Earth is characterized by a relative uniformity of biochemical processes, i.e., domination by a few elements—carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS)—and the same processes being continuously repeated in many places. Before life began, there must have been a period of chemical evolution in which the primordial chemical diversity underwent a process of selection that eventually led to self-organizing chemical systems. The low temperatures found in the outermost regions of the solar system inhibit chemical interactions, and such regions thus are likely to preserve evidence of this diversity.
Kuiper Belt objects may be repositories of evidence about the process of solar system formation and may offer insights on how the biogenic elements (CHNOPS) and their compounds can affect the formation of the planets themselves. The Kuiper Belt should be the place to find evidence of carbonaceous materials that may have played a role in the aggregation of grains in the early solar nebula. Even though a direct relationship between these carbon compounds and the chemistry of life has not been demonstrated and remains controversial, it is possible that some of the compounds essential for life may have been essential to planetary formation as well. As such, it may be that the existence of Kuiper Belt objects about a star could be an indication that prebiotic chemical evolution might also be occurring in that solar system. Observational programs designed to detect extrasolar planets might someday provide information that could be correlated with the distribution of life elsewhere in the galaxy.
Objects from the trans-neptunian region are periodically perturbed into the inner solar system, where they are likely to play an important role as a source of volatiles and as potential impactors.
There is mounting evidence, both from modeling and from measurements of terrestrial samples carrying mantle-derived gases (e.g., basalts from mid-ocean ridges), that the initial noble-gas inventories on and in Earth were similar to those of the Sun.37 If so, where did these gases come from? The noble gases trapped in most meteorites, for example, have a nonsolar isotopic composition. Many possible sources have been suggested. These include accreted material irradiated by an early solar wind; direct adsorption of solar-composition nebular gases on planetary accretion cores or on preaccretion dust; and low-temperature occlusion of nebular gases in icy bodies from the outer solar system which were later perturbed inward and rained down on the inner planets. While the concept of an icy-planetesimal source has many supporters, it is somewhat ad hoc because the noble-gas abundances of trans-neptunian objects have not been observed directly.
In addition to their role as possible conveyors of volatile materials to the inner solar system, the trans-neptunian objects perturbed into the inner solar system (i.e., comets) have a finite probability of colliding with planetary surfaces. In doing so, these impactors not only influence the formation and evolution of planetary surface features, but also may possibly influence the evolution of living organisms. A prime example of this is the theory that the mass extinction of dinosaurs and other species at the end of the Cretaceous period was triggered by a cometary impact.
1. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 33–34.
2. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995–2010, National Academy Press, Washington, D.C., 1994, pp. 13–14.
3. D.P. Cruikshank, ed., Neptune and Triton, University of Arizona Press, Tucson, Arizona, 1995.
4. For a current review see, for example, S.A. Stern and D.J. Tholan, eds. Pluto and Charon, University of Arizona Press, Tucson, Arizona, 1997.
5. For a recent review of the Kuiper Belt see, for example, P. Weissmann, “The Kuiper Belt,” Annual Reviews of Astronomy and Astrophysics 33:327, 1995.
6. P. Weissman and H.F. Levison, “The Population of the Trans-Neptunian Region: The Pluton-Charon Environment,” Pluto and Charon, S.A. Stern and D.J. Tholen, eds., University of Arizona Press, Tucson, Arizona, 1997, p. 559.
7. I.P. Williams et al., “The Slow-Moving Objects 1993 SB and 1993 SC,” Icarus 116:180, 1995.