Parametric study of cryogenic-specific reddish coloration induced by cryoplasma

Icy bodies in the outer solar system such as trans-neptunian objects and Centaurs can be characterized by their color diversity, which can provide significant clues that shed light on the formation and evolution processes of the solar system. Laboratory experiments by irradiation of analog materials have suggested that the reddish coloration of such bodies is due to the presence of complex refractory organic compounds such as tholins [1]. However, as such complex organic compounds are non-volatile even at room temperature, it is inadequate in explaining the absence of ultra-red objects near the Sun. Thus, the causes of the color diversity remain debatable. Through the use of cryoplasma, where the gas temperature can be controlled continuously between 273 – 5 K [2], as an energetic activation source, our group has established a reddish coloration that is stable only at cryogenic temperatures [3]. This result provides experimental support to the idea that the reddish material is thermally unstable at high temperatures, hence ultra-red objects are not found near the Sun [4]. In this presentation, we will focus on the balance of production and disappearance rates of reddish coloration on a methanol- and water-containing ice, which should be environmental-condition dependent. Our results indicate that while the reddish coloration can be generated and sustained at cryogenic temperatures, it cannot be formed at higher temperatures like 130 K with our experimental conditions. We will also demonstrate the role of gaseous N₂ in the appearance of reddish coloration by comparing with no gaseous N₂ case [3]. Further details will be presented at the meeting.References [1] H. Imanaka et al., Icarus 168, 344 (2004) [2] N. Sakakibara & K. Terashima, J.Phys.D 50, 22LT01 (2017) [3] N. Sakakibara et al., ApJL, in press [4] D.C. Jewitt, Science 367, 980 (2020)