Abstract
A new model for terrestrial planet formation (Hansen [2009]. Astrophys. J., 703, 1131–1140; Walsh, K.J., et al. [2011]. Nature, 2011, 206–209) has explored accretion in a truncated protoplanetary disk, and found that such a configuration is able to reproduce the distribution of mass among the planets in the Solar System, especially the Earth/Mars mass ratio, which earlier simulations have generally not been able to match. Walsh et al. (Walsh, K.J., et al. [2011]. Nature, 2011, 206–209) tested a possible mechanism to truncate the disk—a two-stage, inward-then-outward migration of Jupiter and Saturn, as found in numerous hydrodynamical simulations of giant planet formation. In addition to truncating the disk and producing a more realistic Earth/Mars mass ratio, the migration of the giant planets also populates the asteroid belt with two distinct populations of bodies—the inner belt is filled by bodies originating inside of 3AU, and the outer belt is filled with bodies originating from between and beyond the giant planets (which are hereafter referred to as ‘primitive’ bodies).One implication of the truncation mechanism proposed in Walsh et al. (Walsh, K.J., et al. [2011]. Nature, 2011, 206–209) is the scattering of primitive planetesimals onto planet-crossing orbits during the formation of the planets. We find here that the planets will accrete on order 1–2% of their total mass from these bodies. For an assumed value of 10% for the water mass fraction of the primitive planetesimals, this model delivers a total amount of water comparable to that estimated to be on the Earth today. The radial distribution of the planetary masses and the dynamical excitation of their orbits are a good match to the observed system. However, we find that a truncated disk leads to formation timescales more rapid than suggested by radiometric chronometers. In particular, the last giant impact is typically earlier than 20Myr, and a substantial amount of mass is accreted after that event. This is at odds with the dating of the Moon-forming impact and the estimated amount of mass accreted by Earth following that event. However, 5 of the 27 planets larger than half an Earth mass formed in all simulations do experience large late impacts and subsequent accretion consistent with those constraints.
Highlights
The timeline for the important processes of terrestrial planet formation extends from the condensation of the first solids $4.567–4.568 Gyr ago (Amelin et al, 2002; Bouvier and Wadhwa, 2010; Connelly et al, 2012) until the end of heavy bombardment of the inner Solar System around 4.1–3.8 Gyr ago (Tera et al, 1974; Chapman et al, 2007; Bottke et al, 2012)
There are additional aspects of the terrestrial planet formation process that were not analyzed in detail in Walsh et al (2011) that we will include here, such as chronological constraints related to the accretion timescales of the terrestrial planets, and the planetesimal accretion following the last giant impact, which is relevant to the Earth’s ‘‘late veneer’’ of highly siderophile elements
The inward-thenoutward migration of Jupiter in that scenario provides a unique mechanism for truncating the disk of embryos and planetesimals in the inner Solar System, which has been shown by Hansen (2009) to lead to a more realistic distribution of terrestrial planets, and for delivering primitive planetesimals to the terrestrial planets from beyond the orbit of Jupiter
Summary
The timeline for the important processes of terrestrial planet formation extends from the condensation of the first solids $4.567–4.568 Gyr ago (Amelin et al, 2002; Bouvier and Wadhwa, 2010; Connelly et al, 2012) until the end of heavy bombardment of the inner Solar System around 4.1–3.8 Gyr ago (Tera et al, 1974; Chapman et al, 2007; Bottke et al, 2012). Due to their dynamically cold orbits, planetesimals collide with low relative velocities, amenable to accretion, and grow into larger planetary embryos. The starting point for the final stage of terrestrial planet formation is a suite of planetary embryos in a sea of smaller remnant planetesimals This has typically been modeled as an idealized bimodal mass distribution of planetesimals and embryos, with approximately equal mass in each population (Chambers, 2001; O’Brien et al, 2006; Raymond et al, 2009). While the initial conditions of Hansen (2009) were ad hoc and not based on any physical model, the work was inspiring and led to a new exploration of possible terrestrial planet formation scenarios
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