The Unaccreted


“Accreted planets are alike...unaccreted planets are different in how they are unaccreted.”

In my 2010 paper I come to various conclusions regarding the importance of unaccreted impactors, the ‘ones that got away’.  Just read the abstract, it’s good for you:

    “It is assumed in models of terrestrial planet formation that colliding bodies simply merge. From this the dynamical and chemical properties (and habitability) of finished planets have been computed, and our own and other planetary systems compared to the results of these calculations. But efficient mergers may be exceptions to the rule, for the similar-sized collisions (SSCs) that dominate terrestrial planet formation, simply because moderately off-axis SSCs are grazing; their centers of mass overshoot. In a ‘‘hit and run’’ collision the smaller body narrowly avoids accretion and is profoundly deformed and altered by gravitational and mechanical torques, shears, tides, and impact shocks. Consequences to the larger body are minor in inverse proportion to its relative mass. Over the possible impact angles, hit- and-run is the most common outcome for impact velocities vimp between ~1:2 and 2.7 times the mutual escape velocity vesc between similar-sized planets. Slower collisions are usually accretionary, and faster SSCs are erosive or disruptive, and thus the prevalence of hit-and-run is sensitive to the velocity regime during epochs of accretion. Consequences of hit-and-run are diverse. If barely grazing, the target strips much of the exterior from the impactor—any atmosphere and ocean, much of the crust—and unloads its deep interior from hydrostatic pressure for about an hour. If closer to head-on, a hit-and-run can cause the impactor core to plow through the target mantle, graze the target core, and emerge as a chain of diverse new planetoids on escaping trajectories. A hypothesis is developed for the diversity of next-largest bodies (NLBs) in an accreting planetary system—the bodies from which asteroids and meteorites derive. Because nearly all the NLBs eventually get accreted by the largest (Venus and Earth in our terrestrial system) or by the Sun, or otherwise lost, those we see today have survived the attrition of merger, evolving with each close call towards denser and volatile-poor bulk composition. This hypothesis would explain the observed density diversity of differentiated asteroids, and of dwarf planets beyond Neptune, in terms of episodic global-scale losses of rock or ice mantles, respectively. In an event similar to the Moon-forming giant impact, Mercury might have lost its original crust and upper mantle when it emerged from a modest velocity hit and run collision with a larger embryo or planet. In systems with super-Earths, profound diversity and diminished habitability are predicted among the unaccreted Earth-mass planets, as many of these will have been stripped of their atmospheres, oceans and crusts.”


Planets collide and the impactor gets radically transformed, even loses its mantle and any atmosphere (potential biosphere) when the collision is off-axis, as is usually the case.  Red is iron and blue is rock. Download our article here (Asphaug et al. 2006).

In hit and run collisions the impactor “bounces off”, severely transformed and stripped of much of its volatiles and mantle, and thermophysically altered by pressure release and shock.  The figure above is the 1:2 mass ratio collision at 45° near the center of the green oval.

The target body feels a solid blow, to be sure, but doesn’t lose much mass in a hit-and-run.  Yet there is still a net mass exchange over time.

Two bodies that hit-and-run have a high likelihood of colliding again. If accretion eventually results, the impactor is swallowed up.  But if accretion does not result, the stripped material goes back into the local disk and is preferentially swept up by the larger, which has much greater gravitational cross section.  If the smaller body avoids being accreted after a few close calls, it survives to the present day as one of the cosmic oddballs.  The larger bodies grows fat with atmospheres and volatiles, crust-forming minerals, and other goodies.