Objective:  To understand the origin of compositional differences among the planets of the solar system.

 Sequence of events leading to the origin of the solar system:

I. Big bang, origin of the unverse 13.2 billion years ago.

II. Early formation of stars, galaxies, and star clusters.

     A. Stars form by condensation of nebula  (the primordial nebula or cloud is  primarily             hydrogen).

     B. Uneven distribution of material in nebula leads to formation of  galaxies and clusters of stars.

     C. Each star has a distinctive evolutionary path while it is burning its H fuel by  fusion to He.

III. Supernova explosion in the vicinity of our solar system 4.5 - 4.7 billion years ago.

     A. The the life cycle of a star depends on its mass .  The most massive stars exist  the shortest time (some as short as 100,000 years), exploding as supernovae when  their nuclear fuel is exausted.

     B.  Fusion up to Fe is possible in the most massive stars, but processes needed  to synthesize elements heavier than iron only occur at the pressures and  temperatures achieved in supernovae explosions (click here to view the periodic table of elements).

IV.  Our solar nebula condenses.  4.5 -4.7 billion years ago. The complete process of formation of the sun and all of the planets of the solar system takes less than 20 million years.

     A. Nebula flattens. Random collisions of nebular material become organized to  impart a primary direction of rotation to the nebula.  The nebula flattens into a disk  shape.

     B.  Nuclear fusion begins at the center of the nebula -- the sun is born.
When the pressures and pressures at the center of the nebula become high enough  for fusion to occur, out sun is formed.

     C. The planets condense and accrete. (condensation -- transition from gas to liquid  or solid; accretion -- collisions driven by mutual graviational forces between  particles several millimeters to kilometers in diameter)

          1. Local concentrations of material in the nebula condense and accrete to    form planets.  Since the nebula is now a flattened disk, the orbits of the planets all lie close to a common plane.

          2. The condensation of different elements is strongly influenced by a    condensation sequence based on temperature.  Planets close to the sun are   depeleted, compared to solar abundances in volatiles.  Planets further away   from the sun are closer to solar composition.

          3. Concentrations of material at the farthest reaches of the solar nebula condense and accrete to form comets before the rotating nebula has flatted. Comet formation occurs before the nebula has flattened into a disk shape, giving rise to the more spherical distribution of cometary orbits in the Oort Cloud.

 D.  As soon as the the sun forms, a solar wind will exist, blowing  radially outward from the center of the nebula.

          1. The solar wind has an electromagetic component and a particle component. The electromagnetic component also behaves as if it is     composed of particles (light has both wavelike behaver and particle like behavior).  The true particle component of the solar wind is composed of    ions and primarily electrons, which have been acelerated past the escape    velocity at the surface of the sun.

          2.The solar wind acts to sweep away any remaining, uncondensed or    accreted material in the solar system, leaving the regions between planets    and moons primarily as a vacuum.

          3. (Note: some solar wind effects blow material out of the solar system;  other effects act to make small particles spiral into the sun.)