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J. Stansberry, R.H. Brown, J. Lunine, D. Trilling, W. Grundy (U of Arizona)
Recent observations of a near-IR absorption feature near 2.3\mum suggest the presence of methane or higher hydrocarbons on grains (~10\mum diameter) in the circumstellar disk of 55 Cancri (Trilling and Brown (1998) Nature 395, 775). Other observations (Brown et al. (1998) Science 276, 937) suggest the presence of methane or higher hydrocarbons on the surface of the large Kuiper Belt Object 1993SC. Several processes set timescales for the persistence of volatiles in the outer reaches of a dust disk or in the Kuiper Belt: Jeans escape, energy limited escape, ballistic escape, photolysis/radiolysis, and cold-trapping. Likewise there are processes which set lifetimes for dust and surface processing under relevant conditions: radiation pressure, impacts, and fragmentation. We examine the timescales for the above processes in the context of our Kuiper Belt and the circumstellar environment of 55 Cancri and in the presence of methane and/or more carbon-rich molecules derived from it. We also examine the optical properties of grains composed of H2O, CH4, and a neutral absorber to place constraints on the abundance of methane (if that is in fact the absorber responsible for the 2.3\mum feature) in the 55 Cancri disk. Devolatilization of grains smaller than 1mm is very efficient even in the cold environment of the 55 Cancri disk. The presence of methane or its photolytic products may require sequestration in grain interiors. Conversely, on large bodies such as 1993SC cold trapping of volatiles offers ample opportunity for photolytic/radiolytic processing of methane, and probably can even stabilize methane itself over timescales comparable to the age of the Solar System. We discuss implications of these processes for nitrogen as well, although there is very little hope of detecting it in such environments on human timescales unless the Pluto--Kuiper Express mission becomes a reality. The potential for addressing issues such as these via observations with SIRTF will also be addressed.