Processing of methane and acetylene ices by galactic cosmic rays and implications to the color diversity of Kuiper Belt objects

Chaojiang Zhang; Cheng Zhu; Andrew M Turner; Ivan O Antonov; Adrien D Garcia; Cornelia Meinert; Leslie A Young; David C Jewitt; Ralf I Kaiser

doi:10.1126/sciadv.adg6936

Processing of methane and acetylene ices by galactic cosmic rays and implications to the color diversity of Kuiper Belt objects

Sci Adv. 2023 Jun 2;9(22):eadg6936. doi: 10.1126/sciadv.adg6936. Epub 2023 May 31.

Authors

Chaojiang Zhang^{1

2}, Cheng Zhu^{1

2}, Andrew M Turner^{1

2}, Ivan O Antonov^{1

2}, Adrien D Garcia³, Cornelia Meinert³, Leslie A Young⁴, David C Jewitt⁵, Ralf I Kaiser^{1

2}

Affiliations

¹ Department of Chemistry, University of Hawaii at Mānoa, Honolulu, HI 96822, USA.
² W.M. Keck Laboratory in Astrochemistry, University of Hawaii at Mānoa, Honolulu, HI 96822, USA.
³ Université Côte d'Azur, Institut de Chimie de Nice, UMR 7272 CNRS, Nice 06108, France.
⁴ Department of Space Studies, Southwest Research Institute, Boulder, CO 80302, USA.
⁵ Department of Earth and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA.

Abstract

Kuiper Belt objects exhibit a wider color range than any other solar system population. The origin of this color diversity is unknown, but likely the result of the prolonged irradiation of organic materials by galactic cosmic rays (GCRs). Here, we combine ultrahigh-vacuum irradiation experiments with comprehensive spectroscopic analyses to examine the color evolution during GCR processing methane and acetylene under Kuiper Belt conditions. This study replicates the colors of a population of Kuiper Belt objects such as Makemake, Orcus, and Salacia. Aromatic structural units carrying up to three rings as in phenanthrene (C₁₄H₁₀), phenalene (C₉H₁₀), and acenaphthylene (C₁₂H₈), of which some carry structural motives of DNA and RNA connected via unsaturated linkers, were found to play a key role in producing the reddish colors. These studies demonstrate the level of molecular complexity synthesized of GCR processing hydrocarbon and hint at the role played by irradiated ice in the early production of biological precursor molecules.