M. E. Palumbo

Organics captured from comet 81P/Wild 2 by the Stardust spacecraft

Organics found in comet 81P/Wild 2 samples show a heterogeneous and unequilibrated distribution in abundance and composition. Some organics are similar, but not identical, to those in interplanetary dust particles and carbonaceous meteorites. A class of aromatic-poor organic material is also present. The organics are rich in oxygen and nitrogen compared with meteoritic organics. Aromatic compounds are present, but the samples tend to be relatively poorer in aromatics than are meteorites and interplanetary dust particles. The presence of deuterium and nitrogen-15 excesses suggest that some organics have an interstellar/protostellar heritage. Although the variable extent of modification of these materials by impact capture is not yet fully constrained, a diverse suite of organic compounds is present and identifiable within the returned samples.

Infrared Spectroscopy of Comet 81P/Wild 2 Samples Returned by Stardust

Infrared spectra of material captured from comet 81P/Wild 2 by the Stardust spacecraft reveal indigenous aliphatic hydrocarbons similar to those in interplanetary dust particles thought to be derived from comets, but with longer chain lengths than those observed in the diffuse interstellar medium. Similarly, the Stardust samples contain abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene. The presence of crystalline silicates in Wild 2 is consistent with mixing of solar system and interstellar matter. No hydrous silicates or carbonate minerals were detected, which suggests a lack of aqueous processing of Wild 2 dust.

Formation of compact solid water after ion irradiation at 15 K

We used infrared absorption spectroscopy to study the effects of ion irradiation on the morphology/porosity of amorphous water ice. Thin icy films (about 0.25 μm) of amorphous water were irradiated with 200 keV protons at 15 K. Both the behaviour of the OH dangling bond feature and the ability to trap carbon monoxide (CO) were used to investigate the evolution of icy samples after ion irradiation. We show that the intensity of the OH dangling bond feature decreases after ion irradiation and that the amount of absorbed carbon monoxide decreases as the fluence of impinging ions increases. The results obtained indicate that the porosity of amorphous water ice decreases after ion irradiation. Furthermore, icy mixtures such as H 2 O:CO 2 , H 2 O:CO, and H 2 O:CH 4 were irradiated with 200 keV H + , 30 and 200 keV He + ions. Also in these cases, the intensity of the OH dangling bond band decreases after ion irradiation. However, when a second molecular species is present in the ice sample, this decrease is slower. Here we present the experimental results and discuss their relevance to our understanding of the properties of interstellar water ice. In particular, we suggest that, because of cosmic ion bombardment, water ice in interstellar grain mantles is compact in structure.

CO2 synthesis in solid CO by Lyman-α photons and 200 keV protons

We have studied the synthesis of carbon dioxide from solid carbon monoxide at 16 K induced by photolysis with Lyman-α photons and by irradiation with 200 keV protons to quantitatively compare the effects of photolysis and ion irradiation on CO ice and to determine the importance of these processes in interstellar ice grains. The CO and CO2 concentrations during irradiation of an initially pure CO film evolve with fluence to a saturation value, a behaviour that is explained by a two-state model. Our results indicate that the initial CO2 production rates for both radiation processes are similar when normalized to the absorbed energy and that the solid CO2 abundance observed in the interstellar ices cannot be explained only by radiolysis and photolysis of pure solid CO.

A comparison of ion irradiation and UV photolysis of CH4 and CH3OH

We have studied by infrared absorption spectroscopy the effects induced by fast ions (30 keV) and Lyman-α photons (10.2 eV) on some molecular ices at low temperature (10-20 K). It is well known that in both cases the physical and chemical properties of the ices are modified. However while the energy released by ions depends mainly on their energy and on the target species, the effects induced by photons also depend on the optical properties of the sample. Here we show that the effects of ion irradiation and UV photolysis are comparable on fresh ices (i.e. at low doses) but are increasingly different as processing is continued (i.e. at high doses).

Formation of CO and CO2 Molecules by Ion Irradiation of Water Ice-covered Hydrogenated Carbon Grains

We present the results of experiments aimed at studying the influence of the type of grain on the chemical composition of the ice mantles during energetic processing under simulated dense medium conditions. Formation of CO and CO2 molecules occurs when hydrogenated carbon grains with a water ice cap are irradiated with 30 keV He+ ions at low temperature. The fraction of carbon in the grains converted to CO and CO2 by ions is at least 0.03 and 0.02, respectively. An estimation of the formation cross section of these molecules by 30 keV He+ ions has been derived from the intensity increase of their infrared stretching bands as a function of the ion fluence. On the basis of the laboratory results, it has been possible to evaluate the contribution of CO and CO2 produced on carbon grain by cosmic rays to the observed column densities of these molecules for dense clouds whose visual extinction is known. The mechanism we have studied does not dominate other CO2 formation processes; however, its contribution is in addition to other processes occurring on ice mantles. The spectral profile and the contribution to the observed column densities make solid CO formed by cosmic-ray irradiation of ice-layered carbon grains a good candidate for the red component of the interstellar CO stretching feature, which is generally attributed to CO mixed in with water ice. As a consequence of the formation of CO and CO2 molecules on carbon grains, a slow chemical erosion of the particles takes place.

Evolution of icy surfaces : an experimental approach

Abstract We present new laboratory results on some effects induced by ion irradiation (30 keV He+ and 60 keV Ar++) of frozen NH3, CH4, H2O, and their mixtures. These species have been chosen in view of their possible presence on the surface of Saturnian satellites and rings. In fact these surfaces are exposed to intense irradiation by magnetospheric and⧹or solar energetic particles. We find that the abundance ratio ammonia⧹water in irradiated mixtures H2O : NH3 (sime;2 : 1) decreases as irradiation dose increases. The decrease is greater when the temperature of the bombarded target is higher. New species have been synthesized after irradiation of mixtures H2O : NH3 : CH4 (sime;2.5 : 1 : 2) and some firmly identified : C2H6, CO, and CO2. Refractories containing OCN groups and, possibly, amino acids, are also observed. The results are discussed in view of their potential interest in the Saturnian environment. In particular they confirm that ion irradiation produces a decrease of the ammonia⧹ water molecular number ratio. However IR signatures of ammonia could be found on the surfaces of the bright Saturnian moons (Mimas, Enceladus, Tethys, Dione, and Rhea), especially those in the spectral region of the fundamental bands, covered by the instruments on board Cassini. We also suggested that the presence of ammonia could be indirectly sustained from the observation of XCN compounds on the surfaces of the dark satellites (Hyperion, Iapetus, Phoebe). Prebiotic chemistry could also be activated by particle irradiation on Saturnian moons.

Formation of methyl formate after cosmic ion irradiation of icy grain mantles

Context. Methyl formate (HCOOCH3) is a complex organic molecule detected in hot cores and hot corinos. Gas-phase chemistry fails to reproduce its observed abundance, which usually varies between 10 −7 and 10 −9 with respect to H2. Aims. Laboratory experiments were performed in order to investigate a solid-state route of methyl formate formation, to obtain an estimate of the amount that can be formed, and to verify whether it can account for the observed abundances. Methods. Several solid samples (16 K) of astrophysical interest were analyzed by infrared spectroscopy in the 4400−400 cm −1 range. The infrared spectral characteristics of frozen methyl formate were studied by deriving their band strength values. The effects produced upon warm-up of the samples were analyzed comparing the spectra taken at different temperatures. In order to study the formation and destruction mechanism of methyl formate in the interstellar ices, a binary mixture of methanol (CH3OH) and carbon monoxide (CO) ice and a sample of pure methanol were irradiated at 16 K with 200 keV protons. Methyl formate was identified through its fundamental mode (CH3 rocking) at about 1160 cm −1 . Results. We present the mid-infrared methyl formate ice spectrum showing both the amorphous (16 K) and the crystalline (110 K) structure. We report novel measurements of the band strength values of the six main methyl formate bands. We prove the formation and the destruction of methyl formate after irradiation of CH3OH and a CO:CH3OH mixture. Extrapolating our results to the interstellar medium conditions we found that the production timescale of methyl formate agrees well with the evolutionary time of molecular clouds. The comparison with the observational data indicates that the amount of methyl formate formed after irradiation can account for the observed abundances. Conclusions. The present results allow us to suggest that gas phase methyl formate observed in dense molecular clouds is formed in the solid state after cosmic ion irradiation of icy grain mantles containing CO and CH3OH and released to the gas phase after desorption of icy mantles.

A Raman study of ion irradiated icy mixtures

In this paper we present a Raman study of pure CH4 ,H 2O:CH4:N2 and CH3OH:N2 frozen films before and after ion irradiation at 12 K, 100 K and 300 K. By means of Raman spectroscopy, we monitor the structural evolution of each film, whose chemical and physical properties are deeply modified by the interaction with the ion beam. For the two methane containing samples, Raman spectra show that the initial ice is partially converted into a refractory residue, which under further irradiation evolves towards an amorphous carbon (AC) with a band near 1560 cm 1 (G line) and a shoulder at about 1360 cm 1 (D line). No evidence of the AC Raman band is seen in the spectra of the methanol-containing mixture. By means of Lorentzian fits, we have determined the specific parameters of the AC band (G and D line peak positions, widths and relative intensities) in our spectra after ion irradiation and we have compared them with the corresponding parameters of the band as observed in the spectra of 11 IDPs (Interplanetary Dust Particles). Here we present the experimental results and discuss their contribution to our knowledge of the origin and evolution of IDPs.

Novel measurements of refractive index, density and mid-infrared integrated band strengths for solid O2, N2O and NO2: N2O4 mixtures

We present novel measurements of the refractive index, density and integrated band strengths of mid-infrared features of solid N(2)O at 16K and of NO(2) and N(2)O(4) in two frozen NO(2):N(2)O(4) mixtures deposited at 16 and 60K. The refractive index and density measurements were performed also for frozen O(2) deposited at 16K. In this case, the integrated band strength values could not be determined since O(2) is a homonuclear molecule and therefore its fundamental mode is not infrared active. The solid samples were analysed by infrared spectroscopy in the 8000/800cm(-1) range. The sample thickness was measured by the interference curve obtained using a He-Ne laser operating at 543nm. The refractive index at this laser wavelength was obtained, by numerical methods, from the measured amplitude of the interference curve. The density values were obtained using the Lorentz-Lorenz relation. Integrated band strength values were then obtained by a linear fit of the integrated band intensities plotted versus column density values. The astrophysical relevance of these novel measurements is briefly discussed.