Deep search for carbon monoxide in cometary precursors using millimeter wave spectroscopy

Abstract

Carbon monoxide is abundant in comets and is volatile throughout most of the planetary region of the solar system, raising the possibility that it might be detectable in the cometary precursor classes known as the Centaurs and the Kuiper belt objects (KBOs). In this paper we present a search for carbon monoxide in Centaurs and Kuiper belt objects focused on the J = 2-1 rotational transition at 230 GHz. No CO emission is detected. We use upper limits from the radio spectra to infer that the surfaces of these objects are strongly depleted in supervolatile ices: CO can cover no more than 0.1-1% of the surface area. Two possibilities for the survival of sub-surface CO ice are considered. First, bulk CO ice could survive undetected at depths much greater than the annual thermal skin depth. Inward drift of the perihelion distance would allow heat conducted from the surface to reach the CO ice, driving outgassing through short-lived vents at rates generally too small to be detected. Second, CO might be physically trapped within a porous, amorphous ice matrix and released where the local temperatures are sufficient to promote the transformation of amorphous into crystalline ice. In either case, the non-detections in our data would reflect the large perihelia and low temperatures of the sampled objects but would not set useful constraints on the interior abundances of CO or other supervolatiles. © 2008. The American Astronomical Society. All rights reserved.