Water Delivery and Giant Impacts in the 'Grand Tack' Scenario (1407.3290v1)

Abstract: A new model for terrestrial planet formation (Hansen 2009, Walsh et al. 2011) 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. 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 3 AU, and the outer belt is filled with bodies originating from between and beyond the giant planets (which are hereafter referred to as `primitive' bodies). We find here that the planets will accrete on order 1-2% of their total mass from primitive planetesimals scattered onto planet-crossing orbits during the formation of the planets. 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. While the radial distribution of the planetary masses and the dynamical excitation of their orbits are a good match to the observed system, we find that the last giant impact is typically earlier than 20 Myr, and a substantial amount of mass is accreted after 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 the dating of the Moon-forming impact and the estimated amount of mass accreted by Earth following that event.

• The paper reveals that gas giant migration truncates the planetesimal disk, facilitating the formation of a smaller Mars and effective water delivery to Earth.
• The simulations show that Earth-like planets acquire 1–2% of their mass from volatile-rich planetesimals during late-stage accretion.
• The study highlights a mismatch between rapid simulated impact timelines and radiometric dating, underscoring the need for refined models in early solar system dynamics.

An Analysis of Terrestrial Planet Formation and Water Delivery in the 'Grand Tack' Scenario

The paper, "Water Delivery and Giant Impacts in the ‘Grand Tack’ Scenario," by O’Brien et al., revisits the complexities of terrestrial planet formation with a particular emphasis on explaining the unique Earth/Mars mass ratio and the water content on Earth. The researchers build upon new models of planet formation which involve large-scale planetary migration described in the ‘Grand Tack’ model and analyze numerical simulations to understand these phenomena.

Key Features of the 'Grand Tack' Scenario

The central hypothesis of the paper leverages the ‘Grand Tack’ scenario, which suggests that Jupiter and Saturn underwent significant migration during the early solar system, marked by an initial inward journey followed by an outward migration. This migration worked to truncate the distribution of planetesimals in the protoplanetary disk, effectively creating conditions conducive to the formation of planetary systems akin to our own. The researchers' simulations incorporate the dynamics of this migration to examine its impact on planetary formation and water delivery to the terrestrial planets.

Numerical Simulations and Outcomes

The simulations build on earlier work by Hansen (2009) and Walsh et al. (2011) and explore various initial conditions for the distribution of embryos and planetesimals. It is observed that this inward-then-outward migration of the gas giants facilitated two significant outcomes:

1. Terrestrial Planet Formation: The chaotic interactions resulting from the disk truncation reinforce conditions conducive to forming a smaller Mars, overcoming earlier simulation shortcomings which produced Mars analogs far more massive than the real planet. However, the outcomes consistently fail to produce viable Mercury analogs, suggesting the need for further paper or additional constraints, such as the role of giant impacts, in explaining Mercury’s current mass and density.
2. Water Delivery to Earth: Intriguingly, the simulations reveal that the inner planets accrete about 1-2% of their total mass from the so-called 'primitive planetesimals,' which are sourced from beyond the gas giants' original positions. For Earth-like planets, these primitive bodies potentially introduce significant water content, compatible with estimates of the Earth's current water inventory. The timing of accretion suggests that these deliveries occur predominantly in the latter stages of planetary mass accumulation, facilitating retention of water and volatiles.

Challenges and Considerations

While the incorporation of the giant planets’ migrations provided a mechanism consistent with forming a realistic inner Solar System, the timing constraints imposed by radiometric dating and geochemical evidence remain challenging. The simulations indicate accretion timelines more rapid than evidence suggests. For instance, simulation outcomes frequently feature giant impacts earlier than 20 Myr, conflicting with Moon-forming impact chronologies, suggesting that the current simulation parameters might undervalue the impact timescales.

Moreover, the ability to produce Mercury analogs remains limited, potentially necessitating an adjustment of initial conditions or a consideration of other processes that can account for Mercury's unique characteristics, possibly including varying disk surface density or the composition of impacting bodies.

Implications and Future Directions

The implications of this research are multifaceted. From a theoretical standpoint, the 'Grand Tack' scenario provides a compelling framework that may resolve long-standing issues in planetary formation theories, primarily the size discretization of inner planets and the transport of volatile-rich bodies to the inner solar system. From a practical perspective, understanding these processes enhances the ability to model the conditions necessary for habitability in exoplanetary systems.

Future research avenues may include expanding the parameter space of initial conditions to refine terrestrial formation scenarios further and incorporating additional physical processes, such as differentiated impact modeling or varied chemical compositions of accreting bodies. Detailed studies may also explore the compositional diversity of asteroids and their evolving role in planetary accretion.

Overall, this paper advances our comprehension of the formation and growth of terrestrial planets and enriches our understanding of how our Earth acquired its complement of water and the features of the solar system's dynamic history.