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Project Progress

Survivability on icy Worlds investigation examines the survivability of biological compounds under simulated icy world surface conditions, and compares the degradation products to abiotically synthesized compounds resulting from the radiation chemistry on icy worlds.

Summary of Objectives and Research
Investigation 2 focuses on survivability. As part of our Survivability investigation, we examine the similarities and differences between the abiotic chemistry of planetary ices irradiated with ultraviolet photons (UV), electrons, and ions, and the chemistry of biomolecules exposed to similar conditions. Can the chemical products resulting from these two scenarios be distinguished? Can viable microbes persist after exposure to such conditions? These are motivating questions for our investigation.

Project Progress

Co-Investigator Murthy Gudipati and his group continued studying the photodegradation of PAH molecules in ices using UV radiation and electrons. In particular, they found that electrons (up to 2 keV studied here) can damage organic molecules far deeper in ice that what models predict, as shown in Figure 2.1 (bottom), where the laboratory data is well reproduced with simple empirical models involving sigmoidal penetration depth profiles for electrons. Organic probe molecules (Pyrene in this case) are completely destroyed after electron irradiation, no significant UV absorption due to any new product could be detected as seen in the UV-VIS absorption spectrum in Figure 2.1 (top), indicating that the PAHs are not only ionized in the first step, but then also degraded through consecutive radiation induced chemistry.

Gudipati and team have also been working on obtaining MALDI mass spectroscopic data for radiation processed PAHs in ices. We built a QMS system to do selective mass filtering for further studies, as a part of a different project. Earlier studies showed that we couldn’t use the same system for understanding radiation-induced chemistry of PAHs using MALDI techniques. Hence we decided to procure a TOF-MS system this year. This system has been installed recently and has shown very high sensitivity of IR induced ice ablation and UV induced ionization of the plume molecules as shown in Figure 2.2.

Several publications are in preparation. This work has been presented at DPS, AGU, AOGS, and ACAS conferences.

Investigators Paul Johnson, Robert Hodyss and Isik Kanik have continued studying the photolysis of organic molecules in icy matrices relevant to solar system bodies. In this work our goal is to determine the precise chemical pathways that various molecules evolve in analog solar system ices as a function of temperature and wavelength. This involves photolysis of organics in ice, pure organics, matrix isolated organics, and organics under a layer of peroxide (to provided a source of hydroxyl radicals). Addition baseline spectra are also taken of suspected products to confirm identifications. They are measuring the photolytic lifetimes of hydrocarbons in both water and nitrogen matrices. They are also studying the photolytic chemistry of amino acid (glycine) and lipid (oleic acid and SLPC) degradation through matrix isolation experiments.

An extensive set of data have been collected on pholoysis of glycine, phenylalanine, oleic acid and SLPC. This work has involved overcoming the technical challenge of vapor deposition these large, non-volatile organics from a heated crucible without destruction of the molecule. These data are now being prepared for publication.

Photochemical studies of thin cryogenic ice films composed of N₂ and CH₄ in ratios analogous to those on the surfaces of Neptune’s largest satellite, Triton, and on Pluto have been completed. Experiments were performed using a hydrogen discharge lamp, which provides an intense source of ultraviolet light in order to elucidate the solar induced photochemistry of these icy bodies. Initial characterization via infrared spectroscopy showed that C₂H₆, C₂H₄ and C₂H₂ are formed by the dissociation of CH₄ into H, CH₂ and CH₃ and the subsequent reaction of these radicals within the ice (see figure 2.3). Acetylene, C₂H₂,, is further photodissociated into C₂. Other radical species, such as C₂ C₂-, CN, NCN and CNN are observed in the visible and UV regions of the spectrum. These species imply a rich chemistry based on reactions of atomic carbon with the N2 matrix. The appearance of these radical species is consistent with a chemistry based on the formation of atomic carbon and its subsequent reaction with the N₂ matrix. Carbon atoms are most likely produced photolytically from C₂. Ultimately, this work suggests that C₂-, CN, and CNN may be found in significant quantities on the surfaces of Triton and Pluto and that new observations of these objects in the appropriate wavelength regions are warranted.

In addition, a study of UV stimulated fluorescence of benzene, toluene and the isomers of xylene has been conducted. These results have implications for exploiting such phenomena for in situ detection of organics is solar system ices. This work is summarized in a manuscript that will be submitted to Astrobiology this fall.

In the upcoming year, the Johnson/Hodyss team will continue to explore the detailed, wavelength dependant photolysis of amino acids and lipids.

Co-I Paul Cooper of George Mason university has developed methodology to deposit hydroxy radicals into icy planetary surfaces and used these OH radicals to understand the formation of methanol in solar system ices.

The first project is a novel and innovative approach to understanding the composition of the both the surface and atmospheres of icy bodies in the outer solar system. Icy satellites are surrounded by tenuous atmospheres that contain chemical species such as H₂O, OH, O₂ and O. These species can deposit onto the surface, and with subsequent chemical reactions, form new chemical species. This is a radical departure from all previous work that has assumed that the composition of both the surface and atmosphere of an icy satellite is determined by the primordial composition in addition to radiolytic chemical processing. This process may be especially important at the Saturnian system due to the large neutral OH torus that exists due to cryovolcanic activity at Enceladus. Neutral OH can sweep through the system and deposit onto other icy satellites.

OH radicals were generated in a Tesla coil discharge of water vapor diluted in neon. The resulting ice is a result of depositing OH and H₂O. The formation of hydrogen peroxide in the ice is strongly temperature dependent as shown by the decrease in the 2850 cm^-1 band in Fig 2.4. We estimate that the OH/H₂O percentage by number in the gas-phase prior to deposition in our experiments is ~12-15%. Recent modeling by Tim Cassidy (JPL) has shown that this OH abundance is comparable to the tenuous atmospheres of the icy Saturnian satellites. At 30 K we calculate there is ~ 6 % H₂O₂ relative to H₂O, which is an order of magnitude greater than H₂O₂ abundances produced in irradiation experiments. At 60 K there is ~ 0.6%.

The lack of H₂O₂ at 80 K is likely a result of chemical destruction with depositing OH to form O₂. We observe some O₂ that is trapped in the ice that we have yet to quantify but there will also be some (perhaps the majority?) that is lost to the vacuum. In the case of a planetary surface this would be a source of O₂ in an atmosphere. This would be applicable to both icy satellites and icy ring systems throughout the solar system.
Further work will quantify the O₂ trapped in the surface and the O₂ lost to the vacuum as the ice grows. This data will then be used to model surface OH reactions as an unaccounted source of O₂ in tenuous atmospheres surrounding icy satellites and ring systems. Additional work will attempt to vary the OH/H₂O ratio and determine the affect of surface impurities that may act as a sink for OH.

A manuscript to Ap. J. was submitted with the preliminary data showing the above results. The results have also been presented at Fall AGU in an invited speaker oral presentation and at DPS 2010.

In collaboration with colleagues at The University of Western Australia, we have shown in matrix-isolation experiments that methanol formation can occur in H₂O and CH₄ mixtures by the unusual O atom insertion reaction with CH₄. This has been shown by using various isotopologues of water and methane. Previous reports of irradiated ices have assumed a reaction between the methyl and OH radicals. In light of these results we are preparing to begin electron-irradiation experiments to conclusively determine the reaction mechanism in an irradiated ice sample. CH₄, CD₄, D₂O and H₂¹⁸O have been purchased with Icy Worlds funds and we have measured the spectra of these pure substances and some mixtures. By the end of October 2010, we will have begun irradiation experiments.

A manuscript describing the matrix-isolation data is currently in preparation. The new experimental work ice work should be completed by the end of the year with a manuscript likely to be submitted in the first quarter of 2011.

The following experiments and data analysis were also performed by co-I Strazzulla during 2010:

- Ion irradiation of methanol and methanol/CO mixture

- Determination of survival rate of Deinococcus radiodurans against ion bombardment

- Ion irradiation of sulfur bearing icy species.

Survival curves of Deinococcus radiodurans to irradiation with 200 keV protons. NC=naked cells, SST=cells mixed with sandstone grains, B=cells mixed with basalt grains, SID and SIL=cells deposited on olivine films. (Paulino-Lima, Baratta, Brucato, Cockell, Olsson-Francis, Spinella, Strazzulla, Mason, Lage, in preparation)
In 2011 we plan to study the role of energetic processing in the formation of methilformate, glycolaldheyde and other pre-biotic molecules in comets (Modica, Palumbo, Strazzulla, in preparation). The survival of bacteria mixed with grains to solar ion irradiation will be investigated.

Co-Investigators Ponce and Hand continued to study the behavior of microbial spores under energetic conditions, Investigators Kevin Hand and Adrian Ponce have installed the monodispersed spores deposition chamber. The irradiation chamber is ready for samples. Dr. Wanwan Yang, a new Caltech postdoctoral fellow, is preparing samples using chamber. We expect samples to be ready for irradiation in 2 months.