Department of Biochemistry & Integrative Medical Biology, School of Medicine, Keio University
Gas: Mementos of Creation
      Out of whose womb came the ice ? Job
  "As the nebula contacts and its density increases,collisions of grains with one another become more frequent. In part because of the organic contents of these grains,when they collide they tend to stick. Big grains gobble up smaller ones. But all this does not go on in the dark. The primitive Sun has begun brightly shining. In the outer parts of the disc,it is still sufficiently cold for exotic ices such as methane or CO to be perfectly stable in the growing condensations of matter.
But in the very inner solar system,it is too hot even for water ice; the ices on the grains evaporate and dissipate,and what survives in mainly made of silicates. You have to carry a rock very close to the Sun for it to boil. As a result of all this,the chemistry in the outer solar system must have differed from the chemistry in the outer solar system - silicates predominating inside,and ices with a few percent of organics outside." (cited from COMET by Carl Sagan and Ann Druyan)