How Big Were the First Planetesimals? Does Size Matter?

Introduction: The terrestrial planets formed by collisional accretion of small planetesimals [1,2]. Stochastic coagulation and runaway growth yield an “oligarchy” of embryos with similar sub-planetary masses [3] that then collide to yield planets [4-5]. A common assumption is km-sized initial planetesimals, based on the scale of gravitational instability in a dust layer in a quiescent solar nebula [6-7]. This mechanism is untenable due to shear-induced turbulence [8]. Alternative mechanisms [9-11] depend on poorly constrained properties of particles (sticking efficiency, impact strength) and/or the nebula (turbulent velocity, eddy timescale), and do not predict planetesimal sizes from first principles. Here we investigate by numerical modeling effects of initial planetesimal size on outcomes of accretion. C o llis io na l O utco m es: C o ll is ion between planetesimals yield net growth or destruction, depending on size, strength, and impact velocity. Impact strength Q* is defined as specific energy to remove half of the target's mass. This involves fracturing the target and partitioning kinetic energy to fragments that escape their mutual gravity. Small bodies are dominated by material strength; large ones by self-gravity. Scaling laws yield Q* with a minimum at roughly km size [12]. We assume 1% of impact energy is partitioned into fragment kinetic energy. Collisional Modeling: We use the PSI multi-zone accretion code [13]. The swarm has surface density