H. Genda^a, R. Brasser^a, S. J. Mojzsis^b,c
Earth and Planetary Science Letters 478, 143-149 Link to Article [https://doi.org/10.1016/j.epsl.2017.09.041]
^aEarth-Life Science Institute, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
^bDepartment of Geological Sciences, University of Colorado, UCB 399, 2200 Colorado Avenue, Boulder, CO 80309-0399, USA
^cInstitute for Geological and Geochemical Research, Research Center for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 45 Budaörsi Street, H-1112 Budapest, Hungary
Copyright Elsevier

Overabundances in highly siderophile elements (HSEs) of Earth’s mantle can be explained by conveyance from a singular, immense (D~3000km) “Late Veneer” impactor of chondritic composition, subsequent to lunar formation and terrestrial core-closure. Such rocky objects of approximately lunar mass (∼0.01 M_⊕) ought to be differentiated, such that nearly all of their HSE payload is sequestered into iron cores. Here, we analyze the mechanical and chemical fate of the core of such a Late Veneer impactor, and trace how its HSEs are suspended – and thus pollute – the mantle. For the statistically most-likely oblique collision (∼45°), the impactor’s core elongates and thereafter disintegrates into a metallic hail of small particles (∼10 m). Some strike the orbiting Moon as sesquinary impactors, but most re-accrete to Earth as secondaries with further fragmentation. We show that a single oblique impactor provides an adequate amount of HSEs to the primordial terrestrial silicate reservoirs via oxidation of (<m-sized) metal particles with a hydrous, pre-impact, early Hadean Earth.

S. Q. Yan¹ et al. (>10)
Astrophysical Journal 848, 98 Link to Article [https://doi.org/10.3847/1538-4357/aa8c74]
¹China Institute of Atomic Energy, P.O. Box 275(10), Beijing 102413, P. R. China

The ⁹⁵Zr(n, γ)⁹⁶Zr reaction cross section is crucial in the modeling of s-process nucleosynthesis in asymptotic giant branch stars because it controls the operation of the branching point at the unstable ⁹⁵Zr and the subsequent production of ⁹⁶Zr. We have carried out the measurement of the ⁹⁴Zr(¹⁸O, ¹⁶O) and ⁹⁰Zr(¹⁸O, ¹⁶O) reactions and obtained the γ-decay probability ratio of ⁹⁶Zr* and ⁹²Zr* to determine the ⁹⁵Zr(n, γ)⁹⁶Zr reaction cross sections with the surrogate ratio method. Our deduced Maxwellian-averaged cross section of 66 ± 16 mb at 30 keV is close to the value recommended by Bao et al., but 30% and more than a factor of two larger than the values proposed by Toukan & Käppeler and Lugaro et al., respectively, and routinely used in s-process models. We tested the new rate in stellar models with masses between 2 and 6 M _⊙ and metallicities of 0.014 and 0.03. The largest changes—up to 80% variations in ⁹⁶Zr—are seen in models of mass 3–4 M _⊙, where the ²²Ne neutron source is mildly activated. The new rate can still provide a match to data from meteoritic stardust silicon carbide grains, provided that the maximum mass of the parent stars is below 4 M _⊙, for a metallicity of 0.03.

Ziyan Xu^1,2, Xue-Ning Bai^3,4,5, and Ruth A. Murray-Clay⁶
Astrophysical Journal 846, 52 Link to Article [https://doi.org/10.3847/1538-4357/aa8620]
¹Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
²Department of Astronomy, Peking University, Beijing 100871, China
³Institute for Advanced Study, Tsinghua University, Beijing 100084, China
⁴Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, China
⁵Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA 02138, USA
⁶Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA

It has been realized in recent years that the accretion of pebble-sized dust particles onto planetary cores is an important mode of core growth, which enables the formation of giant planets at large distances and assists planet formation in general. The pebble accretion theory is built upon the orbit theory of dust particles in a laminar protoplanetary disk (PPD). For sufficiently large core mass (in the “Hill regime”), essentially all particles of appropriate sizes entering the Hill sphere can be captured. However, the outer regions of PPDs are expected to be weakly turbulent due to the magnetorotational instability (MRI), where turbulent stirring of particle orbits may affect the efficiency of pebble accretion. We conduct shearing-box simulations of pebble accretion with different levels of MRI turbulence (strongly turbulent assuming ideal magnetohydrodynamics, weakly turbulent in the presence of ambipolar diffusion, and laminar) and different core masses to test the efficiency of pebble accretion at a microphysical level. We find that accretion remains efficient for marginally coupled particles (dimensionless stopping time ) even in the presence of strong MRI turbulence. Though more dust particles are brought toward the core by the turbulence, this effect is largely canceled by a reduction in accretion probability. As a result, the overall effect of turbulence on the accretion rate is mainly reflected in the changes in the thickness of the dust layer. On the other hand, we find that the efficiency of pebble accretion for strongly coupled particles (down to ) can be modestly reduced by strong turbulence for low-mass cores.