Coagulation theory has often been used to model the growth of planets, where primordial bodies formed by streaming instability or other processes accrete pairwise, with runaway growth happening beyond a certain threshold condition. These models assume perfect merging, leading to a hierarchy of growth that has been useful in framing the problem. However, about half of “small giant impacts” between planetesimals are in fact non-accretions. Because accretions dispose of the smaller objects, the advancing hierarchy of mergers leads to strong survivorship bias among the unaccreted, of which the major asteroids are sparse samples in my model. I will end with some implications for the primordial planetesimals themselves, and the source regions of the major asteroids and their diversity, and will address the so-called missing mantle paradox.
Small giant impacts: the complex yet self-similar pathway from early planetesimals to asteroids
Join us in listening to Erik Asphaug, planetary science professor in the Lunar and Planetary Laboratory at the University of Arizona!
Outcome of a “small giant impact”, including strength, from the database of Emsenhuber, Asphaug, et al., (PSJ 2024). In this case, a mostly accretionary, low-velocity (vcoll=1.3vesc), typical-impact-angle (43°) collision ended up acquiring 59% of the projectile onto the target and capturing most of the rest of it into stable orbit. The final rotation of the target and solid strength support massive iron blobs from the projectile in the upper mantle (yellow) and an offset iron core (red).
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Institute of Theoretical Astrophysics
Published Oct. 16, 2026 1:54 PM
- Last modified Oct. 16, 2026 1:54 PM