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Space Exposure of Amino Acids and Their Precursors

in the Tanpopo Mission Using the International Space Station

By Kensei Kobayashi1), Hajime Mita2), Hikaru Yabuta3), Kazumichi Nakagawa4), Yukinori Kawamoto1), Kazuhiro Kanda5), Eiichi Imai6), Hirofumi Hashimoto7), Shin-ichi Yokobori8),

Akihiko Yamagishi8), and Tanpopo WG7) (5mm)

1)Yokohama National University, Yokohama, Japan 2)Fukuoka Institute of Technology, Fukuoka, Japan

3)Osaka University, Toyonaka, Japany 4)Kobe University, Kobe, Japan

5)University of Hyogo, Kamigori-cho, Japan 6)Nagaoka University of Technology, Nagaoka, Japan

7)JAXA/ISAS, Sagamihara, Japan 8)Tokyo University of Pharmacy and Life Science, Hachioji, Japan

A wide variety of organic compounds have been found in space, and their relevance to the origin of life is discussed. Interplanetary dust particles (IDPs) are most promising carriers of extraterrestrial organic compounds, but presence of bioorganic compounds are controversial since they are so small and were collected in the terrestrial biosphere. In addition, IDPs are directly exposed to cosmic and solar radiation. Thus, it is important to evaluate the stability of organics in IDPs in space environment. We are planning a novel astrobiology mission named Tanpopo by utilizing the Exposed Facility of Japan Experimental Module (JEM/EF) of the International Space Station (ISS). Two types of experiments will be done: Capture experiments and exposure experiments. In the exposure experiments, organics and microbes will be exposed to the space environments to examine possible alteration of organic compounds and survivability of microbes. Selected targets for the exposure experiments of organic compounds are as follows: Amino acids (glycine and isovaline), their possible precursors (hydantoin and 5-ethyl-5-methyl hydantoin) and complex precursors “CAW” synthesized from a mixture of carbon monoxide, ammonia and water by proton irradiation. In addition to them, powder of the Murchison meteorite will be exposed to examine possible alteration of meteoritic organics in space. We will show the results of preparatory experiments on ground by using a UV lamp, synchrotron facilities, and a heavy ion irradiation facility.

Key Words: Amino Acid Precursors, Exposure, Origins of Life, Interplanetary Dust Particles, International Space Station

1. Introduction It has been reported that a wide variety of organic compounds are contained in carbonaceous chondrites and in comets. Their relevance to the emergence of terrestrial life is widely discussed. It was suggested that more organic carbons were delivered to the early Earth by interplanetary dust particles (IDPs) than by meteorites or comets1). IDPs (or micrometeorites (MMs)) have been collected in ocean sediments, Antarctic ices, and air in stratosphere. Though presence of bioorganic compounds in IDPs/MMs is expected, it is difficult to judge it since they are so small and were collected in the terrestrial biosphere. Thus it would be of importance to collect IDPs out of the terrestrial biosphere. We are planning a novel astrobiology mission named Tanpopo by utilizing the Exposed Facility of Japan Experimental

Module (JEM-EF) on the International Space Station (ISS). Two types of experiments will be done in the Tanpopo Mission: Capture experiments and exposure experiments. In order to collect cosmic dusts (including IDPs) on the ISS, we are going to use extra-low density aerogel, since both cosmic dusts and ISS are moving at 8 km s-1 or over. We have developed novel aerogel whose density is 0.01 g cm-3. In the exposure experiments, organics and microbes will be exposed to the space environments to examine possible alteration of organic compounds and survivability of microbes. Here we will report on the selection of organic compounds for the space exposure, and the results of preparatory experiments on ground. A number of amino acids were detected in water extract of carbonaceous chondrites. It is controversial whether meteorites contain free amino acids or amino acid precursors.

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When dusts are formed from meteorites or comets in interplanetary space, they are exposed to high-energy particles and photons. In order to evaluate stability and possible alteration of amino acid-related compounds, we chose amino acids (glycine and isovaline) and hydantoins (precursors of amino acids), and products of proton irradiation of a mixture of CO, NH3 and H2O (CAW; containing high molecular weight precursors of amino acids2)). We performed ground simulation experiments by using accelerators (HIMAC, NIRS, Japan and NewSUBARU, University of Hyogo, Japan), which showed that amino acid precursors were much more stable than free amino acids against radiation. The ground simulation also showed that solar UV is more lethal than cosmic rays for organic compounds in interplanetary space. The Tanpopo Mission is the first Japanese astrobiology space experiment, which is now scheduled to start in 20143). Samples will be retrieved 1-3 years after launch. We can expect to have the first IDPs sampled in space to see what kind of organics can be delivered by IDPs. In addition to this capture experiments, we are going to expose selected organic compounds and meteorite powders to space environments to examine stability and alteration of possible extraterrestrial organic compounds in space. In this paper, we described objectives of the organics-exposure experiments in the Tanpopo Mission, and showed preliminary results of ground simulation experiments for the preparation of the mission, together with experimental setups in space. 2. Origins of Organic Compounds in IDPs It was suggested that organics in extraterrestrial bodies such as meteorites, comets and IDPs were originally formed in ice mantles of interstellar dusts in dense cloud, since isotopic studies of organic compounds in meteorites and comets suggested that they were formed in quite cold environments4). Irradiation of frozen mixture of possible interstellar molecules including CO (and/or CH3OH), NH3 and H2O with high-energy particles5,6) or ultraviolet light7,8) gave amino acid precursors (molecules that give amino acids after hydrolysis) with high molecular weights [1]. Such complex organic molecules were taken in planetesimals or comets in the early solar system. In prior to the generation of the terrestrial life, extraterrestrial organics were delivered to the primitive Earth by such small bodies as meteorites, comets and space dusts. These organics would have been altered by cosmic rays and solar radiation (UV, X-rays) before the delivery to the Earth. We examined possible alteration of amino acids, their precursors and nucleic acid bases in interplanetary space by irradiation with high-energy photons and heavy ions. A mixture of CO, NH3 and H2O was irradiated with high-energy protons from a van de Graaff accelerator (TIT, Japan). The resulting products (hereafter referred to as CAW) are complex precursors of amino acids. CAW, amino acids (DL-isovaline, glycine), hydantoins (amino acid

precursors) and nucleic acid bases were irradiated with continuous emission (soft X-rays to IR; hereafter referred to as soft X-rays irradiation) from BL-6 of NewSUBARU synchrotron radiation facility (Univ. Hyogo). They were also irradiated with heavy ions (eg., 290 MeV/u C6+) from HIMAC accelerator (NIRS, Japan). After soft X-rays irradiation, water insoluble materials were formed. After irradiation with soft X-rays or heavy ions, amino acid precursors (CAW and hydantoins) gave higher ratio of amino acids were recovered after hydrolysis than free amino acids. Nucleic acid bases showed higher stability than free amino acids. Complex amino acid precursors with high molecular weights could be formed in simulated dense cloud environments. They would have been altered in the early solar system by irradiation with soft X-rays from the young Sun, which caused increase of hydrophobicity of the organics of interstellar origin. They were taken up by parent bodies of meteorites or comets, and could have been delivered to the Earth by meteorites, comets and cosmic dusts. Cosmic dusts were so small that they were directly exposed to the solar radiation, which might be critical for the survivability of organics in them. The stability of IDPs’ organic compounds in space environments will be tested in the exposure experiments in the Tanpopo Mission. 3. Ground Simulation of Alteration of Amino Acid-Related Compounds in Earth Orbit 3.1. Selection of target molecules It has been shown that amino acid precursors can be formed in interstellar environments5-8), and that amino acids or their precursors are present in carbonaceous chondrites9,10). It was also reported that glycine was found in cometary dusts returned by the Stardust Mission11). Thus it is possible that IDPs that has been ejected from asteroids or comets have amino acids or their precursors. Among a great number of amino acids found in carbonaceous chondrites, we selected glycine and isovaline to study the stability of amino acids in space. The former is one of the simplest and the most abundant protein amino acids. The latter is non-protein amino acids and enantiomeric excesses were reported in isovaline extracted from carbonaceous chondrites 12). Their possible precursors, hydantoin and 5-ethyl-e-methyl hydantoin were also added to the target list, together with complex amino acid precursors, CAW. 3.2. Irradiation of target molecules with UV, γ-rays and Heavy Ions In the space environments, UV-light and cosmic rays (high-energy ions and γ-ray) will cause the alteration of organic compounds. Therefore, experiments to examine possible photolysis and radiolysis of organic compounds in space environments were performed. In order to examine the stability of the target molecules against VUV, Xe-excimer lamp (at 172 nm; Ushio standard

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type excimer light emission unit) was used. Solution of exposure samples were added into the small hole (1 mmφ) on a SUS plate and dried under reduced pressure. Then the plate was placed in the vacuum chamber and were irradiated for 6.2 h at 4 × 1014 photons s-1 cm-2 and the irradiation was equivalent for 4 days irradiation around the ISS orbital. After irradiation, samples were extracted with water and analyzed by HPLC. We also irradiated the target molecules with synchrotron radiation soft X-rays at NewSUBARU BL06 (University of Hyogo, Japan), with γ-rays from a 60Co source (Quantum Beam, JAEA, Japan) and with carbon ion beam (290 MeV/u) at the HIMAC biology beam line (NIRS, Japan). In each irradiation experiment, amino acids or their precursors were irradiated in a solid state. Thermal stability at 100°C of the compounds were also tested. 3.3. Stability of Amino Acids and Their Precursors against High-Energy Photons and Particles γ-Rays and heavy ion beam irradiation with dose of ISS environment for one year induced little decomposition of organic compounds. However, UV irradiation was critical for organic compounds. Although almost all glycine and isovaline were decomposed, recovery of hydantoin and 5-ehtyl-5-methyl hydantoin were approximately 29 % and 72%, respectively, with UV dose of ISS environment for one year. Furthermore, CAW was more stable than hydantoins. Amino acids precursors, especially, complex organics were more stable than free amino acids in space environments. Soft X-rays irradiation experiments showed the same tendency: Free amino acids (glycine and isovaline) decomposed more rapidly than their precursors (hydantoins and CAW). Table 1 shows estimated recovery ratios after 1 year’s exposure to space environments in the exposed facility of the international space station (EF/ISS). Here the flux of VUV on ISS orbit was estimated to be 0.12 J m-2 s-1 for 120 nm to 200 nm radiation13). Thus, the yearly dose of VUV172 nm in LEO was estimated to 3.8 × 103 kJ m-2. It can be seen that the most fatal factor for organic compounds in low Earth orbit (LEO) is UV, and effects of the other factors (cosmic rays and heat) would be negligible. We

can expect that appreciable percentage of amino acid precursors would survive after 1 year’s exposure in LEO, while very limited part of the original free amino acids would be recovered. 4. Exposure of meteorite grains for understanding of space weathering effect on asteroidal regolith Space weathering on the surface regolith of asteroids could have been one of the important processes for chemical evolution in the early Solar System. However, it has been difficult to identify the records from meteorite samples that are lack of asteroid geologic information. For the first time, the preliminary examination of Itokawa asteroid particles returned by Hayabusa mission has revealed the evidence of space weathering on the mineral particles14) and the irradiation history of the asteroid regolith15). The space weathering effect on organic compounds has been unknown since organic compounds have not been detected from Itokawa particles16), but we will consider more seriously having information about the processes in the world future asteroid sample return missions, such as Hayabusa-2, OSIRIS-REx, and Marco Polo-R, which will go sampling the surface regolith of carbonaceous asteroids. Kanuchova et al.17) have shown the changes induced by energetic ion irradiation in the ultraviolet-visual-near-infrared (UV-Vis-NIR) reflectance spectra of olivine pellets covered by polystyrene. However, polystyrene has quite different compositions to those of organic materials in asteroids (meteorites), and thus it will be necessary to examine more realistic samples. In Tanpopo project, we propose to conduct an exposure experiment of meteorite grains on ISS in order to understand the space weathering alteration of molecular, isotopic, and morphological compositions of organic materials. The Low Earth Orbit environment satisfies the similar conditions to those on the surface regolith of asteroids, e.g., multiple irradiation energies (e.g., UV, radiation) and temperature cycle in sunrise and sunset, which are difficult to reproduce in a laboratory experiment. For a sample preparation, suspended Murchison meteorite particles in water will be dropped and fixed onto a silicon nitride membrane window TEM grid and the grid will be placed on the exposure plate. After 1-3 years of exposure at ISS, the returned samples

Table 1. Estimated recoveries after 1 year irradiation in Earth Orbit UV irradiation Cosmic rays Heat Total 172 nm 60Co γ-ray Carbon ion 100 oC Glycine 2 × 10-5 1.0 1.0 1.0 2 × 10-5 Isovaline 3 × 10-5 > 0.99 > 0.99 1.0 3 × 10-5 Hydantoin 0.29 1.0 1.0 1.0 0.29 Ethylmethylhydanotin 0.72 > 0.99 > 0.99 >0.99 0.72 Complex organics (CAW) N.A. 1.0 1.0 1.0 N. A. N.A.: not available

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will be analyzed by micro-X-ray absorption near edge structure (XANES), transmission electron microscope (TEM), and nano-secondary ion mass spectrometry (SIMS). The obtained molecular and isotopic compositions will be compared with those of pre-exposed meteorite samples. De Gregorio et al.18) has demonstrated the increase of the isotopic ratio of deuterium to hydrogen (D/H) of polymer resins after the electron irradiation. Dworkin et al.19) has synthesized organic vesicular structures by UV photolysis of interstellar ice analogs with a repetitive warming-freezing process. According to these experiments, the alteration of chemical and/or morphologic compositions of meteoritic organics will be expected after the experimental space weathering. Finally, it will be expected to learn the role of minerals for the alteration of organics in meteorite matrix, which is a complementary study with SEVO experiment in

O/OREOS mission which investigates the effects of photocatalytic role of mineral surface20). 5. Exposure Facility for the Tanpopo Mission An apparatus for exposure experiments is shown in Fig. 1. This is a rectangular solid of 100 mm × 100 mm × 20 mm. It can divide into 20 independent small chamber and we can independently use each chamber. The cross section of a small chamber is shown in Fig. 2. An MgF2 window of 16mm in diameter is attached at the top of the small chamber, and a filter of 0.3 mm is at the bottom. We can expose some samples, including organic compounds and microorganisms, without contamination of surroundings. 6. Vacuum Ultraviolet Dosimetry and Photochemistry of Alanine Film In order to develop the vacuum ultraviolet dosimeter, it is necessary to find material of which main absorption band is at vacuum ultraviolet VUV region. Here we will try an alanine film deposited on a quartz or MgF2 windows. Because absorption cross-section is negligible small at the wavelength region longer than 200 nm as shown in Fig. 3, this alanine dosimeter is sensitive at the region of 160 < λ < 190 nm when

Figure 1. An apparatus for space exposure experiments of microbes and organic compounds in Tanpopo mission. This is a rectangular solid of 100 mm × 100 mm × 20 mm. It can divide into 20 independent small chamber and we can independently use each chamber (Figure 2).

Figure 2. The cross section of a unit cell. An MgF2 (or quartz) window with 16mm in diameter is attached at the top of the unit, and 0.3 µm filter is at the bottom to contain the microbe while the sample

is exposed to the space vacuum.

Figure 3. Absorption spectra of alanine film. An arrowshows the cut-off wavelength of SiO2.

Figure 4. 172 nm photolysis curve of alanine film. Lamp intensity I0 was 5.2 x 1015 photons s-1.

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a SiO2 window is chosen. According to the photolysis experiment with 172 nm excimer lamp by Izumi et al.21), alanine film was decomposed as shown in Fig. 4 with the rate constant of k =1.8 × 10-18 photon-1; dn/dt = -knI0, where n the number of survived alanine molecules and I0 the number of absorbed photons per unit time. Using these data, we will develop a VUV dosimeter as shown in Fig. 5. In the figure, the mesh was used to guard and the coating material is used to prevent the sublimation loss of alanine molecule from the film. Survival rate of the alanine film after irradiation at the location of the international space station was roughly estimated from the solar irradiance spectrum observed by satellite13) and from an appropriate attenuation factor 0.015 by the mesh and a neutral density filter, to be 70, 49 and 34 % after one, two and three years, respectively. Photochemical reaction will be studied through the analysis of returned samples. One of important target molecule is alanylalanine. According to Izumi et al. 21), quantum yield of alanylalanine production was roughly estimated to be 1.3 × 10-3 photon-1 at the beginning stage of irradiation. Detection of alanylalanine is the direct evidence of chemical evolution resulted from solar irradiation at the space. 7. Conclusions

The Tanpopo Mission is the first astrobiological mission by utilizing ISS/JEM. Among a number of subthemes of the Tanpopo Mission, “exposure of organic compounds in space” aims to test the hypothesis that extraterrestrial organics played important roles in the generation of the first terrestrial life. IDPs could deliver organic compounds more safely than large comets and meteorites, but they had been exposed strong solar and cosmic radiation in interplanetary space. Amino acids in free forms are not so stable against radiation and heat, but their precursors are much more stable than free amino acids. In the present subtheme of the mission, we will compare the stability between free amino acids and amino acid precursors in space. In addition, we are going to study the alteration processes of extraterrestrial complex organic compounds in space.

We selected 2 free amino acids (glycine and isovaline) and

3 amino acid precursors (hydantoin, 5-ethyl-5-methyl hydantoin, and complex amino acid precursors “CAW” synthesized from CO, NH3 and H2O by proton irradiation. They were exposed to solar VUV and cosmic radiation on JEM-EF for 1-3 years. Powder of the Murchison meteorite will also be exposed. Dose will be monitored with the alanine VUV dosimeter. After the laboratory simulation experiments, we estimate that at least amino acid precursors will be survived after a few years’ expose in space. Detailed analysis of the exposed samples after return to the Earth, we will be able to discuss on the roles of extraterrestrial organics in chemical evolution and generation of life on the Earth or elsewhere. Acknowledgments The present author thank to Dr. Katsunori Kawasaki, Dr. Yoshiyuki Oguri, Dr. Hitoshi Fukuda (Tokyo Institute of Technology), Dr. Issay Narumi (Toyo University) for their kind help in irradiation experiments. They also thank to Mr. Takeo Kaneko and Dr. Yumiko Obayashi for their collaboration in analysis. The present work was partly supported by JSPS KAKENHI (Grant No. 24654181) and by JAXA Space Utilization WG program.

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