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Atmospheric oscillations with daily periodicity are observed in in situ near-surface pressure, temperature, and winds observations and also in remotely sensed temperature and pressure observations of the Martian atmosphere. Such oscillations are interpreted as thermal tides driven by the diurnal cycle of solar radiation and occur at various frequencies, with the most prominent being the diurnal, semidiurnal, terdiurnal, and quadiurnal tides. Mars global circulation models reproduce these tides with varying levels of success. Until recently, both the MarsWRF and newly developed MarsMPAS models were able to produce realistic diurnal and semidiurnal tide amplitudes but predicted higher-order mode amplitudes that were significantly weaker than observed. We use linear wave analysis to show that the divergence damping applied within both MarsWRF and MarsMPAS is responsible for suppressing the amplitude of thermal tides with frequency greater than 2 per sol, despite being designed to suppress only acoustic wave modes. Decreasing the strength of the divergence damping in MarsWRF and MarsMPAS allows for excellent prediction of the higher-order tidal modes. This finding demonstrates that care must be taken when applying numerical dampers and filters that may eliminate some desired dynamical features in planetary atmospheres.

We report the first detailed study of the diurnal thermal structure of upper mesosphere/lower thermosphere (∼80 to 160 km) of Mars from stellar occultation observations by the Imaging Ultraviolet Spectrograph (IUVS) aboard the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Due to stray light contamination, analyses of this data set to date have been confined to the nighttime events. This study makes use of a revised algorithm for removal of stray light from occultation spectra to retrieve the dayside events as well. The dayside is observed to be warmer than the nightside, with the maximum day/night difference of ∼30 K in the lower thermosphere, ∼20 K around the mesopause, with little diurnal variations at lower altitudes. This is consistent with the radiative time constant which is of the order of 1 Mars day in the urn:x-wiley:21699097:media:jgre22064:jgre22064-math-0001 to urn:x-wiley:21699097:media:jgre22064:jgre22064-math-0002 Pa region. The data also shows that the regions at pressure less than urn:x-wiley:21699097:media:jgre22064:jgre22064-math-0003 Pa are under strong solar control with no prominent migrating tidal signatures. In contrast, on Earth, the radiative time constant near the mesopause is ∼10 Earth days and the temperature variations due to tides are quite large. The Mars Climate Database shows a diurnal trend opposite to the data in the mesosphere, with the dayside mesopause predicted to be cooler than the nightside by ∼10 K along with signatures of a vertically propagating tide. The IUVS data set provides an unprecedented constraint on the structure of the Martian mesosphere. © 2022. American Geophysical Union. All Rights Reserved., The MAVEN mission is supported by NASA through the Mars Exploration Program in association with the University of Colorado and NASA's Goddard Space Flight Center. F.G.-G. is funded by the Spanish Ministerio de Ciencia, Innovación y Universidades, the Agencia Estatal de Investigación and EC FEDER funds under project RTI2018-100920-J-I00, and acknowledges financial support from the State Agency for Research of the Spanish MCIU through the Center of Excellence Severo Ochoa award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709).

This is the data set to accompany our publication, "Neutral Composition and Horizontal Variations of the Martian Upper Atmosphere from MAVEN NGIMS" in Journal of Geophysical Research: Planets. The repository contains .csv files. Within the files are the data shown in Figures 4 through 21 of the manuscript. This work was supported by funding from the MAVEN mission, a part of the NASA Mars Exploration Program. The MAVEN NGIMS data set is publicly available on the NASA Planetary Data System (PDS, see https://doi.org/10.17189/1518931) and the MAVEN Science Data Center (SDC, see https://lasp.colorado.edu/maven/sdc/public/pages/datasets/ngims.html).

In situ measurements of ionospheric and thermospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to the cold gas and plasma. We present

We employ a linearized full-wave model to show that the rovibrational damping (hereafter, RV damping) of gravity waves is an important source of atmospheric heating, which needs to be taken into account when modeling the wave processes in the jovian thermospheres. We find that RV damping of internal gravity waves becomes strong when the wave frequencies approach the thermal collision frequency. RV damping, compared to the damping by molecular viscosity and thermal diffusivity, can effectively lower the damping altitudes for waves with large frequencies, and requires larger upward wave energy fluxes to produce prescribed wave amplitudes for these waves. Particularly, the energy deposited by RV damping, viscous and thermal dissipation of the two waves identified in measurements by the Atmospheric Structure Instrument (ASI) on the Galileo Probe can produce a maximum temperature about 40 K higher than those in previous studies. We further demonstrate RV damping effect on the mesoscale gravity wave observed during the New Horizons flyby on Jupiter. Under the influence of RV damping, this wave has the vertical wavelength and damping altitude comparable to those of wave 2 retrieved from the ASI observations. The energy dissipation of these three observed waves, if coexist, should provide sufficient heating to produce the observed mean temperature between 357 km and 700 km above 1 bar.

We present a one-dimensional coupled ion-neutral photochemical kinetics and diffusion model to study the atmospheric composition of Titan in light of new theoretical kinetics calculations and scientific findings from the Cassini–Huygens mission. The model extends from the surface to the exobase. The atmospheric background, boundary conditions, vertical transport and aerosol opacity are all constrained by the Cassini–Huygens observations. The chemical network includes reactions between hydrocarbons, nitrogen and oxygen bearing species. It takes into account neutrals and both positive and negative ions with masses extending up to 116 and 74 u, respectively. We incorporate high-resolution isotopic photoabsorption and photodissociation cross sections for N2 as well as new photodissociation branching ratios for CH4 and C2H2. Ab initio transition state theory calculations are performed in order to estimate the rate coefficients and products for critical reactions. Main reactions of production and loss for neutrals and ions are quantitatively assessed and thoroughly discussed. The vertical distributions of neutrals and ions predicted by the model generally reproduce observational data, suggesting that for the small species most chemical processes in Titan’s atmosphere and ionosphere are adequately described and understood; some differences are highlighted. Notable remaining issues include (i) the total positive ion density (essentially HCNH + ) in the upper ionosphere, (ii) the low mass negative ion densities ( CN − , C 3 N − / C 4 H − ) in the upper atmosphere, and (iii) the minor oxygen-bearing species (CO2, H2O) density in the stratosphere. Pathways towards complex molecules and the impact of aerosols (UV shielding, atomic and molecular hydrogen budget, nitriles heterogeneous chemistry and condensation) are evaluated in the model, along with lifetimes and solar cycle variations.

International audience; Measurements by multiple Mars Atmosphere and Volatile Evolution mission instruments, obtained between November 2014 and November 2017, are analyzed to produce deuterium properties in the upper atmosphere of Mars. We show here, for the first time, the seasonal distribution and variability of D densities, temperatures, and estimated Jeans escape rates at the exobase (200 km). Within the data constraints, it is found that the variations in D properties are similar for the northern and southern hemispheres, and peak near southern summer solstice. Trends in the D Lyman‐α brightness, temperature, density, and escape rate are increasing during the beginning of the dust storm season, peak near southern summer solstice, and decrease toward the end of the dust storm season. This suggests that seasonal drivers at Mars cause deuterium in the upper atmosphere to become globally enhanced when Mars is closest to the Sun and during the martian dust season when water is provided to the upper atmosphere by subsurface, hydrological, and dust storm dynamics.

National Natural Science Foundation of China [41525015, 41774186, 41525016]; Science and Technology Development Fund of Macau SAR [039/2013/A2, 119/2017/A3]; National Aeronautics and Space Administration (NASA); Swedish National Space Agency [135/13, 166/14]; Swedish Research Council (VR grant) [621-2013-4191]

We construct Saturn equatorial neutral temperature and density profiles of H, H$_2$, He, and CH$_4$, between 10$^{-12}$ and 1 bar using measurements from Cassini's Ion Neutral Mass Spectrometer (INMS) taken during the spacecraft's final plunge into Saturn's atmosphere on 15 September 2017, combined with previous deeper atmospheric measurements from the Cassini Composite InfraRed Spectrometer (CIRS) and from the UltraViolet Imaging Spectrograph (UVIS). These neutral profiles are fed into an energy deposition model employing soft X-ray and Extreme UltraViolet (EUV) solar fluxes at a range of spectral resolutions ($\Delta\lambda=4\times10^{-3}$ nm to 1 nm) assembled from TIMED/SEE, from SOHO/SUMER, and from the Whole Heliosphere Interval (WHI) quiet Sun campaign. Our energy deposition model calculates ion production rate profiles through photo-ionisation and electron-impact ionisation processes, as well as rates of photo-dissociation of CH$_4$. The ion reaction rate profiles we determine are important to obtain accurate ion density profiles, meanwhile methane photo-dissociation is key to initiate complex organic chemical processes. We assess the importance of spectral resolution in the energy deposition model by using a high-resolution H$_2$ photo-absorption cross section, which has the effect of producing additional ionisation peaks near 800 km altitude. We find that these peaks are still formed when using low-resolution ($\Delta\lambda=1$ nm) or mid-resolution ($\Delta\lambda=0.1$ nm) solar spectra, as long as high-resolution cross sections are included in the model., Comment: 21 pages, 13 figures, 5 tables, accepted for publication in Icarus

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International audience; We report a new set of stellar occultation measurements for nightside temperature profiles made by the MAVEN/IUVS that provide evidence for a recurring layer of warm air between 70‐90 km altitudes in the nightside mesosphere of Mars during Ls = 0° ‐ 180° in Martian Year 33‐34. The nightside profiles reveal a recurring peak of atmospheric temperature around 80 km over the equator to the middle latitudes in the northern hemisphere. The predictions of the Mars Climate Database have a warm layer with much smaller amplitudes. The observed peak amplitudes are larger than those predicted by the model by up to 90 K. Wavenumber‐3 structures are seen in the warm layer that are potentially signatures of thermal tides or stationary planetary waves, with amplitudes two‐times larger than predicted.

Martian sub-solar electron temperatures obtained below 250 km are examined using data obtained by instruments on the Mars Atmosphere Evolution Mission (MAVEN) during the three sub-solar deep dip ca...

The Cassini spacecraft's last orbits directly sampled Saturn's thermosphere and revealed a much more chemically complex environment than previously believed. Observations from the Ion and Neutral Mass Spectrometer (INMS) aboard Cassini provided compositional measurements of this region and found an influx of material raining into Saturn's upper atmosphere from the rings. We present here an in-depth analysis of the CH$_4$, H$_2$O, and NH$_3$ signal from INMS and provide further evidence of external material entering Saturn's atmosphere from the rings. We use a new mass spectral deconvolution algorithm to determine the amount of each species observed in the spectrum and use these values to determine the influx and mass deposition rate for these species., Comment: 24 pages, 7 figures, 3 tables, accepted for publication in the Journal of Geophysical Research: Planets

International audience; This work offers the first in‐depth study of the global characteristics of wave perturbations in temperature profiles at 20 ‐ 140 km altitudes derived from the Imaging Ultraviolet Spectrograph (IUVS) onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. The peak amplitudes of waves seen in temperature profiles exceed 20 % of the mean background, especially on the nightside, which is larger than those in Earth’s mesosphere and thermosphere. The wave perturbations generate an instability layer around 70‐100 km on the nightside, which potentially causes wave‐breaking and turbulences. Our results highlighted a seasonal variation in the latitudinal distribution of nightside perturbations. Amplitudes of wave perturbations were found to be large in the northern low‐latitude region and the southern polar region during the first half of the year (Ls = 0° ‐ 180°). An increase of waves in the spectral density was found in southern low‐latitude regions in the latter half of the year (Ls = 180° ‐ 360°). Vertical wavenumber spectral density in the Martian middle atmosphere shows a power‐law dependence with a logarithmic spectral slope of ‐3, similar to the features seen in the Earth’s atmosphere. The derived spectral power density suggests the longer waves growing with height while the effective dissipation of shorter waves occurs. The strong CO2 15‐micron band cooling can effectively dissipate shorter waves. In contrast, the spectral power density at longer waves suggests an amplitude growth with height of unsaturated waves up to the lower thermosphere.

International audience; The analysis of Lyman α observations in the exosphere of Mars has become limited by the assumption of spherical symmetry in the modeling process, as the models are being used to analyze increasingly detailed measurements. In order to overcome this limitation, a two-dimensional density model is presented, which better emulates the density distribution of deuterium and hydrogen atoms in the exosphere of Mars. A two-dimensional radiative transfer model developed in order to simulate multiple scattering of solar Lyman α photons by an asymmetric, non-isothermal hydrogen exosphere, is also presented here. The models incorporate changes in density and temperature structure of the martian atmosphere with radial distance and solar zenith angle. The 2-D models were applied to the MAVEN-IUVS echelle observations of deuterium and hydrogen Lyman α as well as HST Lyman α observations of hydrogen at Mars. The asymmetric 2-D model provided better fits to the data and smaller thermal escape rates in comparison to the symmetric 1-D model for the exosphere of Mars. However, intensity differences between both models became small above ~2.5 martian radii indicating that the exosphere of Mars approaches spherical symmetry at higher altitudes, in agreement with earlier studies. In addition, a new cross calibration of the absolute sensitivities of two instruments on the Hubble Space Telescope and the MAVEN-IUVS echelle mode is presented based on near-simultaneous observations of the geocorona and Mars.

International audience; We analyze Saturn Lyman α system scans observed by the Cassini/UVIS instrument probing non-auroral airglow emissions from Saturn's disk, the ring atmosphere and the Saturn hydrogen torus. We constrain variations in disk brightness with latitude and local time, relating them to models of Saturn's upper atmosphere. We compare these results with Voyager observations of similar viewing geometry. The Cassini disk brightness is almost four time lower than the Voyager brightness. This difference cannot be explained by changes in the solar Lyman α flux or atmospheric structure. The change in solar flux is modest and atmospheric structure retrieved from occultation data is relatively stable over time. The disk brightness observed by Cassini can be explained by resonant scattering of solar Lyman α radiation and the best-fit hydrogen abundance in the thermosphere is consistent with atmospheric structure retrieved from the occultation data as well as photochemical model predictions. Our results support the recently proposed recalibration of the Voyager/UVS instruments that would bring the Voyager disk brightness in line with our model predictions and provide a consistent interpretation of Saturn Lyman α emissions over time. We also explore the extended emission in the Saturn system, centered near the ring plane in edge-on viewing. The extended emission can be explained by a distribution of atomic hydrogen that is symmetric about the ring plane. We do not find evidence for a plume of hot hydrogen atoms escaping from Saturn's sunlit atmosphere that was proposed previously. Instead, the structure earlier interpreted as a plume is most simply explained by solar illumination from below the ring plane during southern summer combined with the edge-on viewing geometry.

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.

Dust storms cause Mars to lose water Mars was once a wet planet, but it has lost most of its water through reactions that produce hydrogen, which escapes from the upper atmosphere into space. Stone et al. used data from the Mars Atmosphere and Volatile Evolution spacecraft to study how water is transported to the upper atmosphere and converted to hydrogen. They found that water can reach higher altitudes than previously thought, especially during global or regional dust storms. Photochemical modeling shows that this process dominates the current loss of water from Mars and influenced the evolution of its climate. Science , this issue p. 824

International audience; We report the detection of bright aurora spanning Mars’ nightside during the space weather event occurring in September 2017. The phenomenon was similar to diffuse aurora detected previously at Mars, but 25 times brighter and detectable over the entire visible nightside. The observations were made with the Imaging UltraViolet Spectrograph (IUVS), a remote sensing instrument on the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft orbiting Mars. Images show that the emission was brightest around the limb of the planet, with a fairly uniform faint glow against the disk itself. Spectra identified four molecular emissions associated with aurora, and limb scans show the emission originated from an altitude of ~60 km in the atmosphere. Both are consistent with very high energy particle precipitation. The auroral brightening peaked around 13 September, when the flux of solar energetic electrons and protons both peaked. During the declining phase of the event, faint but statistically significant auroral emissions briefly appeared against the disk of the planet in the form of narrow wisps and small patches. These features are approximately aligned with predicted open field lines in the region of strong crustal magnetic fields in Mars’ southern hemisphere.

The MAVEN mission has measured an electron temperature spike at altitudes where the Martian atmosphere becomes optically thick and changes occur in the atmospheric chemistry. The temperature spike ...

On 1 February 2016 Titan occulted the three stars in Orion’s belt, as seen from the Casssini spacecraft. The ultraviolet spectrometer on Cassini observed this event with the stars arranged along the instrument’s entrance slit. These three stellar occultations probed different latitudes in Titan’s northern hemisphere but similar longitudes. The occultations were also observed simultaneously in the extreme ultraviolet and far ultraviolet regions of the spectrum. Analysis of these measurements provides information on the atmospheric composition and thermal structure. We determine altitude profiles of N 2 , CH 4 , C 2 H 2 , C 2 H 4 , C 2 H 6 , C 4 H 2 , C 6 H 6 , HCN, and HC 3 N densities along with aerosol extinction. We infer significant variations in latitude for C 2 H 2 , C 2 H 4 , C 4 H 2 , aerosols, and temperature, providing clues to global circulations patterns. The inferred mole fractions of C 2 H 2 , C 2 H 4 , C 4 H 2 , C 6 H 6 , HCN, and HC 3 N are roughly consistent with photochemical model predictions, but C 2 H 2 and C 2 H 4 are over predicted by tens of percent at pressures greater than 10−5 hPa, C 4 H 2 is under predicted by ∼ x2 at most levels, and C 6 H 6 is under predicted by ∼ x2 at pressures less than 10−5 hPa.

We present a study of the SOIR/Venus Express CO2 and CO profiles in order to constrain the eddy diffusion profile in the Venus mesosphere (80 to 130 km of altitude). We compute the mean number density profile of both species, and the hydrostatic temperature based on the CO2 mean density profile. We use a 1D-photochemical/diffusion model to adjust the eddy diffusion profile such that the 1D-model and SOIR CO profiles best agree. We compare our results with eddy diffusion profiles found in the literature. We use our approach to revisit the He and CO2 profiles from Pioneer Venus probe mass spectrometer in order to derive an eddy diffusion profile extending from 80 to 200 km of altitude. Our eddy diffusion profile is different in shape and in magnitude compared to the Pioneer Venus profile: it is equal to 3.6 × 106 cm2/s at 110 km, 7.5 × 106 cm2/s at 120 km, 0.4 × 106 cm2/s at 130 km, and 201 × 106 cm2/s at 140 km, and constant below 110 km and above 140 km. This new eddy diffusion profile can be used in Venus Global Circulation Models as well as 1D-photochemical models.

We provide a comprehensive update of photochemical escape rates of atomic carbon from the present-day Martian atmosphere using a one-dimensional photochemical model and a Monte Carlo escape model. The photochemical model incorporates new results relevant to carbon photochemistry at Mars, including new cross sections for photodissociation of CO 2 into C and O 2 (Lu et al. 2014) and electron impact dissociation of CO (Ajello et al. 2019). We find the newly included channel of CO 2 photodissociation to be the largest contributor to C escape, at 34%–58%. CO photodissociation and CO + dissociative recombination, which have been discussed extensively in the literature, also show up as significant sources of hot C atoms, with respective contributions of 15%–23% and 7%–10%. Electron impact dissociation of CO 2 (11%–15%) and photoionization of CO (6%–20%) are also important channels. Overall, escape rates vary over 3– 11 × 1 0 23 s−1, with an increase of 70% at perihelion compared to aphelion, and a much larger increase of 133% at solar maximum compared to solar minimum. While these present escape rates give a total integrated escape of only 1.3 mbar of CO 2 when multiplied by 3.6 billion years, the better characterization of carbon photochemistry and escape from this study will enable us to more reliably extrapolate backwards in time to when conditions of the Martian atmosphere were significantly different from those of today.

We present an analysis of gravity waves in Mars' upper atmosphere above 120 km. Using in-situ data from NGIMS onboard MAVEN we have been able to characterise waves from nearly 4000 orbits. We have used temperature and density profiles to extract perturbations and interpret these as vertically propagating gravity waves which we characterise by their amplitude and wavelength. In this region of the atmosphere gravity waves have amplitudes of around 10%. Vertical wavelengths are found to be around 10–30 km. We observe an increase in gravity wave amplitudes with increasing solar zenith angle. Gravity wave amplitudes appear invariant in altitude on the dayside, however increase with altitude on the nightside.

Atmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere). Waves may be generated from instabilities, convective storms or other atmospheric phenomena. The inferred wave amplitudes change little with height within the sampled region, raising the possibility of the waves being damped, which in turn may enhance the eddy friction within the thermosphere. Using our Saturn Thermosphere Ionosphere General Circulation Model, we explore the parameter space of how an enhanced Rayleigh drag in different latitude regimes would affect the global circulation pattern within the thermosphere and, in turn, its global thermal structure. We find that Rayleigh drag of sufficient magnitude at midlatitudes may reduce the otherwise dominant Coriolis forces and enhance equatorward winds to transport energy from poles toward the equator, raising the temperatures there to observed values. Without this Rayleigh drag, energy supplied into the polar upper atmosphere by magnetosphere‐atmosphere coupling processes remains trapped at high latitudes and causes low‐latitude thermosphere temperatures to remain well below the observed levels. Our simulations thus suggest that giant planet upper atmosphere global circulation models need to include additional Rayleigh drag in order to capture the effects of physical processes otherwise not resolved by the codes.

International audience; This paper reports the first detections of two high‐altitude nighttime CO2 clouds on Mars during southern summer (Ls = 264° and Ls = 330°) with stellar occultation measurements by the Imaging Ultraviolet Spectrograph on board the Mars Atmosphere and Volatile Evolution spacecraft. Interpretation of the transmission spectra with Mie theory indicates particle radii of ~90‐110 nm assuming a mono‐disperse distribution. The altitude profile of extinction indicates that the cloud layers are confined horizontally to sizes less than ~ 500‐700~km. Examination of the CO2 density and temperature profiles reveals strong wave‐like perturbations. Supersaturated temperatures occur at the maximum negative extent of these wave‐like perturbations, which are organized in longitude with a dominant m=3 zonal harmonic. This suggests that tides are important in the formation of CO2 clouds.

We have simulated the 3D atomic hydrogen density in the Martian upper atmosphere and associated Jeans escape rate during Martian years 28 and 29. The coronal Lyman-α brightness is computed using a 3D radiative transfer model which accounts for the monthly average hydrogen density for these two years and is compared to a large set of observations by Mars Express/SPICAM. The simulated brightness is generally in good agreement with the observations for Ls 330° for Martian year 28 and Ls < 270°, Ls > 340° for Martian year 29, but the model strongly underestimated the brightness for 230 < Ls < 330° for Martian year 28 and 270 < Ls < 340° for Martian year 29. In these simulations the transport of water vapor contributes to the production of hydrogen at high altitudes during southern summer. A possible explanation for the model discrepancy is an underestimate of this water transport, associated with an underestimate of the hygropause altitude and/or an underestimate of the supersaturation of the mesosphere. Considering this discrepancy, we estimate the hydrogen escape rate during these two Martian years to vary by almost two orders of magnitude, between ~10 to 6 × 10 s (equivalent to a global layer of water ~33 to 2000 mm deep every billion years), in agreement with the seasonal variations estimated directly from the fit of the SPICAM observations during the Martian year 28 by Chaffin et al. (2014). Our analysis suggests that episodic dust storms and associated enhancements at high altitude near perihelion are a major factor in the H escape estimates averaged over one martian year or longer periods, but the accumulated water lost at this rate for 4 billion years is much lower than the amount of water needed to form the flow channels observed on Mars.© 2019 Elsevier Inc., This project was partially funded by the Programme National de Planetologie and Programme National Soleil Terre and by the Centre National d'Etudes Spatiales. This work has been partially funded by the European Union Horizon 2020 Programme (H2020 Compet -08-2014) under grant agreement UPWARDS-633127. FGG has been partly funded by the Spanish Ministerio de Ciencia e Innovación , the Agencia Estatal de Investigacion and EC FEDER funds under projects ESP2017-87143-R and RTI2018-100920-J-I00 . We would like to thank G. Lacombe and L. Baggio for their support on the calibration. We thank Allyson Leffler for her careful review of grammatical aspects of this paper. The SPICAM/MEX data used in this study are available through the ESA Planetary Science Archive (PSA, https://archives.esac.esa.int/psa/ ). The monthly average 3D simulated hydrogen exosphere can be found on: https://owncloud.latmos.ipsl.fr/index.php/s/AdUgv9PdipA2rim Appendix A, With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

We provide a comprehensive characterization of the photochemistry behind atomic carbon in the Mars atmosphere. Using a one-dimensional photochemical model, with an extensive reaction list incorporating new high-resolution photodissociation cross-sections ( Heays et al., 2017 ) and the recently experimentally confirmed CO 2 + h ν → C + O 2 ( Lu et al., 2014 ), we investigate the dominant channels for the production and loss of atomic carbon, against a subsolar background atmosphere based on MAVEN Deep Dip 2 observations. We confirm the results from previous studies that CO photodissociation and CO + dissociative recombination are important contributors to atomic C production, and that reaction with O 2 to form CO is the main loss channel. However, we also find significant contributions from CO 2 + h ν → C + O 2 , HCO + + e → C + OH and charge exchange of C + with CO 2 . These additional production channels give rise to significantly higher C densities than have been previously reported, with a peak at 4 × 1 0 5 cm−3 at a CO 2 density of 1 . 7 × 1 0 10 cm−3 ( ∼ 146 km altitude). We find the C densities to vary with H 2 O densities over a Martian year, with the wetter perihelion season having 13% lower C column densities. Contrary to Anbar et al. (1993) , we find C densities to be relatively insensitive to the temperature-dependence of cross-sections for CO 2 and CO photodissociation. A good understanding of carbon photochemistry in the present-day Martian atmosphere provides the essential foundational framework for determining the fate of atmospheric carbon in the study of Mars’ climate evolution.

Using the Mars Atmospheric and Volatile EvolutioN mission (MAVEN) Imaging Ultraviolet Spectrograph (IUVS), we found periodic longitudinal variations in CO2 density in the Martian atmosphere. The variations exhibit significant structure with longitudinal wave numbers 1, 2, and 3 in an effectively constant local solar time frame, and we attribute this structure to nonmigrating tides. The wave-2 component is dominated by the diurnal eastward moving DE1 tide at the equator and the semidiurnal stationary S0 tide at the midlatitudes. Wave-3 is dominated by the diurnal eastward moving DE2 tide, with possibly the semidiurnal eastward moving SE1 tide causing an amplitude increase at the midlatitudes. Structure in the wave-1 component can be explained by the semidiurnal westward moving SW1 tide.

The Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS) provides sensitive detections of neutral gas and ambient ion composition. NGIMS measurements of nine atomic and molecular neutral species, and their variation with altitude, latitude, and solar zenith angle are reported over several months of operation of the MAVEN mission. Sampling NGIMS signals from multiple neutral species every several seconds reveals persistent and unexpectedly large amplitude density structures. The scale height temperatures are mapped over the course of the first few months of the mission from high down to midlatitudes. NGIMS measurements near the homopause of 40Ar/N2 ratios agree with those reported by the Sample Analysis at Mars investigation and allow the altitude of the homopause for the most abundant gases to be established.

We report the results of the observations of the ionosphere of Mars by the Neutral Gas and Ion Mass Spectrometer. These observations were conducted during the first 8 months of the Mars Atmosphere and Volatile EvolutioN mission (MAVEN). These observations revealed the spatial and temporal structures in the density distributions of 22 ions: H2+, H3+, He+, O2+, C+, CH+, N+, NH+, O+, OH+, H2O+, H3O+, N2+/CO+, HCO+/HOC+/N2H+, NO+, HNO+, O2+, HO2+, Ar+, ArH+, CO2+, and OCOH+. Dusk/dawn and day/night asymmetries in the density distributions were observed for nearly all ion species. Additionally, high-density fluctuations were detected on the nightside and may reflect the effect of the partial screening of the atmosphere of Mars by the weak intrinsic magnetic field of the planet. The two first MAVEN “deep dip” campaigns were used to investigate the location of the primary ion peak. This peak was detected at 190 km near the terminator but was below the spacecraft altitude of 130 km near the subsolar point.

We retrieved the density and temperature profiles in Saturn’s thermosphere from 26 stellar occultations observed by the Cassini/UVIS instrument. These results expand upon and complement the previous analysis of 15 Cassini/UVIS solar occultations by Saturn’s upper thermosphere. We find that the exospheric temperatures based on the stellar occultations agree with the solar occultations and range from 380 K to 590 K. These temperatures are also consistent with the recent re-analysis of the Voyager/UVS occultations. The retrieved density profiles support our earlier inference that the shape of the atmosphere at low pressures is consistent with a meridional trend of increasing temperatures with absolute latitude. This implies a high-latitude heat source, such as auroral heating, although the existing circulation models that include auroral heating still underestimate the equatorial temperatures by overestimating the meridional temperature gradient. This suggests either that the circulation models are somehow incomplete or there is some other heat source at low to mid latitudes that is relatively less efficient than high-latitude heating. We also find evidence for the expansion of the exobase by about 500 km between 2006 and 2011 near the equator, followed by possible contraction after 2011. The expansion appears to be caused by significant warming of the lower thermosphere that anti-correlates with solar activity and may be connected to changes in global circulation. Lastly, we note that our density profiles are in good general agreement with the Voyager/UVS data. In particular, the Voyager density profiles are most consistent with the Cassini/UVIS stellar occultations from late 2008 and early 2009 that roughly coincide in season with the Voyager flybys.

The history of Mars’ atmosphere is important for understanding the geological evolution and potential habitability of the planet. We determine the amount of gas lost to space through time using measurements of the upper-atmospheric structure made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. We derive the structure of 38Ar/36Ar between the homopause and exobase altitudes. Fractionation of argon occurs as a result of loss of gas to space by pickup-ion sputtering, which preferentially removes the lighter atom. The measurements require that 66% of the atmospheric argon has been lost to space. Thus, a large fraction of Mars’ atmospheric gas has been lost to space, contributing to the transition in climate from an early, warm, wet environment to today’s cold, dry atmosphere.

Electrodynamics plays an important role in controlling circulation and energy balance in Saturn's auroral thermosphere, but less is known about its effects at lower latitudes. Recent observations by the Cassini magnetometer instrument, taken around the equator during the Grand Finale tour, revealed azimuthal magnetic field perturbations of the order of 10–30 nT associated with electrodynamics in Saturn's upper atmosphere. In order to investigate the implications of these observations, we develop a steady-state, axisymmetric model adapted from terrestrial studies to simulate wind-driven electrodynamics at low to middle latitudes in Saturn's thermosphere. Our results demonstrate, based on rigorous theory, that the magnetic field observations can be reproduced by a wind dynamo generating electric currents in the ionosphere of the order 10−9 − 10−7 Am−2 provided that eastward zonal winds of the order of 100 ms−1 exist in the equatorial thermosphere. The resistive (Joule) heating rate based on the equatorial current system, however, is significantly lower than that required to explain the temperatures in Saturn's thermosphere. In spite of this, we find that resistive heating and ion drag due to a mid-latitude wind dynamo have the potential to alter the energy balance and general circulation in Saturn's thermosphere and should be treated self-consistently in future global circulation models.

On 19 October 2014, comet C/2013 A1 (Siding Spring) had a close encounter with Mars and deposited cometary dust particles into the Martian atmosphere. We report a comprehensive analysis of the resulting meteor shower and its perturbation on Mars' atmosphere and ionosphere. Using Mars Atmosphere and Volatile EvolutioN/Imaging Ultraviolet Spectrograph observations of ablated meteoric metallic species, we show this shower lasted less than 3 hr and was therefore limited to one hemisphere. Meteoric ablation occurred in a narrow altitude layer, with Mg+, Mg, Fe+, and Fe deposited between about 105 and 120 km, consistent with comet Siding Spring's relative velocity of 56 km/s. We find that 82 ± 25 t of dust was deposited, improving previous measurements and a thousand times larger than model expectations. With regular observations over two Mars days, we show that horizontal winds globally redistribute this material and also suggest new vertical transport mechanisms for metallic ions. Such transport is inconsistent with diffusion and may be related to electrodynamic processes. The rapid loss of neutral species and presence of ions at high altitudes indicate that our understanding of existing Martian meteoric chemistry modeling and ionospheric dynamics is incomplete.

Cassini's final phase of exploration The Cassini spacecraft spent 13 years orbiting Saturn; as it ran low on fuel, the trajectory was changed to sample regions it had not yet visited. A series of orbits close to the rings was followed by a Grand Finale orbit, which took the spacecraft through the gap between Saturn and its rings before the spacecraft was destroyed when it entered the planet's upper atmosphere. Six papers in this issue report results from these final phases of the Cassini mission. Dougherty et al. measured the magnetic field close to Saturn, which implies a complex multilayer dynamo process inside the planet. Roussos et al. detected an additional radiation belt trapped within the rings, sustained by the radioactive decay of free neutrons. Lamy et al. present plasma measurements taken as Cassini flew through regions emitting kilometric radiation, connected to the planet's aurorae. Hsu et al. determined the composition of large, solid dust particles falling from the rings into the planet, whereas Mitchell et al. investigated the smaller dust nanograins and show how they interact with the planet's upper atmosphere. Finally, Waite et al. identified molecules in the infalling material and directly measured the composition of Saturn's atmosphere. Science , this issue p. eaat5434 , p. eaat1962 , p. eaat2027 , p. eaat3185 , p. eaat2236 , p. eaat2382

International audience; The Imaging UltraViolet Spectrograph (IUVS) instrument of the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has acquired data on Mars for more than one Martian year. During this time, beginning with March 2015, hundreds of stellar occultations have been observed, in 12 dedicated occultation campaigns, executed on average every two to three months. The occultations cover the latitudes from 80° S to 75° N and the full range longitude, and local times with relatively sparse sampling. From these measurements we retrieve CO2, O2, and O3 number densities as well as temperature profiles in the altitude range from 20 to 160 km, covering eight order of magnitudes in pressure from ∼2 × 101 to ∼4 × 10−7 Pa. These data constrain the composition and thermal structure of the atmosphere. The O2 mixing ratios retrieved during this study show a high variability from 1.5 × 10−3 to 6 × 10−3; however, the mean value seems to be constant with solar longitude. We detect ozone between 20 and 60 km. In many profiles there is a well defined peak between 30 and 40 km with a maximum density of 1 – 2 × 109 cm−3. Examination of the vertical temperature profiles reveals substantial disagreement with models, with observed temperatures both warmer and colder than predicted. Examination of the altitude profiles of density perturbations and their variation with longitude shows structured atmospheric perturbations at altitudes above 100 km that are likely non‐migrating tides. These perturbations are dominated by zonal wavenumber 2 and 3 with amplitudes greater than 45 %.

We simulate the hydrogen density near the exobase of Mars, using the 3-D Martian Global Circulation Model of Laboratoire de Météorologie Dynamique, coupled to an exospheric ballistic model to compute the downward ballistic flux. The simulated hydrogen distribution near the exobase obtained at two different seasons—Ls = 180° and Ls = 270°—is close to Zero Net Ballistic Flux equilibrium. In other words, the hydrogen density near the exobase adjusts to have a balance between the local upward ballistic and the downward ballistic flux due to a short lateral migration time in the exosphere compared to the vertical diffusion time. This equilibrium leads to a hydrogen density n near the exobase directly controlled by the exospheric temperature T by the relation nT5/2 = constant. This relation could be used to extend 1-D hydrogen exospheric model of Mars used to derive the hydrogen density and escape flux at Mars from Lyman-α observations to 3-D model based on observed or modeled exospheric temperature near the exobase, without increasing the number of free parameters. ©2018. American Geophysical Union. All Rights Reserved., This project was partly funded by the Programme National de Planetologie and Programme National Soleil Terre and by the Centre National d'Etudes Spatiales. This work has been partially funded by the European Union Horizon 2020 Programme (H2020 Compet-082014) under grant agreement UPWARDS-633127.