[M-IS19] Aqua planetology

[MIS19-P06] Adhesion and chemical properties of organic material analogs in the early Solar System

*Yusaku Sadakane¹, Eito Hirai¹, Yasuhito Sekine^1,2,3, Shuya Tan⁴, Yoko Ochiai¹, Li Yamei¹, Hiroyuki Kagi⁵ (1.Tokyo Institute of Technology, 2.Kanazawa University, 3.Tohoku University, 4.Japan Agency for Marine-Earth Science and Technology, 5. Geochemical Research Center, Graduate School of Science, University of Tokyo)

Keywords:protoplanetary disk, planetesimals, collisional growth, complex organic materials, adhesion force, surface energy

The evolution of dust in a protoplanetary disk is controlled by transport and collisional growth. Previous work suggested that effective collisional growth of dusts may have been attributed from organic matter covering dust particles. However, adhesion force of organic matter in a protoplanetary disk has been poorly investigated experimentally.
In this study, we measure adhesion force of organic matter synthesized by different processes in a protoplanetary disk; that is, UV-high energy particle irradiations into the surface of protoplanetary disks and hydrothermal reactions within icy planetesimals. The objective of this study is to understand the relationship between the chemical structure of the organic matter and its adhesion force. We simulated chemical reactions in a disk surface induced by UV-high energy particles through cold plasma irradiations onto a gas mixture of H₂, CO, and N₂ (hereinafter referred to as disk organic matter). We also simulated hydrothermal synthesis of organic matter from solutions containing formaldehyde, glycolaldehyde, and ammonia (hereinafter referred to as hydrothermal organic matter). We analyzed the produced organic matter using infrared spectroscopy, UV-visible spectroscopy, Raman spectroscopy, elemental analysis, gas chromatography, and liquid chromatography. Adhesion force of the organic matter was measured with an atomic force microscopy.
Our results show that the disk organic matter contains more aliphatic compounds, nitriles, and methyl compounds than the hydrothermal organic matter. On the other hand, hydrothermal organic matter contains relatively large amounts of aromatic compounds, including nitrogen and oxygen. The hydrothermal organic matter has higher adhesion force and surface energy than the disk organic matter. These results suggest that organics containing more polycyclic aromatic structures have higher adhesion force, consistent with previous studies in the field of physical chemistry. The surface energies of the organic matter produced in this study are several times higher than those used in the previous microphysical models of dust collisions in a protoplanetary disk. This suggests that organic matter generated in a protoplanetary disk can induce efficient dust growth through collisional processes.