Aleksandra Borysow

Gaseous abundances and methane supersaturation in Titan's troposphere

Various properties of Titans troposphere are inferred from an analysis of Voyager 1 infrared spectrometer (IRIS) data between 200 and 600 cm−1. Two homogeneous spectral averages acquired at widely separated emission angles are chosen for the analysis. Both data sets are associated with northern low latitudes very close to that of the radio science ingress occultation point. Solutions require simultaneous nonlinear least-squares fits to the two IRIS data sets, coupled with iteration of the radio occultation refractivity data. Values and associated 1-σ uncertainties of several parameters are inferred from our analysis. These include mole fractions for molecular hydrogen (∼0.0011), argon (small), and methane near the surface (∼0.057). Solutions are also obtained for the hydrogen para-fraction (close to equilibrium, with considerable uncertainty), air temperature near the surface (∼93 K), surface temperature discontinuity (∼1 K), and maximum degree of methane supersaturation in the upper troposphere (∼ 1.5). Actual values for the above-mentioned parameters depend on the amount of ethane cloud near the tropopause. There is no evidence for methane clouds in the upper troposphere, nor is their presence compatible with large degrees of supersaturation. A wave number dependence for the stratospheric haze opacity is inferred similar to that found for a polymeric residue created in laboratory discharge experiments. This haze appears to be uniformly distributed with latitude between altitudes of 40 and 160 km, provided those nighttime data at southern high latitudes that are subject to possible systematic calibration errors are discounted. Assuming uniform haze distribution, both the air temperature and methane vapor mole fraction near the surface are symmetrically distributed about the equator, with lower values at higher latitudes. Either the tropopause temperature or the maximum degree of methane supersaturation is asymmetrically distributed about the equator. In either case, the data are consistent with a decrease of methane supersaturation toward the poles, which suggests an increase in mean annual precipitation at high latitudes compared with the equatorial region. If methane vapor is in saturation equilibrium with Titans surface, the derived latitudinal gradient of the near surface methane vapor mole fraction implies that the liquid content of the surface is ethane-enriched near the poles.

Modeling of collision-induced infrared absorption spectra of H2-H2 pairs in the fundamental band at temperatures from 20 to 300 K

Abstract A numerical method is presented which generates the rotovibrational (free-free) collision-induced absorption (RV CIA) spectra of H 2 -H 2 pairs in the fundamental band of hydrogen, at temperatures from 20 to 300 K. Computed synthesized spectra are found to be in close agreement with all presently known CIA spectra of H 2 -H 2 and reproduce the results of the quantum mechanical computations to within a few percent. The spectral lineshape parameters are obtained from the lowest three quantum mechanical spectral moments computed from first principles. The model RV CIA spectra of H 2 -H 2 can thus be computed easily even on small computers. The work is of interest for the modeling of the atmospheres of the outer planets in the near infrared region of the spectrum.

Theoretical collision-induced rototranslational absorption spectra for the outer planets - H2-CH4 pairs

The modeling of the collision-induced rototranslational absorption spectra for H2-CH4 molecules is described. The intermolecular potentials of the molecular complex are analyzed. The absorption coefficient is dependent on the molecular structure and the line-shape formulas for induction by hydrogen and methane are provided. Dipoles induced by an electric multipole field are examined. The measurements of the collision-induced absorption spectrum of the H2-CH4 system at 195 and 297 K are fitted to the Sutter et al. data. The relation between spectral components and H2-CH4 intermolecular potential are studied using the Hanley-Klein potential model (1972). Absorption coefficients of H2-CH4 are presented. 51 references.

Collision-induced infrared spectra of H2-He pairs involving 0-1 vibrational transitions and temperatures from 18 to 7000 K

Theoretical estimates are given of the absorption spectra of collisional pairs of hydrogen and helium (H2-He). The range of temperatures considered is from 18 to 7000 K. The resulting spectra, which can be computed in seconds even on small computers, are in close agreement with presently known laboratory measurements and extrapolate these dependably to higher as well as to lower temperatures, and to higher frequencies. This work is of interest for the modeling of the atmospheres of the outer planets and certain cool stars that contain neutral molecular hydrogen and helium in the near-IR region of the spectrum. 38 references.

New model of collision-induced infrared absorption spectra of H2He pairs in the 2–2.5 μm range at temperatures from 20 to 300 K: An update

A revised numerical method to calculate the collision-induced absorption rotovibrational (RV CIA) spectra of H2He pairs in the fundamental band of hydrogen, at temperatures from 20 to 300 K is presented. The paper corrects few inaccuracies which exist in the previous modeling of the H2He absorption in this frequency range. New computations account for the previously neglected J-dependence of the vibrational matrix elements of the induced dipoles. Model spectra are presented which reproduce closely the latest quantum mechanical results and are found to agree with most experimental CIA coefficients of H2He to within a few percent. The numerical model uses simple analytical model lineshapes and can generate the RV CIA spectra of H2He extremely efficiently. A FORTRAN program, H2HE01, utilizing the described method is available upon request from the author. This work is of interest for the modeling of the atmospheres of the outer planets in the near-infrared region of the spectrum.

A new computation of the infrared absorption by H2 pairs in the fundamental band at temperatures from 600 to 5000 K

A new computation of the absorption spectrum of H2 pairs in the fundamental band is given for temperatures from 600 to 5000 K. It is based on advanced interaction potentials and recent induced dipole components which were shown to be consistent with the existing laboratory measurements of low-temperature absorption spectra. The absorption is greater by factors of 2-3 than previous estimates and show a different band profile at the higher temperatures. 11 refs.

Collision-induced infrared spectra of H2-He pairs at temperatures from 18 to 7000 K. II: Overtone and hot bands

The three lowest spectral moments of the collision induced absorption (CIA) spectra of H2-He pairs have been computed from first principles for temperatures T from 18 to 7000 K for a number of hydrogen overtone and hot bands involving vibrational quantum numbers nu = 0, 1, 2, 3 yields nu-prime = 0, 1, 2, 3. The data are given in a form suitable for the computation of CIA spectra of H2-He as function of frequency and temperature, using simple computer codes and model line shapes. The work is of interest for the spectroscopy of the atmospheres of the outer planets and of stars that contain neutral molecular hydrogen and helium (late stars, white dwarfs, and Population II stars) in the infrared and visible region of the spectrum. 13 refs.

Computer Simulation of the far Infrared Collision Induced Absorption Spectra of Gaseous CO2

Far infrared collision induced absorption spectra of gaseous CO2 were computed using molecular dynamics simulations. The quadrupole and hexadecapole multipolar induction, through the trace, and the anisotropy of the molecular polarizability were found to be insufficient to represent properly the dipole induction mechanism. For a detailed analysis of the induction process the spectra obtained were decomposed into components resulting from different terms of the induced dipole. Based on this decomposition, an additional overlap contribution for each term was proposed. When spectra were recomputed including such overlap, good agreement between experiment and simulation was achieved over the temperature range at which measurements exist (233-400K). The use of an anisotropic intermolecular potential was found to be of crucial importance for obtaining the right shape of the far wings of the spectra.

On the problem of detailed balance and model lineshapes in collision-induced rotovibrational bands: H2-H2 and H2-He

When account is taken of the vibrational dependence of the potential function, the usual condition of detailed balance taken with respect to the vibrational-rotational frequencies does not apply, and model spectral functions satisfying this condition are inaccurate. A model correlation function is derived that deals with the case of vibration-dependent potentials. The parameters of the model are computed from relations involving the spectral moments. This model is shown to give spectral shapes, for a number of examples involving the fundamental and first overtone bands of H2-He and H2-H2 at room temperature and 1000 K, in excellent agreement with spectra calculated quantum mechanically from first principles. The model is useful for predicting the collision-induced vibrational spectra for these and other systems for which the vibration-dependent induced dipole and potential are known.

Modelling of collision-induced absorption spectra

A simple formalism allows the computation of complete collision-induced translational-rotational-vibrational absorption spectra from interaction potential and induced dipole moment, with a precision of a few per cent and at a fraction (∼1/20) of the cost and complexity of full quantum line shape calculations. The reduced mass of the collisional complex and the temperatures must be sufficiently small so that the collisional pair may be considered a quantum system. A comparison of exact H2-He collision-induced absorption spectra with models generated according to the new prescription shows satisfactory agreement at temperatures from 50 to 300 K and suggests important applications, such as the modeling of radiative transfer in planetary atmospheres.