Shane Wesley Stone

NASA’s Mars Atmosphere and Volatile EvolutioN spacecraft carries an extensive suite of instruments for characterizing the Mars upper atmosphere. Of these, the NGIMS, IUVS, and EUVM instruments produce datasets for CO ₂ density and neutral atmosphere temperature. The different instruments and retrieval methods utilized provide an expansive view of the Mars upper atmosphere. To make full use of the geophysical coverage offered by these datasets, we undertake a systematic comparison of the datasets to understand where they are different and how any biases between datasets can be removed. We conduct pairwise comparisons between datasets, binning the data by geophysical and forcing parameters, to develop adjustment factors that can be used to adjust the measured CO ₂ density and neutral temperature of one dataset to nominal agreement with another. The determined adjustment factors are reported for use by the wider Mars aeronomy community.

MAVEN's 10 Deep Dip campaigns, when periapsis altitude was lowered from 150 km to 120 km for short periods in 2015–2019, have made major contributions to current understanding of the thermosphere and ionosphere of Mars, as well as of present escape and implications for past climate. Yet the available literature on the Deep Dip campaigns is fragmentary, naturally tending to emphasize the scientific results of the individual research project being reported. In order to view the Deep Dip campaigns in their totality, to integrate their main findings coherently, and to identify promising opportunities for further exploitation of their data, here we present a comprehensive overview of MAVEN's Deep Dip campaigns. For the thermosphere, MAVEN Deep Dip observations have revealed: the homopause altitude and its notable variability; gravity waves and their dissipation; thermospheric composition, its day‐to‐day variability (exemplified by the O/C ratio), and its response to changing solar irradiance; and the thermal structure of the lower thermosphere between the cold mesopause and the hot, isothermal upper thermosphere. For the ionosphere, MAVEN Deep Dip observations have revealed: ionospheric composition, its day‐to‐day variability (exemplified by the /C ratio), and its response to changing solar irradiance; the dependence of ionization efficiency upon altitude; electron and ion temperatures; and the presence of multiple species of metal ions. Solar conditions and atmospheric dust conditions, relevant observations by MAVEN remote sensing instruments and by other missions, and potential avenues for future analyses of Deep Dip data are also addressed.

Metallic ions deposited in planetary atmospheres via meteoroid ablation are an invaluable tool for understanding electric fields, atmospheric winds, and minor ion transport. At Mars, metallic ion distributions are poorly understood. We analyze MAVEN/NGIMS distributions in the Martian ionosphere over the period of 2015–2020 at altitudes ∼120–200 km. The vertical structure observed during individual low‐altitude MAVEN Deep Dip campaigns is highly variable likely due to variations in the ion magnetization altitude and corresponding ion transport conditions. Deep Dip campaigns on or near the martian nightside show evidence for in situ production of ions via electron precipitation. On average, ions are globally distributed in the martian ionosphere at altitudes 120 km with only slight decreases on the martian nightside and in the southern hemisphere. We find a similar, albeit less intense, decrease in the densities in the northern hemisphere near perihelion as has been reported for .

Introduction: The homopause defines the boundary between two distinct regions of the atmosphere: the homosphere and the heterosphere. In the homosphere, located below the homopause, strong turbulent mixing causes atmospheric gases to vary vertically according to a common scale height, determined by the mean molecular mass of the atmosphere. In contrast, above the homopause, in the heterosphere, molecular diffusion dominates, resulting in the diffusive separation of species and each gas varying vertically according to its own scale height, which depends on its own molecular weight.In one-dimensional diffusion models, the altitude of the homopause is typically defined as the level at which the eddy diffusion coefficient equals the molecular diffusion coefficient. While the eddy diffusion coefficient is a useful parameterisation of turbulent mixing in these models, it remains largely unconstrained.The homopause from the Mars PCM: The Mars Planetary Climate Model (PCM) enables simulations of the physical, chemical and dynamical processes in the atmosphere of Mars. In this work, we estimate the value of the homopause altitude using the Ar/N2 densities in the model. The ratio of these two species, which are largely chemically inert, is approximately constant in the homosphere, while it varies due to diffusive separation above the homopause, given the different molecular weight of these two species.Figure 1 shows the climatology of the homopause altitude as predicted from the Mars PCM for Martian Years (MY) 34 and 35. The homopause altitude shows distinct seasonal variations, peaking close to the solstice periods (LS = 90 and 270˚) and finding relative minima during the equinoxes. Latitudinally, the highest homopause altitudes are found in the summer hemispheres (i.e, northern hemisphere for LS = 90˚ and southern hemisphere for LS = 270˚), while absolute minima are found in the winter hemispheres at these solar longitudes. Additionally, these simulations suggest that the presence of dust events such as the Global Dust Storm (GDS) in MY34 can produce a substantial increase of the homopause altitude.Figure 1: Climatology of the homopause altitude estimated using simulations of the Ar/N2 ratio from the Mars PCM for MY34 (top) and MY35 (bottom). Estimation of the Eddy diffusion coefficient: Aiming to constrain the range of values suitable for parameterising turbulent diffusion in 1-dimensional models of the atmosphere of Mars, we use the predictions from the Mars PCM to estimate values for the Eddy diffusion coefficient (K) relevant for different times and locations. In particular, we build a 1-dimensional diffusion model and run simulations to find the value of K that provides a best fit to the Ar/N2 ratio profiles from the Mars PCM. We will present the results from this work, showing the range of values of the Eddy diffusion coefficient relevant for distinct seasons, locations, and dust loading scenarios.

The upper atmosphere of Mars, including the mesosphere (60-120 km) and thermosphere (120 up to the exobase, around 180-230 km), is no doubt the less explored region of the red planet. Two recent and on-going missions to Mars are investigating these altitudes with new instrumentation and unprecedented detail: (1) the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN), entirely devoted to the uppermost layers and the escape to space, started its science measurements in 2014; and (2) the ESA-Roscosmos Trace Gas Orbiter (TGO), which started its routine science operations in 2018 after a long period of aerobraking, and which contains two solar occultation instruments, NOMAD and ACS, which permit sounding from the ground up to upper thermosphere, about 180 km.This work is devoted to the ambitious goal of combining datasets from diverse instruments on board these two missions, of exploiting the sinergy and complemetariety between them, and of obtaining a joint picture of the thermal structure of the whole Martian atmosphere, from the ground up to the exobase. This is part of an ISSI project entitled “A multi-mission approach to close the gaps in understanding of the structure and variability in the Mars upper atmosphere”. We will present the results of a broad team of scientists working for the past two years on this project, with focus on the global, planet encircling climatology of the Mars thermal structure (temperature and CO2 densities), and also its variability. The combination of MAVEN and TGO data permits to obtain the regular patterns of the Martian temperature profiles and how these vary with season, latitude, longitude and local time at different altitudes. Also we can combine the two missions’ datasets to explore year-to-year variations, dust storm impacts and solar-rotation effects at such a wide range of altitudes as those covered jointly by MAVEN and TGO, for the first time.This work combining MAVEN and TGO is also very informative for the validation of Mars Global Climate Models, given the complementariety of the two missions from the observational point of view, with very different coverage and sampling in the key magnitudes like local time, longitude, latitude and season. A small number of other companion contributions to this conference tackle some of these specific aspects.

The study of the coupling between the lower and the upper atmosphere has been hindered by the lack of continuous temperature measurements covering the whole atmosphere from the surface to the exobase. Thus, a large fraction of our knowledge on the topic relies on Global Climate Models (GCM). Here we combine temperature measurements from different instruments on board the ExoMars TGO, MAVEN, and Mars Express missions to cover altitudes from 50 to 250 km. The combined measurements are then used to validate the predictions of the Mars-PCM, a ground-to-exobase GCM. The comparison with the GCM also allows identifying potential biases and differences between datasets. We will pay special attention to the local time variation of temperatures, affected by local heating processes and by propagating tides, which previous studies have shown not to be well captured by M-PCM, at least at the mesopause.

We make use of photochemical modelling and measurements by the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard the Mars Atmosphere and Volatile EvolutioN mission (MAVEN) in an attempt to identify the ion species that dominate the sub-population of ions in the Martian ionosphere characterized by a mass of 41 atomic mass units (amu). Previous photochemical reaction networks used for the ionospheres of Mars and Venus include only a single species, namely ArH+, to account for ions with that mass. We highlight that the straightforward photochemical modeling of ArH+ yields predicted concentrations that are two orders of magnitude lower than the observed 41 amu ions densities in the dayside ionosphere of Mars. We hypothesize that the 41 amu ion population is instead dominated by protonated dicarbon monoxide, HC2O+, and we advocate for experimental investigations into relevant ion-neutral reactions.

Mars’ nightside ionosphere is highly variable and poorly understood, with several internal, external, and local factors affecting multiple sources of ionospheric plasma, all combining to drive highly complex behavior. The three primary plasma sources are a) planetary rotation and cross-terminator ion winds transporting dayside plasma to the nightside, particularly at dusk, and impact ionization when magnetic topology connects nightside upper atmosphere to b) dayside suprathermal photoelectrons c) solar wind electrons (accelerated or not) precipitating from the magnetotail. These sources are impacted by different subsets of: season, local time, subsolar longitude, IMF direction (which determines tail draping configuration), solar wind pressure, solar EUV flux, and the local strength and geometry of crustal magnetic fields that combine with global draping patterns to guide electrons toward or away from certain areas. We utilize 130.4 nm auroral observations from the Emirates Mars Ultraviolet Spectrograph, simulations of terminator ion winds, and in situ measurements from MAVEN of ion and electron density and temperature and suprathermal electron precipitation, as well as upstream measurements and proxies of IMF clock angle and solar wind pressure. Analysis reveals some interesting features. First, in regions of weak or no crustal magnetic fields, surprisingly, pre-dawn (~4 am) plasma densities vary by up to a factor of 30 in response to solar EUV varying by only a factor of ~3. Second, 5R aurora occurrence rate in a given region can vary by up to a factor of 7 with IMF orientation, generally highest for southward IMF and lowest for westward IMF, with higher auroral occurrence post-dusk, decreasing toward and past midnight before increasing again towards dawn. A broad diversity of IMF and local time dependence of plasma density and aurora occurrence is observed across Mars’ crustal magnetic fields. The polarity of a given crustal field determines whether its field lines preferentially magnetically connect to dayside ionospheric or nightside magnetotail electron sources in the post-dusk or pre-dawn sectors. These results highlight the complex and dynamic nature of Mars’ magnetic topology in controlling auroral electron access. Future studies integrating two-dimensional ionospheric models with in situ electron measurements with synoptic and limb auroral imaging will further constrain the relative contributions of the many factors controlling Mars’ enigmatic nightside ionosphere.

The Nadir and Occultation for MArs Discovery (NOMAD) spectrometer has been collecting Mars observations since 2018, providing a massive amount of information regarding its atmospheric composition, its vertical structure and bridging the gap between the previous knowledge of the lower atmosphere and the data from other missions (e.g., MAVEN) regarding atmospheric escape. The capability of the Solar Occultation (SO) channel to map the vertical structure of the atmosphere with a very high (>1000) signal-to-noise ratio, at a very high spectral resolution (>17000) and a high vertical sampling (0.5 to 2 km) is valuable in many contexts, ranging from the search for trace species in the lower atmosphere (10 to 40 km) to mapping the isotopic composition of the main atmospheric constituents (H2O, CO2, CO) or exploring the vertical structure of dust, water ice and CO2 ice clouds.Aerosols are some of the main drivers of the Martian climate, and the study of their spatial distribution and microphysical properties can advance our knowledge of their impact on the climate of the planet and on their formation and dynamics. This work will show the extension of previous investigations focused on dust, water ice and CO2 ice using NOMAD data, by presenting the mapping of these atmospheric components on a global scale over 3 Martian Years (MY34 Ls 160 to MY37 Ls 170). The acquisition by NOMAD of several diffraction orders during a single occultation allows in fact to obtain spectrally broad information that can be used to map dust and water ice vertical distributions and particle sizes. The information content of NOMAD data about particle sizes of water ice has been demonstrated to be particularly high and to give important information about the nucleation processes of water ice. NOMAD data can also be used to look for CO2 ice by combining broad spectral information with localized CO2 ice features at 3600 and 3710 cm-1, which are well identifiable in the NOMAD spectra.Besides presenting the climatology of aerosols, we will illustrate specific features occurring during the Martian Year and their repeatability; more specifically, we will look into the differences between MY 34, characterized by a Global Dust Storm, and following years, to highlight the impact of dust-induced heating over cloud formation. We will also give some insights into CO2 ice cloud formation, which was confirmed to be surprisingly heterogeneous compared to results obtained before TGO operations.

Mars’ water history is fundamental to understanding Earth-like planet evolution. Water escapes to space as atoms, and hydrogen atoms escape faster than deuterium giving an increase in the residual D/H ratio. The present ratio reflects the total water Mars has lost. Observations with the Mars Atmosphere and Volatile Evolution (MAVEN) and Hubble Space Telescope (HST) spacecraft provide atomic densities and escape rates for H and D. Large increases near perihelion observed each martian year are consistent with a strong upwelling of water vapor. Short-term changes require processes in addition to thermal escape, likely from atmospheric dynamics and superthermal atoms. Including escape from hot atoms, both H and D escape rapidly, and the escape fluxes are limited by resupply from the lower atmosphere. In this paradigm for the escape of water, the D/H ratio of the escaping atoms and the enhancement in water are determined by upwelling water vapor and atmospheric dynamics rather than by the specific details of atomic escape.