John Cooper
Goddard Space Flight Center
Space Weathering of Icy Bodies in the New Outer Solar System
Since the discovery of Pluto in 1930, its moon Charon in 1978, and the first Kuiper Belt Object in 1992, our remote observational knowledge of Kuiper Belt Objects has been rapidly expanding, even as the Voyager 1 and 2 spacecraft have been directly exploring the space weathering environment of these bodies. The Pluto/Charon system, primary target of the recently launched New Frontiers mission, is now literally recognized merely as the “tip of the iceberg” for potentially dozens of outermost solar system bodies in this size class, notably 2003 UB313 (Xena), a thousand smaller (100 km) KBOs already known, and ten of thousands yet undetected. Some of the known Scattered Disk Objects have passed innumerable times through the heliosheath region, now being traversed by Voyager 1 and likely soon by Voyager 2, and even far out into the local interstellar plasma environment. The relatively stable configuration of the outer solar system for the past four billion years requires that the surface, and any atmospheric, chemical composition must have long ago reached equilibrium with respect to space weathering effects. The most relevant heliospheric interactions include radiolytic chemistry induced by energy loss of energetic particles and surface erosion arising from ion sputtering and micrometeoroid impacts. A delicate balance between erosional processes and radiolysis induced by cosmic ray ionization may account for mostly red colors of the dynamically ‘cold old’ KBOs. A greater diversity of colors for ‘hot’ KBOs and other dynamical classes requires a broader range of surface processing effects by irradiation, erosion, and collisions. In the context of increasing cosmic ray fluxes beyond the Classical KBOs to the heliosheath, as measured by the Voyagers, and projected into the local interstellar environment, high albedo of large distant objects such as 2003 UB313 requires resurfacing processes suggestive of interior activity. Cumulative radiolytic inputs from cosmic rays may alternatively be considerable as exogenic energy sources for these processes. Remotely measurable sudden transients in neutral atom and x-ray emissions from surfaces and any extended atmospheres would be expected as the more eccentric Scattered Disk Objects are irradiated at greatly different intensities upstream and downstream of the termination shock.