Volatile loss from accreting icy protoplanets

by: Stevenson, D. J.

ABSTRACT

A large self-gravitating body does not easily lose significant mass because
the escape velocity is much larger than the sound speed of atmosphere-for-
ming species under ambient thermal conditions. The most significant excep-
tions to this are giant impacts or impact jetting by fast-moving projectiles.
A very small object (e.g. a comet) also does not easily lose significant
volatile mass upon formation because the energy release associated with
its accretion is so small. (It can however lose a great deal of mass if it
is subsequently moved closer to the Sun.) I argue that there is an inter-
mediate mass range (corresponding to bodies with radii of approximately
300-800 km) for which the ambient steady-state mass loss is a maximum. By
ambient, I mean those conditions pertaining to the formation region of the
body. By steady state, I mean to exclude infrequent traumas (giant impacts).
The existence of a preferred intermediate mass arises through the compe-
tition of growing gravitational containment and growing energy release by
accretion it corresponds typically to GM (Rc(sub s)(exp 2)) approximately
equals 2 to 4, where M is the protoplanet mass of radius R, and c(sub s)
is the sound speed. Several factors determine the amount of volatile loss
is this vulnerable zone during accretion but in general the loss is a subs-
tantial fraction of the volatiles, sometimes approaching 100 percent. The
principal implication is that bodies larger than a few hundred kilometers
in radius will not have a 'primitive' (i.e. cometary) composition. This
is relevant for understanding Triton, Pluto, Charon, and perhaps Chiron.

Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science
Conference. Part 3 N-Z p 1355-1356, N94-20636 05-91, 1994.