The Color Diversity of Kuiper Belt Objects

Objects in the Kuiper Belt exhibit the reddest known surfaces and reveal a wider range of colors than any other Solar System population. The origin of this extraordinary color diversity is unknown, but likely the result of the prolonged irradiation of organic materials by Galactic Cosmic Rays (GCRs). However, laboratory experiments simulating irradiation processes have provided conflicting results and are highly dependent on the assumed initial composition, GCR flux, and exposure time. Here, we combine ultrahigh vacuum irradiation experiments with comprehensive spectroscopic analyses to examine the GCR processing of simple hydrocarbon surfaces of methane and acetylene under Kuiper Belt conditions. This study efficiently replicates the color diversity shown by Kuiper Belt Objects located at distances from 39 to 44 AU from the Sun such as Makemake, Orcus, and Salacia, and indicates effective exposure ages of at least 1,100 million years. Aromatic structural units carrying up to three rings as in phenanthrene (C₁₄H₁₀), phenalene (C₉H₁₀), and acenaphthylene (C₁₂H₈), of which some carry structural motives of nitrogen bases of DNA and RNA connected via unsaturated linkers, play a key role in producing the reddish colors. These studies demonstrate the level of molecular complexity synthesized by GCR processing and hint at the role played by irradiated ice in the early production of biological precursor molecules. Extrasolar counterparts to the Kuiper belt are known in the population of debris disks such as those around Fomalhaut and Vega, where these processes must also be relevant.