・Elucidation of the meaning of the anomalous ortho-to-para ratio (OPR)¹ of water existing in comets and interstellar clouds, which has been an open question for many years.

・Discovery that it is not true that the OPR shows the temperature at the time when the water formed in space, as was previously thought.

・Proposal of a reinterpretation of past observational results, and a revision of the theory, regarding the origin of water in space and in the solar system.

Water (H₂O) exists ubiquitously in space such as in the interstellar clouds in which stars are born, in proto-planetary disks, in the comets and icy satellites of the solar system, and on the Earth. Therefore, water is a molecule that is related to evolution from interstellar clouds to the solar system. By studying the properties of water that have been created in space it is possible to understand the environment of the interstellar clouds and comets and to explore how the solar system formed.

In particular, the ortho-to-para ratio (OPR) of water in space has recently received attention. Water (H₂O) molecules can be classified into two types: ortho-H₂O in which the spin directions of the nuclei of the hydrogen atoms are parallel, and para-H₂O in which the directions are antiparallel (Fig. 1). At the temperature conditions of the Earth (27°C = 300 K), the OPR of H₂O is 3:1 based on its quantum mechanical properties. However, at low temperatures below -223°C (= 50 K) the amount of para-H₂O increases. It is known that the OPR of water in interstellar clouds and comets has more para-H₂O than on Earth (Fig. 1). For example, on comets the OPR is 2.5, which corresponds to -243°C (= 30 K). Ortho-para conversion in the gas phase is extremely slow, so in astronomy it has been believed that the OPR of water in space is determined according to the time when the water was formed, so if the OPR is observed then one can learn about the temperature at the time when the water was formed in the past and therefore that the water in comets was formed in an environment of -243°C (= 30 K) 4.6 billion years ago.

However, experiments to investigate whether the OPR really does show the temperature at the time that the water was formed had not been performed until now, because separately detecting ortho-H₂O and para-H₂O with high sensitivity is difficult. Consequently, the true meaning of the anomalous OPR of the water in space is unclear, despite the accumulation of observational studies spanning thirty years.

In space, water forms first as ice through the chemical reaction of oxygen and hydrogen on the surfaces of small and extremely low-temperature dust grains called interstellar dust² (Fig. 2). This ice-covered dust is then irradiated with strong light in interstellar clouds and proto-planetary disks, or is heated in comets, and the water is desorbed into a gaseous phase and its OPR can be observed. Our research has faithfully reproduced in the laboratory the process by which ice is formed in space and desorbed into a gaseous phase, and has directly measured the OPR using a method called resonance-enhanced multiphoton ionization spectroscopy³ (Fig. 3). Our result show that even if ice is formed at -263°C (= 10 K), the OPR of the water desorbed from the ice is not the OPR of 0.3 that corresponds to 10 K, but rather corresponds to the same high-temperature condition as that of the Earth (OPR = 3). In other words, we revealed that this water OPR does not show the temperature at the time of formation.

Our research can greatly advance the understanding of how water formed during the evolution of the solar system from an interstellar cloud, and how water became incorporated in celestial bodies such as comets. For example, the hypothesis that comets formed 4.6 billion years ago at 30 K based on the OPR of cometary water is not correct, so a new theory is needed to understand the temperature conditions at the time when the solar system was formed.

Also, we have learned that the chemical reactions that occur after water desorbs from ice are much more important than previously thought for explaining the origin of the anomalous OPR observed in space. Chemical reactions greatly depend on the physical environment (such as temperature and pressure) of a celestial body. Therefore in the future, by investigating the relationship between the OPR of water in space and the chemical reactions that occur at such sites, it will be possible to understand in detail the physical environments of celestial bodies. The results of this research will also be very helpful in correctly understanding the latest observational results that have been obtained, such as from the ALMA telescope and the Rosetta spacecraft.

1. Ortho-to-Para Ratio (OPR): The nuclei of hydrogen atoms that form water (H₂O) molecules rotate like tops, and is called “spin.” H₂O can be classified into two types: ortho-H₂O in which the spin directions of the nuclei of the two hydrogen atoms are parallel, and para-H₂O in which the directions are antiparallel, and they have different magnetic properties. The OPR of water on the Earth is 3:1.

2. Interstellar Dust: Fine particles of about 0.1 μm that exist in interstellar clouds. Their nuclei may contain minerals or carbonaceous materials, and their surfaces are covered with ice. This interstellar dust covered with ice is the raw material from which the bodies of the solar system (such as comets and planets) are formed.

3. Resonance-Enhanced Multiphoton Ionization Spectroscopy: When molecules are irradiated with concentrated laser light the molecules are ionized and can be detected as an electrical signal (photoionization spectroscopy). In the usual photoionization spectroscopy it is impossible to separately detect ortho-H₂O and para-H₂O. However, by adjusting laser light to a specific wavelength and using resonance-enhanced multiphoton ionization spectroscopy, it is possible to selectively ionize and perform high-sensitivity detection of just ortho-H₂O, or of just para-H₂O.

Astrophysical Chemistry and Ice and Planetary Science Group, Institute of Low Temperature Science, Hokkaido University

Assist. Prof. Tetsuya HAMA

TEL：+81-11-706-5474

FAX：+81-11-706-7142

hama[at]lowtem.hokudai.ac.jp