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Geology 130F

 Lecture Five

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
Radial Position Temperature (K) Dominant Solid
1. 1700 Refractory minerals, (CaO, Al2O3 TiO)
2. 1470 Metals (Fe, Ni, Co, and their alloys)
3. 1450 Magnesium rich silicates
4. 1000 Alkali feldspars (silicates abundant in alkali elements (Na, K, Rb)
5. 700 Iron sulfide FeS (triolite)
6. 400 Fe condenses
7. ~350 Hydrated minerals rich in calcium
8. ~300 Hydrated minerals rich in Iron and Magnesium
9. 273 Water ice
10. 150  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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