1. surveyed to date. While telescopes are showing us that trans-neptunian objects are relatively common and are providing information about their disk-averaged surface composition, spacecraft missions are necessary to explore the detailed nature of these icy bodies.
  2. Reservoirs of primitive materials. While KBOs may not be pristine relics of the original solar nebula, it is in the outer solar system that we might expect to find the least-modified materials as well as samples that have suffered a range of degrees of modification. These bodies can provide the links for understanding the relationships among the interstellar medium, the solar nebula, and current materials in the solar system.
  3. Processes that reveal the solar system's origin and evolution. The observable characteristics of objects tell us about the processes they have experienced. The distribution of a population of objects in orbital phase space provides clues about their origins and the dynamical processes that control them over long periods. The distribution of sizes within a population reveals the relative importance of accretion versus collisional erosion. The wide range of sizes and different collisional histories among objects in the trans-neptunian region implies varying degrees of internal differentiation. Surface geology provides important constraints on an object's thermal history. Surface chemistry and atmospheric properties reveal processes of outgassing, photochemistry, transport, and redeposition of volatiles.
  4. Links to extrasolar planets. Studies of early stars similar to the Sun have shown that some are surrounded by disks of dust that are thought to be derived from collisions between comets. It is natural, therefore, to relate such dust disks to the Kuiper Belt. Applying knowledge of the Kuiper Belt to stellar dust disks suggests that the inner boundary exhibited by some disks may be an indication of the existence of planets. Comparisons of the Kuiper Belt with these dust disks is an important component of the new field of comparative studies of solar systems.
  5. Prebiotic chemistry. As remnants of the early solar system, trans-neptunian objects can provide critical clues about processes of prebiotic chemistry and about the materials that would have been delivered to the early Earth and may have formed the source of volatile materials from which life arose here and possibly on other planets of this and other solar systems.

These five themes are not on an equal footing. The first three are well-established areas of scientific investigation and are backed up by a substantial body of observational and theoretical understanding. The last two, however, are more speculative. They are included here because they raise a number of interesting possibilities that seem particularly suited to an interdisciplinary approach uniting planetary scientists with their colleagues in the astrophysical and life science communities.

Although not considered in any detail in this report, the distant outer solar system also has direct relevance to Earth and the other terrestrial planets because it is the source of comets that bring volatiles into the inner solar system. The resulting inevitable impacts between comets and other planetary bodies can play a major role in the evolution of planetary surfaces and atmospheres. Indeed, comets can also play major roles in the evolution of life as suggested by, for example, the Cretaceous-Tertiary boundary bolide and the extinction of the dinosaurs.

Trans-Neptunian Objects.

The five major themes described above involve general scientific issues that apply to the trans-neptunian region as a whole. Below COMPLEX summarizes the current knowledge and outstanding issues of the separate major types of objects in the trans-neptunian region.

Triton

Triton is by far the best-explored icy body in the distant outer solar system,4 and, as such, sets the context for the discussion of the other bodies. Triton is thought to be a planetary body that was captured by Neptune in the distant past. Voyager 2's flyby of Triton demonstrated the wealth of information available only from a spacecraft mission. Triton's density suggests that it has a rock core (70% by mass) surrounded by ice. Tidal heating due to orbital evolution and/or collision(s) with other satellites probably caused differentiation of the interior. Geological



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