By measuring the regular oscillations of the density of CO2 in the upper atmosphere (between 120 and 190 km), the mass spectrometer MAVEN/NGIMS (Atmosphere and Volatile EvolutioN/Neutral Gas Ion Mass ...Spectrometer) reveals the local impact of gravity waves. This yields precious information on the activity of gravity waves and the atmospheric conditions in which they propagate and break. The intensity of gravity waves measured by MAVEN in the upper atmosphere has been shown to be dictated by saturation processes in isothermal conditions. As a result, gravity waves activity is correlated to the evolution of the inverse of the background temperature. Previous data gathered at lower altitudes (∼95–∼150 km) during aerobraking by the accelerometers on board MGS (Mars Global Surveyor), ODY (Mars Odyssey) and MRO (Mars Reconnaissance Orbiter) are analyzed in the light of those recent findings with MAVEN. The anti-correlation between GW-induced density perturbations and background temperature is plausibly found in the ODY data acquired in the polar regions, but not in the MGS and MRO data. MRO data in polar regions exhibit a correlation between the density perturbations and the Brunt-Väisälä frequency (or, equivalently, static stability), obtained from Global Climate Modeling compiled in the Mars Climate Database. At lower altitude levels (between 100 and 120 km), although wave saturation might still be dominant, isothermal conditions are no longer verified. In this case, theory predicts that the intensity of gravity waves is no more correlated to background temperature, but to static stability. At other latitudes in the three aerobraking datasets, the GW-induced relative density perturbations are correlated with neither inverse temperature nor static stability; in this particular case, this means that the observed activity of gravity waves is not only controlled by saturation, but also by the effects of gravity-wave sources and wind filtering through critical levels. This result highlights the exceptional nature of MAVEN/NGIMS observations which combine both isothermal and saturated conditions contrary to aerobraking measurements.
•Gravity wave activity causes density perturbations in the Martian thermosphere.•MAVEN found a correlation between GW activity and inverse background temperature.•Lower-altitude aerobraking data do not show this correlation, except for Mars Odyssey.•Aerobraking data and GCMs suggest instead wave activity correlated with Static stability.•When no such correlation, a mix of saturation, critical levels and sources is suspected.
Water escape on Mars has recently undergone a paradigm shift with the discovery of unexpected seasonal variations in the population of hydrogen atoms in the exosphere where thermal escape occurs and ...results in water lost to space. This discovery led to the hypothesis that, contradicting the accepted pathway, atomic hydrogen in the exosphere was not only produced by molecular hydrogen but mostly by high altitude water vapor. Enhanced presence of water at high altitude during southern spring and summer, due to atmospheric warming and intensified transport, favors production of H through photon‐induced ion chemistry of water molecules and thus appears to be the main cause of the observed seasonal variability in escaping hydrogen. This hypothesis is supported by the observation of large concentrations of water vapor between 50 and 150 km during the southern summer solstice and global dust events. Using a simplified yet representative air parcel transport model, we show that in addition to the formation of atomic hydrogen from water photolysis above 80 km, a major fraction of the exospheric hydrogen is formed at altitudes as low as 60 km and is then directly advected to the upper atmosphere. Comparing the injection modes of a variety of events (global dust storm, perihelion periods, and regional storm), we conclude that southern spring/summer controls H production and further ascent into the upper atmosphere on the long term with direct implication for water escape.
Plain Language Summary
Numerous lines of evidence suggest that Mars' water inventory was much larger in the past than it is today. The loss of this inventory has been driven by the formation of hydrated minerals on the surface, as well as by the escape of water to space. The first part of the escape process comprises the formation of H atoms, which may escape the planet once they reach the uppermost layers of the atmosphere. Here, we investigate one mechanism by which the H atoms may reach these high altitudes: the breakdown of water molecules by solar ultraviolet photons in the middle atmosphere (60–70 km above the surface), and the posterior ascent of the newly formed H atoms to the upper altitudes. We use a model that reveals that this process is the dominant contributor of atomic H to the upper atmosphere during periods of strong atmospheric circulation. In particular, we find that this mechanism is most efficient during the spring/summer season in the Southern Hemisphere, when Mars is closest to the Sun. Given that this season occurs every Martian year, our calculations suggest that this process has been the dominant contributor to water escape in the long term.
Key Points
We decipher hydrogen production and migration to Mars' upper atmosphere using a box model for a variety of elevated water vapor cases
H atoms formed between 60 and 80 km supply a dominant fraction of hydrogen to the upper atmosphere
Our results suggest that perihelion climate has a key role in the hydrogen transfer to the upper atmosphere overall
Exposed scarps images and ice‐penetrating radar measurements in the North Polar Layered Deposits (NPLD) of Mars show alternating layers that provide an archive of past climate oscillations, that are ...thought to be linked to orbital variations, akin to Milankovitch cycles on Earth. We use the Laboratoire de Météorologie Dynamique Martian Global Climate Model to study paleoclimate states to enable a better interpretation of the NPLD physical and chemical stratigraphy. When a tropical ice reservoir is present, water vapor transport from the tropics to the poles at low obliquity is modulated by the intensity of summer. At times of low and relatively constant obliquity, the flux still varies due to other orbital elements, promoting polar layer formation. Ice migrates from the tropics toward the poles in two stages. First, when surface ice is present in the tropics, and second, when the equatorial deposit is exhausted, from ice that was previously deposited in mid‐high latitudes. The polar accumulation rate is significantly higher when tropical ice is available, forming thicker layers per orbital cycle. However, the majority of the NPLD is sourced from ice that temporary resided in the mid‐high latitudes and the layers become thinner as the source location moves poleward. The migration stages imprint different D/H ratios in different sections in the PLDs. The NPLD is isotopically depleted compared to the South Polar Layered Deposits in all simulations. Thus we predict the D/H ratio of the atmosphere in contact with NPLD upper layers is biased relative to the average global ice reservoirs.
Plain Language Summary
In this work we run simulations of a Global Climate Model for Mars with a broad range of orbital elements, and ice initially placed in the tropics, we calculate the growth rate and hydrogen isotopic composition of the polar caps. These simulations help to understand the migration of ice from the tropical region to the polar caps, as believed to have occurred in the recent past. Ice transport to the poles occurs at two stages. The first is when ice is present in the tropics. The second is after the tropical reservoir has been exhausted, and the source of vapor that reaches the poles is from ice accumulated in mid‐high latitudes during the first stage. The polar caps growth rate during the first stage is larger and results in thicker layers. We find that both physical and chemical records are expected in the polar caps, controlled by the orbital elements and the surface ice distribution. The chemical record also depends on the source enrichment. These results should help to interpret ice records in order to decode past climate variations, and suggest the current hydrogen isotopic composition of the atmosphere is not representative of the total ice reservoirs on Mars.
Key Points
The North Polar Layered Deposits growth rate strongly depends on perihelion position, in late stages accumulation can be of order a meter over a precession cycle
The isotopic stratigraphic signal in the polar caps experiences secular evolution, in addition to oscillations due to orbital elements
Northern ice deposits are depleted in deuterium compared to the south, biasing the isotopic composition of the present‐day atmosphere
We present simulations of the thermal and nonthermal escape processes for H and D, under atomic, molecular and ion forms at Mars during spring equinox. These processes include Jeans escape, several ...photochemical reactions and the escape associated to the solar wind interaction with Mars. While the hydrogen escape is dominated by the atomic Jeans escape, we find that the deuterium escape is dominated by the photochemical atomic escape. Ions escape represent only 10% of the total escape for both species and is mostly due to charge exchange between neutral and solar wind protons. Including all the processes, we find a D/H fractionation factor (D/H escape ratio divided by the D/H atmospheric ratio) f = 0.04, with a main uncertainty associated to the elastic collisional cross sections needed to accurately derive the photochemical escape rate. Using this fractionation factor and considering a 30 m exchangeable reservoir of water, the average hydrogen escape rate needed to fractionate the Martian water from its primordial value to its current D/H value during the last 4.5 Gyr is ∼1.0 × 1028 s−1 which is larger than the current average escape rate (∼ 2 × 1026 s−1).
The choice of stones by the ancient Greeks to build edifices remains an open question. If the use of local materials seems generalized, allochthonous stones are usually also present but lead to ...obvious extra costs. The current work aims to have an exhaustive view of the origins of the stones used in the Sanctuary of Delphi. Located on the Parnassus zone, on the hanging wall of a large normal fault related to the Corinth Rift, this Apollo Sanctuary is mainly built of limestones, breccia, marbles, as well as more recent poorly consolidated sediments generally called poros in the literature. To overpass this global view, the different lithologies employed in the archaeological site have been identified, as well as the local quarries, in order to find their origins. The different limestones are autochthons and come from the Upper Jurassic-Cretaceous carbonate platform of the Tethys Ocean involved in the Hellenides orogen. Those limestones of the Parnassus Massif constitute the majority of the rock volume in the site; a specific facies of Maastrichtian limestone called "Profitis Ilias limestone" has been used for the more prestigious edifices such as the Apollo Temple. The corresponding ancient quarry is located few kilometers west of the sanctuary. Then, slope breccia has been largely used in the sanctuary: it crops out in and around the site and is laying on top of the carbonates. Finally, the poros appear to be very variable and seven different facies have been documented, including travertine, oolitic grainstone, marine carbonates and coarse-grained sandstones. All these recent facies exist in the south-east shore of the Gulf of Corinth, although--except for the grainstone--the quarries are not yet known.
Objective: To investigate the long-term performance of aortic valve repair, we analyzed the results obtained in a 22-year period in patients who underwent repair of nonsevere rheumatic aortic valve ...disease during other valvular procedures.
Methods: Fifty-three patients (mean 40 ± 11.6 years of age) with predominant rheumatic mitral valve disease had concomitant aortic valve disease in association with serious tricuspid valve disease in 25 of them. Preoperatively, aortic valve disease was considered moderate in 47.2% of the patients and mild in 52.8%. All patients underwent reparative techniques of the aortic valve (free edge unrolling, 44; subcommissural annuloplasty, 40; commissurotomy, 36) at the time of mitral or mitrotricuspid valve surgery. The completeness of follow-up during the closing interval was 100%, with a mean follow-up of 18.8 years (range 8 to 22.5 years).
Results: Hospital mortality rate was 7.5%. Of 49 surviving patients, 26 (53.1%) died during late follow-up. The actuarial survival curve including hospital mortality was 35.4% ± 8.7% at 22 years. For patients who underwent mitral and aortic valve surgery, the actuarial survival curve at 22 years was 32.3% ± 13%, whereas for patients who had a triple-valve operation the survival was 37.0% ± 10.1% (
p = 0.07). Twenty-five patients underwent an aortic prosthetic valve replacement. Actuarial free from aortic structural deterioration and valve-related complications at 22 years was 25.3% ± 9.3% and 12.7% ± 4.8%, respectively.
Conclusions: Long-term functional results of reparative procedures of nonsevere aortic valve disease in patients with predominant rheumatic mitral valve disease have been inadequate at 22 years of follow-up. According to these data, conservative operations for rheumatic aortic valve disease do not seem appropriate. (J Thorac Cardiovasc Surg 1998;115:1130-5)
Exposed scarps images and ice-penetrating radar measurements in the North Polar Layered Deposits (NPLD) of Mars show alternating layers that provide an archive of past climate oscillations, that are ...thought to be linked to orbital variations, akin to Milankovitch cycles on Earth. We use the Laboratoire de Meteorologie Dynamique (LMD) Martian Global Climate Model (GCM) to study paleoclimate states to enable a better interpretation of the NPLD physical and chemical stratigraphy. When a tropical ice reservoir is present, water vapor transport from the tropics to the poles at low obliquity is modulated by the intensity of summer. At times of low and relatively constant obliquity, the flux still varies due to other orbital elements, promoting polar layer formation. Ice migrates from the tropics towards the poles in two stages. First, when surface ice is present in the tropics, and second, when the equatorial deposit is exhausted, from ice that was previously deposited in mid-high latitudes. The polar accumulation rate is significantly higher when tropical ice is available, forming thicker layers per orbital cycle. However, the majority of the NPLD is sourced from ice that temporary resided in the mid-high latitudes and the layers become thinner as the source location moves poleward. The migration stages imprint different D/H ratios in different sections in the PLDs. The NPLD is isotopically depleted compared to the SPLD in all simulations. Thus we predict the D/H ratio of the atmosphere in contact with NPLD upper layers is biased relative to the average global ice reservoirs.
By measuring the regular oscillations of the density of CO\(_{2}\) in the upper atmosphere (between 120 and 190~km), the mass spectrometer MAVEN/NGIMS (Atmosphere and Volatile EvolutioN/Neutral Gas ...Ion Mass Spectrometer) reveals the local impact of gravity waves (GWs). This yields precious information on the activity of GWs and the atmospheric conditions in which they propagate and break. The intensity of GWs measured by MAVEN in the upper atmosphere has been shown to be dictated by saturation processes in isothermal conditions. As a result, GWs activity is correlated to the evolution of the inverse of the background temperature. Previous data gathered at lower altitudes (\(\sim\)95 to \(\sim\)150~km) during aerobraking by the accelerometers on board MGS (Mars Global Surveyor), ODY (Mars Odyssey) and MRO (Mars Reconnaissance Orbiter) are analyzed in the light of those recent findings with MAVEN. The anti-correlation between GW-induced density perturbations and background temperature is plausibly found in the ODY data acquired in the polar regions, but not in he MGS and MRO data. MRO data in polar regions exhibit a correlation between the density perturbations and the Brunt-V\"{a}is\"{a}l\"{a} frequency, obtained from Global Climate Modeling. At lower altitude levels (between 100 and 120~km), although wave saturation might still be dominant, isothermal conditions are no longer verified. In this case, theory predicts that the intensity of GWs is no more correlated to background temperature, but to static stability. At other latitudes in the three aerobraking datasets, the GW-induced relative density perturbations are correlated with neither inverse temperature nor static stability; in this particular case, this means that the observed activity of GWs is not only controlled by saturation, but also by the effects of GWs sources and wind filtering through critical levels.
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