Loss of water from Mars:: Implications for the oxidation of the soil

The evolution of the martian atmosphere with regard to its H₂O inventory is influenced by thermal loss processes of H, H₂, nonthermal atmospheric loss processes of H⁺, H₂⁺, O, O⁺, CO₂, and O₂⁺ into space, as well as by chemical weathering of the surface soil. The evolution of thermal and nonthermal escape processes depend on the history of the intensity of the solar XUV radiation and the solar wind density. Thus, we use actual data from the observation of solar proxies with different ages from the Sun in Time program for reconstructing the Sun's radiation and particle environment from the present to 3.5 Gyr ago. The correlation between mass loss and X-ray surface flux of solar proxies follows a power law relationship, which indicates a solar wind density up to 1000 times higher at the beginning of the Sun's main sequence lifetime. For the study of various atmospheric escape processes we used a gas dynamic test particle model for the estimation of the pick up ion loss rates and considered pick up ion sputtering, as well as dissociative recombination. The loss of H₂O from Mars over the last 3.5 Gyr was estimated to be equivalent to a global martian H₂O ocean with a depth of about 12 m, which is smaller than the values reported by previous studies. If ion momentum transport, a process studied in detail by Mars Express is significant on Mars, the water loss may be enhanced by a factor of about 2. In our investigation we found that the sum of thermal and nonthermal atmospheric loss rates of H and all nonthermal escape processes of O to space are not compatible with a ratio of 2:1, and is currently close to about 20:1. Escape to space cannot therefore be the only sink for oxygen on Mars. Our results suggest that the missing oxygen (needed for the validation of the 2:1 ratio between H and O) can be explained by the incorporation into the martian surface by chemical weathering processes since the onset of intense oxidation about 2 Gyr ago. Based on the evolution of the atmosphere-surface-interaction on Mars, an overall global surface sink of about 2×10⁴² oxygen particles in the regolith can be expected. Because of the intense oxidation of inorganic matter, this process may have led to the formation of considerable amounts of sulfates and ferric oxides on Mars. To model this effect we consider several factors: (1) the amount of incorporated oxygen, (2) the inorganic composition of the martian soil and (3) meteoritic gardening. We show that the oxygen incorporation has also implications for the oxidant extinction depth, which is an important parameter to determine required sampling depths on Mars aimed at finding putative organic material. We found that the oxidant extinction depth is expected to lie in a range between 2 and 5 m for global mean values.     

Nonthermal escape of hydrogen and deuterium from Venus and implications for loss of water

Nonthermal escape processes responsible for the escape of hydrogen and deuterium from Venus are examined for present and past atmospheres. Three mechanisms are important for the escape of hydrogen from the present atmosphere: (a) charge exchange of plasmaspheric H⁺ with exospheric H, (b) impact of exospheric hot O atoms on H, and (c) ion molecule reactions involving O⁺ and H₂. However, in the past when the H abundance was higher, the charge-exchange mechanism would be the strongest. The H escape flux increases rapidly with increasing hydrogen abundance in the upper atmosphere and saturates at a value of 1 × 10¹⁰ cm^?2 sec^?1 emerging primarily from the day side when the H mixing ratio at the homopause is 2 × 10^?3. This corresponds to an H₂O mixing ratio of 1 × 10^?3 at the cold trap and ～15% at the surface. Deuterium would also escape by the charge-exchange mechanism and a D/H enrichment by a factor of ～1000 over the nonthermal escape regime is expected, which could have lasted over the last 3 billion years. Coincidentally, the onset of hydrodynamic flow leading to efficient H escape occurs just at the H₂O mixing ratio at which the charge-exchange escape flux saturates. Thus it is possible that Venus has lost an Earth-equivalent ocean of water over geologic time. If so, either the D/H enrichment has been kept low by modest outgassing of juvenile water or Venus started out with a D/H ratio of ～4.0 × 10^?6.     

Influence of Inelastic Collisions with Hydrogen Atoms on Non-LTE Oxygen Abundance Determinations

We present the results of our modeling of the O I line formation under non-LTE conditions in the atmospheres of FG stars. The statistical equilibrium of O I has been calculated using Barklem’s quantum-mechanical rates of inelastic collisions with hydrogen atoms. We have determined the non-LTE oxygen abundance from atomic O I lines for the Sun and 46 FG stars in a wide metallicity range, ?2.6 < [Fe/H] < 0.2. The application of accurate atomic data has led to an increase in the departures from LTE and a decrease in the oxygen abundance compared to the use of Drawin’s theoretical approximation. The change in the non-LTE abundance from the infrared O I 7771-5 Å triplet lines is 0.11 dex for solar atmospheric parameters and diminishes in absolute value with decreasing metallicity. We have revised the [O/Fe]–[Fe/H] relationship derived by us previously. The change in [O/Fe] is small in the [Fe/H] range from ?1.5 to 0.2. For stars with [Fe/H] < ?1 the [O/Fe] ratio has increased so that [O/Fe] = 0.60 at [Fe/H] = ?0.8 and rises to [O/Fe] = 0.75 at [Fe/H] = ?2.6.     

Ion-acoustic double layers in magnetized positive-negative ion plasmas with nonthermal electrons

The nonlinear ion-acoustic double layers (IADLs) in a warm magnetoplasma with positive-negative ions and nonthermal electrons are investigated. For this purpose, the hydrodynamic equations for the positive-negative ions, nonthermal electron density distribution, and the Poisson equation are used to derive a modified Zakharov–Kuznetsov (MZK) equation, in the small amplitude regime. It is found that compressive and rarefactive IADLs strongly depend on the mass and density ratios of the negative-to-positive ions as well as the nonthermal electron parameter. Also, it is shown that there are one critical value for the density ratio of the negative-to-positive ions (ν), the ratio between unperturbed electron-to-positive ion density (μ), and the nonthermal electron parameter (β), which decide the existence of positive and negative IADLs. The present study is applied to examine the small amplitude nonlinear IADL excitations for the (H⁺, O₂^-)(\mathrm{H}^{+}, \mathrm{O}_{2}^{-}) and (H⁺,H⁻) plasmas, where they are found in the D- and F-regions of the Earth’s ionosphere. This investigation should be helpful in understanding the salient features of the nonlinear IADLs in either space or laboratory plasmas where two distinct groups of ions and non-Boltzmann distributed electrons are present.     

Chemical Evolution of the Magellanic Clouds

Previous models for the chemical evolution of the Magellanic Clouds have assumed either a steepened IMF compared to the solar neighbourhood or preferential expulsion of oxygen and α-particle elements by selective galactic winds. These assumptions were largely motivated by a belief that the O/Fe ratio in the Clouds is substantially lower than in the Galaxy, but the difference appears to have been exaggerated: Galactic supergiants have a similar O/Fe ratio as Cloud supergiants, there is no corresponding effect in Mg and other α-elements and a combination of data from planetary nebulae, H II regions and supernova remnants indicates an O/Fe ratio more or less equal to solar. Consequently new analytical models for the chemical evolution of the Magellanic Clouds have been developed, assuming chemical yields and time delays identical to those we previously assumed for the solar neighbourhood, but assuming (in addition to infall) non-selective galactic winds and burst-like modes of star formation represented by discontinuous variations in the star formation rate per unit gas mass. We find adequate agreement with age-metallicity relations and element:element ratios within their substantial uncertainties, whereas our LMC model turns out to give an excellent fit to the anomalous Galactic halo stars discovered by Nissen and Schuster (1997). It also gives an enhanced SNIa/SNII ratio compared to the solar neighbourhood, due to the assumption that the SFR has declined in the past 1 to 2 Gyr. This revised version was published online in July 2006 with corrections to the Cover Date.     

Multifrequency emission analysis of TeV blazars H 2356-309 and 1ES 1218+304

The multiband nonthermal emissions in radio,X-ray,and very high-energy (VHE)γ-ray bands from two distant blazars,H 2356-309 and 1ES 1218+304,have been detected,and,especially from recent observations with the Suzaku,MAGIC and VERITAS telescopes,clearly reveal nonthermal power-law spectra. We study the broadband nonthermal spectra of the two sources by using a combination of a onezone homogeneous synchrotron self-Compton (SSC) model and an inhomogeneous conical jet model,where the new external background light (EBL) model is taken into account. The results show that (1) the nonthermal emissions of the two blazars,ranging from X-rays to VHE γ-rays,are from the homogeneous zone whereas the emissions in the radio bands can be explained as the radiation from the inhomogeneous conical jet; (2) a strict lower-limit EBL model can be used to explain their observed spectra well.     

羟基的切仑科夫射电发射谱线

本文提出用切仑科夫射电谱线发射机制解释分子云的非热发射谱线。相对论电子在某种粒子数反转布居的分子云中穿行时,可以产生具有反常强度比的微波发射谱线。对与超新星遗迹成协的Ⅱ_(?)类OH源的非热平衡发射谱线所作的计算表明,这种非热辐射机制可能成为解决天体脉塞理论困难的一种有希望的途径。     

D/O and D/H ratios in the local interstellar medium from FUSE observations

Since HI, OI, and DI have nearly the same ionization potential, the deuterium-to-oxygen ratio (D/O) is an important tracer of the D/H ratio and of its putative spatial variations. D/O is indeed very sensitive to astration, both because of deuterium destruction and oxygen production. Moreover, D/O measurements are less subject to systematic errors than D/H.A survey of D/O in the interstellar medium is performed by the Deuterium Working Group of the FUSE PI Team. The first results show that D/O is a better D/H proxy than D/N, and that D/O is homogeneous in the local interstellar medium, at least within about 100pc. The stability of D/O appears as a strong argument which supports both D/H and O/H spatial stability in the local interstellar medium.     

Oxygen and carbon isotope ratios in the martian atmosphere

Oxygen and carbon isotope ratios in the martian CO₂ are key values to study evolution of volatiles on Mars. The major problems in spectroscopic determinations of these ratios on Mars are uncertainties associated with: (1) equivalent widths of the observed absorption lines, (2) line strengths in spectroscopic databases, and (3) thermal structure of the martian atmosphere during the observation. We have made special efforts to reduce all these uncertainties. We observed Mars using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope. While the oxygen and carbon isotope ratios on Mars were byproducts in the previous observations, our observation was specifically aimed at these isotope ratios. We covered a range of 6022 to 6308 cm⁻¹ with the highest resolving power of ν/δν=3.5×10⁵ and a signal-to-noise ratio of 180 in the middle of the spectrum. The chosen spectral range involves 475 lines of the main isotope, 184 lines of ¹³CO₂, 181 lines of CO¹⁸O, and 119 lines of CO¹⁷O. (Lines with strengths exceeding 10⁻²⁷ cm at 218 K are considered here.) Due to the high spectral resolution, most of the lines are not blended. Uncertainties of retrieved isotope abundances are in inverse proportion to resolving power, signal-to-noise ratio, and square root of the number of lines. Laboratory studies of the CO₂ isotope spectra in the range of our observation achieved an accuracy of 1% in the line strengths. Detailed observations of temperature profiles using MGS/TES and data on temperature variations with local time from two GCMs are used to simulate each absorption line at various heights in each part of the instrument field of view and then sum up the results. Thermal radiation of Mars' surface and atmosphere is negligible in the chosen spectral range, and this reduces errors associated with uncertainties in the thermal structure on Mars. Using a combination of all these factors, the highest accuracy has been achieved in measuring the CO₂ isotope ratios: ¹³C/¹²C = 0.978 ± 0.020 and ¹⁸O/¹⁶O = 1.018 ± 0.018 times the terrestrial standards. Heavy isotopes in the atmosphere are enriched by nonthermal escape and sputtering, and depleted by fractionation with solid-phase reservoirs. The retrieved ratios show that isotope fractionation between CO₂ and oxygen and carbon reservoirs in the solid phase is almost balanced by nonthermal escape and sputtering of O and C from Mars.     

Oxygen abundance in giant H II galaxies

A sample of 93 emission-line high luminosity galaxies from the Sloan Digital Sky Survey (SDSS) has been investigated. Line intensities have been measured in 116 SDSS spectra. Oxygen abundance has been determined in the studied galaxies. Since the auroral line of twice ionized oxygen [O III] λ 436.3 nm cannot be detected in the spectra of the sample, the intensity ratio of nebular to auroral lines needed to determine the electron temperature is calculated using the ff-relation. The oxygen abundances obtained in SDSS high luminosity galaxies are 0.2–0.5 dex lower than the maximum attainable value. This is caused by the fact that the sample includes only the gas-rich galaxies in which intense bursts of star formation occur. The equivalent number of O7 V stars which are responsible for excitation of luminescent H II regions in the studied sample is two or three orders of magnitude more than the number of stars which cause the luminescence of the brightest H II regions in nearby galaxies, and it exceeds by one order of magnitude the number of stars which cause gas in SBS 0335-052 E to glow.     

New Views on the Early Evolution of Oxygen in the Galaxy

We discuss results on the oxygen abundance in a sample of 23 metal-poor (?3.0≤ [Fe/H] ≤ ?0.3) unevolved stars and one giant. High resolutionspectroscopy of OH lines in the near UV allowed us to trace the early evolution of oxygenversus metallicity. Contrary to previous expectations, we find that oxygen abundances derived from these low excitation lines agree well withthose derived from the high excitation lines of the OI IR triplet and from the [OI] λ 6300 Å line. Our new oxygen abundances show a smooth extension of previouslyknown trends of [O/Fe] versus [Fe/H] in disk stars to much lower metallicities, with a slope of ?0.31± 0.11. The [O/Fe] ratio increasesfrom 0.6 to 1 between [Fe/H] =?1.5 and ?3.0. Comparison with oxygen abundances in giant stars of the same metallicity imply that the lattermay have suffered a process of oxygen depletion. We briefly discussthe impact of these results on the yields of Type II SNe in the early Galaxy and on the age of globular clusters.     

Japanese VLBI Network Observations of a Gamma-Ray Narrow-Line Seyfert 1 Galaxy 1H 0323+342

We made simultaneous single-dish and VLBI observations of a gamma-ray narrow-line Seyfert 1 (NLS1) galaxy 1H 0323+342. We found significant flux variation at 8 GHz on a time scale of one month. The total flux density varied by 5.5% in 32 days, corresponding to a variability brightness temperature of 7.0 × 10¹¹ K. We also obtained brightness temperatures of greater than 5.2 × 10¹⁰ K from the VLBI images. These high brightness temperatures suggest that the source has nonthermal processes in the central engine. The source structure could be modelled by two elliptical Gaussian components on the parsec scales. The flux of the central component decreases in the same way as the total flux density, showing that the short-term variability is mainly associated with this component.     

Gas-phase mean opacities for varying [M/H], N/O and C/O

We present a set of gas-phase Planck mean and Rosseland mean opacity tables applicable for simulations of star and planet formation, stellar evolution and disc modelling at various metallicities in hydrogen-rich environments. The tables are calculated for gas temperatures between 1000 and 10 000 K and total hydrogen number densities between 10² and  10¹⁷ cm⁻³  . The carbon-to-oxygen ratio is varied from 0.43 to well above 2.0, the nitrogen-to-oxygen ratio between 0.14 and 100.0. The tables are calculated for a range of metallicities down to  [M/H]'= log  N _M/ N _H=−7.0  . We demonstrate how the mean opacities and the abundances of the opacity species vary with C/O, N/O and  [M/H]'  . We use the element abundances from Grevesse et al., and we provide additional tables for the oxygen abundance value from Caffau et al. All tables will be available online at http://star-www.st-and.ac.uk/~ch80/datasources.html .     

Stochastic models of hot planetary and satellite coronas: A photochemical source of hot oxygen in the upper atmosphere of Mars

The processes of the kinetics and transport of hot oxygen atoms in the upper atmosphere of Mars are studied. A reaction of dissociative recombination of the main ionospheric ion O ₂ ⁺ with thermal electrons is considered as a photochemical source of suprathermal oxygen atoms. Oxygen atoms are formed in the dissociative recombination reaction with an excess of kinetic energy of about 0.4–4 eV and lose that energy in elastic and inelastic collisions with the ambient thermal atmospheric gas. The altitude distributions of the concentrations of neutral and ionized components, as well as their temperatures, were taken from Krasnopolsky (2002). Unlike the models published earlier, detailed calculations of the formation, collisional kinetics, and transport of suprathermal oxygen atoms in the thermosphere-exosphere transition region of the upper atmosphere of Mars have been made for the first time. For this, we used a stochastic model of the formation of a hot planetary corona (Shematovich, 2004). It has been shown that the considered photochemical source of suprathermal oxygen leads to the formation of the hot corona and to higher nonthermal losses of oxygen from the upper atmosphere of Mars due to escape fluxes. The detailed energy spectra of the fluxes of suprathermal atomic oxygen were calculated for the thermosphere-exosphere transition region of the Martian atmosphere.Translated from Astronomicheskii Vestnik, Vol. 39, No. 1, 2005, pp. 26–37.Original Russian Text Copyright © 2005 by Krestyanikova, Shematovich.     

Evolution of the titanium and oxygen abundances from observations of FGK dwarfs in a wide metallicity range

For a sample of dwarf stars close to the Sun with well-known atmospheric parameters and an iron abundance in the range ?2.6 < [Fe/H] < 0.2, we have determined the titanium and oxygen abundances by taking into account the departures from LTE. The dependence of the [O/Fe] and [Ti/Fe] abundance ratios on [Fe/H] has been refined in comparison with the published data. We have established that [O/Fe] increases from ?0.2 to 0.6 as the metallicity [Fe/H] decreases from 0.2 to ?0.8 and remains constant at a lower metallicity. A similar behavior has been found for [Ti/Fe], but the plateau is formed by stars with [Fe/H] > ?0.7, and the titanium overabundance relative to iron is 0.3. The results confirm that not only oxygen but also titanium are synthesized in the α-process. Our data can be used to test the Galactic chemical evolution models.     

发射线星系[OⅡ]λ3727/Hα谱线流量比的研究

利用从斯隆数字巡天(Sloan Digital Sky Survey,简称SDSS)第4次释放的光谱数据中选取的10~5个发射线星系样本,研究了[O_Ⅱ]λ3727/Hα流量比与星系尘埃消光、气体电离态和金属丰度的关系.发现尘埃消光改正对[O_Ⅱ]λ3727/Hα谱线流量比影响显著,消光改正前、后的[O_Ⅱ]λ3727/Hα谱线流量比的中值分别为0.48和0.89;尘埃消光改正后,F([O_Ⅱ]λ3727)-F(Hα)的弥散显著减小.贫金属星系的[O_Ⅱ]λ3727/Hα谱线流量比随星系气体的电离度增高而减小,而富金属星系不存在这种关系.另外,[O_Ⅱ]λ3727/Hα流量比与星系金属丰度相关.当12+lg(O/H)8.5时,星系[O_Ⅱ]λ3727/Hα流量比随金属丰度增加而下降;12+lg(O/H)8.5的星系,谱线流量比与金属丰度正相关.最后,利用气体电离度参数和星系的金属丰度,给出了计算不同类型星系[O_Ⅱ]λ3727/Hα流量比的公式.LAMOST望远镜将观测到大量红移z0.4的星系光谱,利用该公式可以给出星系的[O_Ⅱ]λ3727/Hα流量比,从而可以利用[O_Ⅱ]λ3727谱线流量计算z0.4星系的恒星形成率.     

Dust acoustic solitary waves in dusty plasma with nonthermal ions

In this paper, the characteristics of the dust acoustic solitary waves in dusty plasmas are studied. The distribution of ions is nonthermal, and the nonthermal parameter is treated as a variable. The pseudo-potential method has been used to investigate the possibility of soliton formation. We show that for some values of the nonthermal parameter there is no soliton.     

Nonthermal velocities in the solar transition zone observed with the high-resolution telescope and spectrograph

Data obtained during the first rocket flight of the NRL High Resolution Telescope and Spectrograph (HRTS) have been used to study nonthermal velocities for spectral lines primarily covering the temperature range 10⁴ to 2 × 10⁶ K. The high spectral and spatial resolution, combined with an enhanced dynamic intensity range of the reduced data, has enabled us to study the distribution of the nonthermal velocities for quiet and active regions. Average values of the nonthermal velocities peak at about 27 km s^–1 at 10⁵ K for the quiet regions, with a wide distribution of nonthermal velocities for each line. The active region nonthermal velocities have a narrower distribution which is weighted towards higher values. The SiIV and C IV line profiles are not well described by a single Gaussian, indicating that high-velocity components (above 30 km s^–1) are present in the quiet-Sun spectra. The radiative losses for all plasma above l0⁵ K have been calculated for the quiet Sun, an active region and a coronal hole. These have been compared with the acoustic wave flux inferred from the nonthermal line widths. There appears to be a sufficient flux of waves to heat these regions of the atmosphere.     

The impact of the nitrogen-to-oxygen ratio on ionized nebula diagnostics based on [N ii] emission lines

We study the relation between nitrogen and oxygen abundances as a function of metallicity for a sample of emission-line objects for which a direct measurement of the metallicity has been possible. This sample is representative of the very different conditions in ionization and chemical enrichment that we can find in the Universe. We first construct the N/O versus O/H ratio diagram, and discuss its large dispersion at all metallicity regimes. Using the same sample and a large grid of photoionization models covering very different values of the N/O ratio, we then study the most widely used strong-line calibrators of metallicity based on [N  ii ] emission lines, such as N2 and O3N2. We demonstrate that these parameters underestimate the metallicity at low N/O ratios and vice versa. We also investigate the effect of the N/O ratio on different diagnostic diagrams used to discriminate narrow-line active galactic nuclei from star-forming regions, such as the [O  iii ]/Hβ versus [N  ii ]/Hα, and show that a large fraction of the galaxies catalogued as composite in this diagram can be, in fact, star-forming galaxies with a high value of the N/O ratio. Finally, using strong-line methods sensitive to the N/O abundance ratio, like N2O2 and N2S2, we investigate the relation between this ratio and the stellar mass for the galaxies of the Sloan Digital Sky Survey. We find, as in the case of the mass–metallicity relation, a correlation between these two quantities and a flattening of the relation for the most massive galaxies, which could be a consequence of the enhancement of the dispersion of N/O ratio in the high-metallicity regime.     

Gas emission spectrum in the Irr galaxy IC 10

Observations of the dwarf Irr galaxy IC 10 have been performed at the 6-m Special Astrophysical Observatory telescope with the SCORPIO focal reducer in the mode of a slit spectrograph. The ionized-gas emission spectrum in the region of intense current star formation has been investigated. The relative oxygen, N⁺, and S⁺ abundances in about twenty H II regions and the synchrotron superbubble have been estimated. The galaxy-averaged oxygen abundance is 12 + log(O/H) = 8.17 ± 0.35 and the metallicity is Z = 0.18 ± 0.14Z _⊙. The metallicity found by comparing diagnostic diagrams with photoionization models is shown to be less reliable than its estimate based on strong oxygen lines.