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Project Progress

In preparation for the OREOcube experiment, we have performed laboratory characterizations of five classes of candidate organic compounds (see Table 2) in combination with inorganic catalysts. Amino acids, the building blocks of proteins, are generally the most abundant single class of molecules in bacterial cells followed by nucleobases (associated with DNA and RNA). Both represent important target molecules in the search for life. PAHs are abundant and ubiquitous in galactic and extragalactic regions, protoplanetary disks, and solar system objects. Their spectral measurements indicate the presence of a large variety of individual compounds, clusters, and PAH-related molecules. A class of redox-active aromatic molecules that play roles in biological electron transfer processes, the quinones, has been formed in numerous laboratory simulations of UV-photolyzed interstellar and planetary ices containing PAHs.

Representative compounds from four of the organic sample classes (porphyrins, quinones, PAHs, amino acids) listed in Table 1 have been investigated during the Organism/Organic Exposure to Orbital Stresses (O/OREOS) Mission. Three of these compounds isoviolanthrene (IVA; PAH class), iron-5,10,15,20-tetraphenyl-21H,23H-porphine iron(III) chloride (FeTPP; porphyrin class), and 1,5-dihydroxyanthraquinone (anthrarufin; quinone class) will be used as OREOcube samples. Anthrarufin is a partially oxidized form of 9,10-anthraquinone, which is present in plants, fungi, lichens, and insects. FeTPP is representative of a large class of redox-active organometallic compounds, typified by porphyrins and phthalocyanines, that enable biological processes from photosynthesis to respiration. By using these samples in OREOcube inorganic catalysis experiments, O/OREOS science will be expanded. Additionally, OREOcube will provide data sets which will be used to examine the dependence of reaction rates and products on parameters related to ISS and O/OREOS satellite orbit, primarily the time-varying pattern of light/dark exposures and temperatures.

We present here results from experiments conducted with iron oxide thin films in combination with a thin film of the quinone anthrarufin. Iron oxide films were prepared for laboratory studies by physical vapor deposition of reduced iron in a high-vacuum deposition chamber (~ 1 × 10^-4 Pa). After deposition, the films were converted to magnetite or hematite by heating for 4 h, in the presence of oxygen atmosphere, at 175 °C or 575 °C, respectively. The thickness of the iron films prior to oxidation was 2 nm. Anthrarufin (51 nm) thin films were deposited as described above either on uncoated fused silica (SiO₂) or on top of hematite or magnetite deposited on SiO₂. Anthrarufin (Sigma-Aldrich, St. Louis, MO, USA) was pre-purified by heating under vacuum prior to film deposition. UV exposures were performed using a Hanovia 1000 W Xe arc lamp with an IR water filter to prevent sample heating, providing an integrated (200 – 400 nm) UV flux of 62 ± 0.5 W/m².

The sample-coated windows were not sealed into sample cells, but instead the inorganic/organic films were exposed, as a single lot, in sample chamber with a Suprasil fused silica optical port. A 800 Pa CO₂ (99.99%) atmosphere, with a mass flow controlled 0.15 sccm flow rate, was maintained using an oil-free diaphragm pump. The dewpoint was held between -50° and -60° C. The overall objective of these experiments was to examine UV photolysis rates under Mars-relevant conditions (i.e., Mars pCO₂ and pH₂O in combination with iron oxide catalysis) as a selection criterium for samples for the OREOcube ISS experiment (see Figure 1).

The results of Figure 1 highlight the power of time-resolved spectral measurements that record not only starting and ending points, but also dynamic changes and alteration rates. For OREOcube on the ISS, the same sort of kinetic analysis will be possible, but the time between measurements will be more regular and there will be no intervening air exposures; the experiment will be fully autonomous and remotely controlled. Our pre-flight simulation experiment with magnetite and hematite thin films demonstrates that kinetic differences in the photolyzed samples are measurable. Hematite with its known semiconducting photo-catalytic properties showed the most rapid decrease in integrated absorbance due to anthrarufin degradation while anthrarufin alone showed the slowest photodegradation.

In addition to pre-flight experiments, baseline flight and ground control samples were prepared with organic thin films of isoviolanthrene, iron tetraphenylporphyrin and anthrarufin on various catalytic substrates including Cr₂O₃, Fe-Ni, TiO₂ and Fe₃O₄ and Fe₂O₃. All films were deposited onto magnesium fluoride (MgF₂) windows, which were characterized prior to film deposition by UV-vis spectroscopy. Window cleaning procedures were tested on a subset of MgF₂ windows and measured in the far-UV region at the ASTRID synchrotron facility in Aarhus, Denmark (see Figure 2).

All flight samples have been sealed within flight sample cells containing argon. Baseline UV-Vis spectra have been recorded while Argon leak rate testing of all flight cells is still ongoing. Flight samples will be launched (anticipated in 2015) to and installed on the outside of the International Space Station (ISS).