Terrestrial Planet Formation: Constraining the Formation of Mercury

Abstract: The formation of the four terrestrial planets of the solar system is one of the most fundamental problems in the planetary sciences. However, the formation of Mercury remains poorly understood. We investigated terrestrial planet formation by performing 110 high-resolution N-body simulation runs using more than 100 embryos and 6000 disk planetesimals representing a primordial protoplanetary disk. To investigate the formation of Mercury, these simulations considered an inner region of the disk at 0.2-0.5 au (the Mercury region) and disks with and without mass enhancements beyond the ice line location, aIL, in the disk, where aIL = 1.5, 2.25, and 3.0 au were tested. Although Venus and Earth analogs (considering both orbits and masses) successfully formed in the majority of the runs, Mercury analogs were obtained in only nine runs. Mars analogs were also similarly scarce. Our Mercury analogs concentrated at orbits with a ~ 0.27-0.34 au, relatively small eccentricities/inclinations, and median mass m ~ 0.2 Earth masses. In addition, we found that our Mercury analogs acquired most of their final masses from embryos/planetesimals initially located between 0.2 and ~1-1.5 au within 10 Myr, while the remaining mass came from a wider region up to ~3 au at later times. Although the ice line was negligible in the formation of planets located in the Mercury region, it enriched all terrestrial planets with water. Indeed, Mercury analogs showed a wide range of water mass fractions at the end of terrestrial planet formation.

• The paper uses high-resolution N-body simulations to explore terrestrial planet formation with a focus on Mercury's origin.
• It identifies nine Mercury analogs with eccentric orbits and masses around 0.2 times Mercury, emphasizing local accretion over external contributions.
• The study underscores a local formation scenario for Mercury, challenging scattering hypotheses and offering new insights into disk dynamics and volatile delivery.

Terrestrial Planet Formation: Constraining the Formation of Mercury

The research conducted by Lykawka and Ito provides a comprehensive examination of terrestrial planet formation, with an emphasis on the enigmatic process behind Mercury's development. Utilizing high-resolution N-body simulations, the study explores multiple scenarios involving a primordial protoplanetary disk comprised of over 100 embryos and 6000 disk planetesimals. Their investigation focuses on the inner region, designated from 0.2 to 0.5 astronomical units (au), and integrates variables such as mass enhancements beyond the ice line to evaluate their influence on planet formation, including Mercury's.

Summary of Methods and Findings

The simulations considered several variations of the protoplanetary disk's mass distribution, including scenarios with differing ratios of mass contributions from embryos versus planetesimals (r values) and the presence of ice lines at different radial locations. The objective was to identify conditions conducive to the concurrent formation of Mercury, Venus, Earth, and Mars analogs.

The outcomes yielded nine Mercury analogs with orbital parameters closely resembling the planet itself, albeit with some discrepancies in mass. Notably, the Mercury analogs were typically found to have eccentric orbits with semimajor axes ranging between 0.27 and 0.34 au, and an average mass of approximately 0.2 Mercury masses (M_☿). These analogs primarily accumulated mass from a feeding zone extending from 0.2 to roughly 1.5 au, indicating that local accretion played a dominant role compared to any significant material influx from beyond 1.5 au.

An unexpected yet revealing result involved the negligible role of the ice line in Mercury's formation. Despite this, the presence of the ice line contributed to water enrichment across terrestrial planets, underscoring the importance of this feature in volatile distribution beyond the Mercury region.

Implications and Future Directions

The study emphasizes that reproducing Mercury's specific orbit and mass remains challenging due to the underlying complexity of disk conditions and dynamics during the early solar system. However, it delineates several promising disk configurations that may yield better analog results. These include considering a disk with a reduced mass within the Mercury region, restructuring the inner and outer edges of this region, and varying the initial conditions of Jupiter and Saturn to better align with their current positions.

Theoretical implications from these findings suggest that a local origin, rather than an external scattering hypothesis, better explains Mercury's formation under the scenarios tested. Practically, insights into the delivery mechanisms of volatiles like water could refine our understanding of planetary habitability and chemical composition beyond Earth.

Looking forward, extending simulation conditions to explore narrower or truncated disks may illuminate the mechanistic intricacies behind Mercury's evolution. Further studies incorporating fragmentation processes and alternative configurations for giant planets could refine these models, enhancing the fidelity of Mercury, Venus, and even Mars analog formations. This ongoing research is crucial for bridging current gaps in terrestrial planet formation theories and establishing a more complete portrayal of our solar system's genesis.

In conclusion, Lykawka and Ito's study advances the dialogue on terrestrial planet formation, specifically Mercury's role and origins. Their findings lay a foundation for further detailed explorations into the dynamics of early solar system materials and the intricate processes that determined the unique architectures of the planets we observe today.