Ozone formation in icy mantles

In this thesis the results of a detailed experimental study of the chemistry induced by irradiation of ice maniies by electrons and ions are presented. This research is directed towards gaining a better understanding of the mechanisms by which molecules are Synthesised in the Interstellar Medium (ISM) and lunar surfaces. Several experiments explored the formation of ozone in such ice mantles, since the ozone in any planetary or lunar atmosphere has been suggested as biomarker in die search for life. However recent space missions have revealed ozone on several Saturnian moons and Jupiter's Ganymede. In this thesis we show that ozone is readily produced abiotically by irradiation of oxygen, carbon dioxide, nitrous oxide and nitrogen dioxide ices. These results have important implications for the use of ozone as a biomarker in the search for life on extraterrestrial planets

Communication: Vacuum ultraviolet photoabsorption of interstellar icy thiols

Following the recent identification of ethanethiol in the interstellar medium (ISM) we have carried out Vacuum UltraViolet (VUV) spectroscopy studies of ethanethiol (CH3CH2SH) from 10 K until sublimation in an ultrahigh vacuum chamber simulating astrochemical conditions. These results are compared with those of methanethiol (CH3SH), the lower order thiol also reported to be present in the ISM. VUV spectra recorded at higher temperature reveal conformational changes in the ice and phase transitions whilst evidence for dimer production is also presented

Possible detection of hydrazine on Saturn’s moon Rhea

We present the first analysis of far-ultraviolet reflectance spectra of regions on Rhea’s leading and trailing hemispheres collected by the Cassini Ultraviolet Imaging Spectrograph during targeted flybys. In particular, we aim to explain the unidentified broad absorption feature centred near 184 nm. We have used laboratory measurements of the UV spectroscopy of a set of candidate molecules and found a good fit to Rhea’s spectra with both hydrazine monohydrate and several chlorine-containing molecules. Given the radiation-dominated chemistry on the surface of icy satellites embedded within their planets’ magnetospheres, hydrazine monohydrate is argued to be the most plausible candidate for explaining the absorption feature at 184 nm. Hydrazine was also used as a propellant in Cassini’s thrusters, but the thrusters were not used during icy satellite flybys and thus the signal is believed to not arise from spacecraft fuel. We discuss how hydrazine monohydrate may be chemically produced on icy surfaces

New Signatures of Bio-Molecular Complexity in the Hypervelocity Impact Ejecta of Icy Moon Analogues

Impact delivery of prebiotic compounds to the early Earth from an impacting comet is considered to be one of the possible ways by which prebiotic molecules arrived on the Earth. Given the ubiquity of impact features observed on all planetary bodies, bolide impacts may be a common source of organics on other planetary bodies both in our own and other solar systems. Biomolecules such as amino acids have been detected on comets and are known to be synthesized due to impact-induced shock processing. Here we report the results of a set of hypervelocity impact experiments where we shocked icy mixtures of amino acids mimicking the icy surface of planetary bodies with high-speed projectiles using a two-stage light gas gun and analyzed the ejecta material after impact. Electron microscopic observations of the ejecta have shown the presence of macroscale structures with long polypeptide chains revealed from LCMS analysis. These results suggest a pathway in which impact on cometary ices containing building blocks of life can lead to the synthesis of material architectures that could have played a role in the emergence of life on the Earth and which may be applied to other planetary bodies as well. View Full-Tex

A systematic mid-infrared spectroscopic study of thermally processed H2S ices

The positive identification of the molecular components of interstellar icy grain mantles is critically reliant upon the availability of laboratory-generated mid-infrared absorption spectra which can be compared against data acquired by ground- and space-borne telescopes. However, one molecule which remains thus far undetected in interstellar ices is H S, despite its important roles in astrochemical and geophysical processes. Such a lack of a detection is surprising, particularly in light of its relative abundance in cometary ices which are believed to be the most pristine remnants of pre-solar interstellar ices available for study. In this paper, we present the results of an extensive and quantitative mid-infrared spectroscopic characterisation of H S ices deposited at 20, 40, and 70 K and thermally processed to sublimation in an ultrahigh-vacuum system. We anticipate our results to be useful in confirming the detection of interstellar H S ice using high-resolution and high-sensitivity instruments such as the James Webb Space Telescope, as well as in the identification of solid H S in icy environments in the outer Solar System, such as comets and moons

A systematic mid-infrared spectroscopic study of thermally processed SO2 ices

The use of mid-infrared spectroscopy to characterise the chemistry of icy interstellar and Solar System environments will be exploited in the near future to better understand the chemical processes and molecular inventories in various astronomical environments. This is, in part, due to observational work made possible by the recently launched James Webb Space Telescope as well as forthcoming missions to the outer Solar System that will observe in the mid-infrared spectroscopic region (e.g., the Jupiter Icy Moons Explorer and the Europa Clipper missions). However, such spectroscopic characterisations are crucially reliant upon the generation of laboratory data for comparative purposes. In this paper, we present an extensive mid-infrared characterisation of SO2 ice condensed at several cryogenic temperatures between 20 and 100 K and thermally annealed to sublimation in an ultrahigh-vacuum system. Our results are anticipated to be useful in confirming the detection (and possibly thermal history) of SO2 on various Solar System bodies, such as Ceres and the icy Galilean moons of Jupiter, as well as in interstellar icy grain mantles

A systematic IR and VUV spectroscopic investigation of ion, electron, and thermally processed ethanolamine ice

The recent detection of ethanolamine (EtA, HOCH2CH2NH2), a key component of phospholipids, i.e. the building blocks of cell membranes, in the interstellar medium is in line with an exogenous origin of life-relevant molecules. However, the stability and survivability of EtA molecules under inter/circumstellar and Solar System conditions have yet to be demonstrated. Starting from the assumption that EtA mainly forms on interstellar ice grains, we have systematically exposed EtA, pure and mixed with amorphous water (H2O) ice, to electron, ion, and thermal processing, representing ‘energetic’ mechanisms that are known to induce physicochemical changes within the ice material under controlled laboratory conditions. Using infrared (IR) spectroscopy we have found that heating of pure EtA ice causes a phase change from amorphous to crystalline at 180 K, and further temperature increase of the ice results in sublimation-induced losses until full desorption occurs at about 225 K. IR and vacuum ultraviolet (VUV) spectra of EtA-containing ices deposited and irradiated at 20 K with 1 keV electrons as well as IR spectra of H2O:EtA mixed ice obtained after 1 MeV He+ ion irradiation have been collected at different doses. The main radiolysis products, including H2O, CO, CO2, NH3, and CH3OH, have been identified and their formation pathways are discussed. The measured column density of EtA is demonstrated to undergo exponential decay upon electron and ion bombardment. The half-life doses for electron and He+ ion irradiation of pure EtA and H2O:EtA mixed ice are derived to range between 10.8 − 26.3 eV/16u. Extrapolating these results to space conditions, we conclude that EtA mixed in H2O ice is more stable than in pure form and it should survive throughout the star and planet formation process

Temperature-dependent Formation of Ozone in Solid Oxygen by 5 keV Electron Irradiation and Implications for Solar System Ices

We have investigated the formation of ozone by electron impact in solid molecular oxygen ices between 11 and 30 K. The amount of ozone formed is shown to be strongly dependent on the sample temperature. As the ice temperature increases, the column density of the ozone monomer is found to diminish. This is ascribed to the loss of oxygen atoms by recombination with a neighboring oxygen atom to “recycle” molecular oxygen. In the “warm-up” phase after irradiation, two additional temperature-dependent reaction mechanisms were observed to synthesize ozone: (1) a reaction of oxygen atoms from a [O3…O] complex with a neighboring oxygen molecule and (2) a reaction of trapped oxygen atoms with oxygen molecules to yield the ozone monomer. These experiments have important implications to the oxygen chemistry in icy satellites throughout our solar system

Astrochemical model to study the abundances of branched carbon-chain molecules in a hot molecular core with realistic binding energies

Straight-chain (normal-propyl cyanide, n - C3H7CN) and branched-chain (iso-propyl cyanide, i - C3H7CN) alkyl cyanides are recently identified in the massive star-forming regions (Sgr B2(N) and Orion). These branched-chain molecules indicate that the key amino acids (side-chain structures) may also be present in a similar region. The process by which this branching could propagate towards the higher order (butyl cyanide, C4H9CN) is an active field of research. Since the grain catalysis process could have formed a major portion of these species, considering a realistic set of binding energies are indeed essential. We employ quantum chemical calculations to estimate the binding energy of these species considering water as a substrate because water is the principal constituent of this interstellar ice. We find significantly lower binding energy values for these species than were previously used. It is noticed that the use of realistic binding energy values can significantly change the abundance of these species. The branching is more favourable for the higher order alkyl cyanides with the new binding energies. With the inclusion of our new binding energy values and one essential destruction reaction (i - C3H7CN + H -> CH3C(CH3)CN + H-2 , having an activation barrier of 947 K), abundances of t - C4H9CN dramatically increased