¹Ryuki Hyodo,²Hidenori Genda,²Ramon Brasser
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.114064]
¹ISAS, JAXA, Sagamihara, Japan
²Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-8550, Japan
Copyright Elsevier

Late accretion is a process that strongly modulated surface geomorphic and geochemical features of Mercury. Yet, the fate of the impactors and their effects on Mercury’s surface through the bombardment epoch are not clear. Using Monte-Carlo and analytical approaches of cratering impacts, we investigate the physical and thermodynamical outcomes of late accretion on Mercury. Considering the uncertainties in late accretion, we develop scaling laws for the following parameters as a function of impact velocity and total mass of late accretion: (1) depth of crustal erosion, (2) the degree of resurfacing, and (3) mass accreted from impactor material. Existing dynamical models indicate that Mercury experienced an intense impact bombardment (a total mass of ∼8 × 1018 − 8 × 1020 kg with a typical impact velocity of 30 − 40 km s−1) after 4.5 Ga. For this parameter range, we find that late accretion could remove 50 m to 10 km of the early (post-formation) crust of Mercury, but the change to its core-to-mantle ratio is negligible. Alternatively, the mantles of putative differentiated planetesimals in the early solar system could be more easily removed by impact erosion and their respective core fraction increased, if Mercury ultimately accreted from such objects. Although the cratering is notable for erasing the older geological surface records on Mercury, we show that ∼40 − 50wt. % of the impactor’s exogenic materials, including the volatile-bearing materials, can be heterogeneously implanted on Mercury’s surface as a late veneer (at least 3 × 1018 − 1.6 × 1019 kg in total). About half of the accreted impactor’s materials are vaporized, and the rest is completely melted upon the impact. We expect that the further interplay between our theoretical results and forthcoming surface observations of Mercury, including the BepiColombo mission, will lead us to a better understanding of Mercury’s origin and evolution.

¹C.Xiang,¹A.Carballido,¹L.S.Matthews,¹T.W.Hyde
Icarus (in Press) Linkto Article [https://doi.org/10.1016/j.icarus.2020.114053]
¹Center for Astrophysics, Space Physics, and Engineering Research, Baylor University, Waco, TX 76798-7316, USA
Copyright Elsevier

In order to characterize the early growth of fine-grained dust rims (FGRs) that commonly surround chondrules, we simulate the growth of FGRs through direct accretion of monomers of various sizes onto the chondrule surfaces. Dust becomes charged to varying degrees in the radiative plasma environment of the solar nebula (SN), and the resulting electrostatic force alters the trajectories of colliding dust grains, influencing the structure of the dust rim as well as the time scale of rim formation. We compare the growth of FGRs in protoplanetary disks (PPD) with different turbulence strengths and plasma conditions to previous models which assumed neutral dust grains (Xiang, C., Carballido, A., Hanna, R.D., Matthews, L.S., Hyde, T.W., 2019). We use a combination of a Monte Carlo method and an N-body code to simulate the collision of dust monomers of radii 0.5 – m with chondrules whose radii are between 500 and m: a Monte Carlo algorithm is used to randomly select dust particles that will collide with the chondrule as well as determine the elapsed time interval between collisions; at close approach, the detailed collision process is modeled using an N-body algorithm, Aggregate Builder (AB), to determine the collision outcome, as well as any restructuring of the chondrule rim. For computational expediency, we limit accretion of dust monomers to a small patch of the chondrule surface. The collisions are driven by Brownian motion and coupling to turbulent gas motion in the protoplanetary disk. The charge distribution of the dust rim is modeled, used to calculate the trajectories of dust grains, and then analyze the resulting morphology of the dust rim. In a weakly turbulent region, the decreased relative velocity between charged particles causes small grains to be repelled from the chondrule, causing dust rims to grow more slowly and be composed of larger monomers, which results in a more porous structure. In a highly turbulent region, the presence of charge mainly affects the porosity of the rim by causing dust particles to deviate from the extremities of the rim and reducing the amount of restructuring caused by high-velocity collisions.