Laboratory research has revealed how carbon atoms diffuse on the surface of interstellar ice grains to form complex organic (carbon-based) compounds that are critical for unraveling the complexity of cosmic chemistry. The list of organic molecules detected in space and understanding of how they interact is steadily expanding thanks to continued improvements in direct observation techniques, however, laboratory experiments that reveal complex processes can also provide important clues.
Artistic depiction of the formation of organic compounds on interstellar ice. Source: MasashiTsuge
Researchers at Hokkaido University, working with colleagues at the University of Tokyo in Japan, report new laboratory-based insights into the central role of carbon atoms on interstellar ice grains in the journal Nature Astronomy.
Some of the most complex organic molecules in space are thought to be created at extremely low temperatures on the surfaces of interstellar ice particles. It is understood that ice particles suitable for this purpose abound in the universe.
All organic molecules are based on a skeleton of bonded carbon atoms. Most carbon atoms are initially formed through nuclear fusion reactions in stars, and are eventually dispersed into interstellar space when the star dies in a supernova explosion. But to form complex organic molecules, carbon atoms need a mechanism to clump together on the surface of ice particles, meet partner atoms, and form chemical bonds with them. New research suggests a possible mechanism.
Above 30 Kelvin (minus 243 degrees Celsius / minus 405.4 degrees Fahrenheit), carbon atoms diffuse and combine together to form the diatomic carbon C2. Source: MasashiTsuge et al., Nature Astronomy. September 14, 2023
Masashi Tsuge, a chemist at Hokkaido University's Institute of Cryogenic Science, said: "In our study, we reproduced feasible interstellar conditions in the laboratory and were able to detect the diffusion reaction of weakly bonded carbon atoms on the surface of ice particles and generate C2 molecules. C2, also known as diatomic carbon, is a molecule in which two carbon atoms are bonded together; its formation is concrete evidence of the presence of diffused carbon atoms on interstellar ice particles."
The study found that this diffusion can occur at temperatures above 30 Kelvin (minus 243 degrees Celsius / 405.4 degrees above zero Fahrenheit), and in space, it only takes 22 Kelvin (minus 251 degrees Celsius / 419.8 degrees Fahrenheit above zero) to activate the diffusion of carbon atoms.
Masashi Tsuzuki (left), the first author and corresponding author of the paper, and Naoki Watanabe (right), the co-author. Source: MasashiTsuge
Zhezhi said the findings put a previously overlooked chemical process into the explanatory framework of how more complex organic molecules are built by continually adding carbon atoms. He believes that these processes may occur in protoplanetary disks around stars, from which planets form. The required conditions can also develop in so-called translucent clouds, which eventually evolve into star-forming regions. This could also explain the origin of chemicals on Earth that might have given rise to life.
In addition to questions about the origin of life, this research adds a fundamental new process to the variety of chemical reactions that may have, and may still, structure carbon-based chemistry throughout the universe.
The authors also summarize the current understanding of the formation of complex organic chemicals in space more generally, and consider how reactions driven by diffusing carbon atoms might change the current situation.