April 28, 2004

The Origin of the Solar System II

Last time dealt with processes of condensation in the protoplanetary nebula. Given this gaseous disk, solid matter would begin to precipitate out.

We can put two and two together to come to an important conclusion. Last time we saw that at different temperatures, different materials would condense out of the vapor phase. We also discussed the fact that in the inner solar system, the temperature of the solar nebula would have been high, and in the outer solar system it would be low. This then indicates that different materials “rained out” at different parts of the solar system. This is illustrated in Figure 18.18 of the text.

This precipitated material 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 artist’s conception is shown of a similar event, occurring right now around the star Fomalhaut.

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.

That is, initially there were billions and billions of small planetesimals, each one of which you could have carried around in your backback. Over time these coalesced into a few hundred. Then these few hundred coaleseced into a few dozen. Then to five, then to two, and finally the largest planetesimal ate the second largest planetesimal.

Question: Can you think of any feature of the solar system, which we have discussed in class this semester, which is consistent with this picture?

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.

Sweep-Up of the Last Planetesimals

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.

Evidence for Planetesimals

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.

Asteroids, more or less examples of planetesimals.
The existence of craters on solar system objects.
The moons of Mars, Phobos and Deimos, are generally believed to be planetesimals that were captured by Mars. A picture of Phobos is shown below.

The existence of water on Earth(?!) “Most of the biosphere was brought on the primitive Earth by an intense bombardment of comets”.