Title:  Planetesimal vs. Pebble Accretion

A major issue in planet formation theory is whether the terrestrial planets formed mostly by accreting asteroid-sized (~100 km) planetesimals, or mostly by accreting small (~cm) pebbles. In either theory, planetesimals are the first steps in formation, and these may accrete into Mars-sized planets within the ≈2 Myr lifetime of the inner disk. Pebble accretion has been rejected for various reasons, but those reasons don't actually discriminate between the theories. One objection is that even mm-sized objects are commonly thought to drift into the Sun in < 2 Myr due to aerodynamic drag. This would prevent terrestrial planets from accreting pebbles with "NC" (inner-disk) isotopic composition. But it would also be a problem for formation of planetesimal parent bodies of chondrites. However, I'll show that the combination of the pebble trap at the dead zone inner edge, plus turbulent diffusion of chondrules, is quite favorable for retention of chondrule-sized pebbles, enabling both pebble accretion and planetesimal accretion. Another objection is the geochemical constraint from Hf-W dating that Earth's core's formed over ~10^8 yr, consistent with planetesimal accretion. However, nebular H, He, and Ne in Earth's deep mantle demand ingassing into a magma ocean, and rapid core formation in ~ 10^6 yr. This geochemical constraint demands the Earth's Hf-W age was reset during the Moon-forming impact and does not discriminate between the scenarios. I outline a scenario of the Moon's formation. What might discriminate between the formation scenarios are moderately volatile element (MVE) abundances. Whereas Mars has MVE patterns consistent with planetesimal accretion of chondrites, Earth is depleted in MVEs. I describe a scenario by which Earth's embryos mostly grew by accretion of MVE-depleted chondrule-sized pebbles. Growth of embryos  larger than Mars (proto-Earth, Theia, Venus) may be dominated by pebble accretion.