Scientific Background

The Solar Nebula

Vesta and the Dawn mission

The Jovian Early Bombardment

The Solar Nebula

One of the less understood phases in the history of the Solar System is that of the Solar Nebula, when the young Sun was surrounded by its circumstellar disk of gas and dust. The Solar Nebula phase can be identified as the time interval starting with the condensation of the first solids (the Ca-Al-rich inclusions, CAIs in the following) about 4568 Ma ago (Bouvier & Wadhwa 2010) and ending at most 10 Ma later with the dispersal of the gas of the circumstellar disk surrounding the Sun (see e.g. Meyer 2009). The Solar Nebula is thus a relatively short phase, yet across this timespan several important processes were taking place. Planetesimals were forming and, in some cases, differentiating across the whole Solar System due to the decay of short-lived radionuclides. In the inner Solar System planetary embryos were accreting, reaching mass values ranging between those of the Moon and Mars. In the outer Solar System the planetary cores of the giant planets formed and accreted the nebular gas before its dispersal. We know from meteoritic constraints (see e.g. Scott 2007 and references therein) and theoretical models (see e.g. Morbidelli et al. 2009, Weidenschilling 2011) that the progenitors of the present-day asteroids formed in the Solar Nebula on a 1 Ma timescale. Jupiter and the other giant planets should have appeared later, about 3-5 Ma after CAIs (see e.g. Bottke et al. 2005, Scott 2006). The differentiation of the primordial asteroids took place extremely early in the history of the Solar System, i.e. about 1-2 Ma after the formation of CAIs (Baker et al. 2005; Bizzarro et al. 2005), therefore on the same timescale as the accretion of the asteroids themselves. Such primordial differentiation was due to the presence of short-lived radionuclides, mainly 26Al and 60Fe (Urey 1955) in bodies varying between tens and hundreds of kilometres in radius (Scott 2007, Yang et al. 2007).
The actual scenario for the formation of the planetesimals, its efficiency and the resulting initial size-frequency distribution, however, are still poorly constrained (see e.g. Morbidelli et al. 2009, Weidenschilling 2011 for a discussion). The same holds true also for the timescale and the formation region of the giant planets (see e.g. Lissauer & Stevenson 2007 for a discussion) and the extent and timescale of their migration due to the interaction with the Solar Nebula (see e.g. Papaloizou et al. 2007 and references therein). Moreover, it has been recently proposed that the evolution of the early Solar System could have followed a different path than the one generally assumed, with Jupiter penetrating into the inner Solar System and causing the dispersal of the planetesimals originally residing in the asteroid belt. In their "Grand Tack" scenario, Walsh et al. (2011) hypothesize Jupiter being initially located at about 3 AU and migrating first inward to about 1.5 AU due to its exchange of angular momentum with the gas in the Solar Nebula and then outward to its present orbit due to its locking in the 3:2 resonance with Saturn. According to the authors, this scenario would explain both the depletion of mass of the present asteroid belt and the small mass of Mars. In the "Grand Tack" scenario, the original population of the asteroid belt would be scattered away by Jupiter while migrating; the migration of the giant planet would also cause two new populations of planetesimals to take their place. The planetesimals that would populate what would become the inner asteroid belt initially formed at 1.5-2 AU. Similarly, the asteroids in the outer asteroid belt originated in the region extending beyond 3.5 AU.

Vesta and the Dawn mission

As discussed in Coradini et al. (2011a), one of the main issues in searching for signatures of such ancient events is that most planetary bodies used to study the records of the evolution of the Solar System, like the Moon or Mars, formed or completed their geological evolution after the dispersal of the Solar Nebula. As a consequence, we have little observational constrains on the relative timescale and the interplay of the different processes that shaped the early Solar System. In this context Vesta, one of the two targets of the NASA Dawn mission, occupies a unique position in the whole Solar System. Vesta, presently orbiting at a = 2.36 AU from the Sun, has long been identified as the possible parent body of the HED (Howardite, Eucrite and Diogenite) meteorites based on spectroscopic comparison with ground-based telescopes (McCord et al. 1970) and the realization that these basaltic meteorites are derived from the surface of a parent body whose dunite interior is not sampled in our meteorite collection and thus most likely is still intact (Consolmagno and Drake, 1977). This hypothesis has been recently strongly supported by the first results of the DAWN spacecraft (Coradini et al. 2011b), presently orbiting the asteroid, which confirmed the spectroscopic matching between HED meteorites and Vesta and showed that, on a global scale, the composition of its surface is consistent with that of eucritic and howarditic meteorites. Eucritic and diogenitic meteorites are basaltic achondrites (Hess & Henderson 1949; see also McSween et al. 2011 and references therein), i.e. fragments originating from a differentiated planetary body (Consolmagno & Drake 1977; see also Zuber et al. 2011 and references therein), while howarditic meteorites are composed by a mixture of eucritic and diogenitic material (Jérome & Goles 1971; see also McSween et al. 2011 and references therein). According to the analysis of Greenwood et al. (2005), the HED meteorite suite formed during early, global-scale melting events. If Vesta is the parent body of the HED meteorites, it differentiated extremely early in its history: according to the results of Bizzarro et al. (2005) in studying eucritic meteorites, Vesta should have differentiated about 3 Ma after CAIs. Early modelling of the thermal evolution and differentiation of Vesta by Ghosh & McSween (1998) suggested that, due to its size, the asteroid cooled down and solidified on a 10-100 Ma long timescale. Recent results by Schiller et al. (2011) on the crystallization timescale of the diogenitic meteorites (i.e. the material supposed to form the upper mantle of Vesta) suggest instead that the asteroid underwent a rapid cooling and that its geologically active phase ended in a few Ma. When compared with the timescale of formation of Jupiter (see e.g. Bottke et al. 2005; Scott 2006), these results would imply that Vesta ended its geological evolution before or contemporary to the birth of the giant planet.
While meteoritic and spectroscopic data suggest that Vesta could be the best preserved witness of the evolution of the early Solar System identified to date, the first estimates of the cratering record on the asteroid seem to indicate that crater counting cannot give us information about times earlier than 4 Ga ago (Neukum et al. 2011). Such age limit has been obtained by applying to Vesta the lunar crater production function rescaled to the asteroid belt (Neukum et al. 2011) and it is presently debated whether this age limit is real (e.g. due to a major impact that reset the surface of Vesta at a global scale, Schmedemann et al. 2012) or is due to the fact that the crater production function for Vesta is different from the one for the Moon (O'Brien et al. 2012). Moreover, there are indications that crater saturation could affect significantly the cratering record on Vesta and impose an age limit to the potential resolution of the crater counting technique (Turrini, Magni & Coradini 2011; Turrini, unpublished). As a consequence, a different approach has to be devised to investigate the ancient past of this asteroid.

The Jovian Early Bombardment

It was recently showed that the formation of Jupiter triggers a phase of primordial bombardment in the asteroid belt due to the rapid mass increase and the inward radial migration of the giant planet (Turrini, Magni & Coradini 2011, Turrini, Coradini & Magni 2012). This event, named the Jovian Early Bombardment (JEB in the following) results from the appearance of the mean motion resonances with Jupiter in the asteroid belt and from the scattering of planetesimals from the outer Solar System (Turrini, Magni & Coradini 2011). The bulk of the JEB is due to planetesimals affected by the resonances with Jupiter, in particular the 3:1 and the 2:1 resonances that are respectively located at about 2.5 AU and 3.3 AU when Jupiter is orbiting at a = 5.2 AU from the Sun. The intensity of the JEB on a given target body varies as a function of its orbital position relative to these resonances (Turrini, Magni & Coradini 2011, Turrini, Coradini & Magni 2012): a planetesimal located between the 3:1 and the 2:1 resonances (e.g. Ceres) would receive a flux of impactors far greater than the one affecting a planetesimal located just inside the 3:1 resonance (e.g. Vesta). Since the Jupiter is extremely efficient in exciting the orbits of the planetesimals affected by these resonances, the duration of the JEB is limited, being of the order of 0.5-1 Ma (Turrini, Magni & Coradini 2011). However, due to the greater population of bodies residing at the time in the asteroid belt relative to the present one (Weidenschilling 1977; see also Morbidelli et al. 2009; Weidenschilling 2011 for a discussion), the JEB is extremely violent. In particular, across the JEB collisional erosion played a more important role than catastrophic disruption in determining the fate of the planetesimals (Turrini, Coradini & Magni 2012).
Turrini, Magni & Coradini (2011) originally estimated that the JEB would completely erode the primordial crust of Vesta and cause large scale effusive phenomena analogous to the Lunar maria. Such results, however, were based on simplistic assumptions concerning the excavation efficiency of the impacts and did not take into account the fact that the re-accretion of the excavated material plays an important role due to the relatively high gravity of Vesta. In their improved analysis, Turrini et al. (2012) suggested two possible scenarios for the collisional evolution of Vesta across the JEB. In the first scenario (the cratering scenario) the JEB would saturate the original crust of Vesta with craters but would excavate it only on a local scale (i.e. at the location of the major craters, whose diameter exceeds about 150 km). In the second scenario (the resurfacing scenario), the JEB would erode the crust of Vesta and either expose the molten mantle or trigger effusive phenomena at a regional scale, causing the global resurfacing of the asteroid. The surface erosion of Vesta is strictly linked to the extent of Jupiter's migration (see Table 1): as a consequence, the present surface composition of the asteroid can allow us to constrain the dynamical evolution of Jupiter and the intensity of the JEB. As an example, in the scenario where Jupiter migrates by 1 AU (see Table 1), the JEB would possibly strip the eucritic material from the surface of Vesta, inconsistently with the data supplied by the Dawn mission.
As showed by Turrini, Magni & Coradini (2011), the JEB is triggered by the formation of Jupiter: the migration of the giant planet can enhance the intensity of the bombardment but is not required to start it. As a consequence, the JEB should have taken place also in the scenario suggested by Walsh et al. (2011). The inner orbit on which Jupiter is assumed to form in the "Grand Tack" scenario and the extensive migration of the giant planet could have major implications for the collisional erosion caused by the JEB. In this scenario, Vesta likely formed closer to the Sun, possibly between 1.5-2 AU (Morbidelli, pers. comm.), and was delivered to its orbital region when the asteroid belt was re-populated by the migration of the giant planets. Estimating the effects of the JEB on Vesta in the "Grand Tack" scenario would thus allow us to assess its viability as an evolutionary path for the early Solar System.

Table 1: the effects of the JEB on Vesta from Turrini et al. (2012). The table shows the estimated number of impacts (N_coll), the eroded mass expressed in terms of the thickness of the excavated shell using the scaling law from (Svetsov 2011), the number of high-energy (0.01 ≤ Q_D/Q*_D < 1) and critical (Q_D/Q*_D ≥ 1) impacts. Q*_D is the critical disruption energy as defined by Benz & Asphaugh (1999). The ISS and OSS labels indicate respectively the impactors from inner and outer Solar System.