Chemical origin of the colors of objects in the outer solar system and the Kuiper belt connection

Abstract

The common small objects in the Kuiper belt come in two colors: red and very red. Such a strong bifurcation in what must be chemical properties is extremely hard to explain through standard ideas of outer solar system mixing, accretion and formation. We have proposed that, instead, all objects start with essentially identical chemical composition, but, after formation has ended and the nebula has dispersed, icy outer solar system objects all go through a cometary phase where surface volatiles are removed. Volatile removal is an exponential function of surface temperature, thus volatile evaporation lines will be imposed across the outer solar system. We find that the H2S evaporation line occurs in the middle of the proposed Kuiper belt object (KBO) formation region. Thus, some fraction of KBOs will retain surface H_2S, while the remainder will have none. Subsequent irradiation will yield involatile residues -- which give rise to the KBO colors -- of two types: ones with, and ones without H_2S. We have performed laboratory experiments simulating the formation and irradiation and such ices and find, indeed, that the irradiated H2S containined residues are significantly more red.