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The Complex History of Trojan Asteroids

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The Complex History of Trojan Asteroids 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 primi- tive 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 signifi- cant 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. 1. INTRODUCTION the term Trojan eventually came to be used for any object trapped in the L4 or L5 region of any body. Nevertheless, only Originally considered as simply an extension of the main Jupiter Trojans are named from the Iliad, and when used with- belt, Trojan asteroids have become recognized as a large out a designator, “Trojan” refers either specifically to Jupiter and important population of small bodies. Trojans share Ju- Trojans or sometimes to the collection of all bodies in stable piter’s orbit around the Sun, residing in the L4 and L5 stable Lagrange points. Several other solar system bodies also sup- Lagrange regions. Leading and trailing Jupiter by 60°, these port stable Trojan populations, including Mars, Neptune, and are regions of stable equilibrium in the Sun-Jupiter-asteroid two satellites of Saturn (Tethys and Dione). The populations three-body gravitational system. The moniker “Trojan” is coorbiting with Mars and the two saturnian moons appear to an artifact of history — the first three objects discovered in be quite small, but Neptune’s family of Trojans is thought Jupiter’s Lagrange regions were named after heroes from the to be extensive (e.g., Sheppard and Trujillo, 2010). Planets Iliad. The naming convention stuck for Jupiter’s swarms, and can destabilize each other’s Lagrange regions. For instance, 203 204 Asteroids IV Saturn and Uranus do not have stable Trojan populations to explain the capture of Trojans settled on two potential because the other planets perturb the orbits on timescales that mechanisms as most likely: gas drag in the early nebula are short relative to the age of the solar system. The Jupiter (e.g., Peale, 1993) and capture during the growth of Jupiter Trojans, which are the focus of this chapter, are estimated (Marzari and Scholl, 1998a). Both mechanisms predict that to be nearly as populous as the main belt and have stability the present-day Trojans formed in the middle of the solar timescales that exceed the age of the solar system. nebula, near where they currently reside. Since there is no The history of the exploration of Trojan asteroids begins other reservoir of material available for study from this with Max Wolf, who, in the late nineteenth century, was region, the Trojans would, in this case, be an exciting win- the first to turn to wide-field astrophotography for asteroid dow into the conditions of the solar nebula near the snow discovery (Tenn, 1994). In early 1906 he detected an object line and near Jupiter’s formation region. However, neither near Jupiter’s L4 point, marking the first observational con- mechanism fully explains the current orbital properties of firmation of Lagrange’s three-body solution. An object was Trojans, particularly the high inclinations. detected near L5 later in 1906 by August Kopf, then another More recently, Morbidelli et al. (2005) proposed the near L4 in early 1907. These were later named (588) Achilles, capture of Trojans from the same population from which (617) Patroclus, and (624) Hektor, respectively (Nicholson, the Kuiper belt originated. The Nice model postulates that 1961). As physical studies of asteroids accelerated in the resonant interactions between Jupiter and Saturn temporarily 1970s and 1980s, the Trojans were included, and the first destabilize the orbits of Uranus and Neptune, which move sizes, albedos, rotation periods, and (visible wavelength) into the primordial Kuiper belt, scattering material widely spectra were published (e.g., Dunlap and Gehrels, 1969; Crui- across the solar system. In this framework, Jupiter’s primor- kshank, 1977; Hartmann and Cruikshank, 1978; Chapman dial Trojan population is lost and the Lagrange regions are and Gaffey, 1979). Gradie and Veverka (1980) established repopulated with this scattered Kuiper belt material. Dotto the paradigm, which is still commonly invoked, that the et al. (2008) include a description of this capture scenario low albedo and red spectral slopes are due to the presence and a discussion of the implications for Trojans. This mecha- of complex organic molecules on Trojan surfaces. By 1989, nism predicts that Trojans formed much farther out in the when the Asteroids II book was published, 157 Trojans were solar nebula (~20–35 AU). In this case, the Trojans would known, from which Shoemaker et al. (1989) estimated a total represent the most readily accessible repository of Kuiper population comparable to that of the main belt — an esti- belt material. In the years since those reviews, some aspects mate that still stands, to within a factor of a few. Discovery of the Nice model have been reworked, and refinements to and characterization accelerated rapidly for Trojans (as with this newer mechanism for Trojan capture have been made. all asteroids) through the end of the twentieth century, and Unraveling the complex history of the Trojans promises by the time of Asteroids III in 2002, 993 Trojans had been key insight into solar system evolution. As primitive objects, discovered. The number now stands at 6073. Trojan compositions provide direct indicators of the condi- Summarizing the state of knowledge of the physical tions of the nebula in the region(s) in which they formed. properties of Trojans at the turn of the twenty-first century, As a population of small bodies, Trojans act as unique Barucci et al. (2002) describe a population that is far more probes of the history, interaction, and physical processing homogeneous than the main belt, with uniformly low albedos of the solar system. In this chapter, we review the physical (pv ~ 0.03 to 0.07) and featureless, red-sloped spectra at vis- properties of Trojan asteroids and scenarios for their origin ible and near-infrared (VNIR) wavelengths (0.4–2.5 µm). A and evolution. We rely heavily on previous reviews for much later review by Dotto et al. (2008) reports additional spectral of the early work (Shoemaker et al., 1989; Barucci et al., observations, particularly of members of potential collisional 2002; Marzari et al., 2002a; Dotto et al., 2008; Slyusarev families (Dotto et al., 2006; Fornasier et al., 2007), the and Belskaya, 2014), focusing here on new observations and detection of signatures of fine-grained silicates (Emery et recent advances in the knowledge of Trojans. al., 2006), and the first bulk-density measurement Marchis( et al., 2006). From these properties and their locations at 2. PHYSICAL PROPERTIES 5.2 AU, Trojans have generally been inferred to contain a large fraction of H2O ice hidden from view by a refractory 2.1. Size Distribution mantle, and a higher abundance of complex organic mol- ecules than most main-belt asteroids (MBAs). Since those Most asteroid surveys are conducted in visible (reflected) reviews, significant strides have been made in the physical light, from which it is not possible to derive the size unless characterization of Trojans, which in turn provide new in- the albedo is known. Studies of size distributions, therefore, sights into the nature of these enigmatic bodies. often use absolute magnitude (Hv) as a proxy for size. For Marzari et al. (2002a) review models for the capture of a population like the Trojans, where the albedo distribution Trojans and the stability of the Lagrange regions that had is very uniform (see section 2.2), the Hv distribution should developed up to that point.
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