Previous: Lecture Four
Formation of Condensates
(Ch. 23 in The New Solar System, pp. 28-30 in Exploring the Planets)
The solar nebula was originally gas, but as the density of the gas increased
solid material began to condense out. The process is the inverse of sublimation,
in which a solid such as ice goes directly to the gas phase (water vapor
in this example). A solid formed by condensation is called a condensate.
Relative locations and temperatures of condensates
||Refractory minerals, (CaO, Al2O3 TiO)
||Metals (Fe, Ni, Co, and their alloys)
||Magnesium rich silicates
||Alkali feldspars (silicates abundant in alkali elements (Na, K, Rb)
||Iron sulfide FeS (triolite)
||Hydrated minerals rich in calcium
||Hydrated minerals rich in Iron and Magnesium
||Other ices (NH3, H2O, etc)
Accretion of Condensates
From dust to planetesimals
The condensates take the form of (1 micron size) dust grains in the solar
disk. These grains will settle to the disk midplane since they are heavier
than the H and He gas. What happens next is uncertain. One possibility
is that the thin disk of dust is gravitationally unstable, leading to the
formation of roughly 1 kilometer size objects known as planetesimals. Another
possibility is that the flow in the disk is turbulent, so that the dust
cannot settle out and form an unstable thin disk. In this picture the dust
grains collide with each other and stick to form slightly larger bodies,
which in turn collide to form yet larger bodies. This picture suffers from
the difficulty that bodies between the size of dust and planetesimals suffer
the effects of drag, and so tend to spiral into the sun.
From planetesimals to planets
Collisions between small planetesimals will build up larger bodies over
relatively short times (10,000-100,000 years).
The number of planetesimals of a given mass follows a power law, where
most of the planetesimals are small (and so have small masses), N(m)=m-b
, where b is a positive number. The question is, what happens next?
Collisions between planetesimals should lead to accretion, in which some
planetesimals grow by eating their smaller brethern.
Numerical simulations of this process suggest two phases of growth.
Initially there is a period of "runaway growth", in which the largest bodies
grow most rapidly. Once the largest planetesimals reach sizes comparable
to that of the Earth, a second phase, called oligarchic
growth takes over. In this phase, the growth of the largest bodies
is slowed dramatically, while smaller bodies continue to accrete. This
leads to the formation of several to tens of Mars to Earth mass objects.
However, when most of the planetesimals have been incorporated into
50-500 protoplanets, the time between collisions becomes very long, at
least in the simple models employed up to now. The question of planet formation
is still an area of active research.
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