Origin, Internal Structure and Evolution of 4 Vesta

Abstract

Asteroid 4 Vesta is the only preserved intact example of a large, differentiated protoplanet like those believed to be the building blocks of terrestrial planet accretion. Vesta accreted rapidly from the solar nebula in the inner asteroid belt and likely melted due to heat released due to the decay of 26 Al. Analyses of meteorites from the howardite-eucritediogenite (HED) suite, which have been both spectroscopically and dynamically linked to Vesta, lead to a model of the asteroid with a basaltic crust that overlies a depleted peridotitic mantle and an iron core. Vesta's crust may become more mafic with depth and might have been intruded by plutons arising from mantle melting. Constraints on the asteroid's moments of inertia from the long-wavelength gravity field, pole position and rotation, informed by bulk composition estimates, allow tradeoffs between mantle density and core size; cores of up to half the planetary radius can be consistent with plausible mantle compositions. The asteroid's present surface is expected to consist of widespread volcanic terrain, modified extensively by impacts that exposed the underlying crust or possibly the mantle. Hemispheric heterogeneity has been observed by poorly resolved imaging of the surface that suggests the possibility of a physiographic dichotomy as occurs on other terrestrial planets. Vesta might have had an early magma ocean but details of the early thermal structure are far from clear owing to model uncertainties and paradoxical observations from the HEDs. Petrological analysis of the eucrites coupled with thermal evolution modeling recognizes two possible mechanisms of silicate-metal differentiation leading to the formation of the basaltic M.T. Zuber et al.achondrites: equilibrium partial melting or crystallization of residual liquid from the cooling magma ocean. A firmer understanding the plethora of complex physical and chemical processes that contribute to melting and crystallization will ultimately be required to distinguish among these possibilities. The most prominent physiographic feature on Vesta is the massive south polar basin, whose formation likely re-oriented the body axis of the asteroid's rotation. The large impact represents the likely major mechanism of ejection of fragments that became the HEDs. Observations from the Dawn mission hold the promise of revolutionizing our understanding of 4 Vesta, and by extension, the nature of collisional, melting and differentiation processes in the nascent solar system. Large Asteroids as Planetary EmbryosThe largest asteroids are central to understanding nascent planetary evolution, and among these 4 Vesta plays a unique role in the reconstruction of the physical and chemical processes that comprise collectively terrestrial planet accretion. Vesta is alone among the largest asteroids in having a basaltic crust (McCord et al. 1970) indicative of silicate volcanic and/or plutonic processes (Binzel et al. 2003). Observations of surface spectra (McCord et al. 1970;Binzel and Xu 1993) in concert with information on ...

The rotational motion of Vesta

“…As discussed in section 2.2 the determination of the three moments of inertia of Vesta (Eq. 1) is underestimated from the two gravity coefficients C 20 and C 22 obtained by Dawn, and we develop a two-layer model as suggested by Zuber et al (2011) and Russell et al (2012) to fix the value of the polar moment of inertia C/MR 2 . The value of C/MR 2 ranges between 0.38 and 0.42 (∼ 10%) depending on the size of the core and the jump density between the core and the mantle.…”

“…We compute the polar moment of inertia of Vesta for two sets of interior models: ρ c = 7100 kg/m 3 , r c = 113 km and ρ c = 7800 kg/m 3 , r c = 107 km (Russell et al 2012). By conserving the mass, we obtain a density for the mantle of 3148.2 kg/m 3 and 3148.4 kg/m 3 that is in the range of mantle density determined in Zuber et al (2011). The resulting polar moment of inertia is C/MR 2 = 0.4168 and C/MR 2 = 0.4161, which are very close.…”

Vesta and Ceres: Crossing the History of the Solar System

“…This connection, if confirmed, would imply that the asteroid is differentiated (see e.g. Drake 2001; Keil 2002, and references therein and the chapters by McSween et al 2010 andZuber et al 2011). Moreover, the 40 Ar-39 Ar ages of the oldest HED meteorites (see Keil 2002; Table 1 Collisional erosion of Vesta due to primordial planetesimals formed in quiescent (Coradini et al 1981) and turbulent disks (Morbidelli et al 2009;Chambers 2010) and due to collisionally evolved planetesimals (Morbidelli et al 2009) in the four migration scenarios of Jupiter considered by Turrini et al (2011).…”

“…Federico et al 2011). Several attempts were made in order to model the thermal evolution of Vesta: we refer the readers to Keil (2002) and to the chapter by Zuber et al (2011) for a detailed discussion. Based on geochemical and mineralogical observations, generally it is assumed that Vesta underwent an extensive differentiation, and that was completely melted and homogenized.…”

The dynamical environment of asteroid 21 Lutetia according to different internal models

“…The layers size and density are constrained to the model of internal structures of Vesta discussed in Park et al (2014); Konopliv et al (2014) and Zuber et al (2011). We preserve the total mass of Lutetia by fixing the medium density at 3.4 g.cm −3 .…”