We describe the Mars ionosphere with unprecedented detail in 3‐D, as simulated by a Mars general circulation model (the Laboratoire de Météorologie Dynamique Mars GCM), and compare it with recent ...measurements. The model includes a number of recent extensions and improvements. Different simulations for a full Martian year have been performed. The electron density at the main ionospheric peak is shown to vary with the Sun‐Mars distance and with the solar variability, both in the long‐term (11 year solar cycle) and on shorter temporal scales (solar rotation). The main electronic peak is shown to be located at the same pressure level during all the Martian year. As a consequence, its altitude varies with latitude, local time, and season according to the natural expansions and fluctuations of the neutral atmosphere, in agreement with previous models. The model predicts a nighttime ionosphere due only to photochemistry. The simulated ionosphere close to the evening terminator is in agreement with observations. No effort has been made to explain the patchy ionosphere observed in the deep nightside. We have compared the modeled ionosphere with Mars Global Surveyor and Mars Advanced Radar for Subsurface and Ionosphere Sounding data. The model reproduces the solar zenith angle variability of the electron density and the altitude of the peak, although it underestimates the electron density at the main peak by about 20%. The electron density at the secondary peak is strongly underestimated by the model, probably due to a very crude representation of the X‐ray solar flux. This is one of the aspects that needs a revision in future versions of the model.
Key Points
3D simulations of the Martian ionosphere during a full Martian year
Results in agreement with MGS and MARSIS observations
Nightside ionosphere due to photochemistry is simulated
The superrotation of the atmospheres of slowly rotating bodies is a long‐standing problem yet unsolved in atmospheric dynamics. On Venus, the most extreme case known of superrotation, this is ...accompanied and influenced by a recurrent planetary‐scale cloud structure, known as the Y feature. So far, no model has simultaneously reproduced its shape, temporal evolution, related wind field, nor the relation between its dynamics and the unknown UV‐absorbing aerosol that produces its dark morphology. In this paper we present an analytical model for a Kelvin‐like wave that offers an explanation of these peculiarities. Under Venus cyclostrophic conditions, this wave is equatorially and vertically trapped where zonal winds peak and extends 7 km in altitude, and its vertical wind perturbations are shown to produce upwelling of the UV absorber. The Y‐feature morphology and its 30 day evolution are reproduced as distortions of the wave structure by the Venus winds.
Key Points
First analytical solution for equatorial waves in cyclostrophic regimes
Explanation for Y feature's morphology, darkness, and time evolution
Analytical model applicable to exoplanets with atmospheric superrotation
We present the extension to the thermosphere of a Martian general circulation model, the first able to self‐consistently study the whole Martian atmosphere from the surface to the exosphere. We ...describe the parameterizations developed to include physical processes important for thermospheric altitudes. The results of a simulation covering 1 full Martian year are presented, focusing on the seasonal, diurnal, and day‐to‐day variability of the temperatures in the exobase region. The seasonal variation of the zonal mean temperatures in the upper atmosphere is of about 100 K, mostly due to the variation of the solar forcing. The temperature of the mesopause ranges between 115 and 130 K, with little seasonal and day‐night variations. Its pressure level undergoes significant seasonal and day‐night variations. Comparisons with SPICAM observations show that the modeled mesopause is too low and too warm. A similar study for the homopause shows that it is located higher in the atmosphere during solstices, owing to reinforced mixing by a stronger circulation. Important day‐night temperature differences are found in the thermosphere, ranging from about 60 K at aphelion to 110 K at perihelion. This diurnal cycle is slightly perturbed by the day‐to‐day variations of temperature, dominated by waves with periods of 2 to 6 sols and amplitude of 30 K. The model reproduces the observed solar cycle variation in temperatures when using a UV heating efficiency of 16%, slightly lower than the theoretical value. The seasonal variation of temperatures is overestimated by the model, in comparison with the available measurements.
Many independent measurements have shown that extremely cold temperatures are found in the Martian mesosphere. These mesospheric “cold pockets” may result from the propagation of atmospheric waves. ...Recent observational achievements also hint at such cold pockets by revealing mesospheric clouds formed through the condensation of CO2, the major component of the Martian atmosphere. Thus far, modeling studies addressing the presence of cold pockets in the Martian mesosphere have explored the influence of large‐scale circulations. Mesoscale phenomena, such as gravity waves, have received less attention. Here we show through multiscale meteorological modeling that mesoscale gravity waves could play a key role in the formation of mesospheric cold pockets propitious to CO2 condensation.
Key Points
Mesoscale gravity waves permit subcondensation mesospheric cold pockets
Regions with observed CO2 clouds feature propitious conditions for GW activity
Mesoscale modeling appears as a necessary complement to global scale models
Warming of the martian lower thermosphere (100–130 km) at north polar latitudes near the perihelion/winter solstice (Ls = 270) was recently observed. No analogous warming has been observed within the ...south polar thermosphere during its aphelion/winter season (Ls ∼ 90). Detailed global model simulations are required to investigate the physical processes driving these seasonal variations. New simulations are conducted for conditions approximating the atmosphere during these Mars Global Surveyor (MGS) and Odyssey (ODY) aerobraking periods. Strong northern winter polar warming features are calculated near 120 km, yielding nightside mean temperatures 10–15 K warmer than observed ODY values. No southern winter polar warming trend is simulated; however, nightside mean temperatures are 20–30 K warmer than observed by MGS. The stronger interhemispheric circulation during northern winter is clearly driven by stronger insolation and dust heating near perihelion, resulting in subsidence and warmer temperatures in the northern polar night.
► Pyrolysis, combustion and gasification of marine biomass were evaluated. ► Oxygen concentration enhanced the oxidation stage of the NG microalgae. ► N-compounds were evolved in pyrolysis. Sulfur ...compounds are evolved in combustion. ► Combustion and pyrolysis main gas products evolved during the second step. ► H2 production during gasification was enhanced by steam concentration.
Pyrolysis, combustion and gasification characteristics of Nannochloropsis gaditana microalgae (NG microalgae) were investigated by thermogravimetric analysis (TGA). NG microalgae pyrolysis and combustion could be divided into three main stages: dehydration, proteins and polysaccharides degradation and char decomposition. The effects of the initial sample mass, particle size and gas flow on the pyrolysis and combustion processes were studied. In addition, gasification operation conditions such as temperature, initial sample mass, particle size, sweep gas flow and steam concentration, were experimentally evaluated.
The evolved gases were analyzed online using mass spectroscopy (MS). In pyrolysis and combustion processes, most of the gas products were generated at the second degradation step. N-compounds evolution was associated with the degradation of proteins. Furthermore, SO2 release from combustion could be related to sulphated polysaccharides decomposition. The main products detected during gasification were CO2, CO, H2, indicating that oxidation reactions, water gas and water gas shift reactions, were predominant.
Following the recent detection of the oxygen green line airglow on Mars, we have improved the statistical analysis of the data recorded by the NOMAD/UVIS instrument on board the ExoMars Trace Gas ...Orbiter mission by summing up hundreds of spectra to increase the signal‐to‐noise ratio. This led to the observation of the OI 630 nm emission, the first detection in a planetary atmosphere outside the Earth. The average limb profile shows a broad peak intensity of 4.8 kR near 150 km. Comparison with a photochemical model indicates that it is well predicted by current photochemistry, considering the sources of uncertainty. The red/green line intensity ratio decreases dramatically with altitude as a consequence of the efficient quenching of O(1D) by CO2. Simultaneous observations of the green and red dayglow will provide information on variations in the thermosphere in response to seasonal changes and the effects of solar events.
Plain Language Summary
The green and red oxygen emissions at 557.7 and 630 nm, respectively, are among the dominant spectral features of the terrestrial dayglow and aurora. Recently, the presence of the green emission was also observed in the Martian dayglow using the NOMAD/UVIS instrument on board the ExoMars Trace Gas Orbiter mission. The red line was expected to be significantly weaker as the long‐lived upper state of the transition is deactivated by collisions with ambient CO2. Further statistical treatment of the spectra collected during 1.5 years near solar minimum has lead to the discovery of the presence of the 630 nm dayglow emission. The averaged limb profile shows a maximum limb brightness near 150 km about 30 times weaker than the green line. The altitude and brightness of the red emission are in agreement with those simulated with a photochemical model for the same conditions as the observations. The same model also matches the characteristics of the averaged 557.7‐nm dayglow profile observed simultaneously. Variations in the characteristics of the oxygen emissions are related to changes in the composition of the Martian upper atmosphere such as those generated by seasons and energetic solar events.
Key Points
The oxygen 630‐nm emission has been detected in the Mars dayglow with the UVIS‐NOMAD/UVIS instrument on board EXOMARS Trace Gas Orbiter
The 630‐nm emission is broadly distributed in the thermosphere with a maximum intensity at the limb of ∼4 kiloRayleighs near 150 km
Photochemical model simulations predict O(1D) and O(1S) densities in agreement with the observed limb profiles
Aeronomy of the Venus Upper Atmosphere Gérard, J.-C.; Bougher, S. W.; López-Valverde, M. A. ...
Space science reviews,
11/2017, Letnik:
212, Številka:
3-4
Journal Article, Web Resource
Recenzirano
Odprti dostop
We present aeronomical observations collected using remote sensing instruments on board Venus Express, complemented with ground-based observations and numerical modeling. They are mostly based on ...VIRTIS and SPICAV measurements of airglow obtained in the nadir mode and at the limb above 90 km. They complement our understanding of the behavior of Venus’ upper atmosphere that was largely based on Pioneer Venus observations mostly performed over thirty years earlier. Following a summary of recent spectral data from the EUV to the infrared, we examine how these observations have improved our knowledge of the composition, thermal structure, dynamics and transport of the Venus upper atmosphere. We then synthesize progress in three-dimensional modeling of the upper atmosphere which is largely based on global mapping and observations of time variations of the nitric oxide and O
2
nightglow emissions. Processes controlling the escape flux of atoms to space are described. Results based on the VeRA radio propagation experiment are summarized and compared to ionospheric measurements collected during earlier space missions. Finally, we point out some unsolved and open questions generated by these recent datasets and model comparisons.
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.
Plain Language Summary
This study presents new details on the global distribution of Mars water vapor altitude profile based on the daily observations for 3.5 years. We show a strong contrast between northern and southern summer when water vapor is sublimated from the summer polar ice. We find that sublimated water vapor is confined at low altitudes in the northern summer, whereas it directly reaches the upper atmosphere in the southern summer. During solstice periods, global meridional transport from summer to winter hemisphere drives the water vapor distributions. It is suggested that the transport of water vapor by the global circulation is limited by the formation of ice clouds in the northern summer. In contrast, water vapor is more effectively transported to the winter hemisphere in the southern summer because its vertical extent is less constrained by cloud formation in the warm southern summer atmosphere. We show that southern summer is the primary season for the supply of water vapor to the upper atmosphere, in addition to the period of strong dust storms. We also show that water transport from north to south still occurs below 10–20 km in the northern summer season, however it is much reduced relative to the southern summer season.
Key Points
We present global vertical distributions of water vapor in the Mars atmosphere from the observations collected for 3.5 years
We confirm a strong contrast between aphelion and perihelion water vapor vertical distributions
We reveal that water vapor sublimated from the northern polar cap is confined into very low altitudes during the aphelion periods
•Improve the 3D LMD martian GCM to simulate light species such as atomic and molecular hydrogen.•Simulate the diurnal, seasonal and solar activity variations of the hydrogen escape at Mars.•Couple ...the GCM with an exospheric model to simulate the variations of the martian hydrogen exosphere.•Compare results of the simulations with observations.
We present the temporal variability of the atomic and molecular hydrogen density derived from a 3D General Circulation Model describing the martian atmosphere from the surface to the exobase. A kinetic exospheric model is used to compute the hydrogen density above the exobase. We use these models to study the diurnal and seasonal variations of the hydrogen density and the Jeans escape rate as well as their variations with solar activity, assuming a classic dust scenario. We find that the diurnal variations of the hydrogen density are important with a peak in the dawn region during equinoxes and a peak on the nightside during solstices. These features result from the dynamics of the martian upper atmosphere. The variations of the atomic hydrogen Jeans escape with seasons and solar activity are in the range 1.3×1025s−1–4.4×1026s−1. A factor ∼8 is due to the seasonal variations with a maximum during the winter solstice in the northern hemisphere and a minimum during the summer solstice in the northern hemisphere that we attribute to the variation of the Mars–Sun distance. A factor ∼5 is due to the solar cycle with a maximum escape rate at high solar activity. The variations of the molecular hydrogen Jeans escape with seasons and solar activity are in the range 3×1022s−1–6×1024s−1. A factor ∼10 is due to the seasonal variations with a maximum during the winter solstice in the northern hemisphere and a minimum during the summer solstice in the northern hemisphere. A factor ∼20 is due to the solar cycle with a maximum escape rate at high solar activity. If Jeans escape is the major escape channel for hydrogen, the hydrogen escape is never limited by diffusion. The hydrogen density above 10,000km presents seasonal and solar cycle variations similar to the Jeans escape rate at all latitudes and local times. This 3D temporal model of the hydrogen thermosphere/exosphere will be useful to interpret future MAVEN observations and the consequences of the hydrogen corona variability on the martian plasma environment.
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