Characteristics and Origins of the Solar System
Lecture 32
December 3, 2001
Preliminary announcement: We will try observing again Thursday at 8:30PM.
Last time dealt with processes of condensation in the protoplanetary nebula. Given this gaseous disk, solid matter would begin to precipitate out. It would initially be as microscopic specks, later as raindrop-sized pieces, and after years, of clods us big as a few centimeters across. This picture of solar system development is borne out by detailed physics calculation.
An artists conception of this has been painted by William Hartmann.
>>>>> Hartmann picture of the early solar system.
>>>>> Blackboard drawing of protoplanetary disk
These clods are still not planets. A quote from William Hartmann: “The dust grains must have aggregated. If they hadn’t Earth wouldn’t be here and we wouldn’t be here”.
The current best suggestion for how to generate bigger hunks is via what in physics is called an instability. Physics tells us that these clods will settle by gravity into a very thin layer. The instability means that these clods will curdle up into much bigger objects, with sizes of up to 5 kilometers or so. This instability involved the clods. In the inner solar system, the instability gathered together clumps of metals, silicates, and other refractory (hard to melt) substances
In the outer solar system, the instability swept up clumps of ices together with the silicates and hydrated silicates.
It
is important to remember that since oxygen, carbon, and nitrogen are much more
abundant in the Sun than silicon, iron, and magnesium, there was much more
stuff precipitated out in the form of clods and clumps in the outer solar
system (water, ammonia, and methane ice) than in the inner solar system where
we are (silica, metal bits, and silicates of iron, magnesium, and
aluminum).
Following this instability, we believe the entire ecliptic plane was filled with countless asteroid-sized objects. The term we use for these elementary building blocks is planetesimals.
Our understanding of the next phase in the evolution of the solar system comes from solution of the equations which describe these planetesimals colliding with each other, sometimes becoming larger as two of them stick, sometimes shattering into pieces. As planetesimals grow, their gravity will become larger, meaning they are able to draw in smaller planetesimals.
Solution of these equations shows that this process of coalescence causes most of the planetesimals in an annulus of the protoplanetary disk to be swept up in the runaway growth of the largest planetesimal. Thus the mathematical theory can account for the very basic fact that the present day solar system consists of a small number of major planets, widely separated, rather than a universal asteroid belt. The calculations indicate that this process occurred in a few hundred thousand years.
There also occurs a major difference in the development of the inner solar system and the outer solar system. In the inner solar system, where the terrestrial planets presently are, there was the collisional accretion of the planetesimals into a single planet, and that was that. In the outer solar system there was much more material in the form of the planetesimals, so the protoplanets grew to much larger masses. Calculations show that when these planets grew to 10 – 15 times the mass of the Earth, their gravity was strong enough to star pulling in large quantities of the gas which still existed in the disk. Since there were many more kilograms of gas than in solid material, this permitted Jupiter and Saturn to grow into the massive monsters they are today.
After the planets had formed in their present positions and with their present masses, there were still a lot of left-over planetesimals. Over a period of hundreds of millions of years, these would have been swept up by the gravitational force of the planets, and collided with those planets. This is responsible for the Age of Bombardment. When you look at the face of the Moon, you are looking at the crash sites of the building blocks for the planets.
The idea of planetesimals seems somewhat fanciful. It says that the major planets were built from bricks 1 – 10 kilometers in size, in contrast with the handful of large planets we have today. What kind of evidence can we bring to bear to show that these objects actually existed? There are at least four properties of the current solar system that pretty clearly show that these objects must have existed.