases of carbon dioxide cloud detections made by the TIRVIM instrument reveal the simultaneous presence of CO2 ice and H2O ice aerosols. Temperature profiles measured by the Near InfraRed (NIR) channel of ACS are used to calculate CO2 saturation ratio S at locations of carbon dioxide clouds. Supersaturation S > 1 is detected in only 6 out of 19 cases of CO2 cloud layers; extremely low values of S < 0.1 are found in 9 out of 19 cases.

A Global Dataset of Potential Chloride Deposits on Mars as Identified by TGO CaSSIS

Chloride deposits are markers for early Mars’ aqueous past, with important implications for our understanding of the martian climate and habitability. The Colour and Stereo Surface Imaging System (CaSSIS) onboard ESA’s Trace Gas Orbiter provides high-resolution color-infrared images, enabling a planet-wide search for (small) potentially chloride-bearing deposits. Here, we use a neural network to map potentially chloride-bearing deposits in CaSSIS images over a significant fraction of the planet. We identify 965 chloride deposit candidates with diameters ranging from <300 to >3000 m, including previously unknown deposits, 136 (~14%) of which are located in the highlands north of the equator, up to ~36°N. Northern chloride candidates tend to be smaller than in the south and are predominantly located in small-scale topographic depressions in low-albedo Noachian and Hesperian highland terranes. Our new dataset augments existing chloride deposit maps, informs current and future imaging campaigns, and enables future modelling work towards a better understanding of the distribution of near-surface water in Mars’ distant past.

Relationships Between HCl, H2O, Aerosols, and Temperature in the Martian Atmosphere: 2. Quantitative Correlations

AbstractThe detection of hydrogen chloride (HCl) in the atmosphere of Mars was among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Its discovery using the Atmospheric Chemistry Suite mid‐infrared channel (ACS MIR) showed a distinct seasonality and possible link to dust activity. This paper is part 2 of a study investigating the link between HCl and aerosols by comparing gas measurements made with TGO to dust and water ice opacities measured with the Mars Climate Sounder (MCS). In part 1, we showed, and compared, the seasonal evolution of vertical profiles of HCl, water vapor, temperature, dust opacity, and water ice opacity over the dusty periods around perihelion (solar longitudes 180°–360°) across Mars years 34–36. In part 2, we investigated the quantitative correlations in the vertical distribution between each quantity, as well as ozone. We show that there is a strong positive correlation between HCl and water vapor, which is expected due to fast photochemical production rates for HCl when reacting with water vapor photolysis products. We also show a strong positive correlation between water vapor and temperature, but are unable to show any correlation between temperature and HCl. There are weak correlations between the opacities of dust and water ice, and dust and water vapor, but only very low correlations between dust and HCl. We close with a discussion of possible sources and sinks and that interactions between HCl and water ice are the most likely for both, given the inter‐comparison.

Relationships Between HCl, H2O, Aerosols, and Temperature in the Martian Atmosphere: 1. Climatological Outlook

AbstractDetecting trace gases such as hydrogen chloride (HCl) in Mars' atmosphere is among the primary objectives of the ExoMars Trace Gas Orbiter (TGO) mission. Terrestrially, HCl is closely associated with active volcanic activity, so its detection on Mars was expected to point to some form of active magmatism/outgassing. However, after its discovery using the mid‐infrared channel of the TGO Atmospheric Chemistry Suite (ACS MIR), a clear seasonality was observed, beginning with a sudden increase in HCl abundance from below detection limits to 1–3 ppbv in both hemispheres coincident with the start of dust activity, followed by very sudden and rapid loss at the southern autumnal equinox. In this study, we have investigated the relationship between HCl and atmospheric dust by making comparisons in the vertical distribution of gases measured with ACS and aerosols measured co‐located with the Mars Climate Sounder (MCS). This study includes HCl, water vapor, and ozone measured using ACS MIR, water vapor and temperature measured with the near infrared channel of ACS, and temperature, dust opacity, and water ice opacity measured with MCS. In part 1, we show that dust loading has a strong impact in temperature, which controls the abundance of water ice and water vapor, and that HCl is very closely linked to water activity. In part 2, we investigate the quantitative correlations between each quantity and discuss the possible source and sinks of HCl, their likelihood given the correlations, and any issues arising from them.

Upper limits of HO2 in the atmosphere of Mars from the ExoMars Trace Gas Orbiter

ABSTRACT Odd-hydrogen (HO$_\mathrm{x}$) species have a crucial role in regulating the chemistry of the atmosphere of Mars and are important to understand some of the most fundamental aspects regarding its atmospheric composition such as the long-term stability of CO$_2$. Despite the key role of these species for our understanding of the Martian photochemistry, there is little observational evidence constraining their abundances. In this study, we use infrared solar occultation observations from the Atmospheric Chemistry Suite aboard the ExoMars Trace Gas Orbiter to search for spectral signatures of HO$_2$ in the atmosphere of Mars. In our analysis of the data, we retrieve vertical profiles of pressure, temperature, and water vapour mixing ratio, but are unable to confidently detect the presence of HO$_2$ features in the spectra. We report upper limits of 15 ppbv (5$\sigma$), which represents an order of magnitude improvement with respect to previous investigations. Comparing the derived upper limits with the expectations from 3-dimensional Global Climate Models, we find that approximately an order of magnitude improvement in the instrument sensitivity would be required to detect this molecule and/or constrain the models.

Accretion of Meteoric Organic Matter at the Surface of Mars and Potential Production of Methane by Ultraviolet Radiation

In this study, a comprehensive model of the meteoric organic cycle on Mars for the current geological period is developed, which characterizes the ablation of exogenous organic matter in the upper atmosphere, the accretion of intact carbon at the surface, and the potential production of methane by UV photolysis from the surface reservoir. The model accounts for both the latitudinal and seasonal variation of the meteoroids’ input from the most relevant populations in the inner solar system. A recent version of the University of Leeds Chemical Ablation Model, which includes a semiempirical model to describe the pyrolysis kinetics of the meteoric organic matter, is then combined with this meteoroid input function and a semiempirical model that quantifies the UV production of methane. The minimum and maximum accretion rates of organics are between 18 and 90 kg sol−1 at aphelion and 45–134 kg sol−1 at the first crossing of the ecliptic plane. The resulting mixing ratios of carbon, in the top 200 μm of the surface layer, range from 0.09–0.43 ppm at 20°N to 4.8–8.9 ppm around the south pole. To be consistent with the methane upper limit of 0.02 ppbv measured by the NOMAD instrument on the ExoMars Trace Gas Orbiter, the UV photolysis yields for methane production need to be around 3% assuming a meteoric carbon content in comets of 25.6 wt% and an atmospheric lifetime of methane of 329 Earth yr. Alternatively, a laboratory estimate of 20% for the methane production yield would require a lifetime of 60 Earth yr.

The Role and Lifetime of Dissociative Heterogeneous Processes in Improving Simulated Ozone on Mars

AbstractOzone simulated in Mars Global Climate Models (MGCMs) is used to assess the underlying chemistry occurring in the atmosphere. Currently, ozone total column abundance (TCA) is under‐predicted in MGCMs by up to 120%, implying missing or inaccurate chemistry in models. Heterogeneous reactions of hydroxyl radicals (HOX) have been offered as an explanation for some of this bias, because they cause ozone to increase at locations where it's currently under‐predicted. We use four simulations to compare modeled ozone TCA with observations from the UVIS spectrometer aboard the ExoMars Trace Gas Orbiter to improve the representation of heterogeneous processes and their impact on ozone. We use a gas‐phase only run, a dissociative scheme, an adsorbed HOX retention scheme, and a hybrid scheme that combines the dissociative mechanism with the retention of HOX on water ice. We find retention of HOX is dependent on water ice sublimation, and ozone abundance increases when water ice persists for longer periods (1–20 sols). Over time, the loss of HOX causes a depletion in H2O2 concentration (HOX reservoir), and thus allows ozone concentration to increase. When adsorbed HOX are desorbed and dissociate into other by‐products, HOX are not immediately available to destroy ozone. This results in larger ozone concentrations than if desorbed HOX are released directly back into their gaseous states. When using the hybrid scheme, ozone TCA is increased up to 50% where the ozone deficit is greatest, demonstrating the best agreement with observations, and implying that HOX radicals are both retained when adsorbed and dissociate.

Evidence for transient morning water frost deposits on the Tharsis volcanoes of Mars

The present-day water cycle on Mars has implications for habitability and future human exploration. Water ice clouds and water vapour have been detected above the Tharsis volcanic province, suggesting the active exchange of water between regolith and atmosphere. Here we report observational evidence for extensive transient morning frost deposits on the calderas of the Tharsis volcanoes (Olympus, Arsia and Ascraeus Montes, and Ceraunius Tholus) using high-resolution colour images from the Colour and Stereo Surface Imaging System on board the European Space Agency’s Trace Gas Orbiter. The transient bluish deposits appear on the caldera floor and rim in the morning during the colder Martian seasons but are not present by afternoon. The presence of water frost is supported by spectral observations, as well as independent imagery from the European Space Agency’s Mars Express orbiter. Climate model simulations further suggest that early-morning surface temperatures at the high altitudes of the volcano calderas are sufficiently low to support the daily condensation of water—but not CO2—frost. Given the unlikely seasonal nature of volcanic outgassing, we suggest the observed frost is atmospheric in origin, implying the role of microclimate in local frost formation and a contribution to the broader Mars water cycle.

CO2 in the atmosphere of Mars depleted in 13C

We present here vertically resolved measurements of the carbon isotopic composition of CO2 in the Martian atmosphere between the surface and 50 km of altitude. The results are based on data taken by the ExoMars Trace Gas Orbiter with the NOMAD instrument and have been aggregated to derive an average vertical profile of 13C/12C. We find no seasonal or spatial trends with variabilities beyond the sensitivity of the measurements. For the analysis, we developed a method that allows us to estimate whether the observed variability of isotopic measurements is beyond their intrinsic accuracy, by exploiting ab-initio spectroscopy and the radiometric noise of the instrument. Applying this method to our data, we find that atmospheric CO2 is depleted in 13C compared to the Earth standard by 30‰ to 45‰, in line with previous ground-based measurements values of the atmosphere and in contrast with the average value obtained by Curiosity at the surface (46 ± 4‰). These differences in isotopic signatures of CO2 as measured across the atmosphere and near-surface pose new questions when inferring the evolution and history of carbon on Mars, and suggest that processes such as a strong atmosphere-surface interactions may be fractionating the carbon reservoirs on the planet.

Possible influence of Martian surface mineralogy on the detectability of atmospheric trace gases - mid-infrared simulation results

The paper focuses on the influence of the optical properties of Martian surface minerals on remotely detected gaseous components of the Martian atmosphere, when the spectrometer receives a combined signal from the Martian soil and atmosphere. Our considerations are primarily concerned with the detectability of methane, but the problem may also apply to other trace gases. Detections of methane in the Martian atmosphere have been reported from Mars Express (orbiting Mars), the Curiosity rover on the Martian surface, and from Earth. Its presence in the Martian atmosphere is being questioned today. The reason for these doubts is that both spectrometers onboard ExoMars Trace Gas Orbiter have not yet detected any methane in the Martian atmosphere using the very sensitive solar occultation method. The solar occultation method is unable to probe the lowest layers of the atmosphere at mid-latitudes, and so, its presence in this part of the atmosphere is assumed to be due to its possible source in the ground, as suggested by some works.This paper considers whether the spectral characteristics of the soil may hinder the remote detection of methane. One of the examples discussed in the article relates to the possible observation of methane over mineralogical surfaces that may be the source of this gas. The examples of other surface mineralogical compositions are also discussed. The series of numerical simulations carried out in the region of the strong methane absorption band and the examples where the optical properties of the surface change the shape and contrast of this absorption band are shown. The codes used provide estimates of the spectral reflectance/emittance and total radiance of the Martian surface and atmosphere in the mid-infrared spectral region. The surface covered by dust was described by the reflectance and emittance calculated from n,k using Mie and Hapke theories or known from laboratory measurements. The different concentrations of atmospheric trace gases were taken into account.

Climatology of gravity wave activity based on two Martian years from ACS/TGO observations

Context. Gravity waves redistribute energy and momentum between the lower and upper atmosphere, thus providing vertical coupling between layers, and they affect the state, dynamics, and variability of the upper atmosphere. The statistics of gravity wave activity on Mars is poorly explored but is required in order to characterize the atmospheric circulation and to constrain numerical models. Aims. We present the gravity wave statistics accumulated over two Martian years: from the second half of Martian year 34 to the middle of Martian year 36 (May 2018 to February 2022). The statistics includes seasonal and latitude distributions of the wave potential energy and drag, serving to represent the wave activity and impact on the atmospheric dynamics. Methods. The observations were performed by the middle- and near-infrared spectrometers of the Atmospheric Chemistry Suite on board the ExoMars Trace Gas Orbiter. The temperature profiles we obtained independently from both channels during simultaneous measurements show a good agreement, thus providing verification and enhancing confidence in the data. The gravity wave parameters included amplitudes of temperature fluctuations, potential energy per unit mass, and wave drag. These parameters were retrieved at altitudes up to 160 and 100 km from the middle- and near-infrared channels, respectively. Results. A comparison of the data obtained during the global dust storm of Martian year 34 with the corresponding period of Martian year 35 without a storm revealed a reduction of wave activity in mid-latitudes, which is in agreement with previous observations, and enhancement in the polar regions of the southern hemisphere, which was not predicted by simulations with a high-resolution circulation model.

Evidence of rapid hydrogen chloride uptake on water ice in the atmosphere of Mars

In 2020, hydrogen chloride (HCl) in the gas phase was discovered in the atmosphere of Mars with the Atmospheric Chemistry Suite (ACS) onboard the Trace Gas Orbiter (TGO) mission (Korablev et al., 2021). Its volume mixing ratio (VMR) shows a seasonal increase of up to 5 ppbv during the perihelion season, followed by a sudden drop to undetectable levels, contradicting modelling estimates of the HCl lifetime of several months. In the Earth's stratosphere, heterogeneous uptake of HCl onto water ice is known to be a major sink for this species. This reaction is now also considered when modelling HCl abundances in the Martian atmosphere. In this work, we use simultaneous measurements of water ice and HCl obtained by the ACS instrument to find particular structures in the vertical profiles as detached gas layers at ice-free altitudes (“ice-holes”). From these particular examples we conclude that the heterogeneous uptake of HCl onto water ice operates on Mars and is a fast mechanism regulating the HCl abundance in the atmosphere of Mars.

Minimum Noise Fraction Analysis of TGO/NOMAD LNO Channel High-Resolution Nadir Spectra of Mars

NOMAD is a suite of spectrometers on the board of the ESA-Roscosmos Trace Gas Orbiter (TGO) spacecraft and is capable of investigating the Martian environment at very high spectral resolution in the ultraviolet–visible and infrared spectral ranges by means of three separate channels: UVIS (0.2–0.65 μm), LNO (2.2–3.8 μm), and SO (2.3–4.3 μm). Among all channels, LNO is the only one operating at infrared wavelengths in nadir-viewing geometry, providing information on the whole atmospheric column and on the surface. Unfortunately, the LNO data are characterized by an overall low level of signal-to-noise ratio (SNR), limiting their contribution to the scientific objectives of the TGO mission. In this study, we assess the possibility of enhancing LNO nadir data SNR by applying the Minimum Noise Fraction (MNF), a well-known algorithm based on the Principal Components technique that has the advantage of providing transform eigenvalues ordered with increasing noise. We set up a benchmark process on an ensemble of synthetic spectra in order to optimize the algorithm specifically for LNO datasets. We verify that this optimization is limited by the presence of spectral artifacts introduced by the MNF itself, and the maximum achievable SNR is dependent on the scientific purpose of the analysis. MNF application study cases are provided to LNO data subsets in the ranges 2.627–2.648 μm and 2.335–2.353 μm (spectral orders 168 and 189, respectively) covering absorption features of gaseous H2O and CO and CO2 ice, achieving a substantial enhancement in the quality of the observations, whose SNR increases up to a factor of 10. While such an enhancement is still not enough to enable the investigation of spectral features of faint trace gases (in any case featured in orders whose spectral calibration is not fully reliable, hence preventing the application of the MNF), interesting perspectives for improving retrieval of both atmospheric and surface features from LNO nadir data are implied.

Ultraviolet and Visible Reflectance Spectra of Phobos and Deimos as Measured by the ExoMars‐TGO/NOMAD‐UVIS Spectrometer

AbstractSpectroscopic measurements are a powerful tool to investigate the surface composition of airless bodies and provide clues of their origin. The composition and origin of Phobos and Deimos are still unknown and are currently widely debated. We present spectroscopic measurements of Phobos and Deimos at ultraviolet and visible wavelengths (250–650 nm) made by the NOMAD‐Ultraviolet and Visible Spectrometer (UVIS) on the ExoMars TGO mission. These new spectra cover multiple areas on Phobos and Deimos, and are of generally higher spectral resolution and signal‐to‐noise than previous spectra, and extend to lower wavelengths than most previous measurements. The UVIS spectra confirm a red‐sloped spectrum lacking any strong absorption features; however, we confirm the presence of a previously identified absorption feature near 0.65 μm and tentative absorption near 0.45 μm. The observed Phobos and Deimos spectra are similar to D‐ and T‐type asteroids, adding weight to the captured asteroid hypothesis for the moons' origins. We also find, however, that the UVIS Phobos reflectance spectra of Phobos' red unit is a relatively close match to the olivine‐rich, highly shocked Mars meteorite NWA 2737, with a low overall reflectance, a red‐sloped spectrum, and lack of olivine‐associated absorption bands in the UVIS spectral range. This meteorite, however, exhibits spectral features at longer wavelengths that not observed in the Martian moon spectra, indicating a need for further investigation at longer wavelengths to interpret whether this material could inform our understanding of Phobos' origin.

Numerical Simulation of Liulin-MO Instrument for Measuring Cosmic Radiation Onboard ExoMars Trace Gas Orbiter

We present the results from numerical simulation of Liulin-MO instrument for measurement of the space radiation on board Trace Gas Orbiter (TGO) satellite of ExoMars-2016 mission during its transit to Mars in April-September 2016 [Semkova J., T. Dachev, St. Maltchev, B. Tomov, Yu. Matviichuk} et al. (2015) Radiation environment investigations during ExoMars mission to Mars – objectives, experiments and instrumentation, C. R. Acad. Bulg. Sci., 68 (4), 485–496] [Semkova J., R. Koleva, V. Benghin, T. Dachev, Yu. Matviichuk et al. (2018) Charged particles radiation measurements with Liulin-MO dosimeter of FREND instrument aboard ExoMars Trace Gas Orbiter during the transit and in high elliptic Mars orbit, Icarus, 303, 53–66], using the Geant4 package. The methodology for calculating galactic cosmic rays (GCR) linear energy transfer (LET) is described. A methodology for screening false signals in the deposited energy spectra in the Liulin-MO detectors and correspondingly LET spectra is proposed. The contribution of the individual components of the spectrum of GCR in the LET spectrum is estimated. The calculated average radiation quality factor of GCR using the cleaned from false signals LET spectrum is compared with the experimental results obtained during the transit to Mars and a reasonable agreement is observed.

Martian Atmospheric Aerosols Composition and Distribution Retrievals During the First Martian Year of NOMAD/TGO Solar Occultation Measurements: 1. Methodology and Application to the MY 34 Global Dust Storm

AbstractSince the beginning of the Trace Gas Orbiter (TGO) science operations in April 2018, its instrument “Nadir and Occultation for MArs Discovery” (NOMAD) supplies detailed observations of the IR spectrums of the Martian atmosphere. We developed a procedure that allows us to evaluate the composition and distribution's parameters of the atmospheric Martian aerosols. We use a retrieval program (RCP) in conjunction with a radiative forward model (KOPRA) to evaluate the vertical profile of aerosol extinction from NOMAD measurements. We then apply a model/data fitting strategy of the aerosol extinction. In this first article, we describe the method used to evaluate the parameters representing the Martian aerosol composition and size distribution. MY 34 GDS showed a peak intensity from LS 190° to 210°. During this period, the aerosol content rises multiple scale height, reaching altitudes up to 100 km. The lowermost altitude of aerosol's detection during NOMAD observation rises up to 30 km. Dust aerosols reff were observed to be close to 1 μm and its νeff lower than 0.2. Water ice aerosols reff were observed to be submicron with a νeff lower than 0.2. The vertical aerosol structure can be divided in two parts. The lower layers are represented by higher reff than the upper layers. The change between the lower and upper layers is very steep, taking only few kilometers. The decaying phase of the GDS, LS 210°–260°, shows a decrease in altitude of the aerosol content but no meaningful difference in the observed aerosol's size distribution parameters.

Thermal Sounding of the Martian Atmosphere Using the ACS TIRVIM FT-IR Spectrometer on Board ExoMars TGO: Method for Solving the Inverse Problem

This paper presents a method for solving the inverse problem of thermal sounding using calibrated data from the ACS TIRVIM experiment on board the ExoMars Trace Gas Orbiter. The 1.7–17 µm range TIRVIM Fourier spectrometer as part of the ACS instrument complex aboard the ExoMars TGO operates in the nadir and solar occultation modes in orbit around Mars. The main scientific goal of TIRVIM in the nadir observation mode is the long-term constant monitoring of the thermal structure of the Martian atmosphere and the general content of aerosols and water vapor from measurements in the range of 5–16.7 µm (600–2000 cm–1). To process the TIRVIM nadir measurements, an algorithm was developed, allowing the retrieval of the vertical temperature profile from the surface to 60 km, the surface temperature, and the general content of dust and water ice in the atmosphere from the TIRVIM spectrum in the range of 600–1250 cm–1, as well as the water vapor column abundance according to measurements in the range of 1250–1830 cm–1. The processing method widely uses the achievements of previous similar experiments, taking into account the features of the TIRVIM spectra. Using the developed method 2.28 × 106 spectra obtained by TIRVIM in nadir by regular measurements, were processed with retrieval of the thermal structure up to 60 km altitude and the aerosol content in the atmosphere as well as additional 2.3 × 105 specially averaged TIRVIM spectra, were processed with retrieval of the water vapor column abundancein the Martian atmosphere.

Water Vapor Vertical Distribution on Mars During Perihelion Season of MY 34 and MY 35 With ExoMars‐TGO/NOMAD Observations

AbstractThe water vapor in the Martian atmosphere plays a significant role in the planet's climate, being crucial in most of the chemical and radiative transfer processes. Despite its importance, the vertical distribution of H2O in the atmosphere has not still been characterized precisely enough. The recent ExoMars Trace Gas Orbiter mission, with its Nadir and Occultation for MArs Discovery instrument, has allowed us to measure the H2O vertical distribution with unprecedented resolution. Recent studies of vertical profiles have shown that high dust concentration in the atmosphere, in particular during dust storms, induces an efficient transport of the H2O to higher altitudes, from 40 km up to 80 km. We study the H2O vertical distribution in a subset of solar occultations during the perihelion of two Martian years (MYs), including the 2018 Global Dust Storm (GDS), in order to compare the same Martian season under GDS and non‐GDS conditions. We present our state‐of‐the‐art retrieval scheme, and we apply it to a combination of two diffraction orders, which permits sounding up to about 100 km. We confirm recent findings of H2O increasing at high altitudes during Ls = 190°–205° in MY 34, reaching abundances of about 150 ppmv at 80 km in both hemispheres not found during the same period of MY 35. We found a hygropause's steep rising during the GDS from 30 up to 80 km. Furthermore, strong supersaturation events have been identified at mesospheric altitudes even in presence of water ice layers retrieved by the IAA team.

Impact of Mars GDS 2018 on the Chemistry of Water, Nitrogenated and Deuterated Cluster Ions: NOMAD Observations

AbstractThe ExoMars Trace Gas Orbiter instruments, Nadir and Occultation for Mars Discovery and Atmospheric Chemistry Suite have provided vertical profiles of H2O and HDO mixing ratios in the presence and absence of Global Dust Storm 2018. These observations are carried out in the lower atmosphere at SZA ∼ 71° of northern mid latitude (∼55°N, 130°E). Using NOMAD observations, we developed chemical models 1 and 2 for dust and non‐dust storm conditions, respectively. The production/loss rates of H2O, HDO and D2O and the densities of water, nitrogenated and deuterated cluster ions (H2O+, D2O+, HDO+, H+(H2O)n, D+(D2O)n, H+(HDO)n, CO4−, CO3−, CO3−(H2O)n, CO3−(D2O)n, CO3−(HDO)n, NO2−(H2O), NO2−(HDO), and NO2−(D2O) (for n = 1 to 5)) are estimated from these models between 10 and 70 km. We have used two ionization sources: (a) Galactic Cosmic Rays (GCR) and (b) hard X‐ray in both models. The D peak density produced by GCR is lower by a factor of ∼5 than that produced by the hard X‐rays. In model 1, the electron densities were increased by a factor of ∼2–3 than that produced by model 2. The estimated production/loss rates of H2O, HDO, and D2O are also larger by an order of magnitude in model 1 than that estimated by model 2.