Why is the Kuiper belt so colorful? Scientists find answers at the molecular level

The Kuiper belt's colors recreated in a lab

The new paper, published in the journal Science Advances, details how the team used ultrahigh vacuum irradiation experiments to analyze the color evolution of objects in the Kuiper belt on the molecular level.

They found, for example, that aromatic structural units carrying up to three rings and connected by hydrogen-deficient bridges were likely to produce a reddish hue.

"This research is a critical first step to systematically unravel the carriers of the molecular units responsible for hydrocarbon-rich surfaces of Kuiper Belt objects," Professor Ralf I. Kaiser, study lead, explained in a press statement.

"Since astronomical detections also detected, e.g., ammonia, water, and methanol, on the surfaces of Kuiper Belt objects," Kaiser continued, "Further experiments on the cosmic ray processing of these ices hopefully reveal the nature of the true color diversity of Kuiper Belt objects on the molecular level."

The Kuiper belt and NASA's New Horizons mission problem

Even more insight into the colors of the Kuiper belt could have come from NASA's New Horizons spacecraft mission. Unfortunately, though, the mission is currently in limbo as NASA has scaled it back and rejected part of the science team's proposal to conduct an extensive and ambitious exploration mission of the distant region.

New Horizons launched aboard one of United Launch Alliance's Atlas V rockets in 2006. It flew by Pluto in 2015 and it is expected to continue traversing and operating from the Kuiper Belt beyond 2028.

When NASA rejected part of the New Horizons science team's proposal, Alan Stern, principal investigator for the mission, said "We need to finish the Kuiper Belt. We finally got a spacecraft here. We're going to leave the Kuiper Belt in a few years. Why so impatient over pennies out of the planetary budget?"

Study 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.