The diffusion of hydrogen atoms on dust grains is a key process in the formation of interstellar H2 and some hydrogenated molecules such as formaldehyde and methanol. We investigate the adsorption ...and diffusion of H atoms on pure solid CO as an analog of dust surfaces observed toward some cold interstellar regions. Using a combination of photostimulated desorption and resonance-enhanced multiphoton ionization methods to detect H atoms directly, the relative adsorption probabilities and diffusion coefficients of the H atoms are measured on pure solid CO at 8, 12, and 15 K. There is little difference between the diffusion coefficients of the hydrogen and deuterium atoms, indicating that the diffusion is limited by thermal hopping. The activation energies controlling the H-atom diffusion depend on the surface temperature, and values of 22, 30, and ∼37 meV were obtained for 8, 12, and 15 K, respectively.
Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark ...interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions. Aims. We present here the first experimental evidence of water synthesis by such a process on a realistic analogue of grain surface in dense clouds, i.e., amorphous water ice. Methods. Atomic beams of oxygen and deuterium are aimed at a porous water ice substrate (H2O) held at 10 K. Products are analyzed by the temperature-programmed desorption technique. Results. We observe the production of HDO and D2O, indicating that water is formed under conditions of the dense interstellar medium from hydrogen and oxygen atoms. This experiment opens up the field of a little explored complex chemistry that could occur on dust grains,which is believed to be the site where key processes lead to the molecular diversity and complexity observed in the Universe.
On the basis of previous ground-based and fly-by information, we knew that Titan's atmosphere was mainly nitrogen, with some methane, but its temperature and pressure profiles were poorly constrained ...because of uncertainties in the detailed composition. The extent of atmospheric electricity ('lightning') was also hitherto unknown. Here we report the temperature and density profiles, as determined by the Huygens Atmospheric Structure Instrument (HASI), from an altitude of 1,400 km down to the surface. In the upper part of the atmosphere, the temperature and density were both higher than expected. There is a lower ionospheric layer between 140 km and 40 km, with electrical conductivity peaking near 60 km. We may also have seen the signature of lightning. At the surface, the temperature was 93.65 +/- 0.25 K, and the pressure was 1,467 +/- 1 hPa.
We report on an experimental study on the formation of water, through the D + O2 pathway, on a sample of amorphous solid water, i.e. a realistic analogue of the surface of the interstellar grains of ...dense clouds. For improving our experimental conditions we use deuterium instead of hydrogen. We obtain, using both Temperature-Programmed Desorption Spectroscopy and Infrared Spectroscopy, that the morphology of the nascent water ice is mostly compact. We compare our results with those obtained previously by Oba et al. and show that the first technique is more effective in probing the morphology of amorphous water ice. We propose that the formation of compact ice is due to the heat of reactions which lead to the formation of water molecules. Water is synthesized, through the D + O2 pathway, on a film of compact amorphous solid water and on a porous substrate for comparison purposes. We show that in this latter case a gradual compaction of the sample takes place upon formation of new water molecules. We compare this reduction of water ice porosity with that one obtained by Accolla et al., due to recombination of deuterium atoms sent on the surface of an amorphous porous ice sample, and show that the compaction of the sample as a consequence of the water formation appears clearly more efficient.
Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the ...relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. Using these barriers, we evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H sub(2) formation within a temperature range that is relevant to diffuse interstellar clouds. The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.
PDF
