Hubble Space Telescope detects sub-solar water atmosphere on Ganymede

<p>Ganymede’s tenuous atmosphere is produced by charged particle sputtering and sublimation of its icy surface. Previous far-ultraviolet observations of the OI1356 Å and OI1304 Å oxygen emissions were used to derive sputtered molecular oxygen, O<sub>2,</sub> as an atmospheric constituent. We present a new analysis of high-sensitivity spectra and spectral images of Ganymede’s oxygen emissions acquired by the COS and STIS instruments on the Hubble Space Telescope. The COS eclipse observations constrain atomic oxygen, O, to be at least two orders of magnitude less abundant than O<sub>2</sub>. We then show that dissociative excitation of water vapor, H<sub>2</sub>O, is found to increase the OI1304 Å emissions relative to the OI1356 Å emissions around the sub-solar point, where H<sub>2</sub>O is more abundant than O<sub>2</sub>. Away from the sub-solar region, the emissions are more than two times brighter at OI1356 Å than at OI1304 Å, and O<sub>2</sub> prevails as found in previous analyses. A ~6-fold higher H<sub>2</sub>O/O<sub>2</sub> mixing ratio on the warmer trailing hemisphere compared to the colder leading hemisphere, a spatial concentration at the sub-solar region, and the ratio-estimated H<sub>2</sub>O densities identify icy surface sublimation as a local dayside atmospheric source.<br />Our analysis provides the first evidence for a sublimated atmosphere on an icy moon in the outer solar system.</p>

Electron Induced Emission of Nitrous Oxide in the UV-VIS Spectral Range

The electron impact excitation of N2O was studied using the crossed electron-molecular beams method. Optical emission spectrum initiated by 50 eV electron impact was recorded within the range 200-700 nm. Main emission bands arise from excited ion state N2O+(A2Σ) and dissociative excitation into N2+(B2Σ+u). The rotationally un-resolved excitation-emission cross sections for selected ion transitions were scaled to absolute values and their dependence on electron energy was determined. Several of them were determined for the first time.

Прямое диссоциативное возбуждение гетероядерных и гомоядерных ионов инертных газов электронным ударом

A theoretical description of the process of dissociative excitation of a molecular ion by electron impact in the case of effective population of a huge number of its vibrational-rotational levels is presented. Our consideration is based on the quantal version of the theory of non-adiabatic transitions between the electronic terms of a molecular ion combined with replacing the summation over vJ-levels by integration over v and J. Semianalytical formulas are derived for the integral contribution of the entire vibrational-rotational quasicontinuum to the cross sections, $\sigma_T^{\mathrm{de}}(\varepsilon)$, and the rate constants, $\alpha^{\mathrm{de}}(T, T_e)$ of the process under study in a plasma with temperatures $T_e$ and $T$ of its electronic and ionic components. The developed theory is used to study the processes of dissociative excitation of heteronuclear (HeXe$^{+}$ and ArXe$^{+}$) and homonuclear (Ar$_2^{+}$ and Xe$_2^{+}$) ions of inert gases. We demonstrate a strong dependence of the results on the ion dissociation energy and large differences in the behavior and characteristic values of $\sigma_T^{\mathrm{de}}(\varepsilon)$ and $\alpha^{\mathrm{de}}(T, T_e)$ for these systems in different ranges of electron energy, $\varepsilon$, and temperatures $T_e$ and $T$. The regions of dominance of the contributions of two competing channels are determined: direct dissociative excitation and dissociative recombination into total cross sections and the rate constants of destruction of rare gas molecular ions by electron impact. Analyzing behavior of the differential rate constants of dissociative excitation per unit range of electron energy in the final channel of reaction we demonstrate qualitative differences in the dynamics of this process for weakly bound and moderately bound molecular ions.