Mercury's enigmatic origin
Characterizing the problem: Clement et al. (2019c)
In contrast to that of Mars, Mercury's origin remains the least understood and most mysterious of the solar system's planets. Mercury-like planets are extremely rare within the terrestrial planet formation literature. Namely this concerns Mercury's diminutive size compared to the neighboring Venus, and the two planets' mutual separation (see accompanying figure). In practicality, this is mostly a result of the initial conditions chosen by such studies. However, Mercury's depleted inventory of volatiles and large core suggest that it formed in a different manner than the other terrestrial worlds. While Mercury's high bulk density seems to imply that much of its silicate-rich mantle material was eroded in a massive impact, in a 2019 paper we demonstrated that the various proposed collisional scenarios are highly improbable from a dynamical standpoint. Therefore, we set out to explore more exotic avenues for Mercury's genesis.
Figure reproduced from Clement & Chambers (2021) and Clement et al. (2021). The x-axis depicts the Mercury-Venus orbital period ratio, and the y-axis plots the planets' mass ratio. The acronyms refer to the following papers (which all study terrestrial planet formation from a variety of angles): C18: Clement et al. (2018); Clement et al. (2019a); Ch01: Chambers (2001); LI19: Lykawka & Ito (2019); I15: Izidoro et al. (2015); JM14: Jacobson & Morbidelli (2014).
Mercury as the lone survivor of a system of short-period proto-planets: Clement et al. (2021c)
This first video displays new results from Clement et al. (2021c) where a primordial generation of (in this example) 5 Mercury-mass planets evolve in the presence of the modern solar system. The semi-major axis of each object is plotted against orbital eccentricity (the elliptical-ness of the orbit). Perturbations from the other planets (in particular Jupiter's eccentric precession frequency) destabilize these systems in 10s of million years, leading to a cataclysmic series of high-velocity collisions that tend to eject the proto-planets' mantle material as fragments (green points in the video). In successful realizations, a single planet is left behind on a Mercury-like orbit with a large, iron-rich core (in good agreement with that presumed for the actual Mercury). A consequence of these events is some late material delivery and cratering on Venus', which may or may not conflict with certain aspects of the planet (for example, it's lack of a natural satellite).
In-situ accretion: Clement & Chambers (2021)
This next video depicts a simulation from Clement & Chambers (2021) where Mercury accretes directly (from the ground up) within a mass-depleted component of the terrestrial disk. By tuning the structure of the disk (i.e. its' total mass, surface density profile and size distribution of the accreted material), we were able to find structures that allow Mercury to accrete in dynamical isolation from Venus; thus providing a decent match to the two planets' modern orbital orientation. However, this scenario does not efficiently remove excess mantle-material from Mercury like the lone-survivor model described above. Thus, for this hypothesis to be viable, the Mercury-forming material would need to already be enriched in iron prior to its' accretion. There are several plausible mechanisms to generate these iron-rich initial conditions, and we are continually working to develop new constraints and rule out one model over another.
Venus and Earth's outward migration as the source of : Clement et al. (2021c)
In Clement et al. 2022 we proposed a physical motivation for the successful initial conditions we identified in Clement et al. (2021c) and Clement & Chambers (2021). In our new paper, we argue that, while still imbedded in the Sun's primordial gas nebula, the diminutive proto-Earth and proto-Venus migrated outward towards their modern orbits after having formed interior to Mercury's modern orbit. During this migration phase, Earth and Venus accrete and scatter many of the small pieces of planet-forming material in the Mercury region, and leave behind a remnant distribution of objects similar to the structures found to be successful (albeit without physical motivation) in our previous studies. While numerical simulations of gaseous nebulas have found that outward migration of sub-Earth-mass objects is plausible near the end of the gas disk phase, it is impossible to infer whether the particular structure of the Sun's natal disk supported this type of orbital evolution. However, by assuming that Earth and Venus sculpted the Mercury-forming region a priori, our model explains a number of inner solar system qualities. These include the difference between Earth and Venus' masses, the spacing between Venus and Mercury's orbits, and inferred differences between the compositions of Mars and Earth.
Cartoon depiction of our model. Figure reproduced from Clement et al. 2022