Forming Mercury by Giant Impacts

Monthly Notices of the Royal Astronomical Society

The classical scenario of terrestrial planet formation is characterized by a phase of giant impacts among Moon-to-Mars mass planetary embryos. While the classic model and its adaptations have produced adequate analogues of the outer three terrestrial planets, Mercury’s origin remains elusive. Mercury’s high-core mass fraction compared to the Earth’s is particularly outstanding. Among collisional hypotheses, this feature has been long interpreted as the outcome of an energetic giant impact among two massive protoplanets. Here, we revisit the classical scenario of terrestrial planet formation with focus on the outcome of giant impacts. We have performed a large number of N-body simulations considering different initial distributions of planetary embryos and planetesimals. Our simulations tested the effects of different giant planet configurations, from virtually circular to very eccentric configurations. We compare the giant impacts produced in our simulations with those that are more...

Monthly Notices of the Royal Astronomical Society

We investigate mantle stripping giant impacts (GI) between super-Earths with masses between 1 and $20\, {\rm M}_{\oplus }$. We infer new scaling laws for the mass of the largest fragment and its iron mass fraction, as well as updated fitting coefficients for the critical specific impact energy for catastrophic disruption, $Q_{{\rm RD}}^{*}$. With these scaling laws, we derive equations that relate the impact conditions, i.e. target mass, impact velocity, and impactor-to-target mass ratio, to the mass and iron mass fraction of the largest fragment. This allows one to predict collision outcomes without performing a large suite of simulations. Using these equations we present the maximum and minimum planetary iron mass fraction as a result of collisional stripping of its mantle for a given range of impact conditions. We also infer the radius for a given mass and composition using interior structure models and compare our results to observations of metal-rich exoplanets. We find good ag...

Proceedings of The International Astronomical Union, 2007

We have performed the smoothed particle hydrodynamic (SPH) simulations of collisions between two gas giant planets. Changes in masses of the ice/rock core and the H/He envelope due to the collisions are investigated. The main aim of this study is to constrain the origin and probability of a class of extrasolar hot Jupiters that have much larger cores and/or higher

Planetary and Space Science, 2009

It is often assumed that the terrestrial worlds have experienced identical impact regimes over the course of their formation and evolution, and, as a result, would have started life with identical volatile budgets. In this work, through illustrative dynamical simulations of the impact flux on Venus, the Earth, and Mars, we show that these planets can actually experience greatly different rates of impact from objects injected from different reservoirs. For example, we show scenarios in which Mars experiences far more asteroidal impacts, per cometary impactor, than Venus, with the Earth being intermediate in value between the two. This difference is significant, and is apparent in simulations of both quiescent and highly stirred asteroid belts (such as could be produced by a mutual mean-motion resonance crossing between Jupiter and Saturn, as proposed in the Nice model of the Late Heavy Bombardment). We consider the effects such differences would have on the initial volatilisation of the terrestrial planets in a variety of scenarios of both endogenous and exogenous hydration, with particular focus on the key question of the initial level of deuteration in each planet's water budget. We conclude that each of the terrestrial worlds will have experienced a significantly different distribution of impactors from various reservoirs, and that the assumption that each planet has the same initial volatile budget is, at the very least, a gross over-simplification.

Icarus, 1999

We perform three-dimensional N-body integrations of the final stages of terrestrial planet formation. We report the results of 10 simulations beginning with 22-50 initial planetary embryos spanning the range 0.5-1.5 AU, each with an initial mass of 0.04-0.13M ⊕. Collisions are treated as inelastic mergers. We follow the evolution of each system for 2 × 10 8 years at which time a few terrestrial type planets remain. On average, our simulations produced two planets larger than 0.5M ⊕ in the terrestrial region (1 simulation with one m ≥ 0.5M ⊕ planet, 8 simulations with two m ≥ 0.5M ⊕ planets, and 1 simulation with three m ≥ 0.5M ⊕ planets). These Earth-like planets have eccentricities and orbital spacing considerably larger than the terrestrial planets of comparable mass (e.g., Earth and Venus). We also examine the angular momentum contributions of each collision to the final spin angular momentum of a planet, with an emphasis on the type of impact which is believed to have triggered the formation of the Earth's Moon. There was an average of two impacts per simulation that contributed more angular momentum to a planet than is currently present in the Earth/Moon system. We determine the spin angular momentum states of the growing planets by summing the contributions from each collisional encounter. Our results show that the spin angular momentum states of the final planets are generally the result of contributions made by the last few large impacts. Our results suggest that the current angular momentum of the Earth/Moon system may be the result of more than one large impact rather than a single impact. Further, upon suffering their first collision, the planetary embryos in our simulations are spinning rapidly throughout the final accretion of the planets, suggesting the proto-Earth may have been rotating rapidly prior to the Moon-forming impact event.

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 2001

Nuclides with half-lives of 10 5 -10 8 yr permit the elucidation of nebula time-scales and the rates of accretion of planetesimals. However, the 182 Hf-182 W system with a half-life of 9 ± 2 Myr also provides new and very useful constraints on the formation of the terrestrial planets. This technique allows one to address the timing of metal-silicate equilibration in objects as different as chondrites and the Earth. With improvements in sensitivity and precision, very small time differences in metal segregation in asteroids should be resolvable from measuring iron meteorites. It is already clear that the formation and differentiation of some asteroidal-sized objects was completed in less than 10 Myr. Accretion and core formation were protracted in the case of the Earth (greater than 50 Myr) relative to Mars (probably less than 20 Myr). Indeed, the Martian mantle appears to retain both chemical and isotopic heterogeneities that are residual from the process of core formation. Such early features appear to have been eliminated from the Earth's mantle presumably because of 4.5 Gyr of relatively efficient convective mixing. Tungsten isotope data provide compelling support for the 'giant impact' theory of lunar origin. The Moon is a high Hf/W object that contains a major component of chondritic W. This is consistent with a time of formation of greater than 50 Myr after the start of the Solar System. New highly precise oxygen isotope data are unable to resolve any difference between the source of components in the Earth and Moon. Therefore, the giant impact itself may have produced some of the differences in moderately volatile element budgets between these objects. This finds support in precise Sr isotopic data for early lunar samples. The data are consistent with the proto-Earth and Theia (the impactor) having Rb/Sr ratios that were not very different from that of present day Mars. Therefore, the extended history of accretion, rather than nebular phenomena, may be responsible for some of the major differences between the terrestrial planets.

Journal of Geophysical Research: Planets, 2012

The formation of large impact basins (diameter D ≥ 300 km) was an important process in the early geological evolution of Mercury and influenced the planet's topography, stratigraphy, and crustal structure. We catalog and characterize this basin population on Mercury from global observations by the MESSENGER spacecraft, and we use the new data to evaluate basins suggested on the basis of the Mariner 10 flybys. Forty‐six certain or probable impact basins are recognized; a few additional basins that may have been degraded to the point of ambiguity are plausible on the basis of new data but are classified as uncertain. The spatial density of large basins (D ≥ 500 km) on Mercury is lower than that on the Moon. Morphological characteristics of basins on Mercury suggest that on average they are more degraded than lunar basins. These observations are consistent with more efficient modification, degradation, and obliteration of the largest basins on Mercury than on the Moon. This distinc...

The probability of the collisions and destructions of the planets-giants at various stages of planetary systems evolution is calculated. The flow of the fragments of various sizes and the probability of their observations near the Earth are estimated. Of the particular interest is the case of the fragments of metallic hydrogen under the condition of its metastability at low pressure. The radio bursts, which can be generated at the collapses of the planets-giant's magnetospheres during their collisions, are also discussed.

Planetary and Space …, 2010

Icarus, 2008

Chemical processes associated with meteoroid bombardment of Mercury are considered. Meteoroid impacts lead to production of metal atoms as well as metal oxides and hydroxides in the planetary exosphere. By using quenching theory, the abundances of the main Na-, K-, CaFe Fe-, Al-, Mg-, Si-, and Ti-containing species delivered to the exosphere during meteoroid impacts were estimated. Based on a correlation between the solar photo rates and the molecular constants of atmospheric diatomic molecules, photolysis lifetimes of metal oxides and SiO are estimated. Meteoroid impacts lead to the formation of hot metal atoms (0.2-0.4 eV) produced directly during impacts and of very hot metal atoms (1-2 eV) produced by the subsequent photolysis of oxides and hydroxides in the exosphere of Mercury. The concentrations of impact-produced atoms of the main elements in the exosphere are estimated relative to the observed concentrations of Ca, assumed to be produced mostly by ion sputtering. Condensation of dust grains can significantly reduce the concentrations of impact-produced atoms in the exosphere. Na, K, and Fe atoms are delivered to the exosphere directly by impacts while Ca, Al, Mg, Si, and Ti atoms are produced by the photolysis of their oxides and hydroxides. The chemistry of volatile elements such as H, S, C, and N during meteoroid bombardment is also considered. Our conclusions about the temperature and the concentrations of impact-produced atoms in the exosphere of Mercury may be checked by the Messenger spacecraft in the near future and by BepiColombo spacecraft some years later.