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

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 1−2μm 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.ExoMars is a space mission of ESA and Roscosmos. The ACS experiment is led by IKI, the Space Research Institute in Moscow, assisted by LATMOS in France. The science operations of ACS are funded by Roscosmos and ESA. We are grateful to Michael Wolff, an anonymous reviewer, and Journal of Geophysical Research: Planets editorial board whose comments helped to improve this paper. The early retrievals in 2019 were supported by Ministry of Science and Education of the Russian government. M. Luginin, A. Fedorova, N. Ignatiev, A. Trokhimovskiy, and O. Korablev acknowledge RSF funding of Sections 4 and 5 under grant number 20-42-09035. F. Montmessin acknowl-edges funding from CNES and ANR (PRCI, CE31 AAPG2019)

Martian dust storm impact on atmospheric H₂O and D/H observed by ExoMars Trace Gas Orbiter

Global dust storms on Mars are rare but can affect the Martian atmosphere for several months. They can cause changes in atmospheric dynamics and inflation of the atmosphere, primarily owing to solar heating of the dust. In turn, changes in atmospheric dynamics can affect the distribution of atmospheric water vapour, with potential implications for the atmospheric photochemistry and climate on Mars. Recent observations of the water vapour abundance in the Martian atmosphere during dust storm conditions revealed a high-altitude increase in atmospheric water vapour that was more pronounced at high northern latitudes, as well as a decrease in the water column at low latitudes. Here we present concurrent, high-resolution measurements of dust, water and semiheavy water (HDO) at the onset of a global dust storm, obtained by the NOMAD and ACS instruments onboard the ExoMars Trace Gas Orbiter. We report the vertical distribution of the HDO/H O ratio (D/H) from the planetary boundary layer up to an altitude of 80 kilometres. Our findings suggest that before the onset of the dust storm, HDO abundances were reduced to levels below detectability at altitudes above 40 kilometres. This decrease in HDO coincided with the presence of water-ice clouds. During the storm, an increase in the abundance of H2O and HDO was observed at altitudes between 40 and 80 kilometres. We propose that these increased abundances may be the result of warmer temperatures during the dust storm causing stronger atmospheric circulation and preventing ice cloud formation, which may confine water vapour to lower altitudes through gravitational fall and subsequent sublimation of ice crystals. The observed changes in H2O and HDO abundance occurred within a few days during the development of the dust storm, suggesting a fast impact of dust storms on the Martian atmosphere

No detection of methane on Mars from early ExoMars Trace Gas Orbiter observations

The detection of methane on Mars has been interpreted as indicating that geochemical or biotic activities could persist on Mars today. A number of different measurements of methane show evidence of transient, locally elevated methane concentrations and seasonal variations in background methane concentrations. These measurements, however, are difficult to reconcile with our current understanding of the chemistry and physics of the Martian atmosphere, which-given methane's lifetime of several centuries-predicts an even, well mixed distribution of methane. Here we report highly sensitive measurements of the atmosphere of Mars in an attempt to detect methane, using the ACS and NOMAD instruments onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter from April to August 2018. We did not detect any methane over a range of latitudes in both hemispheres, obtaining an upper limit for methane of about 0.05 parts per billion by volume, which is 10 to 100 times lower than previously reported positive detections. We suggest that reconciliation between the present findings and the background methane concentrations found in the Gale crater would require an unknown process that can rapidly remove or sequester methane from the lower atmosphere before it spreads globally

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

Trace gas retrievals for the ExoMars Trace Gas Orbiter Atmospheric Chemistry Suite mid-infrared solar occultation spectrometer

International audienceExoMars is a two-part mission to Mars jointly led by ESA and Roscosmos. The first phase was launched in March 2016 and consisted of the Trace Gas Or-biter (TGO) and Schiaparelli lander. The TGO successfully entered orbit around Mars in October 2016 and has since begun a crucial aerobreaking campaign to circularize its orbit with a nominal 400 km altitude and 2 hr period. There are four scientific instruments on TGO: the Atmospheric Chemistry Suite (ACS), the Nadir and Occultation for Mars Discovery (NOMAD) spectrometer, the Colour and Stereo Surface Imaging System (CaSSIS), and the Fine-Resolution Epithermal Neutron Detector (FREND). This presentation will focus on trace gas retrievals for the mid-infrared (MIR) channel of the ACS instrument operating in solar oc-cultation mode. ACS is a set of three spectrometers that are designed to better characterize the atmosphere of Mars with unprecedented accuracy. It aims to detect and quantify unknown trace gases diagnostic of active geological or biological processes, to map their distribution and attempt to identify sources, and to refine our knowledge of the vertical distribution of major and minor atmospheric gases. It has three channels: near-infrared (NIR), thermal-infrared (TIRVIM) and MIR. The NIR channel is combination of an echelle grating and an acousto-optical tunable filter (AOTF), and is similar to the Ultraviolet and Infrared Atmospheric Spectrometers for Mars and Venus (SPICAM/V) on Mars Express and Venus Express (Korablev et al., 2006; Bertaux et al., 2007). It has a spectral range of 0.73–1.6 µm and operates in nadir mode. It is intended to provide mapping support to solar occulta-tion measurements. TIRVIM is a small Fourier transform spectrometer with a spectral range of 2–17 µm and resolution of 0.2 cm−1. It has heritage from the Mars Express Planetary Fourier Spectrometer (PFS), operates in both nadir and solar occultation mode, and will be able to measure the physical state of the atmosphere (vertical profiles of temperature, pressure and dust opacity). NOMAD is also a multi-channel spectrometer with complimentary objectives to ACS. It consists of a pair of combination echelle-AOTF spectrometers , much like SPICAM/V and ACS NIR, that operate in both nadir and solar occultation mode. In its original configuration, TGO carried a high-resolution Fourier transform spectrometer (FTS) covering a wide spectral range to detect trace gases (Wennberg et al., 2011), supported by the nadir-viewing NOMAD instrument capable of carrying out trace gas mapping studies. The ACS MIR channel aims to reproduce the capabilities of the FTS using a novel concept for atmospheric studies: a cross-dispersion spectrometer combining an echelle grating with a wide blaze angle and secondary, steerable diffraction grating (Korablev et al., 2017). It is capable of finer resolution than its echelle-AOTF counterparts, but is limited in its instantaneous spectral range compared to its FTS predecessor. The ACS MIR block is thermally isolated from TIRVIM and coupled to NIR, but shares a common electronics block. It consists of an entry telescope and collimator, a large echelle grating (107×240 mm, 3.03 grooves per mm), a steerable pair of secondary grating mirrors, and a Sofradir MCT array detector The low-density echelle grating at a high blaze angle (63.43 °) provides overlapping spectra at high orders. The secondary grating separates the orders and the resulting spectra are recorded by the detector with 640 pixels in the x direction corresponding to wavelength, and 512 pixels in the y direction corresponding to order. Several spectra are recorded for each order on sequential pixel rows. The secondary grating has two reflective gratings mounted side-by-side that can rotated

Mars’ water vapor mapping by the SPICAM IR spectrometer: Five Martian years of observations

International audienceThe SPICAM IR instrument on the Mars Express mission continuously observes the water vapor in the Martian atmosphere starting from 2004 in the 1.38-μm spectral band. The water vapor column abundance is retrieved from nadir observations to characterize its spatial, seasonal and interannual variations. A reference set of SPICAM water vapor column abundances (zonally averaged) covering the time period from 2004 to 2013 (Martian Years 27-31) is available for a grid of 2° Ls x 2° latitude, along with an average reference map of water vapor abundance combining all the Martian years of Mars Express observations. Compared to the previous data retrieval by Fedorova et al. [2006] the new processing algorithm includes many improvements concerning the calibration and assumed parameters. A major improvement is the account for aerosol scattering based on dust and water ice cloud optical depths measured by THEMIS/Mars Odyssey [Smith et al., 2009a]. The account for multiple scattering by aerosol particles increases the retrieved water vapor amount by ∼10% in polar areas during summer, and up to 60-70% for large solar zenith angles. The sensitivity of the results to aerosol properties, surface albedo, solar spectrum, and water vapor vertical distribution has also been studied. The retrieved water vapor reveals nominal annual cycle with maximum abundance of about 60-70 pr. μm for the Northern summer and ∼20 pr. μm for the Southern summer. The annual average amount of water has been estimated to be of 10 to 20 pr. μm, in agreement with other measurements. From year to year the seasonal cycle of water vapor abundance is very stable. An observed decrease during the MY 28 global dust storm cannot be fully attributed to the masking effect of dust, and indicates a real decrease of water amount near or above the surface. No evidence of diurnal variation of column water vapor amount was found, even though the 1.38-μm measurements are sensitive to the few lowermost kilometers above the surface

High-resolution solar spectrum obtained from TGO orbiting Mars reveals new solar lines in the 0.7-1.7 µm range

International audienceThe ACS-NIR spectrometer on board the Trace Gas Orbiter (TGO) is currently used to probe the atmosphere of Mars. It is, however, capable of measuring the near-infrared solar spectrum in the 0.7-1.7 µm domain with high spectral resolution when pointed at the Sun and its line of sight is above the atmosphere of Mars i.e. with its Solar Occultation mode. Specific observations were therefore made during 10 months in order to construct the solar spectrum in this spectral domain. The observations consist in recording all the diffraction orders of ACS-NIR by continuously varying the frequency of its AOTF (Acousto-Optic Tunable Filters, a component used to separate the orders). We will first present how we have treated each order of diffraction to improve the solar spectrum on the 0.7-1.7 µm band by considering for this purpose off-center images attached to certain AOTF frequencies. This method makes it possible to avoid contamination between the successive diffraction orders but also to increase the detection of the solar lines at the ends of each order where the intensity is low due to the Blaze function. We will then show the final version of the solar spectrum that we obtain. It will be compared to the reference spectrum, which is that of Toon. We will finish by showing and discuss some results revealing new solar lines appearing in certain diffraction orders of the ACS-NIR spectrometer and which are not present in the corresponding parts of the reference solar spectrum

Multi‐annual monitoring of the water vapor vertical distribution on Mars by SPICAM on Mars Express

International audienceThe distribution of water vapor with altitude has long remained a missing piece of the observational dataset of water vapor on Mars. In this work, we present the first multi‐annual survey of water vapor profile covering the altitude range from 0 to 100 km based on the SPICAM/Mars Express occultation measurements. During the aphelion season, water remains confined below 40‐60 km for all Martian years observed. The highest altitude where water vapor can be spotted is between 70 and 90 km during the southern summer (Ls=240‐300°; perihelion season), approaching the transition between the middle and upper atmosphere. In this season, years without a global dust storm (GDS) show a significant moistening of the upper atmosphere (∼100 ppmv) in the southern hemisphere, confirming a seasonal impact on the hydrogen escape rate. The two observed GDS, in MY28 and MY34, show a substantial disparity in water vapor response. The storm in MY28, which coincides with the southern summer solstice, creates the largest excess of water in both hemispheres at >80 km. This climatology of water vapor will supply a robust statistical basis to address the long‐term escape processes of water from Mars

Near-infrared high-resolution solar spectrum from ACS NIR onboard TGO

International audienceThe Atmospheric Chemistry Suite (ACS) is Russian contribution to ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. It arrived at Mars in October 2016. In this work, we present preliminary results for high-resolution solar spectra observed by ACS NIR instrument in the near-infrared range

Near-infrared high-resolution solar spectrum from ACS NIR onboard TGO

International audienceThe Atmospheric Chemistry Suite (ACS) is Russian contribution to ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. It arrived at Mars in October 2016. In this work, we present preliminary results for high-resolution solar spectra observed by ACS NIR instrument in the near-infrared range