Chemical modification of insulators by ion implantation, fundamental mechanisms and astrophysical implications

Dust in Dense Clouds

Recent observational and theoretical studies of dust in dense clouds are reviewed with an emphasis on the growth of dust grains through accretion and coagulation. IR reflection nebulae around protostellar objects are useful probes of grain sizes in dense clouds. For example, detailed studies of the IR reflection nebula surrounding OMC 2-IRS 1 show that the (scattering) grains are much larger (Ã 5000 å) than in the diffuse interstellar medium. Likewise, the presence of a weak shoulder at 2.95 μm on the 3.08 μm feature in BN indicates the importance of scattering by icy grains and implies a very similar increase in the grain size.

Theoretical studies of grain surface chemistry predict the possible presence of three distinctly different grain mantle components in dense clouds depending on the physical conditions in the gas phase. These are: 1) A hydrogenated mantle dominated by H2O and CH3OH; 2) An inert grain mantle dominated by CO and O2; and 3) An oxidized grain mantle dominated by CO2. Although the importance of H2O dominated grain mantles was known for 10 yrs, the presence of CH3OH was only recently confirmed. Furthermore, recent studies of the solid CO band have revealed the presence of at least two distinctly different interstellar grain mantle components along the line of sights towards most stars: One dominated by polar and one by non-polar molecules. Although specific identification of the molecules mixed in with the CO in these components is difficult, it is quite possible that the former component is dominated by H2O and the latter by CO itself, as suggested by theoretical models. Finally, the photochemical evolution of icy grain mantles is briefly reviewed and it is suggested that the resulting complex molecular mantles may evolve into amorphous carbon mantles in the diffuse ISM.

Grain-grain collisions can lead to large modifications of the interstellar grain size distribution. At high velocities (v ≳ 1 kms−1) shattering into many small fragments will be important, while at low velocities (v ≲ 10 ms−1) coagulation dominates. Both processes can play a role in dense molecular clouds. The sticking of grains at low velocities is discussed in some detail and it is concluded that coagulation in molecular clouds is only important if the colliding grains are covered by icy grain mantles.

Thus, a model for interstellar dust is proposed in which small (≲ 500 å) silicate and carbonaceous grains are “glued” together in large (Ã 3000å), open conglomerates by a polymerized, all enveloping grain mantle. This structure resembles that of certain interplanetary dust particles collected in the upper stratosphere.

From hot atom chemistry to epithermal chemistry

The rise and fall of hot atom chemistry (HAC) over the years from 1934 to 2004 is reviewed. Several applications are discussed, in particular to astrophysics and the interaction of energetic ions and atoms in space. Epithermal chemistry (ETC) is proposed to substitute the old name, since it better fits the energy range as well as the non-thermal and non-equilibrium character of the reactions. ETC also avoids the strong connexion of HAC to nuclear chemistry and stands for the opening of the field to physical chemistry and astrophysics.

A dose-survival model for low energy ion irradiation

PURPOSE

To explain the dose survival data for low energy ion irradiation in bioorganisms.

MATERIAL AND METHODS

Applying DNA target theory and considering physical processes of low energy ion radiation, a mathematical dose-survival model was derived to analyze the data of low energy ion irradiation of bioorganisms.

RESULTS

The survival fraction S of bioorganisms after a dose D of low energy ion irradiation was: The analysis showed that survival fraction of bioorganisms under low energy ion irradiation can be well fitted with this formula.

CONCLUSIONS

It was shown that the dose-survival effects of low energy ion irradiation in bioorganisms were related to the physical processes of low energy ion irradiation and repair effects that can be explained by momentum transfer processes occurring with low energy ion irradiation.

Primeval procreative comet pond

It is speculated that life originated in a small, shallow body of water containing concentrated prebiotic organic feedstocks, inorganic compounds, and catalytic agents in a diversity of microenvironments. This pond was formed by an improbable, fortuitous soft-landing of a cometary nucleus, or fragment thereof, on the surface of a suitable planet with an atmosphere in an appropriate thermodynamic state, such as Earth.

Coloration and darkening of methane clathrate and other ices by charged particle irradiation: applications to the outer solar system

Methane clathrate is expected to be an important carbon-containing ice in the outer solar system. We investigate the effect of electron irradiation by coronal discharge on several simple hydrocarbons enclathrated in or mixed with H2O or H2O+NH3 in simulation of the effects of the solar wind, planetary magnetospheric particles, and cosmic rays on surfaces containing these ices in the outer solar system and interstellar space. H2O+CH4 clathrate, H2O+C2H6, H2O+CH4+NH3, H2O+C2H6+NH3, and H2O+C2H2 are all initially white ices, and all produce yellowish to brownish organic products upon charged particles irradiation. Significant coloration occurs with doses of 10(9) erg cm-2, corresponding to short interplanetary irradiation times. Uranian magnetospheric electrons penetrate to approximately 1 mm depth and deposit this dose in 8, 30, 65, 200, and 500 years into the surfaces of Miranda, Ariel, Umbriel, Titania, and Oberon, respectively. Further irradiation of the laboratory ice surface results in a progressive darkening and a more subdued color. For a conversion efficiency to solids G approximately equal to 1 molecule keV-1, the upper limit for the time for total destruction of CH4 and other simple hydrocarbons in the upper 1 mm is 5 x 10(4) years (Miranda) to 3 x 10(6) years (Oberon). Remote detection of CH4 is possible only when its replenishment rate exceeds the destruction rate at the depth probed by spectroscopy. Reflection spectroscopy or irradiated H2O+CH4 frost is compared with the spectra of several outer solar system objects and to other relevant organic and inorganic materials. Ultraviolet-visible and infrared transmission spectroscopy of the postirradiation residues is presented. Persistence of color and of CH4 ice bands on Triton and Pluto suggests ongoing surface activity and/or atmospheric haze. Over 4 x 10(9) year time scales, > or = 10 m of satellite and cometary surface material is processed by cosmic rays to a radiation-hardened ice-tholin mixture devoid of CH4. Preaccretional chemistry, exogenous materials, and endogenous organic chemistry all contribute to the spectral properties of icy satellites which accreted simple CH(O) molecules. Radiation darkening traces the deposition of mobilized or impact-exposed carbon-bearing volatiles on these satellites. More exhaustive experiments are necessary to work out the detailed relationships between initial composition, exposure age, and color/albedo.

Radiation stability of organic matter in liquid and frozen H2O, NH3 and water-ammonia mixtures

The redox properties of irradiated liquid and frozen H2O, NH3 and H2O/NH3 mixtures at 298 K and 77 K, resp., towards some simple organic molecules have been checked by injecting carrierfree 11C atoms and analyzing their chemical state by means of radiochromatography. The reactions and the stability of organic products versus radiation dose (in this study by MeV protons) depend on temperature, phase state, mobility of radicals, their concentration and reactivity. Especially dangerous are the reactive OH and O2H radicals which oxidize organic material to inorganic CO2. Highest stability has been found at low temperatures (solid state, reduced mobility of radicals) and for systems containing H-donors (H2O/NH3 mixtures), which reduce the concentration of oxidizing radicals. The fact that many bodies in space consist of H2O-ice with NH3 and CH4 additives at temperatures between 10 and 150 K is promising in view of the survival of organic matter under high doses of radiation.