Naoki Watanabe

Experimental Approach to Understanding the “Evolution of Molecules in Space”

Naoki Watanabe , Professor

Institute of Low Temperature Science/ Graduate School of Science

High school : Tokyo Metropolitan Takenodai High School

Academic background : Doctorate at Tokyo Metropolitan University

Research areas
atomic and molecular physics, interstellar chemical physics
Research keywords
cryogenics, ice particles, chemical evolution, amorphous

What are you aiming to achieve?

I am attempting to explain how and to what extent molecules can become complex under cryogenic circumstances in space through experiments. Recent research has shown that a wide and diverse range of molecules including complex organic substances already exist in the cryogenic regions prior to the birth of stars and planets (molecular clouds: Fig. 1, temperatures of around -263°C). The gas in space is originally atomic, and chemical reaction is required in order for molecules to form and become more complex (known as chemical evolution).

Fig. 1 Image of the Horsehead Nebula (a famous molecular cloud) and surface reaction of ice particles.

Chemical reactions normally do not take place in an ultra-low temperature environment, however. In that case, how are molecules formed, and how do they evolve? One answer to this is in small cosmic ice particles measuring around 0.1 µm in size, which are present in large quantities floating in the molecular clouds. It is believed that most hydrogen molecules, water molecules, carbon dioxide and even organic molecules are produced on the surface of these ice particles. Special reactions only possible on a cryogenic surface take place as a result of quantum mechanics. Our research group is looking into the details of this by experiments regarding reactions on cryogenic ice surfaces, and aiming to completely explain the formation and evolution mechanisms of molecules in space. To date, we have been the first in the world to demonstrate experimentally the formation process for hydrogen molecules, water molecules, formaldehyde and methanol molecules, among others, and we are currently leading the world in this field of research.


What sort of equipment are you using to perform what type of experiments?

Molecular clouds in which ice particles exist are found in relatively high gas density areas of space, but even then the molecular density is around 105 cm-3, an ultrahigh vacuum in comparison with atmospheric pressure on earth (1019 cm-3). In order to recreate the molecular cloud ice particles in the laboratory we use a metal plate installed inside an ultrahigh vacuum experimental apparatus, such as that shown in Fig. 2. The metal surface is cooled by a cryostat to around -263°C, and used to produce analogues of cosmic ice particles (an ice membrane). Ice formed using this method does not form ice crystals such as those seen on earth, but rather the arrangement of water molecules creates an irregular amorphous structure (Fig. 3). We know from astronomical observation that the ice particles in the molecular clouds also have the amorphous structure. Primordial molecules (CO etc.) are vapor deposited on the ice membrane thus formed, and processes that take place in space (hydrogen atomic and UV irradiation) are applied. By measuring changes to molecular composition and the energy condition of the molecules formed at that time, we are able to determine what kind of molecules may be created, and look in detail at the physical and chemical phenomena occurring on the ice surface. We mainly use an infrared absorption spectrometer and the type of laser device shown in Fig. 4 for measurement. In this way, we are able to detect even a small number of molecules on the ice surface.

Fig. 2 Ultra-high vacuum experiment device. Ice is formed at -263°C inside this equipment.

Fig. 3 Pattern diagram of ice structure (the blue dots are water molecules). Upper diagram: Ice crystals found on earth (Ih). Lower diagram: amorphous structure. Ice particles in the molecular cloud and cryogenic ice made in the ultra-high vacuum equipment has an amorphous structure.

Fig. 4 Wavelength tunable dye laser device. Detects minute molecules, and can measure the energy state of molecules.



What areas of your research are particularly interesting?

In particular, the phenomena that we first see in the experiments that could not take place in the natural world on earth, but only in cryogenic space. The question of how complex molecules could have got in the space environment before stars and planets were born is one of the fundamental questions for mankind. Cosmic ice particles are thought to be the origin of complex organic molecules such as amino acids, but we still do not understand the details of atomic and molecular processes that take place on low-temperature ice surfaces. When we carry out these experiments, unexpected reactions occur, or things we expected to happen do not. In this way, our research is important not only for astronomy and earth and planetary sciences, but also fascinating for the worlds of physics and chemistry.


What does the future hold in terms of this research?

I am basically physicist, but our field currently has researchers who are chemists, astronomers, and specialists in earth and planetary sciences and engineering. Based on the specialist knowledge of each of these people, I hope that we can press ahead with multidisciplinary research, stimulating one another. This offers many complex, significant possibilities. Regardless of your major as a university student, if you have an awareness of your objectives and knowledge of basic science, you will be able to start from scratch at graduate school in this field. I look forward to meeting many of you in the future!