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Geological History of Asteroid 4 Vesta: the “Smallest Terrestrial Planet”

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Geological History of Asteroid 4 Vesta: the “Smallest Terrestrial Planet” Keil: Geological History of 4 Vesta 573 Geological History of Asteroid 4 Vesta: The “Smallest Terrestrial Planet” Klaus Keil University of Hawai‘i The asteroid 4 Vesta is the only known differentiated asteroid with an intact internal structure, probably consisting of a metal core, an ultramafic mantle, and a basaltic crust. Considerable evidence suggests that the HED meteorites are impact ejecta from Vesta, and detailed studies of these meteorites in terrestrial laboratories, combined with ever more sophisticated remote sensing studies of the asteroid, have resulted in a good understanding of the geological evolution of this fascinating object. Extensive mineralogical, petrological, geochemical, isotopic, and chro- nological data suggest that heating, melting, and formation of a metal core, a mantle, and a basaltic crust took place in the first few million years of solar system history. It is likely that many more Vesta-like asteroids formed at the dawn of the solar system but were destroyed by impact, with the iron meteorites being remnants of their cores. Such differentiated objects may have played an important role in the accretion and formation of the terrestrial planets, and it is therefore highly desirable to explore by spacecraft this world that can be viewed as the smallest of the terrestrial planets. 1. INTRODUCTION pyroxenites (diogenites), and breccia mixtures principally of these two rock types (howardites) (e.g., Mason, 1962; Takeda The world’s collections contain meteorites from at least et al., 1976; Takeda, 1979; Mittlefehldt et al., 1998, and ref- three sources: asteroids (tens of thousands), Earth’s Moon erences therein) are, in all likelihood, impact-produced frag- (~26), and Mars (~26). Incredible progress has been made ments of Vesta (e.g., McCord et al., 1970; Consolmagno in recent years in the study of asteroidal meteorites, stimu- and Drake, 1977; Drake, 1979, 2001; Binzel and Xu, 1993; lated by the discoveries of thousands of new specimens in Gaffey, 1997). Their detailed study in terrestrial laborato- Antarctica and hot deserts. Among these many thousands ries, combined with the remote sensing data and numerous of meteorites are many new types and rare individuals that modeling studies, have contributed to a reasonably good represent asteroidal parent bodies heretofore unrepresented understanding of the complex history of this unique world. in our collections. Detailed studies of asteroidal meteorites Because much of the geological history of Vesta (i.e., have shown that, based on their mineralogical, chemical, heating, melting, fractionation, extrusion, and solidification and isotopic properties, an astonishing ~135 different aster- of the basaltic crust) took place in the first 10 m.y. of solar oids are represented (Meibom and Clark, 1999). Although system history (e.g., Lugmair and Shukolyukov, 1998; Srini- nearly all asteroids of which we have samples have been vasan et al., 1999; Nyquist et al., 2001), it is of great inter- affected by postaccretionary heating to some degree (Keil, est for understanding the earliest differentiation of solar 2000), many (108 of the 135) were actually partially or com- system bodies at the dawn of the solar system. The aster- pletely melted and differentiated (Meibom and Clark, 1999). oid is also of interest because differentiated objects of this One of the most fascinating asteroids is 4 Vesta. Remote type are thought to have been the embryos that may have sensing shows that it is a differentiated, nearly intact object played an important role in the accretion and formation of with a basaltic crust (e.g., McCord et al., 1970) and ultra- the larger terrestrial planets (e.g., Taylor and Norman, 1990; mafic mantle rocks (pyroxenite; olivine-bearing) exposed in Carlson and Lugmair, 2000). a huge impact crater (e.g., Binzel et al., 1997; Gaffey, 1997; In the present paper, I summarize the most important Thomas et al., 1997a). Modeling (e.g., Righter and Drake, properties of Vesta, based on remote sensing data and the 1997; Dreibus et al., 1997) and density estimates (e.g., study of HEDs. I also use these data, supported by modeling Thomas et al., 1997b) further suggest that Vesta has a metal studies, to outline Vesta’s early geological history. core and thus the asteroid is a differentiated object with crust, mantle, and core, analogous in its structure to the 2. ORBIT, SIZE, SHAPE, MASS, terrestrial planets, albeit much smaller in size. Vesta can AND DENSITY OF 4 VESTA therefore be thought of as the smallest of the terrestrial planets. Fortuitously, we are not restricted to remote sensing Asteroid 4 Vesta was discovered by H. W. Olbers in Bre- data for understanding the geological history of Vesta. The men, Germany, on March 29, 1807 (Pilcher, 1979). It or- howardite-eucrite-diogenite meteorites (or HEDs; Takeda et bits the Sun at a mean heliocentric distance of a = 2.36 AU, al., 1983), a large suite of differentiated basalts (eucrites), has a proper eccentricity of e = 0.097, and a proper sine of 573 574 Asteroids III inclination of sin i = 0.112 (Williams, 1989). Based on Hub- ble Space Telescope (HST) images, Thomas et al. (1997b) concluded that the asteroid is a triaxial ellipsoid of radii 289, 1.1 280, and 229 km, all ±5 km. This compares well with the earlier data of McCarthy et al. (1994) of 278 ± 12, 261 ± 9, 1.0 and 232 ± 5 km (see also Drummond et al., 1998). Its mean radius is 258 ± 12 km and its volume is ~7.19 ± 0.87 × 0.9 107 km3 (Thomas et al., 1997b). The mass of the asteroid has been estimated by Schubart and Matson (1979) to be 0.8 1.38 ± 0.12 × 10–10 M or 2.75 ± 0.24 × 1020 kg, and by Standish and Hellings (1989) as 1.5 ± 0.3 × 10–10 M or 0.7 2.99 ± 0.60 × 1020 kg [note that the recent mass estimate, based on perturbations of Vesta on 26 selected asteroids of Normalized Reflectivity Spectral 0.6 1.36 ± 0.05 × 10–10 M by Michalak (2000), is very close to that of Schubart and Matson (1979)]. Using the Thomas 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 et al. (1997b) volume and the Schubart and Matson (1979) Wavelength (µ) mass yields a mean density of 3800 ± 600 kg/m3, whereas the Standish and Hellings (1989) mass yields a density of 4100 ± 950 kg/m3 [Thomas et al. (1997a) give 3500 ± 400 Fig. 1. Laboratory measurements of the spectral reflectivity of and 3900 ± 800 respectively]. the Nuevo Laredo eucrite (solid line), compared to telescope data points from Vesta, suggesting that Vesta is covered by eucritelike 3. THE HED-VESTA-VESTOID material. Solid circles, data obtained at Cerro Tololo, Chile, De- CONNECTION cember 1969; open circles, data obtained at Mount Wilson, Cali- fornia, October 1968. The standard deviation for each average value is shown as an error bar. Reprinted with permission from The proposal that the HED meteorites are impact ejecta McCord et al. (1970). Copyright 1970, American Association for from Vesta has important implications for understanding the the Advancement of Science. geological history of the asteroid. Detailed studies of the HEDs in terrestrial laboratories with sophisticated analyti- cal techniques have provided a wealth of ground truth data that supplement the “global field geology observations” complete surface of Vesta that is indicative of the presence made possible by ever-improving remote sensing tech- of relatively Ca-rich augite. Recent Earth-based (Gaffey, niques. The link between the HED meteorites and Vesta was 1997) and HST images (Binzel et al., 1997) of Vesta as it originally based on the similarities in the compositions of rotates have confirmed earlier observations of geological the HEDs and the surface mineralogy of Vesta, as deter- diversity across the asteroid (e.g., Gaffey, 1983) and re- mined by spectroscopy. More recently, dynamical consid- vealed a heterogeneous surface, consistent with a compo- erations that connect the so-called “vestoids” of the Vesta sition similar to that of the HED meteorites. Specifically, ν family to Vesta and to the 3:1 and 6 escape hatch reso- Gaffey (1997) noted that Vesta appears to have an old, age- nances have added strong evidence in favor of this propo- darkened surface akin to the howardites and polymict eu- sition (Binzel and Xu, 1993). crites, with fresher rocks akin to diogenites and olivine- McCord et al. (1970) were the first to determine, through bearing material exposed in impact craters. Binzel et al. spectroscopy in the visible and near-infrared using ground- (1997) note that the eastern hemisphere of Vesta is domi- based telescopes, that the surface of Vesta exhibits absorp- nated by what appear to be impact-excavated plutonic rocks tion features typical for low-Ca pyroxene and that it is made of Mg-rich and Ca-poor pyroxene, akin to diogenites, similar in composition to certain basaltic achondrites (i.e., with some units containing a substantial amount of oliv- eucrites) (Fig. 1). Since then, many Earth-based measure- ine. The western hemisphere, on the other hand, is domi- ments in the visible and infrared (e.g., Larson and Fink, nated by Fe-rich and relatively Ca-rich pyroxene, consistent 1975; McFadden et al., 1977; Feierberg et al., 1980; Gaffey, with eucrites. 1983, 1997, and references therein; Cochran and Vilas, One argument against the origin of the HEDs from Vesta 1998) have confirmed the McCord et al. (1970) findings. and the vestoids has been that space weathering will make For example, Feierberg et al. (1980) determined that the basaltic asteroids look like S asteroids. Hence, the sharp, surface of Vesta consists of a mixture of pyroxene (Fs50 ± 5) uniquely defined basaltic reflection spectra of Vesta and the and plagioclase, with a pyroxene/plagioclase ratio of 1.5– vestoids must be due to recent (<10 Ma) impact resurfac- 2.0, consistent with Vesta’s surface being covered by a ing and covering of their surfaces by fresh basalt debris mixture of eucrites and howardites, suggesting that Vesta (Wasson and Chapman, 1996; Wasson et al., 1996).
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