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

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 (C 14 H 10 ), phenalene (C 9 H 10 ), and acenaphthylene (C 12 H 8 ), 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.

Role of Acetylene in the Chemical Evolution of Carbon Complexity

“…Similar processes, including the formation of aromatic molecules from acetylene at low temperatures, can occur not only on Titan but also on other objects of the Solar System, such as the dwarf planets Pluto, Makemake, Ceres, , Orcus, Salacia, Saturn’s satellites Hyperion, Iapetus, and Phoebe, ,, Jupiter’s moons Ganymede and Callisto, satellites Pluto, Charon, Nikta, Hydra, , and others, where regions with dark organic matter have been found. Recently, Kaiser et al showed that the photochemistry of acetylene ice can lead to the formation of PAHs, including five ring molecules, such as dipropylperylene (C 26 H 24 ). , In another experiment simulating the irradiation of methane and acetylene by galactic cosmic rays in the Kuiper belt in an ultrahigh vacuum, Kaiser et al reported the initial formation of acetylene from methane. In this work, approximately 30 resulting organic molecules were identified, including mono- and tricyclic aromatic compounds (substituted benzenes, naphthalenes and biphenylenes, as well as phenanthrene, phenalene, acenaphthylene and 9 H -fluorene).…”

“…Recently, Kaiser et al showed that the photochemistry of acetylene ice can lead to the formation of PAHs, including five ring molecules, such as dipropylperylene (C 26 H 24 ). 401,285 In another experiment simulating the irradiation of methane and acetylene by galactic cosmic rays in the Kuiper belt in an ultrahigh vacuum, Kaiser et al 402 reported the initial formation of acetylene from methane. In this work, approximately 30 resulting organic molecules were identified, including monoand tricyclic aromatic compounds (substituted benzenes, naphthalenes and biphenylenes, as well as phenanthrene, phenalene, acenaphthylene and 9H-fluorene).…”

“…In this work, approximately 30 resulting organic molecules were identified, including monoand tricyclic aromatic compounds (substituted benzenes, naphthalenes and biphenylenes, as well as phenanthrene, phenalene, acenaphthylene and 9H-fluorene). Methyl (CH 3 ), ethyl (C 2 H 5 ), vinyl (C 2 H 3 ), and allylic (C 3 H 5 ) groups were k 2b = 0.055/0.08 amu eV −1 (10/40 K) 402 Galactic CR field a. 3C determined as substituents.…”

“…403 Methane, in turn, can be easily converted by ionizing radiation to acetylene. 159,402 Thus, acetylene becomes a bridge to a variety of complex organic compounds on the hydrocarbon-rich surfaces of planets and their satellites.…”

“…What makes this mechanism even more universal is that methane ices have been found on many of the outer bodies of the Solar System . Methane, in turn, can be easily converted by ionizing radiation to acetylene. , Thus, acetylene becomes a bridge to a variety of complex organic compounds on the hydrocarbon-rich surfaces of planets and their satellites.…”