The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H3O+) and Zundel (H5O2+) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

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[1] We present a study of the formation and distribution of benzene (C 6 H 6 ) on Titan. Analysis of the Cassini Mass Spectrometer (INMS) measurements of benzene densities on 12 Titan passes shows that the benzene signal exhibits an unusual time dependence, peaking $20 s after closest approach, rather than at closest approach. We show that this behavior can be explained by recombination of phenyl radicals (C 6 H 5 ) with H atoms on the walls of the instrument and that the measured signal is a combination of (1) C 6 H 6 from the atmosphere and (2) C 6 H 6 formed within the instrument. In parallel, we investigate Titan benzene chemistry with a set of photochemical models. A model for the ionosphere predicts that the globally averaged production rate of benzene by ion-molecule reactions is $10 7 cm À2 s À1 , of the same order of magnitude as the production rate by neutral reactions of $4 Â 10 6 cm À2 s À1 . We show that benzene is quickly photolyzed in the thermosphere, and that C 6 H 5 radicals, the main photodissociation products, are $3 times as abundant as benzene. This result is consistent with the phenyl/benzene ratio required to match the INMS observations. Loss of benzene occurs primarily through reaction of phenyl with other radicals, leading to the formation of complex aromatic species. These species, along with benzene, diffuse downward, eventually condensing near the tropopause. We find a total production rate of solid aromatics of $10 À15 g cm À2 s À1 , corresponding to an accumulated surface layer of $3 m.

Infrared photodissociation spectroscopy is reported for mass-selected H(+)(H(2)O)(n) complexes and their deuterated analogues with and without argon "tagging." H(+)(H(2)O)(n)Ar(m) and D(+)(D(2)O)(n)Ar(m) complexes are studied in the O-H (O-D) stretching region for clusters in the small size range (n = 2-5). Upon infrared excitation, these clusters fragment by the loss of either argon atoms or one or more intact water molecules. Their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water and in the hydrogen bonding region. Experimental studies are complemented by computational work that explores the isomeric structures, their energetics and vibrational spectra. The addition of an argon atom is essential to obtain photodissociation for the n = 2-3 complexes, and specific inclusion of the argon in calculations is necessary to reproduce the measured spectra. For n = 3-5, spectra are obtained both with and without argon. The added argon atom allows selection of a subset of colder clusters and it increases the photodissociation yield. Although most of these clusters have more than one possible isomeric structure, the spectra measured correspond to a single isomer that is computed to be the most stable. Deuteration in these small cluster sizes leads to expected lowering of frequencies, but the spectra indicate the presence of the same single most-stable isomer for each cluster size.

[1] Measurements of the mole fractions of CH 4 and 40 Ar by the Ion Neutral Mass Spectrometer on the Cassini orbiter are analyzed to determine the rate of vertical mixing in Titan's atmosphere and the escape flux of CH 4 . Analysis of the 40 Ar data indicates an eddy mixing rate of 2-5 Â 10 7 cm 2 s À1 , an order of magnitude smaller than previously determined from analysis of the CH 4 distribution. The eddy profile determined from the 40 Ar data implies that CH 4 distribution is best explained by postulating that it is escaping the atmosphere at the diffusion limited rate of 2.5-3.0 Â 10 9 cm À2 s À1, referred to the surface. This represents a significant loss of atmospheric CH 4 , smaller than but comparable to the photochemical destruction rate. The escape rate is much larger than predicted by the Jeans escape formula and vigorous nonthermal mechanisms are not apparent.

Solar eruptions are spectacular magnetic explosions in the Sun's corona and how they are initiated remains unclear. Prevailing theories often rely on special magnetic topologies which, however, may not generally exist in the pre-eruption source region of corona. Here using fully three-dimensional magnetohydrodynamic simulations with high accuracy, we show that solar eruption can be initiated in a single bipolar configuration with no additional special topology. Through photospheric shearing motion alone, an electric current sheet forms in the highly sheared core field of the magnetic arcade during its quasi-static evolution. Once magnetic reconnection sets in, the whole arcade is expelled impulsively, forming a fast-expanding twisted flux rope with a highly turbulent reconnecting region underneath. The simplicity and efficacy of this scenario argue strongly for its fundamental importance in the initiation of solar eruptions.

[1] We present an in-depth study of the distribution and escape of molecular hydrogen (H 2 ) on Titan, based on the global average H 2 distribution at altitudes between 1000 and 6000 km, extracted from a large sample of Cassini/Ion and Neutral Mass Spectrometer (INMS) measurements. Below Titan's exobase, the observed H 2 distribution can be described by an isothermal diffusion model, with a most probable flux of (1.37 ± 0.01) Â 10 10 cm À2 s À1 , referred to the surface. This is a factor of $3 higher than the Jeans flux of 4.5 Â 10 9 cm À2 s À1 , corresponding to a temperature of 152.5 ± 1.7 K, derived from the background N 2 distribution. The H 2 distribution in Titan's exosphere is modeled with a collisionless approach, with a most probable exobase temperature of 151.2 ± 2.2 K. Kinetic model calculations in the 13-moment approximation indicate a modest temperature decrement of several kelvin for H 2 , as a consequence of the local energy balance between heating/cooling through thermal conduction, viscosity, neutral collision, and adiabatic outflow. The variation of the total energy flux defines an exobase level of $1600 km, where the perturbation of the Maxwellian velocity distribution function, driven primarily by the heat flow, becomes strong enough to raise the H 2 escape flux considerably higher than the Jeans value. Nonthermal processes may not be required to interpret the H 2 escape on Titan. In a more general context, we suggest that the widely used Jeans formula may significantly underestimate the actual thermal escape flux and that a gas kinetic model in the 13-moment approximation provides a better description of thermal escape in planetary atmospheres.Citation: Cui, J., R. V. Yelle, and K. Volk (2008), Distribution and escape of molecular hydrogen in Titan's thermosphere and exosphere,

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