The evolution and history of water on Mars plays a key role in the assessment of the habitability of the planet across time. There is abundant geomorphological evidence suggesting that Mars had a wetter past (Bibring et al., 2006;Carr & Head, 2003), yet the duration and extent of this more humid past remains a topic of substantial debate. For instance, the large deltas, basins, and valleys on Mars are suggestive of large bodies of water that were stable over relatively long periods of time. Some estimates suggest past volumes of water in excess of a 500 m deep Global Equivalent Layer (GEL; Carr & Head, 2003), which is many times larger than the current estimates of labile water on Mars (∼30 m, Lasue et al., 2013).The large enrichments of D/H measured in atmospheric water suggest that a large fraction, beyond 80%, of this water was lost over time (Jakosky, 2021;Villanueva et al., 2015), and Ar and O isotopic ratios measured with MAVEN (Jakosky et al., 2017) and TGO (Alday, Wilson, et al., 2021) indicate that Mars has lost a large fraction of its atmosphere. Because Mars is less massive than Earth, the neutral escape of volatiles is easier on Mars, considering the similar equilibrium temperatures of the two planets, although Mars is obviously colder. Recent results indicate that most of this escape occurred via neutral and nonionized processes (Brain et al., 2015), in which temperature and its variability across geological times were key factors defining the state of the Martian atmosphere. Recent results from dust storms suggest that dust storms can greatly heat the atmosphere, leading to the upward transport and more readily escape of water (
The Open University's repository of research publications and other research outputs Martian atmospheric temperature and density profiles during the 1st year of NOMAD/TGO solar occultation measurements
We present water vapor vertical distributions on Mars retrieved from 3.5 years of solar occultation measurements by Nadir and Occultation for Mars Discovery onboard the ExoMars Trace Gas Orbiter, which reveal a strong contrast between aphelion and perihelion water climates. In equinox periods, most of water vapor is confined into the low‐middle latitudes. In aphelion periods, water vapor sublimated from the northern polar cap is confined into very low altitudes—water vapor mixing ratios observed at the 0–5 km lower boundary of measurement decrease by an order of magnitude at the approximate altitudes of 15 and 30 km for the latitudes higher than 50°N and 30–50°N, respectively. The vertical confinement of water vapor at northern middle latitudes around aphelion is more pronounced in the morning terminators than evening, perhaps controlled by the diurnal cycle of cloud formation. Water vapor is also observed over the low latitude regions in the aphelion southern hemisphere (0–30°S) mostly below 10–20 km, which suggests north‐south transport of water still occurs. In perihelion periods, water vapor sublimated from the southern polar cap directly reaches high altitudes (>80 km) over high southern latitudes, suggesting more effective transport by the meridional circulation without condensation. We show that heating during perihelion, sporadic global dust storms, and regional dust storms occurring annually around 330° of solar longitude (LS) are the main events to supply water vapor to the upper atmosphere above 70 km.
The Solar Occultation (SO) channel of the Nadir and Occultation for Mars Discovery (NOMAD) instrument has been scanning the Martian atmosphere for almost 2 Martian years. In this work, we present a subset of the NOMAD SO data measured at the mesosphere at the terminator. From the data set, we investigated 968 vertical profiles of carbon dioxide density and temperature covering the Martian Year (MY) 35 as well as MY 36 up to a solar longitude (Ls) of 135° and altitudes around 60–100 km. While carbon dioxide density profiles are directly retrieved from the spectral signature in the spectra, temperature profiles are more challenging to retrieve as unlike density profiles, temperature profiles can present some spurious features if the regularization is not correctly managed. Comparing seven regularization methods, we found that the expected error estimation method provides the best regularization parameters. The vertical resolution of the profiles is on average 1.6 km. Numerous warm layers and cold pockets appear in this data set. The warm layers are found in the Northern hemisphere at dawn and dusk as well as in the Southern hemisphere at dawn. Strong warm layers are present in more than 13.5% of the profiles. The Southern hemisphere at dusk does not present any warm layer between Ls 50° and 150°. The height and latitudinal distribution of those warm layers were similar in MY 35 and MY 36 during the first half of the year (Ls = 0°–135°).
We present CO density profiles up to about 100 km in the Martian atmosphere obtained for the first time from retrievals of solar occultation measurements by the Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). CO is an important trace gas on Mars, as it is controlled by CO2 photolysis, chemical reaction with the OH radicals, and the global dynamics. However, the measurements of CO vertical profiles have been elusive until the arrival of TGO. We show how the NOMAD CO variations describe very well the Mars general circulation. We observe a depletion of CO in the upper troposphere and mesosphere during the peak period, LS = 190°–200°, more pronounced over the northern latitudes, confirming a similar result recently reported by Atmospheric Chemistry Suite onboard TGO. However, in the lower troposphere around 20 km, and at least at high latitudes of the S. hemisphere, NOMAD CO mixing ratios increase over 1,500 ppmv during the GDS (Global Dust Storm) onset. This might be related to the downwelling branch of the Hadley circulation. A subsequent increase in tropospheric CO is observed during the decay phase of the GDS around LS = 210°–250° when the dust loading is still high. This could be associated with a reduction in the amount of OH radicals in the lower atmosphere due to lack of solar insolation. Once the GDS is over, CO steadily decreases globally during the southern summer season. A couple of distinct CO patterns associated with the Summer solstice and equinox circulation are reported and discussed.
CO is produced by the photodissociation of CO 2 and recycled to CO 2 by the catalytic cycle involving HOx in the Martian atmosphere (e.g., McElroy & Donahue, 1972). The photochemical lifetime of CO is ∼6 years in the lower atmosphere (Krasnopolsky, 2007). The previous nadir observations revealed latitudinal and seasonal distributions of CO in the lower atmosphere, which indicate CO 2 condensation/sublimation in the polar caps and dynamics (Encrenaz et al., 2006;Smith et al., 2009Smith et al., , 2021. In the middle and upper atmosphere (>∼50 km), the photochemical lifetime of CO becomes much longer due to the decrease in HOx species density. Thus, the characteristic times of production and eddy diffusion of CO are shorter than the photochemical lifetime of CO in
<p align="justify"><strong>*Corresponding author: </strong>ashim@iaa.es</p> <p align="justify"><strong>Abstract</strong>&#160;</p> <p align="justify">NOMAD (<em>Nadir and Occultation for Mars Discovery instrument</em>), is a spectrometer suite onboard Exo Mars Trace Gas Orbiter having within its main scientific objectives the observations of the trace gases in the Martian atmosphere [1]. Here we focus on the retrieval of carbon monoxide (CO) vertical profiles with high vertical resolution. CO is an important trace species which acts as both photochemical and dynamical tracers. We retrieve CO from the solar occultation (SO) observation of the NOMAD orders (186 &#8211; 191) using a state-of-the-art retrieval method [2]. The observational dataset covers a wide range of latitudes and seasons. This permits us to study the impact of different conditions such as dust-storm seasons (local and global), southern summer and winter on the CO vertical profiles over different regions.</p> <p align="justify"><strong>Introduction</strong></p> <p align="justify"><strong> </strong>CO is originated in the upper Martian atmosphere by the photolysis of CO<sub>2</sub> and destroyed by the hydroxyl (OH) radicals in the lower atmosphere. Hydroxyl radicals thus recycle CO into CO<sub>2</sub> [3]. The study of the CO vertical distribution is important to understand the photo-chemical stability of the atmosphere. CO not only links the chemistry of the carbon and odd hydrogen chemical families but is a long-lived species which also serves as a dynamical tracer. By far the current knowledge of CO vertical profiles is largely unconstrained due to lack of systematic measurements. Though the column density of CO has been measured by instruments like CRISM [4] (<em>Compact Reconnaissance Imaging Spectrometer for Mars</em>) for a wide range of latitudes and seasons for multiple Martian years, the lack of its regular mapping in the vertical, limits a full understanding of its distribution and variability. Very recently, CO density profiles were reported from ACS (<em>Atmospheric Chemistry Suite</em>) observations [5] which found a significant depletion in CO mixing ratio during the 2018 global dust storm. NOMAD is performing routine solar occultation measurements since April 2018. Our aim here is to retrieve CO vertical profiles from these measurements with the best achievable precision and resolution and to investigate its distribution and variability through the different seasons and latitudes.</p> <p align="justify"><strong>Retrieval of CO from </strong><strong>NOMAD SO </strong><strong>orders 186 &#8211; 190</strong></p> <p align="justify"><strong> </strong>We present vertical profiles of CO retrieved from a subset of NOMAD solar occultations. The SO channel of NOMAD operates in the 2.3 &#8211; 4.3 &#956;m [3] where strong absorption lines for CO lie. In particular, the diffraction orders 186 (4180.32 cm<sup>-1</sup> - 4213.88 cm<sup>-1</sup>) - 191 (4292.69 cm<sup>-1</sup> - 4327.16 cm<sup>-1</sup>) allow for a good quality CO retrieval from 10 to about 100 km tangent altitudes. However, the recorded spectra suffer from calibration issues [6] such as bending and spectral shifts, in addition to variable systematic and random noise components. At IAA we have developed a cleaning procedure which correct the spectra for possible bending and spectral shift and makes it usable for a precise inversion of CO densities. We use the line-by-line radiative transfer model KOPRA (Karlsruhe Optimized and Precise Radiative transfer Algorithm) [2] as forward model, which was adapted to Mars and to the NOMAD instrument characteristics, in conjunction with an interactive solver (RCP) to retrieve CO from the cleaned spectra. Here we present a summary of this on-going work, which builds on a chain of retrievals of atmospheric aerosols, temperatures and density profiles derived from the same NOMAD scan but different diffraction orders, to obtain vertical profiles of CO in a consistent manner. We will also present first comparisons with Mars GCM results.</p> <p align="justify"><strong>Acknowledgement</strong></p> <p align="justify"><strong> </strong>The IAA/CSIC team acknowledges financial support from the State Agency for Research of the Spanish MCIU through the &#8216;Center of Excellence Severo Ochoa&#8217; award for the Instituto de Astrof&#237;sica de Andaluc&#237;a (SEV-2017-0709). MALV was funded by grant PGC2018-101836-B-100 (MCIU/AEI/FEDER, EU). ExoMars is a space mission of the European Space Agency (ESA) and Roscosmos. The NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy (IASB-BIRA), assisted by Co-PI teams from Spain (IAA-CSIC), Italy (INAF-IAPS), and the United Kingdom (Open University). US investigators were supported by the National Aeronautics and Space Administration.</p> <p><strong>References</strong></p> <p>[1] Neefs, Eddy, et al. "NOMAD spectrometer on the ExoMars trace gas orbiter mission: part 1&#8212;design, manufacturing and testing of the infrared channels."<em>Applied optics </em>54.28 (2015): 8494-8520.</p> <p>[2] Jurado Navarro, &#193;ngel Aythami. "Retrieval of CO<sub>2</sub> and collisional parameters from the MIPAS spectra in the earth atmosphere." (2016).</p> <p>[3] McElroy, Michael B., and Thomas M. Donahue. "Stability of the Martian atmosphere."<em>Science</em><em> </em>177.4053 (1972): 986-988.</p> <p>[4] Smith, Michael D., et al. "The climatology of carbon monoxide and water vapor on Mars as observed by CRISM and modeled by the GEM-Mars general circulation model."<em>Icarus </em>301 (2018): 117-131.</p> <p>[5] Olsen, K. S., et al. "The vertical structure of CO in the Martian atmosphere from the ExoMars Trace Gas Orbiter."<em>Nature Geoscience</em>14.2 (2021): 67-71.</p> <p>[6] Liuzzi, Giuliano, et al. "Methane on Mars: new insights into the sensitivity of CH4 with the NOMAD/ExoMars spectrometer through its first in-flight calibration."&#160;<em>Icarus</em>&#160;321 (2019): 671-690.</p> <p>&#160;</p>
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