A hypothesis for the color diversity of the Kuiper belt

TL;DR

This paper presents a hypothesis that explains the diverse surface colors of Kuiper belt objects through initial composition differences and subsequent irradiation effects, linking formation location to observed color variations.

Contribution

It introduces a new hypothesis connecting formation location, initial composition, and surface colors of Kuiper belt objects, supported by proposed observational and experimental tests.

Findings

Inner belt objects retain H2O and CO2, resulting in dark neutral colors.

Outer belt objects retain CH3OH, leading to brighter redder surfaces.

Cold classical Kuiper belt objects retain NH3, possibly causing their unique colors.

Abstract

We propose a chemical and dynamical process to explain the surface colors of the Kuiper belt. In our hypothesis, the initial bulk compositions of the bodies themselves can be quite diverse -- as is seen in comets -- but the early surface compositions are set by volatile evaporation after the objects are formed. Strong gradients in surface composition, coupled with UV and particle irradiation, lead to the surface colors that are seen today. The objects formed in the inner part of the primordial belt retain only H2O and CO2 as the major ice species on their surfaces. Irradiation of these species plausibly results in the dark neutrally colored centaurs and KBOs. Object formed further in the disk retain CH3OH, which has been shown to lead to brighter redder surfaces after irradiation, as seen in the brighter redder centaurs and KBOs. Objects formed at the current location of the cold classical Kuiper belt uniquely retain NH3, which has been shown to affect irradiation chemistry and could plausibly lead to the unique colors of these objects. We propose observational and experimental tests of this hypothesis.