Ejecta-megaregolith accumulation on planetesimals and large asteroids

Megaregolith accumulation can have important thermal consequences for bodies that lose heat by conduction, as vacuous porosity of the kind observed in the lunar megaregolith lowers thermal conductivity by a factor of 10. I have modeled global average ejecta accumulation as a function of the largest impact size, with no explicit modeling of time. In conjunction with an assumed cratering size-distribution exponent b, the largest crater constrains the sizes of all other craters that significantly contribute to a megaregolith. The largest impactor mass ratio is a major fraction of the catastrophic-disruption mass ratio, and in general the largest crater's diameter is close to the target's diameter. Total accumulation is roughly 1-5% of (and proportional to) the target's radius. Global accumulations estimated by this approach are higher than in the classic Housen et al. (1979) study by a factor of roughly 10. This revision is caused mainly by higher (typical) largest crater size. For b similar to 2, the single largest crater typically contributes close to 50% of the total of new (nonrecycled) ejecta. Megaregolith can be destroyed by sintering, a process whose pressure sensitivity makes it effective at lower temperature on larger bodies. Planetesimals similar to 100 km in diameter may be surprisingly well suited (about as well suited as bodies two to three times larger in diameter) for attaining temperatures conducive to widespread melting. A water-rich composition may be a significant disadvantage in terms of planetesimal heating, as the shallow interior may be densified by aqueous metamorphism, and will have a low sintering temperature.