Abstract. The desorption due to the energy release of free radicals in the ice mantles of a dust grain is investigated theoretically by calculating the ultraviolet radiation field inside the cloud, the free radical accumulation, the cosmic-ray heating of the grain and then the desorption in this situation starting from the cosmic-ray energy spectra. This model can reproduce the observations of the CO gas abundances and level of depletion in dark clouds such as L977 and IC 5146 with a combination of input parameters which are either constrained by independent observations or have been derived independently from laboratory experiments. We investigate other desorption mechanisms and conclude that they cannot explain the observations. The model also shows that the energy input by the cosmic-ray induced ultraviolet field is almost one order of magnitude larger than the direct energy input by cosmic-ray particles. This strengthens the conclusion that desorption due to the energy release by ultraviolet photon produced radicals dominates over direct cosmic-ray desorption.

Amino acids are the essential molecular components of living organisms on Earth, but the proposed mechanisms for their spontaneous generation have been unable to account for their presence in Earth's early history. The delivery of extraterrestrial organic compounds has been proposed as an alternative to generation on Earth, and some amino acids have been found in several meteorites. Here we report the detection of amino acids in the room-temperature residue of an interstellar ice analogue that was ultraviolet-irradiated in a high vacuum at 12 K. We identified 16 amino acids; the chiral ones showed enantiomeric separation. Some of the identified amino acids are also found in meteorites. Our results demonstrate that the spontaneous generation of amino acids in the interstellar medium is possible, supporting the suggestion that prebiotic molecules could have been delivered to the early Earth by cometary dust, meteorites or interplanetary dust particles.

The gas-to-dust mass ratios found for interstellar dust within the Solar System, versus values determined astronomically for the cloud around the Solar System, suggest that large and small interstellar grains have separate histories, and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, R g/d =94 +46−38 . This ratio is dominated by the larger near-micron sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere due to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the Solar System is half this value. Based on in situ data, interstellar dust grains in the of 10 −14 to 10 −13 g mass range are underabundant in the Solar System, compared to an MRN mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of ǫ CMa, give−207 for assumed solar reference abundances, and R g/d =551 +61 −251 for assumed B-star reference abundances. These values exceed the in situ value, suggesting either grain mixing or grain histories are not correctly understood, -4or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen towards λ Sco and 23 Ori, provided B-star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the Solar System exhibits enhanced gas-phase abundances of refractory elements such as Fe + and Mg + , indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe + and Mg + indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe + and H • indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scalelengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ∼150 km s −1 . If solar reference abundances are correct, the low R g/d value towards λ Sco may indicat...

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