Asteroids IV

1. INTRODUCTION Originally considered as simply an extension of the main belt, Trojan asteroids have become recognized as a large and important population of small bodies. Trojans share Jupiter ’s orbit around the Sun, residing in the L4 and L5 stable Lagrange regions. Leading and trailing Jupiter by 60°, these are regions of stable equilibrium in the Sun-Jupiter-asteroid three-body gravitational system. The moniker “Trojan” is an artifact of history — the first three objects discovered in Jupiter’s Lagrange regions were named after heroes from the Iliad. The naming convention stuck for Jupiter’s swarms, and the term Trojan eventually came to be used for any object trapped in the L4 or L5 region of any body. Nevertheless, only Jupiter Trojans are named from the Iliad, and when used without a designator, “Trojan” refers either specifically to Jupiter Trojans or sometimes to the collection of all bodies in stable Lagrange points. Several other solar system bodies also support stable Trojan populations, including Mars, Neptune, and two satellites of Saturn (Tethys and Dione). The populations coorbiting with Mars and the two saturnian moons appear to be quite small, but Neptune’s family of Trojans is thought to be extensive (e.g., Sheppard and Trujillo, 2010). Planets can destabilize each other’s Lagrange regions. For instance, 203 Emery J. P., Marzari F., Morbidelli A., French L. M., and Grav T. (2015) The complex history of Trojan asteroids. In Asteroids IV (P. Michel et al., eds.), pp. 203–220. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch011. The Complex History of Trojan Asteroids Joshua P. Emery University of Tennessee Francesco Marzari Università di Padova Alessandro Morbidelli Lagrange Laboratory, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS Linda M. French Illinois Wesleyan University Tommy Grav Planetary Science Institute The Trojan asteroids, orbiting the Sun in Jupiter’s stable Lagrange points, provide a unique perspective on the history of our solar system. As a large population of small bodies, they record important gravitational interactions in the dynamical evolution of the solar system. As primitive bodies, their compositions and physical properties provide windows into the conditions in the solar nebula in the region in which they formed. In the past decade, significant advances have been made in understanding their physical properties, and there has been a revolution in thinking about the origin of Trojans. The ice and organics generally presumed to be a significant part of Trojan composition have yet to be detected directly, although the low density of the binary system Patroclus (and possibly low density of the binary/moonlet system Hektor) is consistent with an interior ice component. By contrast, fine-grained silicates that appear to be similar to cometary silicates in composition have been detected, and a color bimodality may indicate distinct compositional groups among the Trojans. Whereas Trojans had traditionally been thought to have formed near 5 AU, a new paradigm has developed in which the Trojans formed in the proto-Kuiper belt, and were scattered inward and captured in the Trojan swarms as a result of resonant interactions of the giant planets. Whereas the orbital and population distributions of current Trojans are consistent with this origin scenario, there are significant differences between current physical properties of Trojans and those of Kuiper belt objects. These differences may be indicative of surface modification due to the inward migration of objects that became the Trojans, but understanding of appropriate modification mechanisms is poor and would benefit from additional laboratory studies. Many open questions about this intriguing population remain, and the future promises significant strides in our understanding of Trojans. The time is ripe for a spacecraft mission to the Trojans, to transform these objects into geologic worlds that can be studied in detail to unravel their complex history. 204   Asteroids IV Saturn and Uranus do not have stable Trojan populations because the other planets perturb the orbits on timescales that are short relative to the age of the solar system. The Jupiter Trojans, which are the focus of this chapter, are estimated to be nearly as populous as the main belt and have stability timescales that exceed the age of the solar system. The history of the exploration of Trojan asteroids begins with Max Wolf, who, in the late nineteenth century, was the first to turn to wide-field astrophotography for asteroid discovery (Tenn, 1994). In early 1906 he detected an object near Jupiter’s L4 point, marking the first observational confirmation of Lagrange’s three...