Dynamical Constraints on Mercury's Collisional Origin

Abstract

Of the solar system's four terrestrial planets, the origin of Mercury is perhaps the most mysterious. Modern numerical simulations designed to model the dynamics of terrestrial planet formation systematically fail to replicate Mercury; which possesses just 5% the mass of Earth and the highest orbital eccentricity and inclination among the planets. However, Mercury's large iron-rich core and low volatile inventory stand out among the inner planets, and seem to imply a violent collisional origin. Because most algorithms used for simulating terrestrial accretion do not consider the effects of collisional fragmentation, it has been difficult to test these collisional hypotheses within the larger context of planet formation. Here, we analyze a large suite of terrestrial accretion models that account for the fragmentation of colliding bodies. We find that planets with core mass fractions boosted as a result of repeated hit-and-run collisions are produced in 90% of our simulations. While many of these planets are similar to Mercury in mass, they rarely lie on Mercury-like orbits. Furthermore, we perform an additional batch of simulations designed to specifically test the single giant impact origin scenario. We find less than a 1% probability of simultaneously replicating the Mercury-Venus dynamical spacing and the terrestrial system's degree of orbital excitation after such an event. While dynamical models have made great strides in understanding Mars' low mass, their inability to form accurate Mercury analogs remains a glaring problem.

11 pages, 7 figures, 2 tables, accepted for publication in AJ

References in corpus (21)

• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• Origin of water in the inner Solar System: Planetesimals scattered inward during Jupiter and Saturn's rapid gas accretion
• Close-in planetesimal formation by pile-up of drifting pebbles
• On the evolution of eccentric and inclined protoplanets embedded in protoplanetary disks
• Multiple Impact Origin for the Moon
• Jupiter's Decisive Role in the Inner Solar System's Early Evolution
• Advances in exoplanet science from Kepler
• The early instability scenario: terrestrial planet formation during the giant planet instability, and the effect of collisional fragmentation
• The GENGA Code: Gravitational Encounters in N-body simulations with GPU Acceleration
• A primordial origin for the composition similarity between the Earth and the Moon
• Formation, stratification, and mixing of the cores of Earth and Venus
• The Feeding Zones of Terrestrial Planets and Insights into Moon Formation
• Rapid Formation of Icy Super-Earths and the Cores of Gas Giant Planets
• A Collisional Origin to Earth's Non-chondritic Composition?
• Constraining the giant planets' initial configuration from their evolution: implications for the timing of the planetary instability
• Dynamical Shakeup of Planetary Systems II. N-body simulations of Solar System terrestrial planet formation induced by secular resonance sweeping
• Terrestrial Planet Formation from an Annulus
• Numerically Predicted Indirect Signatures of Terrestrial Planet Formation
• Constraints on the pre-impact orbits of Solar System giant impactors
• Dynamical Evolution of the Earth-Moon Progenitors - Whence Theia?
• Prevalance of Chaos in Planetary Systems Formed Through Embryo Accretion