Major Atmospheric Constituents Research Articles

Kinetic Nonequilibrium Signatures in the Distribution Function of Earth-Escaping Hydrogen Atoms

The lightest atmospheric gas constituents, like H- and He- atoms, are known to escape from planetary gravitational fields to open space. Hereby it counts that not only the uppermost atmospheric layer, the so-called exobase, contributes to this planetary gas escape, but the layers below do contribute as well, especially since from below a nonthermal character of the final particle outflow is induced.We consider the outflow of H - atoms from lower levels of a planetary oxygen-dominated upper atmosphere, stratified by the planet‘s gravitational field - as given in case of the Earth. The terrestrial H-atom outflow is locally modified by elastic collisions of upwards flying H-atoms with the heavy major atmospheric background constituent. This causes a collision–induced velocity-modulation of the upwards directed H-atom flow and induces nonthermal kinetic signatures of the local H ˗ atom distribution function. An important, hitherto unrespected point hereby is that angle-integrated elastic O ˗ H- collision cross sections are velocity-dependent, falling off with increasing velocity v like (1/v). Consequently this modulation influences low velocity H-atoms stronger than high-velocity ones, which changes the kinetic profile of the escaping H-atoms and causes deviations from the classic Jeans escape. Deeply down in the lower thermosphere the local H-atoms, like as well the O-atoms, indeed are in a thermodynamical equilibrium characterized by Maxwell distributions with a common temperature TH = TO. Nevertheless, at the upper exobase border of the atmosphere the resulting H-atom escape flow turns out to be a, non-equilibrium flow with non-thermal escape-relevant properties. Here we describe this collisional modification of the H-escape flow and quantify the upcome of kinetic non-equilibrium features like power laws in the wings of the H-distribution function. This collisional modulation via velocity-dependent collision cross-sections acts as a typical process to convert equilibrium distributions into non-equilibrium kappa-like distribution functions. On the basis of this theoretical approach and stabilizing the upper atmosphere by the type of so-called "iso-baric Kappa functions" which all represent the same H ˗ atom- pressure we can calculate the effective escape flux of H- atoms to open space and can quantify its difference with respect to the classical Jeans escape value. While finding again that the classical Jeans formula slightly overestimates the actual escape flux, we now for the first time taking into account the nonthermal influence of collisional modulations can show that the actual escape with respect to the actual Jeans value is even enhanced by factors 2 to 3.

Nonthermal Messages from Thermal Planetary Atmospheric Systems

Light atmospheric gas constituents tend to evaporate from the planetary gravitational fields. The point is that not only the uppermost atmospheric layer contributes to this gas escape, but the lower layers contribute their share as well and give the outcoming particle flow a nonthermal, non-Maxwellian character. In this article we do study the outflow of hydrogen atoms from a planetary oxygen atmosphere assumed to be one-dimensionally stratified by the action of the planet‘s gravitational field. This outflow is modified by local elastic collisions of upwards flying H-atoms with the heavy major atmospheric background constituent, as in case of the terrestrial atmosphere, the monoatomic oxygen atoms. This shock-modulation of the upwards particle flow produces nonthermal kinetic features of the particle distribution which we want to decribe. Since angle-integrated elastic collision cross sections are velocity-dependent, falling off with the velocity v like (1/v), the occuring collision-modulation of the H-atom flow does change the kinetic velocity profile of the escaping H-atoms. Deeply down in the lower atmophere the local H-atoms, like the O-atoms as well, are in thermodynamical equilibrium characterized by Maxwell Boltzmann distributions with a common temperature TH = TO. Nevertheless, at the upper exobase border of the atmosphere the resulting H-atom escape flow is shown to be a nonMaxwellian, non-equilibrium flow with non-thermal escape-relevant properties. We describe this collisional modification of the H-escape flow and can quantify the upcome of kinetic non-equilibrium features like power laws in the H-distribution function. Thus, as we demonstrate in this article, this collisional modulation effect via velocity-dependent collision cross-sections acts as a typical process to convert equilibrium distributions into non-equilibrium distribution functions. On the basis of this new kinetic theoretical approach we then calculate the effective escape flux of H- atoms to open space, demonstrate its nonthermal character, and quantify its difference with respect to the classical Jeans escape value. We find that with the classical Jeans formula one slightly overestimates the actual escape flux by an amount that varies between 30 - 80 percent, depending on the temperature of the lower atmosphere of the planet which varies with the solar 10.7-cm- radioflux.

Potential for Phototrophy in Venus' Clouds.

We show that solar irradiances calculated across Venus' clouds support the potential for Earth-like phototrophy and that treatment of Venus' aerosols containing neutralized sulfuric acid favor a habitable zone. The phototrophic potential of Venus' atmosphere was assessed by calculating irradiances (200-2000 nm, 15° solar zenith angle, local noon) using a radiative transfer model that accounted for absorption and scattering by the major and minor atmospheric constituents. Comparisons to Earth's surface (46 W m-2, 280-400 nm) suggest that Venus' middle and lower clouds receive ∼87% less normalized UV flux (6-7 W m-2) across 200-400 nm, yet similar normalized photon flux densities (∼4400-6200 μmol m-2 s-1) across 350-1200 nm. Further, Venus' signature phototrophic windows and subwindows overlap with the absorption profiles of several photosynthetic pigments, especially bacteriochlorophyll b from intact cells and phycocyanin. Therefore, Venus' light, with limited UV flux in the middle and lower clouds, is likely quite favorable for phototrophy. We additionally present interpretations to refractive index and radio occultation measures for Venus' aerosols that suggest the presence of lower sulfuric abundances and/or neutralized forms of sulfuric acid, such as ammonium bisulfate. Under these considerations, the aerosols in Venus' middle clouds could harbor water activities (≥0.6) and buffered acidities (Hammett acidity factor, H0 -0.1 to -1.5) that lie within the limits of acidic cultivation (≥H0 -0.4) and are tantalizingly close to the limits of oxygenic photosynthesis (≥H0 0.1). Together, these photophysical and chemical considerations support a potential for phototrophy in Venus' clouds.

Experimental Investigation of the Formation of Formaldehyde by Hadean and Noachian Impacts.

Formaldehyde (FA) is an important precursor in the abiotic synthesis of major biomolecules including amino acids, sugars, and nucleobases. Thus, spontaneous formation of prebiotic FA must have been crucial for the chemical origin of life. The frequent impacts of meteorites and asteroids on Hadean Earth have been considered one of the abiotic synthetic processes of organic compounds. However, the impact-induced formation of FA from CO2 as the major atmospheric constituent has not been confirmed yet. This study investigated the formation of FA in impact-induced reactions among meteoritic minerals, bicarbonate, gaseous nitrogen, and water to simulate the abiotic process experimentally. Products were analyzed with ultra-high-performance liquid chromatography/tandem mass spectrometry and powder X-ray diffraction techniques. The results show the formation of FA and oxidation of metallic iron to siderite in the impact shock experiments. This indicates that this important prebiotic molecule was also synthesized by impacts of iron-bearing meteorites/asteroids on the Hadean oceans. The impact events might have generated spatially and temporally FA-enriched localized environments. Moreover, the impact-induced synthesis of FA may have also occurred on Noachian Mars given the presence of liquid water and a CO2-N2-rich atmosphere on the planet.

Study of Exospheric Neutral Composition of Mars observed from Indian Mars Orbiter Mission

The exospheric composition data of Mars for the period 2014–15 has been extracted and analysed using the observations carried out by the MENCA (Mars Exospheric Neutral Composition Analyser) payload on-board the Mars Orbiter Mission (MOM) launched by India. The latitude, longitude, altitude and solar zenith angle coverage of the partial pressure values of different exospheric constituents are determined and assigned to create a new data-set with orbit-wise data assimilation particularly between 260 and 600 km altitude range. Apart from getting the results on mean individual orbits’ partial pressure profiles, the variations of the total as well as partial pressures are studied with respect to the distribution of the major atmospheric constituents and their dependence on solar activity. In particular, CO2 and O variations are considered together for any differential effects due to photolysis and photo-ionisation. The results on the gradual reduction in densities due to decreasing daily mean sunspot numbers and strong response of CO2 and O pressures to solar energetic particle events like that of 24 December 2014 are presented.

Effects of a Binary Companion Star on Habitability of Tidally Locked Planets around an M-type Host Star

Planets in the "Habitable Zones" around M-type stars are important targets for characterization in future observations. Due to tidal-locking in synchronous spin-orbit rotations, the planets tend to have a hot dayside and a cold nightside. On the cold nightside, water vapor transferred from the dayside can be frozen in ("cold trap") or the major atmospheric constituent could also condense ("atmospheric collapse") if the atmosphere is so thin that the heat re-distribution is not efficient, in the case of a single M-type star. Motivated by the abundance of binary star systems, we investigate the effects of irradiation from a G-type companion star on the climate of a tidally locked planet around an M-type star using the 2D energy balance model. We find that the irradiation from the G-type star is more effective at warming up the nightside of the planet than the dayside. This contributes to the prevention of the irreversible trapping of water and atmosphere on the cold nightside, broadening the parameter space where tidally locked planets can maintain surface liquid water. Tidally locked ocean planets with < ~ 0.3 bar atmospheres or land planets with < ~ 3 bar atmospheres can realize temperate climate with surface liquid water when they are also irradiated by a companion star with a separation of 1 - 4 au. We also demonstrate that planets with given properties can be in the Earth-like temperate climate regime or in a completely frozen state under the same total irradiation.

The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats.

Since the Archean, N2 has been a major atmospheric constituent in Earth's atmosphere. Nitrogen is an essential element in the building blocks of life; therefore, the geobiological nitrogen cycle is a fundamental factor in the long-term evolution of both Earth and Earth-like exoplanets. We discuss the development of Earth's N2 atmosphere since the planet's formation and its relation with the geobiological cycle. Then we suggest atmospheric evolution scenarios and their possible interaction with life-forms: first for a stagnant-lid anoxic world, second for a tectonically active anoxic world, and third for an oxidized tectonically active world. Furthermore, we discuss a possible demise of present Earth's biosphere and its effects on the atmosphere. Since life-forms are the most efficient means for recycling deposited nitrogen back into the atmosphere at present, they sustain its surface partial pressure at high levels. Also, the simultaneous presence of significant N2 and O2 is chemically incompatible in an atmosphere over geological timescales. Thus, we argue that an N2-dominated atmosphere in combination with O2 on Earth-like planets within circumstellar habitable zones can be considered as a geo-biosignature. Terrestrial planets with such atmospheres will have an operating tectonic regime connected with an aerobic biosphere, whereas other scenarios in most cases end up with a CO2-dominated atmosphere. We conclude with implications for the search for life on Earth-like exoplanets inside the habitable zones of M to K stars.

Effect of environment on charge transport properties of polyimide films damaged by high-energy electron radiation

Ground based measurements are critical to understanding the space environment-induced modifications of spacecraft materials and predictive spacecraft modeling. The interaction of high-energy electrons with spacecraft materials, such as polyimide (PI, Kapton-H®), is known to modify the material's chemical and consequently physical properties. Highly stable in its pristine state, radiation-damaged PI becomes chemically reactive due to the formation of species containing unpaired electrons (radicals). As a result, the reaction of residual gases, even at low partial pressures, changes the damaged PI's properties and obscures the understanding of the radiation damage mechanisms. In the presented paper, the authors demonstrated that even very limited air exposure will have a dramatic effect on the charge transport properties of radiation-damaged PI. Further, they also evaluated the effects of several major constituents of the Earth's atmosphere (Ar, N2, O2, and H2O) on the charge transport properties of PI damaged by exposure to 90 keV electrons.

Far ultraviolet instrument technology

AbstractThe far ultraviolet (FUV) spectral range (from about 115 nm to 180 nm) is one of the most useful spectral regions for characterizing the upper atmosphere (thermosphere and ionosphere). The principal advantages are that there are FUV signatures of the major constituents of the upper atmosphere as well as the signatures of the high‐latitude energy inputs. Because of the absorption by thermospheric O2, the FUV signatures are seen against a “black” background, i.e., one that is not affected by ground albedo or clouds and, as a consequence, can make useful observations of the aurora during the day or when the Moon is above the horizon. In this paper we discuss the uses of FUV remote sensing, summarize the various techniques, and discuss the technological challenges. Our focus is on a particular type of FUV instrument, the scanning imaging spectrograph or SIS: an instrument exemplified by the Defense Meteorological Satellite Program Special Sensor Ultraviolet Imager and Thermosphere Ionosphere Mesosphere Energetics and Dynamics Global Ultraviolet Imager. The SIS combines spatial imaging of the disk with limb profiles as well as spectral information at each point in the scan.

STRATOSPHERIC TEMPERATURES AND WATER LOSS FROM MOIST GREENHOUSE ATMOSPHERES OF EARTH-LIKE PLANETS

A radiative-convective climate model is used to calculate stratospheric temperatures and water vapor concentrations for ozone-free atmospheres warmer than that of modern Earth. Cold, dry stratospheres are predicted at low surface temperatures, in agreement with recent 3-D calculations. However, at surface temperatures above 350 K, the stratosphere warms and water vapor becomes a major upper atmospheric constituent, allowing water to be lost by photodissociation and hydrogen escape. Hence, a 'moist greenhouse' explanation for loss of water from Venus, or some exoplanet receiving a comparable amount of stellar radiation, remains a viable hypothesis. Temperatures in the upper parts of such atmospheres are well below those estimated for a gray atmosphere, and this factor should be taken into account when performing 'inverse' climate calculations to determine habitable zone boundaries using 1-D models.

“Water group” ion induced ionization and dissociation of major constituents of atmosphere of Titan

SynopsisProcesses in high-energy collisions of positive and negative “water group” ions with thermal neutral constituents of atmosphere of Titan were studied. It was shown that internal energy deposited in nascent positive ions strongly depends on the nature of incident ions.

Isotope Ratios of H, C, and O in CO 2 and H 2 O of the Martian Atmosphere

Stable isotope ratios of H, C, and O are powerful indicators of a wide variety of planetary geophysical processes, and for Mars they reveal the record of loss of its atmosphere and subsequent interactions with its surface such as carbonate formation. We report in situ measurements of the isotopic ratios of D/H and (18)O/(16)O in water and (13)C/(12)C, (18)O/(16)O, (17)O/(16)O, and (13)C(18)O/(12)C(16)O in carbon dioxide, made in the martian atmosphere at Gale Crater from the Curiosity rover using the Sample Analysis at Mars (SAM)'s tunable laser spectrometer (TLS). Comparison between our measurements in the modern atmosphere and those of martian meteorites such as ALH 84001 implies that the martian reservoirs of CO2 and H2O were largely established ~4 billion years ago, but that atmospheric loss or surface interaction may be still ongoing.

Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover

Volume mixing and isotope ratios secured with repeated atmospheric measurements taken with the Sample Analysis at Mars instrument suite on the Curiosity rover are: carbon dioxide (CO2), 0.960(±0.007); argon-40 ((40)Ar), 0.0193(±0.0001); nitrogen (N2), 0.0189(±0.0003); oxygen, 1.45(±0.09) × 10(-3); carbon monoxide, < 1.0 × 10(-3); and (40)Ar/(36)Ar, 1.9(±0.3) × 10(3). The (40)Ar/N2 ratio is 1.7 times greater and the (40)Ar/(36)Ar ratio 1.6 times lower than values reported by the Viking Lander mass spectrometer in 1976, whereas other values are generally consistent with Viking and remote sensing observations. The (40)Ar/(36)Ar ratio is consistent with martian meteoritic values, which provides additional strong support for a martian origin of these rocks. The isotopic signature δ(13)C from CO2 of ~45 per mil is independently measured with two instruments. This heavy isotope enrichment in carbon supports the hypothesis of substantial atmospheric loss.

CHEMICAL CONSEQUENCES OF THE C/O RATIO ON HOT JUPITERS: EXAMPLES FROM WASP-12b, COROT-2b, XO-1b, AND HD 189733b.

Motivated by recent spectroscopic evidence for carbon-rich atmospheres on some transiting exo-planets, we investigate the influence of the C/O ratio on the chemistry, composition, and spectra of extrasolar giant planets both from a thermochemical-equilibrium perspective and from consideration of disequilibrium processes like photochemistry and transport-induced quenching. We find that although CO is predicted to be a major atmospheric constituent on hot Jupiters for all C/O ratios, other oxygen-bearing molecules like H2O and CO2 are much more abundant when C/O < 1, whereas CH4, HCN, and C2H2 gain significantly in abundance when C/O > 1. Other notable species like N2 and NH3 that do not contain carbon or oxygen are relatively unaffected by the C/O ratio. Disequilibrium processes tend to enhance the abundance of CH4, NH3, HCN, and C2H2 over a wide range of C/O ratios. We compare the results of our models with secondary-eclipse photometric data from the Spitzer Space Telescope and conclude that (1) disequilibrium models with C/O ~ 1 are consistent with spectra of WASP-12b, XO-1b, and CoRoT-2b, confirming the possible carbon-rich nature of these planets, (2) spectra from HD 189733b are consistent with C/O ≲ 1, but as the assumed metallicity is increased above solar, the required C/O ratio must increase toward 1 to prevent too much H2O absorption, (3) species like HCN can have a significant influence on spectral behavior in the 3.6 and 8.0 μm Spitzer channels, potentially providing even more opacity than CH4 when C/O > 1, and (4) the very high CO2 abundance inferred for HD 189733b from near-infrared observations cannot be explained through equilibrium or disequilibrium chemistry in a hydrogen-dominated atmosphere. We discuss possible formation mechanisms for carbon-rich hot Jupiters, including scenarios in which the accretion of CO-rich, H2O-poor gas dominates the atmospheric envelope, and scenarios in which the planets accrete carbon-rich solids while migrating through disk regions inward of the snow line. The C/O ratio and bulk atmospheric metallicity provide important clues regarding the formation and evolution of the giant planets.

EUV-TEC proxy to describe ionospheric variability using satellite-borne solar EUV measurements

Abstract. An updated version of a proxy, termed EUV-TEC, describing the global total primary photoionisation is calculated from satellite-borne EUV measurements assuming a model atmosphere consisting of four major atmospheric constituents. Regional number densities of the background atmosphere are taken from the NRLMSISE-00 climatology. For calculation the Lambert-Beer law is used to describe the decrease of the radiation along their way through the atmosphere. The EUV-TEC proxy thus describes the ionospheric response to solar EUV radiation and its variability. EUV-TEC is compared against the global mean total electron content (TEC), a fundamental ionospheric parameter created from vertical TEC maps derived from GPS data. Strong correlation between these indices is found on different time scales. Results show that the EUV-TEC proxy represents the ionsopheric variability better than the conventional solar index F10.7 does, especially during high and moderate solar activity.

CHARACTERIZING THE ATMOSPHERES OF TRANSITING ROCKY PLANETS AROUND LATE-TYPE DWARFS

Visible and near-infrared spectra of transiting hot Jupiter planets have recently been observed, revealing some of the atmospheric constituents of their atmospheres. In the near future, it is probable that primary and secondary eclipse observations of Earth-like rocky planets will also be achieved. The characterization of the Earth's transmission spectrum has shown that both major and trace atmospheric constituents may present strong absorption features, including important bio-markers such as water, oxygen and methane. Our simulations using a recently published empirical Earth's transmission spectrum, and the stellar spectra for a variety of stellar types, indicate that the new generation of extremely large telescopes, such as the proposed 42-meter European Extremely Large Telescope(E-ELT), could be capable of retrieving the transmission spectrum of an Earth-like planet around very cool stars and brown dwarfs (Teff < 3100 K). For a twin of Earth around a star with Teff around 3100 K (M4), for example, the spectral features of water vapor, methane, carbon dioxide, and oxygen in the wavelength range between 0.9 and 2.4 micron can simultaneously be detected within a hundred hours of observing time, or even less for a late-M star. Such detection would constitute a proof for the existence of life in that planet. The detection time can be reduced to a few hours for a super-Earth type of planet with twice the Earth's radius.

Shortwave radiative fluxes from MODIS: Model development and implementation

A forward inference scheme driven with satellite‐based information is developed to derive top of the atmosphere, surface, and atmospheric spectral shortwave radiative fluxes at global scale. The model takes into account all major atmospheric constituents, the characteristics of water and ice clouds, and the variation of cloud particle effective radius. The multilayered structure accounts for surface elevation effects and for the vertical distribution of the radiative fluxes. Spectral fluxes such as photosynthetically active radiation and near‐infrared radiation can also be estimated. The model is implemented with products from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor both on Terra and Aqua at 1° spatial resolution for a 3‐year period (2003–2005) and at global scale. The derived surface fluxes are evaluated against the globally distributed Baseline Surface Radiation Network (BSRN) measurements and compared with products from independent sources. It is demonstrated that the twice a day MODIS‐based estimates of daily radiative fluxes adjusted for the diurnal cycle and the ground observations have a correlation coefficient greater than 0.96, a relative root‐mean‐square error less than 18%, and a relative bias between 0 and 6% for BSRN sites located in the Antarctic, the Atlantic (islands), North America, Europe, and other continental sites.

Improving the Measurement Accuracy of Water Partial Pressure Using the Major Constituent Analyzer

The Major Constituent Analyzer (MCA) on board the International Space Station (ISS) is a mass spectrometer-based system designed to monitor the major constituents of the ISS internal atmosphere. Of these, nitrogen, oxygen, methane, carbon dioxide, and hydrogen are being monitored as originally designed. Obtaining accurate water vapor measurements, however, has been problematic. The primary issue for water vapor measurement is the adsorption of water onto the interior surfaces of the ISS sampling lines, MCA internal sample transport lines, and the MCA analyzer itself. Whenever the partial pressure of water in the sample stream changes, a new equilibrium between the adsorbed and gas phases must be reached before a measurement will reflect the actual partial pressure. Thus, while changes in the partial pressures of the other major constituents are detected in real-time, accurate measurement of the water partial pressure is delayed. Unfortunately, the time required to reach equilibrium is longer than current MCA operating parameters allow, so the measured values are not accurate.

Isotopic evolution of the major constituents of Titan's atmosphere based on Cassini data

Using Cassini ion neutral mass spectrometer stable isotope observations, we have developed a comprehensive method for modeling the time-evolution of the stable isotopic ratios in Titan's major constituents, N 2, CH 4 and H 2. Our model provides constraints on the initial 14N/ 15N ratio in N 2, the time scale for the outgassing of methane from the interior, and the initial D/H ratio in methane. Over geologic time scales, the isotopes are fractionated by diffusion, atmospheric escape and photochemistry. Diffusion and escape preferentially remove the lighter isotopes for all constituents. Photolysis of methane also removes the lighter isotopes, while photolysis of nitrogen preferentially removes the heavier isotopes. We have found the following: (1) even taking past hydrodynamic escape into consideration, the initial 14N/ 15N ratio in N 2 cannot have changed much from its current value as the result of atmospheric processes. This is due to the large amount of N 2 that must be fractionated. High-rate loss processes, such as hydrodynamic escape, are inefficient fractionators and take a very long time to change the isotopic ratio. On the other hand, low-rate loss processes are efficient fractionators, but also take a very long time to influence a large inventory. (2) The current inventory of methane represents the remnant of methane that, constrained by the 12C/ 13C ratio, began outgassing from the interior more than 60 million years ago, resulting in a total inventory of 3–4 times the current inventory cycling through the system during this time period. Methane production is likely to be ongoing. (3) The initial D/H in methane was found to be 6.96–11.3×10 −5.

Earth’s transmission spectrum from lunar eclipse observations

Of the 342 planets so far discovered orbiting other stars, 58 'transit' the stellar disk, meaning that they can be detected through a periodic decrease in the flux of starlight. The light from the star passes through the atmosphere of the planet, and in a few cases the basic atmospheric composition of the planet can be estimated. As we get closer to finding analogues of Earth, an important consideration for the characterization of extrasolar planetary atmospheres is what the transmission spectrum of our planet looks like. Here we report the optical and near-infrared transmission spectrum of the Earth, obtained during a lunar eclipse. Some biologically relevant atmospheric features that are weak in the reflection spectrum (such as ozone, molecular oxygen, water, carbon dioxide and methane) are much stronger in the transmission spectrum, and indeed stronger than predicted by modelling. We also find the 'fingerprints' of the Earth's ionosphere and of the major atmospheric constituent, molecular nitrogen (N(2)), which are missing in the reflection spectrum.