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Characterizing the abundance of atmospheric hydrogen (H) at Mars is critical for determining the current and, subsequently, the primordial water content on the planet. At present, the atmospheric abundance of Martian H is not directly measured but is simulated using proprietary models that are constrained with observations of H Lyman‐α emission brightness, as well as with observations of other atmospheric parameters, such as temperature and Solar UV irradiance. Publicly available brightness measurements require further processing to have scientific utility. To make the data needed to model H abundances and escape rates more accessible to the community, we use H Lyman‐α emissions made with the Mars Atmosphere and Volatile Evolution (MAVEN) mission. The near decade‐spanning data set is reduced to obtain disk‐pointed averages of the H brightness in the upper atmosphere of Mars and then analyzed for statistical trends across multiple variables. The H Lyman‐α emission brightness is found to be dependent on Solar illumination, Solar cycle, and season. The resulting data trends are used to derive empirical fits to build a predictive framework for future observations or an extrapolative tool for estimates of water content at previous epochs. Data that was intentionally not included in the empirical derivations are used to validate the predictions successfully to within 18% accuracy, on average. This first‐of‐its kind predictive model for H brightness is presented to the community and can be used with atmospheric models to further derive and interpret the abundances and escape rate of H atoms at Mars. Plain Language Summary: The upper atmosphere of Mars contains Hydrogen atoms that can escape into outer space. Since these atoms originate from water, understanding the abundance and variation of these atoms is important to understanding water escape from Mars. There are no direct measurements of H abundances in the upper atmosphere of Mars. However, H atoms emit UV photons, and these photons can be measured and used with models to derive H atom abundances. This work investigates the UV photon emissions of H atoms from the upper atmosphere of Mars using a 9‐year data set by converting the data into scientifically useable form and studying the trends with time. A predictive model is developed to successfully replicate the trends in the UV emissions of H atoms according to illumination, seasonal, and Solar activity conditions. This predictive model can be used with other tools and observed quantities to derive H abundances to infer the present and evolving water content at Mars. Key Points: Nine years of H Ly‐α brightness measurements are found to depend on Solar cycle, illumination, and Martian seasonStatistical fits for H Ly‐α brightness across independent variables are empirically derivedA predictive tool for atmospheric H brightness at Mars is developed and shared with the community [ABSTRACT FROM AUTHOR]

Abstract: In September 2017, an active region of the Sun produced a series of strong flares and a coronal mass ejection that swept past Mars producing enhanced ionization and heating in the upper atmosphere. Emissions from atmospheric hydrogen Lyman‐α were also enhanced at Mars. Temperatures derived from neutral species scale heights were used in conjunction with the H Lyman‐α observations to simulate the effects of this space weather event on Martian hydrogen properties in the exosphere. It was found that hydrogen abundance in the upper atmosphere decreased by ~25% and that the H escape rate increased by a factor of 5, mainly through an increase in upper atmospheric temperature. This significant escape rate variation is comparable to seasonally observed trends but occurred at much shorter timescales. Such solar events would statistically impact extrapolation of Martian water loss over time. [ABSTRACT FROM AUTHOR]

The complex seasonal, dust, and atmospheric circulation patterns at Mars affect the abundance and spatial extent of water and its photo-dissociated atomic byproduct H. Recent observations by the Mars Atmosphere and Volatile Evolution (MAVEN) mission have been used to constrain the H abundance at the exobase (200 km altitude) as well as its thermal and global escape rate. The MAVEN observations span the last seven years and encompass the effects of the declining phase of Solar Cycle 24 and activating phase of Solar Cycle 25 from mid-perihelion of Mars Year (MY) 32 through most of aphelion of MY36. Results show that there are seasonal as well as solar cycle variations in H exobase temperatures, densities, thermal escape rates, and global escape rates. During perihelion of MYs 32–35, the H exobase density fluctuates with a cadence that corresponds to the approach to perihelion as well as the onset and peak of regional dust storms. Additionally, the abundance of H atoms did not show exceptional variability during the MY34 Global Dust Storm, when compared with trends from adjacent years. Seasonal averages of H exobase densities, thermal escape rates, and global escape rates indicated an order of magnitude difference between perihelion and aphelion, and suggest a water global equivalent layer range of 0.02–13 m lost over 4 billon years. H properties are also found to vary with solar cycle to a lesser extent than with seasons across the MAVEN timeline. Trends show that the exobase density varies inversely with solar activity, while both the thermal and global escape rates vary directly with solar activity. It remains to be seen if the effects of a stronger than recent solar cycle on the upper atmospheric H properties would become more pronounced. [ABSTRACT FROM AUTHOR]

The densities of three ion species in the Martian upper atmosphere were compared during the MY33 and MY34 Martian regional and global dust storms (RDS 2016 and GDS 2018, respectively) using data from the neutral gas and ion mass spectrometer of the Mars atmosphere and volatile evolution mission. The trends of the ion species and their relative abundances in altitudes compared to some neutral species were examined from 10 September–4 October 2016 and 27 May–18 June 2018, at altitudes of 160–240 km. Both RDS 2016 and GDS 2018 caused variations in the ion species abundance of the upper atmosphere at their onsets in 18–21 September 2016 and 5–8 June 2018 respectively. The densities of O2+, CO2+, and O+ increased during RDS 2016. Meanwhile, O2+ and O+ densities decreased and CO2+ density increased during GDS 2018. Ion species' relative abundances indicate that during RDS 2016, the increase in O2+ density may be caused by the increase of CO2+ or O+ densities rather than the increase of O or CO2 densities. Meanwhile, the decrease in O2+ density during GDS 2018 may be caused by the decrease of O or O+ densities rather than the decrease in CO2+ or CO2 densities. [ABSTRACT FROM AUTHOR]

The objective of this White Paper, submitted to ESA's Voyage 2050 call, is to get a more holistic knowledge of the dynamics of the Martian plasma system, from its surface up to the undisturbed solar wind outside of the induced magnetosphere. This can only be achieved with coordinated multi-point observations with high temporal resolution as they have the scientific potential to track the whole dynamics of the system (from small to large scales), and they constitute the next generation of the exploration of Mars analogous to what happened at Earth a few decades ago. This White Paper discusses the key science questions that are still open at Mars and how they could be addressed with coordinated multipoint missions. The main science questions are: (i) How does solar wind driving impact the dynamics of the magnetosphere and ionosphere? (ii) What is the structure and nature of the tail of Mars' magnetosphere at all scales? (iii) How does the lower atmosphere couple to the upper atmosphere? (iv) Why should we have a permanent in-situ Space Weather monitor at Mars? Each science question is devoted to a specific plasma region, and includes several specific scientific objectives to study in the coming decades. In addition, two mission concepts are also proposed based on coordinated multi-point science from a constellation of orbiting and ground-based platforms, which focus on understanding and solving the current science gaps. [ABSTRACT FROM AUTHOR]

We provide the first detailed characterization and analysis of the C I 156.1 nm ( 3 D o → 3 P) and 165.7 nm ( 3 P o → 3 P) dayglow in the Martian atmosphere. Using MAVEN/IUVS periapse limb observations from two time periods corresponding to the aphelion and perihelion seasons, we determine through photochemical and radiative transfer modeling that both emissions are driven by electron impact dissociation (EID) of CO 2 and resonant scattering of solar photons by atomic C. EID of CO 2 is dominant at the lower latitudes, while resonant scattering by C is dominant at the higher altitudes, with the cross-over point occurring at ∼ 155 km. The high-altitude C I 156.1 nm dayglow allows us to make the first direct measurement of C densities in the upper Martian atmosphere: at aphelion, (4. 0 ± 0. 3) × 1 0 5 cm−3 at ∼ 165 km altitude, and at perihelion, (4. 4 ± 0. 5) × 1 0 5 cm−3 at ∼ 175 km altitude. These high C densities, while providing support for the addition of CO 2 photodissociation as a major C production channel, suggest that C production rates in photochemical models still have to be higher. Constraints from the dayglow observations also indicate that actual CO 2 + e rates are as much as a factor of 2 compared to photochemical modeling, with implications on slightly higher overall C photochemical escape rates. Contrary to model predictions, we found the altitude of the emission peak at perihelion to be higher than it is at aphelion, and higher for the C I 156.1 nm emission than the C I 165.7 nm emission at the same season. The former seasonal variation can be attributed to low modeled atmospheric temperatures at perihelion. • We analyze the C I 156.1 nm and 165.7 nm dayglow in the Martian atmosphere. • This dayglow is driven by C resonant scattering and CO 2 electron impact dissociation. • We make the first direct measurement of C density in the Martian upper atmosphere. • Observed C densities are significantly higher than modeled. • Observed CO 2 electron impact dissociation rates are also higher than modeled. [ABSTRACT FROM AUTHOR]

The planetary upper atmosphere is the region where most of the solar photons in the far and extreme UV are absorbed and provide heating and ionization of the neutral atmosphere. The use of this absorption during stellar or solar occultation is a standard method to identify species or derive information on the density and temperature of the upper atmosphere. The UV emissions of the interplanetary hydrogen, and of the extended planetary hydrogen coronae can also be absorbed by the planetary upper atmosphere and therefore be used to study the density of the planetary upper atmosphere. This method was used for example by the Mars Express mission to study the seasonal variations of the CO2 density in the Martian upper atmosphere. In June 2018, an intense global dust storm surrounded Mars for a few months, heating the lower atmosphere and leading to an expansion of the Martian atmosphere of few kilometers in the thermosphere. The Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission measured the Lyman-α emission line from the interplanetary hydrogen and from the extended hydrogen corona of Mars at the limb before and throughout this Planet Encircling Dust Event (PEDE). In the nightside IUVS observations, the CO2 absorption of the Lyman-α photons is seen in the lower thermosphere near 110 km. An increase in the altitude of the absorption was observed on 8 June 2018. This altitude shift is attributed to an increase of the CO2 density due heating of the lower atmosphere during the large dust storm at the surface of Mars. In this presentation we will present these observations and the CO2 density variations, at a given altitude. Results from this analysis are compared to other observations of the Martian upper atmosphere obtained at the dayside. [ABSTRACT FROM AUTHOR]