Planetary surfaces

Abstract:

The mid-infrared channel of the Atmospheric Chemistry Suite (ACS MIR) onboard the ExoMars Trace Gas Orbiter is capable of observing the infrared absorption of ozone (O3) in the atmosphere of Mars. During solar occulations, the 003←000 band (3,000-3,060 cm−1) is observed with spectral sampling of ∼0.045 cm−1. Around the equinoxes in both hemispheres and over the southern winters, we regularly observe around 200–500 ppbv of O3 below 30 km. The warm southern summers, near perihelion, produce enough atmospheric moisture that O3 is not detectable at all, and observations are rare even at high northern latitudes. During the northern summers, water vapor is restricted to below 10 km, and an O3 layer (100–300 ppbv) is visible between 20 and 30 km. At this same time, the aphelion cloud belt forms, condensing water vapor and allowing O3 to build up between 30 and 40 km. A comparison to vertical profiles of water vapor and temperature in each season reveals that water vapor abundance is controlled by atmospheric temperature, and H2O and O3 are anti-correlated as expected. When the atmosphere cools, over time or over altitude, water vapor condenses (observed as a reduction in its mixing ratio) and the production of odd hydrogen species is reduced, which allows O3 to build up. Conversely, warmer temperatures lead to water vapor enhancements and ozone loss. The LMD Mars Global Climate Model is able to reproduce vertical structure and seasonal changes of temperature, H2O, and O3 that we observe. However, the observed O3 abundance is larger by factors between 2 and 6, indicating important differences in the rate of odd-hydrogen photochemistry.

Abstract:

To understand the evolving martian water cycle, a global perspective of the combined vertical and horizontal distribution of water is needed in relation to supersaturation and water loss and how it varies spatially and temporally. The global vertical water vapor distribution is investigated through an analysis that unifies water, temperature and dust retrievals from several instruments on multiple spacecraft throughout Mars Year (MY) 34 with a global circulation model. During the dusty season of MY 34, northern polar latitudes are largely absent of water vapor below 20 km with variations above this altitude due to transport from mid-latitudes during a global dust storm, the downwelling branch of circulation during perihelion season and the intense MY 34 southern summer regional dust storm. Evidence is found of supersaturated water vapor breaking into the northern winter polar vortex. Supersaturation above around 60 km is found for most of the time period, with lower altitudes showing more diurnal variation in the saturation state of the atmosphere. Discrete layers of supersaturated water are found across all latitudes. The global dust storm and southern summer regional dust storm forced water vapor at all latitudes in a supersaturated state to 60-90 km where it is more likely to escape from the atmosphere. The reanalysis data set provides a constrained global perspective of the water cycle in which to investigate the horizontal and vertical transport of water throughout the atmosphere, of critical importance to understand how water is exchanged between different reservoirs and escapes the atmosphere.

Abstract:

Temperature and density in the upper Martian atmosphere, above ∼100 km, are key diagnostic parameters to study processes of the species' escape, investigate the impact of solar activity, model the atmospheric circulation, and plan spacecraft descent or aerobraking maneuvers. In this paper, we report vertical profiling of carbon dioxide (CO2) density and temperature from the Atmospheric Chemistry Suite (ACS) solar occultations onboard the ExoMars Trace Gas Orbiter. A strong CO2 absorption band near 2.7 μm observed by the middle infrared spectrometric channel (ACS MIR) allows the retrieval of the atmospheric thermal structure in an unprecedentedly large altitude range, from 20 to 180 km. We present the latitudinal and seasonal climatology of the thermal structure for 1.5 Martian years (MYs), from the middle of MY 34 to the end of MY 35. The results show the variability of distinct atmospheric layers, such as a mesopause (derived from 70 to 145 km) and homopause, changing from 90 to 100 km at aphelion to 120–130 km at perihelion. Some short-term homopause fluctuations are also observed depending on the dust activity.

Abstract:

Carbon monoxide is a non-condensable gas in the Martian atmosphere produced by the photolysis of CO2. Its abundance responds to the condensation and sublimation of CO2 from the polar caps, resulting in seasonal variations of the CO mixing ratio. ACS onboard the ExoMars Trace Gas Orbiter have measured CO in infrared bands by solar occultation. Here we provide the first long-term monitoring of the CO vertical distribution at the altitude range from 0 to 80 km for 1.5 Martian years from Ls = 163° of MY34 to the end of MY35. We obtained a mean CO mixing ratio of ∼960 ppmv at latitudes from 45°S to 45°N and altitudes below 40 km, mostly consistent with previous observations. We found a strong enrichment of CO near the surface at Ls = 100–200° in high southern latitudes with a layer of 3,000–4,000 ppmv, corresponding to local depletion of CO2. At equinoxes we found an increase of the CO mixing ratio above 50 km to 3,000–4,000 ppmv at high latitudes of both hemispheres explained by the downwelling flux of the Hadley circulation on Mars, which drags the CO enriched air. General circulation models tend to overestimate the intensity of this process, bringing too much CO. The observed minimum of CO in the high and mid-latitudes southern summer atmosphere amounts to 700–750 ppmv, agreeing with nadir measurements. During the global dust storm of MY34, when the H2O abundance peaks, we see less CO than during the calm MY35, suggesting an impact of HOx chemistry on the CO abundance.

Abstract:

<p><strong>Introduction:</strong> ESA’s upcoming Comet Interceptor (CI) mission will be the first to visit a long period, potentially dynamically new, comet that will consist of some of the most primitive material from the beginning of our Solar System [1]. Part of CI’s payload includes the Modular InfraRed Molecular and Ices Sensor (MIRMIS) which aims to map the thermal and compositional variation of the comet’s nucleus and coma. MIRMIS is compromised of a near-infrared hyperspectral imager (NIR, 0.9-1.6 µm), a mid-infrared point spectrometer (MIR, 2.5 – 5 µm) and, the focus of this study, a multispectral thermal imager (TIRI, 6-25 µm).</p> <p>TIRI’s instrument design includes one central broadband thermal imaging channel (6-25 µm) and 2 identical sets of eight narrow-band channels situated orthogonal to each other to accommodate the instrument orientation as it changes upon closest approach to the comet (Fig.1). This configuration will allow for optimal comparison between views of the comet, and analysis of photometric effects. </p> <p>To maximize the science return pertinent to the mission objectives, we investigated TIRI’s ability to retrieve both temperature and composition using its originally proposed filter set, described in Table 1, and new alternative filter sets.</p> <p><img src="" alt="" /></p> <p><em>Table 1: Summation of bandpass centers for filter sets. In blue is the original baseline filter set, and those underneath (yellow and green) show proposed filter sets tested for improved science return. Starred set is the proposed ‘new baseline’.</em></p> <p><img src="" alt="" /></p> <p>Figure 1 : Filter layout on TIRI</p> <p><strong>Methods:</strong> To optimize TIRI’s narrow-band filter set, we used both synthetic nucleus spectral targets and laboratory measured analogue minerals and meteorite spectra to understand TIRI’s detection capabilities. In addition to the proposed baseline filter set, several others were proposed that consisted of band-centers evenly spaced along TIRI’s range; band-centers concentrated near 9-11 µm (a region rich in silicate features); or a mix of the two.</p> <p><em>Synthetic Spectral Analysis:</em> Twelve synthetic spectra were generated with the Planetary Spectrum Generator (PSG) [2]. These included six featureless spectra made of single or mixed temperature blackbodies and six spectral mixtures composed of crystalline (fayalite or enstatite) and amorphous (pyroxene) silicates convolved with single or mixed temperature blackbodies. Mixed temperature blackbodies were used to account for nucleus roughness and/or pixel anisothermality. The silicate endmembers used in this study were chosen based on identified minerals of comets Tempel 1 and Hale-Bopp [3-5]. For these simulations we assumed fixed resolving power to create synthesized spectra with a specific pixel-width that already accounted for TIRI noise performance. Six filter sets were tested as defined in Table 1 (yellow).  </p> <p>The simulated retrievals included realistic instrument noise (noise-equivalent-power) and were performed using the Retrieval Module of the PSG [5]. We used this tool to test the ability of each filter set to retrieve a sequence of information about each original synthetic spectrum: 1) temperature; 2) temperature + amorphous pyroxene; 3) temperature + crystalline fayalite; 4) temperature + crystalline enstatite; 5) temperature + crystalline fayalite and enstatite; and 6) temperature + all three silicates (Fig. 2).</p> <p>These retrievals showed that temperature of the featureless spectra was always reliably determined independent of filter set. For the silicate spectra, temperature was always retrieved (±1 K) if >250 K, but spectral shape determination, had a dependence on filter set. Filter set S2 (Table 1, Fig.2 blue) was determined to most accurately identify compositional features.</p> <p><img src="" alt="" /></p> <p><em>Figure 2. Summary of the final retrieval of temperature and composition (T & three silicates) for the 6 spectra with spectral shape (rows) using the 6 filter sets (columns) defined in Table 1. The last column on the right shows the original high-resolution spectra.</em></p> <p><em>Laboratory Spectral Analysis:</em> We examined laboratory spectra of minerals and meteorites likely to be present/analogous to the anticipated primitive comet. These include minerals used in the OSIRIS-REx collection [6] and carbonaceous meteorites from [7]. From these, we identified several key  features for compositional determination generally centered within 8-12 µm (Subset in Table 1).</p> <p>We included a set of mineral mixtures with known amorphous content from [8] to test the filter set ability to identify crystalline content of the material. Differences were subtle at low amorphous content and unlikely to be captured at this spectral resolution. Another set included hydrated/altered meteorites from [7]. Tested filter sets captured the overall spectral shape of the meteorite composition and including longer wavelengths (12-15 µm region) improved differentiation of hydrous alteration. It was determined that TIRI alone would be unable to quantify surface ice content, but hydration of surface mineralogy may be detectable.</p> <p><img src="" alt="" /></p> <p><em>Figure 3. Minerals from the OSIRIS-REx collection [6] and lizardite (unpublished) at laboratory resolution, TIRI baseline, and filter sets defined in Table 1.</em></p> <p><strong>Discussion:</strong> The synthetic analysis showed that filters covering a large range is necessary to capture both temperature and spectral shape of the target comet. The laboratory analysis showed that, while a concentrated filter set was better at distinguishing minute differences between compositions, a wider range of filters can still provide adequate qualitative spectral information to achieve TIRI’s science objectives. We thus propose shifting to the new filter set that encompasses a slightly smaller range (8-22 µm) to retrieve temperatures and to better capture mid-range compositional features (Table 1, starred set).</p> <p>Investigations will continue to incorporate a larger range of compositional spectra into the synthetic analysis model to better understand possible instrument performance and further explore the challenges of compositional unmixing for our target comet.</p> <p><strong>Acknowledgments:</strong> We thank the entire Comet Interceptor team for their inputs into this analysis, and the use of online databases including PSG & RELAB.</p> <p><strong>References:</strong> [1] Snodgrass C. et al (2019) <em>Nat Commun 10,</em> 5418. [2] Villanueva G. L. et al., (2018) <em>JQS&RT 217,</em> 86-104. [3] Lisse C. M. et al., (2006) <em>Icarus, 313,</em> 635-640<em> </em>[4] Lisse C. M. et al., (2007) <em>Icarus, 191(2)</em>, 223-240. [5] Lisse C. M. (2008) <em>Icarus 195(2),</em> 941-944. [6] Donaldson Hanna K. L. et al. (2021) <em>JGRP 126(2)</em> [7] Bates H. J. et al. (2021) <em>JGRP 126</em> [8] Donaldson Hanna K. L. et al. (2018) <em>LPSC 49 #</em>1867</p>