CHAPTER 6

    The accretion theory maintains that the Sun's satellites grew gradually from cold, solid particles that collided and stuck together.
This process of growth, through random collisions, from microscopic grains to asteroids and planets took between 50,000 to 1 million
years, as evidenced from the analysis of radioactive gases in meteorites. Dust grains (silicates, metals, graphite, and hydrocarbons)
are a natural part of the interstellar medium, along with a variety of molecules, such as, ammonia, hydrogen sulfide, formaldehyde,
hydrogen cyanide, methyl cyanide, simple alcohols, carbon monoxide, water, and many others. In addition, supernovae events
enrich the interstellar medium with many differen radioactive elements in much greater abundance than one finds presently in the
Solar System.

    The material that now forms the Solar System separated from the interstellar medium 5 to 6 billion years ago as a relatively dense
and opaque primordial cloud of gas and dust slowly undergoing gravitational contraction. This cloud is called the solar nebula. As
the solar nebula contracted, the conservation of angular momentum demands that the cloud began to rotate faster and faster. This
rotation caused the cloud to dynamically flatten into a disk. See Figure 1. The densest part of the cloud was at the center of the
rotating disk  and this part contracted faster than the outer parts of the disk to become the Sun. Hence, the Sun's rotation indicates
the original direction of rotation of the primordial disk. As the Sun contracted, it left behind dust and gas that continued to revolve
around the Sun in the same direction as the original rotation of the disk. This material eventually accreted to form the planets and,
hence, the planets revolve around the Sun in the same direction that the Sun rotates on its axis.

    The rotational flattening and gravitational contraction (together called the dynamic collapse) of the material  in the
remaining portions of the disk caused the gases to interact chemically to produce more complex  molecules. Sometimes the
microscopic dust particles in the cloud served as catalytic surfaces for such reactions to take place. At the same time, the
dynamical collapse of the primordial cloud caused the dust grains to collide more and more often, resulting in their sticking
together and growing larger and larger and incorporating some of the molecules, mentioned above, into their structures. In
a time somewhere betwee 50,000 years to several hundred thousand years (the exact time is highly debated), the grains
had grown to form many objects that were about 10 to 500 km in diameter called planetesimals (W. K. Hartmann, a planetary
expert at the Univ. of Arizona, defines a planetesimal to range in size from sub-millimeter to 1000 km, but there is no official
IAU definition for the size of a planetesimal).

Sometimes these planetesimals would collide at high speeds and break apart into smaller fragments. It is believed that the
majority of meteoroids that hit the Earth's surface (meteorites) even today are produced by such collisions that continue to
occur in the asteroid belt. However, sometimes two planetesimals would collide at low speeds and stick together to form
even larger objects up to 1000 km in diameter. The result of a slow collision between two planetesimals has sometimes
resulted in a body whose growth was stalled, for some reason. Radar studies of the asteroids have revealed at least two
such objects. One is called Castalia and the other Tautatis. See Figure 2 below, which shows a radar representation of
Castalia. The existence of such bodies is evidence of the accretion process by which the planets formed from smaller
bodies. Objects such as Castalia and Tautatis may have formed late, in a stable solar orbit, after the major planets have
swept up most of the planetesimals and other meteoroids.

      Objects that grew to sizes in excess of 250 to 300 km by numerous fusion collisions reached a critical mass for their growth
and they are referred to as planetesimals.  Some astronomer even refer to them as protoplanets. As these bodies moved in orbit
around the primordial Sun, they began to grow more rapidly by gravitationally attracting smaller bodies that were located near
their orbital paths. The greater the mass they acquired, the greater the gravitational reach they had and the faster they grew.
This process of  growth is called gravitational sweeping.
    The infall of material onto the protoplanets became so intense that the energy released in the collisions melted their surfaces
to a depth of many kilometers. See Figure 3 below.

                                                                    Figure 3. A drawing showing a planetesimal undergoing gravitational sweeping as
                                                                    well as interior melting from the release of energy by the decay of trapped, short
                                                                    lived, radioactive elements. This is a stage similar to panel A in Figure 4.

    At the same time, the interiors of these bodies were heated by the energy released from the decay of trapped radioactive
elements.  From the assumed abundances of such short lived isotopes as Al²⁶, I¹²⁹, and Pu²⁴⁴ , it has been calculated that the
amount of energy released was so great that it caused the interior to melt, if the planetary body was 50 km in diameter or larger.
See panel A in Figure 4. On the other hand, if the diameter was less than 50 km, this heat escaped through the surface of the
object too fast and could not accumulate to melt the interior.

It is believed that these two sources of heat, an external one and an internal one, were:

    1.  An intense meteoroidal bombardment as a result of gravitational sweeping
    2.  The release of energy as a result of the decay of trapped,  short lived, radioactive elements,

These two sources caused the larger planetary bodies to become entirely molten. See  panel B in Figure 4.  In such a state,
gravitation separation of the elements took place thereby giving rise to a chemically differentiated  planet. That is, the heavier
elements sank to the center of the body while the lighter elements rose  to higher layers. Also, their self gravities would cause
them to become spherical in shape, or oblate spheroids if they rotated very fast. Smaller objects, whose surfaces were not melted,
remained whatever shapes they acquired as a result of the random collisions of the accretion process, unless the body had a
sufficiently large mass that its surface was strong enough to overcome rigid body forces and form a spherically shaped object.
That latter critcal size is in the neighborhood of 300 km in diameter.  But this depends on the chemical composition of the body.

Eventually the planetary bodies achieve their final sizes when most of the smaller, solid bodies had
been swept up or the smaller bodies moved in stable orbits out of gravitational reach of the larger
planets. These smaller bodies are the minor planets or asteroids that exist today in the Solar System.

As the amount of infalling material decreased, the planetary surfaces started to cool and solidify. Any
further infall of material then began to form permanent craters on the surfaces of the terrestrial like
planets.  This apparently started about 4.6 billion years ago as evidenced from the oldest rocks found
on our moon. See panel C in Figure 4.

                            Figure 4. Stages in the evolution of a planetary body. Panel A shows a body beginning to
                            undergo gravitational sweeping as the interior melts from energy released by trapped
                            radioactive elements. In B, the body has become entirely molten and gravity gives it a

    Further cooling of the planets produced cracks in the crust, allowing molten material from the
interior to gush upward onto the surface and fill in the lower levels or basin areas. It is believed the
lava plains that are the maria of our moon, other moons, and Mercury were formed in this way. See
panel D in Fig. 4. The infall of material or great meteoroid bombardment had nearly ended 3.5 billion
years ago,  since the rock samples brought back by the Apollo astronauts from the lunar  maria are
this old and the maria show little cratering.

    As the Sun evolved, it became smaller and hotter. Five billion years ago, the Sun was as large
as the orbit of Pluto. Hence, the planets could not have existed at this time. As the Sun became
hotter, radiation pressure and the solar wind blew all the lighter gases, such as hydrogen and helium,
away from the inner regions of the solar system. Hence there is a lack of these light elements in the
compositions of the inner planets today. On the other hand, the Jovian planets were located
sufficiently far from the Sun that they were able to hold onto their extensive primordial atmospheres.
This explains one of the major differences between these two classes of planets.

    In the outer regions of the solar system, where the temperature environment was lower, the more
volatile materials such as water and carbon dioxide were able to remain as ices that were incorpor-
ated into the surfaces of the satellites of the giant planets. In fact, Pluto and its moon Charon are
made of a great deal of ice. The comets, which are also made of a large amount of ice, formed by
accretion in the outermost regions of the solar system. It is estimated that about 100 billion such
objects form a spherical shell around the Sun at a distance of about 50,000 AU. This shell is called
the Oort Cloud.

    Another locus of cometary nuclei, called the Kuiper Belt, has been detected to extend from near
the orbit of Pluto and beyond for some unknown distance less than 50,000 AU.  It is estimated that
the Kuiper belt contains many thousands of small icy bodies and the TNOs. The cometary nuclei
in this belt move within a volume of space shaped like a doughnut or toroid, rather than in a
spherical cloud or flattened disk.  To date, more than 100 individual objects have been discovered
moving in distinct orbits in the Kuiper Belt.

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Copyright 2003. by R. J. Pfeiffer
Professor of Physics and Astronomy
The College of New Jersey