John Robert Brucato

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.

Astrophysical and astrochemical insights into the origin of life

Stellar nucleosynthesis of heavy elements such as carbon allowed the formation of organic molecules in space, which appear to be widespread in our Galaxy. The physical and chemical conditions—including density, temperature, ultraviolet (UV) radiation and energetic particles—determine reaction pathways and the complexity of organic molecules in different space environments. Dense interstellar clouds are the birth sites of stars of all masses and their planetary systems. During the protostellar collapse, interstellar organic molecules in gaseous and solid phases are integrated into protostellar disks from which planets and smaller solar

Temperature Dependence of the Absorption Coefficient of Cosmic Analog Grains in the Wavelength Range 20 Microns to 2 Millimeters

We have measured the absorption coefficient per unit mass of cosmic dust analog grains, crystalline fayalite and forsterite, amorphous fayalite, and two kinds of disordered carbon grains, between 20 μm and 2 mm over the temperature range 295-24 K. The results provide evidence of a significant dependence on temperature. The opacity systematically decreases with decreasing temperature; at 1 mm, it varies by a factor of between 1.9 and 5.8, depending on the material, from room temperature to 24 K. The variations are more marked for the amorphous grains. The wavelength dependence of the absorption coefficient is well fitted by a power law with exponent β that varies with temperature. For the two amorphous carbons, β(24 K) ~1.2 with increases of 24% and 50% with respect to the room-temperature values. A 50% increase is found for amorphous fayalite, characterized by β(24 K) = 2. A less pronounced change of β with temperature, 14% and 10%, is observed for crystalline forsterite, β(24 K) = 2.2, and fayalite, β(24 K) = 2.3, respectively. For amorphous fayalite grains, the millimeter opacity at 24 K is larger by a factor of ~4 than that of the crystalline counterpart. In addition, a temperature dependence of the infrared bands present in the spectrum of the two crystalline silicates is found. The features become more intense, sharpen, and shift to slightly higher frequencies with decreasing temperature. The results are discussed in terms of intrinsic far-infrared-millimeter absorption mechanisms. The linear dependence of the millimeter absorption on temperature suggests that two-phonon difference processes play a dominant role. The absorption coefficients reported in this work can be useful in obtaining a more realistic simulation of a variety of astronomical data concerning dust at low temperatures and give hints to better identify its actual properties. In particular, they are used to discuss the origin of the diffuse far-infrared-millimeter interstellar dust emission spectrum. It is proposed that composite particles formed of silicate and amorphous carbon grains can reproduce the observations. The presence of these particles in the diffuse medium is consistent with the recent interstellar extinction model by Mathis.

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.

C-H Bond Formation in Carbon Grains by Exposure to Atomic Hydrogen: The Evolution of the Carrier of the Interstellar 3.4 Micron Band

We present the results of a systematic study on the interaction of nano-sized carbon grains with atomic hydrogen. The effects of H processing have been analyzed by infrared spectroscopy. The samples were irradiated with fluences ranging between 9.2 × 1016 and 1.3 × 1020 H atoms cm-2. Hydrogen atoms lead to the activation of the aliphatic C–H stretching and bending modes, whose intensity increases with exposure until saturation of hydrogenation. The plateau value of the absorption coefficient per unit mass of material at 3.4 μm is 1.6 × 103 cm2 g-1. The estimated cross section of C–H bond formation by H atoms for carbon particles is σf = (1.9 ± 0.5) × 10-18 cm2 per H atom, as derived from the behavior of the 3.4 μm band intensity as a function of the H atom fluence. We have found that the C–H bond formation depends on the structure of the carbon material that is exposed to atomic hydrogen. In view of the basic role of the hydrogenation of carbon particles by H atoms in interpreting the presence of the 3.4 μm band in the diffuse interstellar medium, the behavior of carbon materials under H processing becomes a fundamental criterion for constraining their applicability as analogs of the interstellar aliphatic component. The 3.4 μm band and the doublet at 6.86 and 7.26 μm of carbon particles processed by H atoms reproduce the peak positions and the relative intensities of those observed in the spectrum of interstellar dust toward the Galactic center. The estimation of the formation cross section of C–H bonds by H atoms, together with the previously determined destruction cross section by UV photons, allows a complete description of the evolution of the interstellar aliphatic carbon component due to UV and H processing. The conclusion of our analysis is that the interstellar component (i.e., the C–H bonds in the CH2 and CH3 groups) responsible for the 3.4 μm stretching band and the associated bending features at 6.85 and 7.25 μm is formed in the diffuse medium, since the carrier readily loses memory of its birthsite, wherever it is, because of interstellar processing, which determines a new equilibrium value for its degree of hydrogenation.

Activation of the 3.4 Micron Band in Carbon Grains by Exposure to Atomic Hydrogen

We present the results of an experiment aimed at studying the interaction of atomic hydrogen with nano-sized carbon grains. The effects induced by H processing have been studied by infrared spectroscopy. C–H stretching and bending modes are activated after H atom exposure. In particular, the 3.4 μm feature fits quite well to the absorption band observed in the diffuse interstellar medium toward the Galactic center as well as in the proto-planetary nebula CRL 618. The estimated efficiency of the process is such that one C–H bond is formed per 16 hydrogen atoms impinging on the sample. The results we obtained have important implications for the formation and evolution of carbon materials responsible for the 3.4 μm absorption band both in the diffuse interstellar medium and in the circumstellar outflows during the transition from the asymptotic giant branch to planetary nebula phases.

Asteroids 2867 Steins and 21 Lutetia: surface composition from far infrared observations with the Spitzer space telescope

Aims. The aim of this paper is to investigate the surface composition of the two asteroids 21 Lutetia and 2867 Steins, targets of the Rosetta space mission. Methods. We observed the two asteroids through their full rotational periods with the Infrared Spectrograph of the Spitzer Space Telescope to investigate the surface properties. The analysis of their thermal emission spectra was carried out to detect emissivity features that diagnose the surface composition. Results. For both asteroids, the Christiansen peak, the Reststrahlen, and the Transparency features were detected. The thermal emissivity shows a clear analogy to carbonaceous chondrite meteorites, in particular to the CO-CV types for 21 Lutetia, while for 2867 Steins, already suggested as belonging to the E-type asteroids, the similarity to the enstatite achondrite meteorite is confirmed.

Forsterite amorphisation by ion irradiation: Monitoring by infrared spectroscopy

We present experimental results on the crystal-amorphous transition of forsterite (Mg2SiO4) silicate under ion ir- radiation. The aim of this work is to study the structural evolution of one of the most abundant crystalline silicates observed in space driven by ion irradiation. To this aim, forsterite films have been synthesised in the laboratory and irradiated with low energy (30-60 keV) ion beams. Structural changes during irradiation with H + ,H e + ,C + ,a nd Ar ++ have been observed and monitored by infrared spectroscopy. The fraction of crystalline forsterite converted into the amorphous form is a function of the energy deposited by nuclear collision by ions in the target. Laboratory results indicate that ion irradiation is a mechanism potentially active in space for the amorphisation of silicates. Physical properties obtained in this work can be used to model the evolution of silicate grains during their life cycle from evolved stars, through different interstellar environments and up to being incorporated in Solar System objects.

Molecular Hydrogen Formation on Amorphous Silicates under Interstellar Conditions

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 H2 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.

Synthesis and degradation of nucleic acid components by formamide and cosmic dust analogues

We show the unprecedented one‐pot synthesis of a large suite of pyrimidines (including cytosine and uracil) and purines from formamide in the presence of cosmic‐dust analogues (CDAs) of olivines. Since the major problem in the origin of informational macromolecules is the instability of their precursors, we also investigate the stabilizing effect of CDAs on the intrinsic instability of oligonucleotides in formamide.