Atmospheric Losses Under Dust Bombardment in the Ancient Atmospheres

We have considered the new process of atmospheric losses - “sputtering” under bombardment by interplanetary dust. It is demonstrated that “sputtering” due to collisions with the interplanetary dust is an effective way of atmospheric gas loss (10^–4–10^–3 of the dust particles' accreting mass) and that it changes the composition of the atmospheric gases. In calculations we have taken that the dust particles collide elastically with the atoms and molecules of the atmosphere. Estimation of the effects of inelastic collisions was also considered. As a result of these collisions a part of the atmospheric atoms and molecules will have “upward” velocity and enough energy to escape. It was considered that escaping atoms can collide with the atoms of the “main” gas of the upper atmosphere. The atmospheric gas composition is assumed to be just as in the modern Martian atmosphere - the “main” gases in the upper atmosphere were taken to be O and CO₂. In our computations we pay particular attention to the abundance of noble gases in planetary atmospheres since these gases are very important for theories of atmospheric origin. We computed that under “sputtering” by the interplanetary dust, atmospheres were enriched by the “heavy” elements and isotopes in the wide range of the upper atmospheric parameters O/CO₂, T/g (O/CO₂– on the level of homosphere;T is temperature of the exosphere,g is gravitational acceleration). However the loss efficiency for “heavy” gases is relatively high compared to other known gas loss processes. In the case of noble gases for the specific parameters of the upper atmosphere (small T/g ratio; high O/CO₂ on the level of homosphere) we have got the unique result: despite the diffusion separation in the upper atmosphere the loss efficiency of Xe > Kr > Ar. The effect of “sputtering” of the planetary atmospheres was strongest during the early stages of the planetary evolution - when the rate of the dust accretion was intrinsically higher than now because of collisions of planetesimals. In light of the new escape process, the main peculiarities of the noble gases abundance in the planetary atmospheres could be explained. This revised version was published online in July 2006 with corrections to the Cover Date.     

<Emphasis Type="Italic">Venus Express</Emphasis>: The presence of turbulence in the mesosphere of venus is confirmed

Among the numerous and valuable results obtained from the Venus Express spacecraft, information from several instruments suggests that turbulence is present in the mesosphere. In this paper, the results of these experiments are interpreted with the use of the available data on turbulence in planetary atmospheres. Accounting for turbulence is necessary for developing models of the structure and dynamics of Venus’ atmosphere.     

Water and related chemistry in the solar system. A guaranteed time key programme for Herschel

“Water and related chemistry in the Solar System” is a Herschel Space Observatory Guaranteed-Time Key Programme. This project, approved by the European Space Agency, aims at determining the distribution, the evolution and the origin of water in Mars, the outer planets, Titan, Enceladus and the comets. It addresses the broad topic of water and its isotopologues in planetary and cometary atmospheres. The nature of cometary activity and the thermodynamics of cometary comae will be investigated by studying water excitation in a sample of comets. The D/H ratio, the key parameter for constraining the origin and evolution of Solar System species, will be measured for the first time in a Jupiter-family comet. A comparison with existing and new measurements of D/H in Oort-cloud comets will constrain the composition of pre-solar cometary grains and possibly the dynamics of the protosolar nebula. New measurements of D/H in giant planets, similarly constraining the composition of proto-planetary ices, will be obtained. The D/H and other isotopic ratios, diagnostic of Mars’ atmosphere evolution, will be accurately measured in H₂O and CO. The role of water vapor in Mars’ atmospheric chemistry will be studied by monitoring vertical profiles of H₂O and HDO and by searching for several other species (and CO and H₂O isotopes). A detailed study of the source of water in the upper atmosphere of the Giant Planets and Titan will be performed. By monitoring the water abundance, vertical profile, and input fluxes in the various objects, and when possible with the help of mapping observations, we will discriminate between the possible sources of water in the outer planets (interplanetary dust particles, cometary impacts, and local sources). In addition to these inter-connected objectives, serendipitous searches will enhance our knowledge of the composition of planetary and cometary atmospheres.     

The Origin and Evolution of the Atmospheres of Venus, Earth and Mars

The chemical compositions of the primordial atmospheres of Venus, Earth and Mars have long been a topic of debate between the experts. Some believe that the original atmospheres were a product of outgassed volatiles from the newly accreted terrestrial planets and that these atmospheres consisted primarily of carbon dioxide, nitrogen, water vapor and residual hydrogen and helium (e.g., Lewis and Prinn, <it>Planets and their Atmospheres,</it> Academic Press, Orlando, FL, 1984, pp. 62–63, 81–84, 228–231, 383). Still others think the earliest atmospheres were composed of the gas components of the solar nebula from which the solar system formed (i.e., hydrogen, helium, methane, ammonia and water). I consider the latter to be the correct scenario. Presented herein is a proposed mechanism by which the original atmospheres of Venus, Earth and Mars were transformed to atmospheres rich in carbon dioxide and nitrogen. An explanation is proposed for why water is so common on the surface of Earth and so scarce on the surfaces of Venus and Mars. Also presented are the effects the “great impact” (single cataclysmic event that was responsible for producing the Earth–Moon system) had upon the early atmosphere of Earth. The origin, structure and composition of the impacting object are determined through deductive analyses.     

Ionization structure of the atmospheres and line profiles in the spectra of Wolf-Rayet stars

The ionization structure of the atmospheres of Wolf-Rayet (WR) and WC stars is studied. The stellar atmospheres were assumed to consist of helium, hydrogen, and carbon. Profiles of the C III l 5696 line are calculated, both for a spherically symmetric atmosphere with a density that decreases monotonically outward and for an atmosphere containing a dense condensation (inhomogeneity). The dependence of line profiles on the parameters of the inhomogeneity is investigated. It is shown that profiles of the C III λ 5696 line calculated assuming no inhomogeneities in the atmosphere are too weak, whereas assuming the existence of inhomogeneities enables one to reconcile the observed and calculated profiles. An equation is obtained relating the mass of an inhomogeneity to the flux in the detail of the total profile of the CIII λ 5696 line formed by that inhomogeneity. This equation is used to construct a stochastic cloud model of the atmosphere of a WR star, consisting of a large number of inhomogeneities in a homogeneous, spherically symmetric stellar wind. In the proposed model, the formation of inhomogeneities was treated as a random process. It is shown that in this model it is possible both to obtain an average line profile corresponding to the observed one and to reproduce the amplitude and overall pattern of variability of profiles in the spectra of Wolf-Rayet stars. Translated from Astrofizika, Vol. 42, No. 3, pp. 373–398, July–September, 1999.     

Analysis of spectra of V471 Tau and HD 115404

We analyze the chemical composition of the atmospheres of a single K-type star HD 115404 and the secondary component of the V471 Tau variable. We use the technique of modeling of synthetic spectra to analyze the high-resolution spectra of these stars, taken with the RTT 150 Russian-Turkish telescope and find the abundances of 23 and 17 elements in the atmospheres of HD 115404 and V471 Tau, respectively. We demonstrate the lack of composition anomalies in the HD 115404 and show it to be consistent with the published data, inferred from equivalent widths of spectral lines. We find the abundances of 15 elements from Na to Ba to be consistent with the metallicity of the atmosphere of V471 Tau ([Fe/H] = −0.22 ± 0.12dex), which differs significantly from the average metallicity of the Hyades cluster. We show the existence of strong carbon and oxygen overabundances (by more than 1dex) due to the enrichment of the secondary by the nucleosynthesis products during the common-envelope stage of the system. On the whole, we demonstrate that V471 Tau and the other precataclysmic variables share similar composition anomalies.     

The "F str 4077" star HD 177645 as a possible barium dwarf star

The abundances of 12 elements in the atmosphere of the "F str 4077" star HD 177645 have been determined from new spectroscopic observations with the CCD camera and the model atmosphere method. The overabundance of nitrogen found for this star indicates its possible relation to barium dwarfs with anomalies in the chemical composition of their atmospheres due to mass transfer from the more evolved companion in a binary system. As an object related to Ba stars and CH subgiants, HD 177645 with an effective temperature T_eff=7150K may also have anomalies in the chemical composition characteristic of diffusion processes in chemically peculiar stars of the upper main sequence, as may be indicated by the overabundance of sulfur in its atmosphere.Translated fromAstrofizika, Vol. 39, No. 2, pp. 201–209, April–June, 1996.     

Synthetic spectra of simulated terrestrial atmospheres containing possible biomarker gases.

NASA's proposed Terrestrial Planet Finder, a space-based interferometer, will eventually allow spectroscopic analyses of the atmospheres of extrasolar planets. Such analyses would provide information about the existence of life on these planets. One strategy in the search for life is to look for evidence of O3 (and hence O2) in a planet's atmosphere; another is to look for gases that might be present in an atmosphere analogous to that of the inhabited early Earth. In order to investigate these possibilities, we have calculated synthetic spectra for several hypothetical terrestrial-type atmospheres. The model atmospheres represent four different scenarios. The first two, representing inhabited terrestrial planets, are an Earth-like atmosphere containing variable amounts of oxygen and an early Earth-type atmosphere containing methane. In addition, two cases representing Mars-like and early Venus-like atmospheres were evaluated, to provide possible "false positive" spectra. The calculated spectra suggest that ozone could be detected by an instrument like Terrestrial Planet Finder if the O2 concentration in the planet's atmosphere is > or = 200 ppm, or 10(-3) times the present atmospheric level. Methane should be observable on an early-Earth type planet if it is present in concentrations of 100 ppm or more. Methane has both biogenic and abiogenic sources, but concentrations exceeding 1000 ppm, or 0.1% by volume, would be difficult to produce from abiogenic sources alone. High methane concentrations in a planet's atmosphere are therefore another potential indicator for extraterrestrial life.     

Quantitative Analysis of H2OComa Images Using a Multiscale MHD Model with Detailed Ion Chemistry

The observation of ions created by ionization of cometary gas, either by ground-based observations or byin situmeasurements can give us useful information about the gas production and composition of comets. However, due to the interaction of ions with the magnetized solar wind and their high chemical reactivity, it is not possible to relate measured ion densities (or column densities) directly to the parent gas densities. In order to quantitatively analyze measured ion abundances in cometary comae it is necessary to understand their dynamics and chemistry. We have developed a detailed ion–chemical network of cometary atmospheres. We include production of ions by photo- and electron impact-ionization of a background neutral atmosphere, charge exchange of solar wind ions with cometary atoms/molecules, reactions between ions and molecules, and dissociative recombination of molecular ions with thermal electrons. By combining the ion–chemical network with the three-dimensional plasma flow as computed by a new fully three-dimensional MHD model of cometary plasma environments (Gombosiet al.1996) we are able to compute the density of the major cometary ions everywhere in the coma. The input parameters for our model are the solar wind conditions (density, speed, temperature, magnetic field) and the composition and production rate of the gas. We applied our model to Comet P/Halley in early March 1986, for which the input parameters are reasonably well known. We compare the resulting column density of H₂O⁺with ground-based observations of H₂O⁺from DiSantiet al.(1990). The results of our model are in good agreement with both the spatial distribution and the absolute abundance of H₂O⁺and with their variations with the changing overall water production rate between two days. The results are encouraging that it will be possible to obtain production rates of neutral cometary constituents from observations of their ion products.     

An atlas of ground UV spectra of selected stars

We present a spectral atlas of 4 B and A stars containing spectra in a poorly studied spectral range of 305–452 nm. The atlas is based on high resolution (R=60 000) spectra obtained with the 6 meter telescope (SAO, Russia) combined with the NES-spectrograph. The procedure of spectral lines identification and compilation of the atlas is discussed in detail. Using the spectral data we thoroughly investigated the velocity field in expanding atmospheres and envelopes of hot evolved stars β Ori, α Cyg and supergiant KS Per with the extreme hydrogen deficiency. The complete atlas and list of the identified spectral lines will be available via the astronomical database SIMBAD.     

Inhomogeneities of the stellar winds of hot supergiants

The causes of variability of line profiles in the spectra of O supergiants are analyzed. It is suggested that the main cause of the variability is the motion in the atmosphere of dense clumps of matter (inhomogeneities or clouds) along the Une of sight between star and observer. The profiles of C IV and Si IV UV resonance lines in the spectra of bright OB supergiants are calculated for spherically symmetric atmospheres and for atmospheres with inhomogeneity along the line of sight. The dependence of the line profiles on the distance of the inhomogeneity from the center of the star is investigated. It is shown that the formation and evolution with time of discrete absorption components (DACs) in the profiles of C IV and Si IV UV resonance lines can be explained within the framework of the proposed model of variability of line profiles. The parameters of the inhomogeneities moving in the atmosphere to produce DACs are estimated. Translated from Astrofizika, Vol. 41, No. 3, pp. 423–441, July–September, 1998.     

Modelling The Irradiated Upper Atmospheres Of Unevolved Companions In Precataclysmic Binaries (Pcb): Evidences On Thermal Instability

We report on our results of modelling the physical processes in the uppermost layers of strongly irradiated atmospheres of low mass unevolved companions in Precataclysmic Binaries (PCBs). Reprocessing of Lyman continuum L _c radiation from the hot subdwarf (sdw) primary is studied in detail. We solve explicitly a set of equations of hydrostatic, ionization and thermal equilibrium to calculate the intensity of the reprocessed emergent radiation in recombinations for an optically thin plasma. We consider contributions from free-free and bound-free transitions. Diffuse radiation is taken into account, but the effects of self-absorption of the re-emitted radiation are neglected. For a purely hydrogenic atmosphere and typical values of incident fluxes, densities, gas pressures for the irradiated upper atmospheric layers, we find that the L _c radiation will be absorbed and re-emitted in recombinations within a column of effective thickness (10⁶–10⁸) cm of HII, depending on the electron density, the effective temperature of the sdw and the separation between the components. One of the non-trivial results of our model computations lies in the overheating of the uppermost layers of the irradiated atmosphere: the equilibrium temperature of the gas turns out to be considerably higher than the blackbody temperature following from the diluted incoming radiation of sdw. Energy balance considerations reveal that roughly 25–50% of the L _c flux is expended on the ionization of H and is re-emitted subsequently in recombinations. The remaining portion of the L _c flux is spent on readjustment of the uppermost layers of the irradiated atmosphere. One of the consequences is the onset of thermal instability in these layers.     

Kronos: exploring the depths of Saturn with probes and remote sensing through an international mission

Kronos is a mission aimed to measure in situ the chemical and isotopic compositions of the Saturnian atmosphere with two probes and also by remote sensing, in order to understand the origin, formation, and evolution of giant planets in general, including extrasolar planets. The abundances of noble gases, hydrogen, carbon, nitrogen, oxygen, sulfur and their compounds, as well as of the D/H, ⁴He/³He, ²²Ne/²¹Ne/²⁰Ne, ³⁶Ar/³⁸Ar, ¹³C/¹²C, ¹⁵N/¹⁴N, ¹⁸O/(¹⁷O)/¹⁶O, ¹³⁶Xe/¹³⁴Xe/¹³²Xe/¹³⁰Xe/¹²⁹Xe isotopic ratios will be measured by mass spectrometry on two probes entering the atmosphere of Saturn at two different locations near mid-latitudes, down to a pressure of 10 Bar. The global composition of Saturn will be investigated through these measurements, together with microwave radiometry determination of H₂O and NH₃ and their 3D variations. The dynamics of Saturn’s atmosphere will be investigated from: (1) measurements of pressure, temperature, vertical distribution of clouds and wind speed along the probes’ descent trajectories, and (2) determination of deep winds, differential rotation and convection with combined probe, gravity and radiometric measurements. Besides these primary goals, Kronos will also measure the intensities and characteristics of Saturn’s magnetic field inside the D ring as well as Saturn’s gravitational field, in order to constrain the abundance of heavy elements in Saturn’s interior and in its central core. Depending on the preferred architecture (flyby versus orbiter), Kronos will be in a position to measure the properties of Saturn’s innermost magnetosphere and to investigate the ring structure in order to understand how these tiny structures could have formed and survived up to the present times. An erratum to this article can be found at     

The influence of forward-scattered light in transmission measurements of (exo)planetary atmospheres

The transmission of light through a planetary atmosphere can be studied as a function of altitude and wavelength using stellar or solar occultations, giving often unique constraints on the atmospheric composition. For exoplanets, a transit yields a limb-integrated, wavelength-dependent transmission spectrum of an atmosphere. When scattering haze and/or cloud particles are present in the planetary atmosphere, the amount of transmitted flux not only depends on the total optical thickness of the slant light path that is probed, but also on the amount of forward-scattering by the scattering particles. Here, we present results of calculations with a three-dimensional Monte Carlo code that simulates the transmitted flux during occultations or transits. For isotropically scattering particles, like gas molecules, the transmitted flux appears to be well-described by the total atmospheric optical thickness. Strongly forward-scattering particles, however, such as commonly found in atmospheres of Solar System planets, can increase the transmitted flux significantly. For exoplanets, such added flux can decrease the apparent radius of the planet by several scale heights, which is comparable to predicted and measured features in exoplanet transit spectra. We performed detailed calculations for Titan’s atmosphere between 2.0 and 2.8 μm and show that haze and gas abundances will be underestimated by about 8% if forward-scattering is ignored in the retrievals. At shorter wavelengths, errors in the gas and haze abundances and in the spectral slope of the haze particles can be several tens of percent, also for other Solar System planetary atmospheres. We also find that the contribution of forward-scattering can be fairly well described by modelling the atmosphere as a plane-parallel slab. This potentially reduces the need for a full three-dimensional Monte Carlo code for calculating transmission spectra of atmospheres that contain forward-scattering particles.     

Chemical modeling of exoplanet atmospheres

The past twenty years have revealed the diversity of planets that exist in the Universe. It turned out that most of exoplanets are different from the planets of our Solar System and thus, everything about them needs to be explored. Thanks to current observational technologies, we are able to determine some information about the atmospheric composition the thermal structure and the dynamics of these exoplanets, but many questions remain still unanswered. To improve our knowledge about exoplanetary systems, more accurate observations are needed and that is why the Exoplanet Characterisation Observatory (EChO) is an essential space mission. Thanks to its large spectral coverage and high spectral resolution, EChO will provide exoplanetary spectra with an unprecedented accuracy, allowing to improve our understanding of exoplanets. In this work, we review what has been done to date concerning the chemical modeling of exoplanet atmospheres and what are the main characteristics of warm exoplanet atmospheres, which are one of the main targets of EChO. Finally we will present the ongoing developments that are necessary for the chemical modeling of exoplanet atmospheres.     

Formation of Zr I and II lines under non-LTE conditions of stellar atmospheres

The formation of Zr I and Zr II lines in stellar atmospheres under non-LTE conditions has been considered for the first time. A model zirconium atom has been composed using 148 Zr I levels, 772 Zr II levels, and the ground Zr III state. Non-LTE calculations have been performed for model atmospheres with T _eff = 5500 and 6000 K, log g = 2.0 and 4.0, [M/H] = −3, −2, −1, 0. In the entire investigated range of parameters, the Zr I levels are shown to be underpopulated relative to their LTE populations in the line formation region. In contrast, the excited Zr II levels are overpopulated, while the ground state and lower excited levels of Zr II retain their LTE populations. Since the non-LTE effects cause the Zr I and Zr II spectral lines being investigated to weaken, the non-LTE corrections to the abundance derived from Zr I and Zr II lines are positive. For Zr II lines, they increase with decreasing metallicity and surface gravity up to 0.34 dex for the model with T _eff = 5500, log g = 2.0, and [M/H] = −2. The non-LTE effects depend weakly on temperature. The non-LTE corrections for Zr I lines reach 0.33 dex for solar-metallicity models. Zr I and Zr II lines in the solar spectrum have been analyzed. The non-LTE zirconium abundances derived from lines in the two ionization stages are shown to agree between themselves within the error limits, while the LTE abundance difference is 0.28 dex. The zirconium abundance in the solar atmosphere (averaged over Zr I and Zr II lines) is log ɛ_Zr,⊙ = 2.63 ± 0.07.     

Lithium in chemically peculiar CP stars with magnetic fields

The problem of lithium in chemically peculiar Ap-CP stars has been the subject of debate for many years. The main reason for this is a lack of spectral observations of Ap stars in the neighborhood of the lithium resonance doublet Li I 6708 Å. An international cooperation project on “Lithium in cool CP stars with magnetic fields” was started in 1996. Systematic observations of CP stars in spectral regions of the 6708 Å and 6103 Å lines at the ZTSh (CrAO), CAT (ESO), Feros (ESO), and the 74″ telescope of the Mount Stromlo Observatory (Australia) have been used to analyze spectra of several CP stars studied by the way the 6708 Å lithium line varies with the stars’ rotational phase. Monitoring of the spectra of the oscillating CP stars (group I) HD 83368, HD 60435, and HD 3980, for which significant Doppler shifts of the Li I 6708 Å line are observed led to the discovery of “lithium spots” on the surface of these stars whose positions are related to the magnetic field structure. Models of the surfaces of these stars with the special program “ROTATE” based on the profiles of the Li I 6708 Å line are used to estimate the size of the spots, their positions on the stars’ surface, and the lithium abundances in these spots. A detailed analysis and modelling of the spectra of slowly rotating oscillating CP stars with strong, invariant lithium 6708 Å emission, including blending with lines of the rare earth elements, reveals an enhanced lithium abundance, with the abundance determined from the lithium 6103 Å line being higher than that determined from the 6708 Å line for all the stars. This may indicate vertical stratification of lithium in the atmospheres of CP stars with an anomalous isotopic composition (⁶Li/⁷Li = 0.2–0.5). HD 101065, an ultraslow rotator (vsini ≈ 1.5) visible from the poles and with powerful oscillations which cause pulsating line broadening in its spectrum, is unique among these stars. The amount of lithium in the atmosphere of HD 101065 logN(Li) = 3.1 on a scale of logN(H) = 12.0 and the isotope ratio ⁶Li/⁷Li ≈ 0.3. The high estimates of ⁶Li/⁷Li may be explained by the production of lithium in spallation reactions and the preservation of surface ⁶Li and ⁷Li by strong magnetic fields in the upper layers of the atmosphere near the magnetic poles. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 463–492 (August 2007).     

On the (anticipated) diversity of terrestrial planet atmospheres

On our way toward the characterization of smaller and more temperate planets, missions dedicated to the spectroscopic observation of exoplanets will teach us about the wide diversity of classes of planetary atmospheres, many of them probably having no equivalent in the Solar System. But what kind of atmospheres can we expect? To start answering this question, many theoretical studies have tried to understand and model the various processes controlling the formation and evolution of planetary atmospheres, with some success in the Solar System. Here, we shortly review these processes and we try to give an idea of the various type of atmospheres that these processes can create. As will be made clear, current atmosphere evolution models have many shortcomings yet, and need heavy calibrations. With that in mind, we will thus discuss how observations with a mission similar to EChO would help us unravel the link between a planet’s environment and its atmosphere.     

Stratification of phosphorus in the atmosphere of the chemically peculiar B-type star HR 1512

A number of empirical relationships are shown to indicate an increase in the abundance of phosphorus logε(P) with height in the atmosphere of HR 1512. These include: (a) a correlation of logε(P) with the observed equivalent width W_obs of PII lines; (b) a correlation of logε(P) with the wavelength of the lines; (c) a systematic divergence in the values of logε(P) for lines with different excitation potentials E_l; in particular, lines with lower E_l correspond on the average to higher abundances logε(P); and, (d) a distinct dependence of logε(P) on the average geometrical height of formation, H_f. In addition, assuming that logε(P) is constant in the star's atmosphere leads to a systematic discrepancy between the theoretical equivalent widths W_th and the observed values W_obs. By a trial and error method we have found a distribution of the phosphorus abundance logε(P) with height H such that the systematic difference between W_th and W_obs vanishes. It turned out, however, that a simpler, step distribution of logε(P) yields equally good agreement between W_th and W_obs. Although the solution is nonunique, both distributions have some features in common, specifically: (1) a sharp rise in logε(P) occurs in the same range of heights H corresponding to optical thicknesses τ ₅₀₀₀ ≈ 10⁻²–10⁻³, i.e., stratification of phosphorus takes place in rather high layers of the atmosphere of HR 1512, and (2) the upper bound, logε _up(P) = 8.9, is the same in both cases, so that in the region of the rise, logε(P) increases by 3.4 dex. A comparison with available data for HgMn, Am, and Ap type stars shows that similar sharp changes in the abundances logε of several elements occur in other CP stars, at the same optical depths or in even higher layers of their atmospheres. __________ Translated from Astrofizika, Vol. 51, No. 2, pp. 239–253 (May 2008).