Irradiated H2O Research Articles

Comparing the radiolytic oxidation of sulfur and chloride within ice on Europa and Mars

The surface of Europa is heavily irradiated by high-energy particles with the energy ranges of 1–100 MeV for primary electrons and ∼10 keV for secondary electrons. These irradiations may cause oxidation of surface materials, such as chlorides and sulfur-bearing species, through reactions with oxidative radicals formed by dissociation of H2O ice. Similar irradiation-induced oxidation could have occurred in near-surface ice on Mars. However, most of previous experiments simulated ∼10 keV electron irradiation onto ice materials on Europa and Mars, and few studies have investigated the oxidation of chlorides and sulfur by MeV electron irradiation. Here we performed laboratory experiments on MeV electron irradiation, as well as 10 keV electron and UV irradiations, onto mixtures of H2O ice and chlorides, or elemental sulfur, at low temperatures. Our experimental results indicate selective oxidation of sulfur against chlorides, with negligible formation of oxychlorides on irradiation of H2O ice and chloride mixtures, regardless of experimental conditions (temperature, electron energy, or presence of oxidants in ice). Sulfate is effectively formed by the electron irradiation. This selective oxidation may be caused by the different stabilities of atoms within irradiated H2O ice. If sulfate on Europa's surface is transported into the subsurface ocean, this could provide energy sources for possible chemoautotrophic life in the ocean. On Europa, sulfur would be one of key elements in redox chemistry on the surface and possibly in the subsurface ocean.

Radiolysis and Photolysis of Icy Satellite Surfaces: Experiments and Theory

The transport and exchange of material between bodies in the outer solar system is often facilitated by their exposure to ionizing radiation. With this in mind we review the effects of energetic ions, electrons and UV photons on materials present in the outer solar system. We consider radiolysis, photolysis, and sputtering of low temperature solids. Radiolysis and photolysis are the chemistry that follows the bond breaking and ionization produced by incident radiation, producing, e.g., O2 and H2 from irradiated H2O ice. Sputtering is the ejection of molecules by incident radiation. Both processes are particularly effective on ices in the outer solar system. Materials reviewed include H2O ice, sulfur-containing compounds (such as SO2 and S8), carbon-containing compounds (such as CH4), nitrogen-containing compounds (such as NH3 and N2), and mixtures of those compounds. We also review the effects of ionizing radiation on a mixture of N2 and CH4 gases, as appropriate to Titan’s upper atmosphere, where radiolysis and photolysis produce complex organic compounds (tholins).

Radiolysis of ammonia-containing ices by energetic, heavy, and highly charged ions inside dense astrophysical environments

Deeply inside dense molecular clouds and protostellar disks, the interstellar ices are protected from stellar energetic UV photons. However, X-rays and energetic cosmic rays can penetrate inside these regions triggering chemical reactions, molecular dissociation and evaporation processes. We present experimental studies on the interaction of heavy, highly charged and energetic ions (46 MeV Ni^13+) with ammonia-containing ices in an attempt to simulate the physical chemistry induced by heavy ion cosmic rays inside dense astrophysical environments. The measurements were performed inside a high vacuum chamber coupled to the heavy ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds) in Caen, France.\textit{In-situ} analysis is performed by a Fourier transform infrared spectrometer (FTIR) at different fluences. The averaged values for the dissociation cross section of water, ammonia and carbon monoxide due to heavy cosmic ray ion analogs are ~2x10^{-13}, 1.4x10^{-13} and 1.9x10^{-13} cm$^2$, respectively. In the presence of a typical heavy cosmic ray field, the estimated half life for the studied species is 2-3x10^6 years. The ice compaction (micropore collapse) due to heavy cosmic rays seems to be at least 3 orders of magnitude higher than the one promoted by (0.8 MeV) protons . In the case of the irradiated H2O:NH3:CO ice, the infrared spectrum at room temperature reveals five bands that were tentatively assigned to vibration modes of the zwitterionic glycine (+NH3CH2COO-).

Formation of Interstellar OCS: Radiation Chemistry and IR Spectra of Precursor Ices

Extensive experimental studies have been performed on the solid-state formation of the OCS molecule in proton-irradiated water-free and water-dominated ices containing CO or CO2 as the carbon source and H2S or SO2 as the sulfur source. In each case OCS is readily formed. Production efficiency follows the trends CO > CO2 and H2S > SO2 as C,O- and S-sources, respectively. In water-dominated ices, OCS production appears to be enhanced for CO : H2S reactants. The mechanism of formation of OCS appears to be the reaction of CO with free S atoms produced by fragmentation of the sulfur parent species. While OCS is readily formed by irradiation, it is also the most easily destroyed on continued exposure. In H2O-dominated ices the half-life of H2S, SO2, and OCS is ~2 eV molecule−1, corresponding to ~7 million years in a cold dense interstellar cloud environment processed by cosmic-ray protons. The spectral profile of the ν3 band of OCS is highly dependent on temperature and ice composition, and changes with radiation processing. These effects can be used in theoretical modeling of interstellar infrared (IR) spectra; a laboratory spectrum of irradiated H2O : CO : H2S, warmed to 50 K, provides a good fit to the 2040 cm−1 feature in the W33A spectrum. The identification of OCS in CO2-dominated ices provides a further challenge, due to the overlap of the OCS band with that of CO3 formed from irradiation of the host ice. The two features can be unraveled by a curve-fitting procedure. It is the width of the 2040 cm−1 band that will help observers determine if features identified in CO2-rich ices are due to OCS or to CO3.

Surface Modification and Direct Bonding of Different Materials Irradiated H2O Ion

Reliable joining technologies are essential for fabrication of microstructures such as micro-machines, and particularly technologies that are capable of jointing different types of materials. However, these technologies cannot be used for materials that have different rates of thermal expansion or in cases where the adhesives have an effect on the properties of the component materials. Our research focused on developing a direct jointing technology which employs hydrogen bonding. In this technology, OH radicals are absorbed into the surface of the material to be bonded by modifying its surface properties by ion irradiation. We studied the modification of the surface properties of two resins, (SU-8 and PMMA) by H2O ion irradiation and Ar cleaning. It was confirmed that the presence of H2O ions on the surface of these resins improved their hydrophilic properties and also the peel strengths of Cu membrane s deposited onto both resins. Based on the results of these studies, a series of experiments were conducted in which two different materials, (copper plus one or other of the resins) were joined directly and the results were evaluated. Each of the resins could be joined to copper by heating to a temperature of 100 °C and pressurizing to 10 MPa. This jointing technology will now be applied to the fabrication of the tilt sensors that we are currently developing.

Infrared Detection of HO2 and HO3 Radicals in Water Ice

Infrared spectroscopy has been used to detect HO(2) and HO(3) radicals in H(2)O + O(2) ice mixtures irradiated with 0.8 MeV protons. In these experiments, HO(2) was formed by the addition of an H atom to O(2) and HO(3) was formed by a similar addition of H to O(3). The band positions observed for HO(2) and HO(3) in H(2)O-ice are 1142 and 1259 cm(-1), respectively, and these assignments were confirmed with (18)O(2). HO(2) and HO(3) were also observed in irradiated H(2)O + O(3) ice mixtures, as well as in irradiated H(2)O(2) ice. The astronomical relevance of these laboratory measurements is discussed.

Production of Complex Molecules in Astrophysical Ices

The inventory of interstellar and solar system molecules now numbers well over 100 species, including ions and molecules both charged and neutral. Gas-phase formation pathways for many of the observed organics are still uncertain, so that solid-phase syntheses are of interest. Low-temperature reactions are thought to occur within interstellar ices, on ice and grain surfaces in the interstellar medium, and on icy surfaces of solar system objects (e.g. Europa, Pluto). Ionizing radiation, such as cosmic rays, and far-UV photons are two possible initiators of such chemistry. In the Cosmic Ice Lab at NASA's Goddard Space Flight Center, we can study both the photo- and radiation chemistries of ices at 8 - 300 K. Our most-recent work has been motivated by the detections of ethylene glycol, HOCH2CH2OH, in an interstellar source (Hollis, Lovas, Jewell, et al. 2002) and in comet Hale-Bopp (Crovisier, Bockel ee-Morvan, Biver, et al. 2002). Ethylene glycol is currently the largest rmly-iden tied cometary molecule as well as one of the larger interstellar organics. This molecule's formation and accompanying chemistry provide challenges and tests for current astrochemical thought. Here we discuss laboratory experiments on ethylene glycol's solid-phase formation and de- struction. Using infrared spectroscopy, we have identied low-temperature radiation-chemical pathways that lead from known interstellar ices, such as either CH3OH or H2O and CO, to ethylene glycol. We also have identied a role for ethylene glycol in the formation of glycolalde- hyde, HOCH2C(O)H, a simple sugar (Hollis, Lovas & Jewell 2000). Related to these results are the 6- and 7-atom molecules formed by oxidation and reduction processes in irradiated H2O + C2H2 ices. Analogous experiments have been conducted on two other triply-bonded molecules in H2O-rich ice, HCN and N2. An important focus of our work is the development of reaction schemes for the formation of complex molecules within astrochemically-relevant ices, and the use of such schemes to predict new molecules awaiting detection.

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.

15O and 11C production in neutron radiotherapy patients

In order to establish the feasibility of performing blood flow measurements following therapeutic neutron irradiation by determining increased 15O disappearance rate from a volume of tissue, knowledge of the probabilities of 15O, 11C and 13N production in tissue by a p(42)Be neutron-beam irradiation is required. Isotope production probability per unit dose (defined as the isotype-yield coefficient) is determined from the measured production of 15O, 13N and 11C in irradiated H2O. The results obtained agree with those calculated using published neutron cross sections and neutron energy spectra, indicating that (n, 2n) reactions are the predominant activation mechanisms.