IGPP Seminar Series

Ejecta-megaregolith accumulation on planetesimals and large asteroids

by Paul Warren


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 conductivity by a factor of 10. I have modeled global average ejecta accumulation as a function of the largest impact crater size, with no explicit modeling of time. In conjunction with an assumed cratering size-distribution exponent b (cf. present-day asteroids), the size of the largest crater implicitly constrains the sizes of all other craters that make significant contributions to a final megaregolith. For any given largest-impactor mass ratio of order 0.001 (at constant b and impact velocity), globally averaged ejecta accumulation thickness is relatively constant over a wide range of d; e.g., for that specific mass ratio and b = 2, results range from 1.0 to 1.3 km for all d between 50 and 800 km. The largest-impactor mass ratio is more likely some consistently major fraction of the catastrophic-disruption mass ratio, which (as a result of increasing gravity effects at larger scales) is a complex function of body size, with an anomalous high at d ~100 km, but in general implies the largest crater’s diameter is close to the d of the target body. Total ejecta accumulation is then roughly proportional to d, and with conservative parameter assumptions (e.g., b = 2) will be 1-5% of the body’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, caused in turn by higher estimated catastrophic-disruption mass ratio. For b = 2, the single largest crater will typically contribute close to 50% of the total of new (non-recycled) ejecta. For modeling of thermal implications, significant stochastic variations probably arise from two effects: concentration of ejecta mass into a relative few large fragments (although if formed relatively early these may be largely eroded by later cratering); and stochastically uneven distribution, especially on relatively small bodies. Megaregolith can be destroyed by sintering. The pressure sensitivity of the sintering process makes it effective at generally far lower temperature on larger (>> 100 km) bodies. Planetesimals ~100 km in d may be surprisingly well-suited (about as well-suited as bodies 2-3 times larger, assuming equal heat production) 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 T. But development of a megaregolith thick and porous enough to have significant thermal evolution consequences is practically inevitable.
Tuesday, 19 January 2010
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM