Well, when the planets formed 4.5 billion yrs ago,
a disk of hot material (Hydrogen, Helium and tiny
bits the other elements) formed a flattened
pancake-like gas disk around the then-forming
sun. This disk is called the ACCRETION DISK
because it was material from this disk that was
falling onto the sun... Thats how the sun gathered
Anyway, there was a strong radial
temperature gradient in this disk.Imagine that you
can look down onto the disk from the top. you see
then ascent sun (protosun) at the center. Then you
see this debris pancake of stuff. There was a
Temperature gradient as you radially go away from
the protosun. Temp was HIGH close to the proto sun
and temp was cooler far away.Hence, when the
accretion disk of gas cooled, it was easy to get
condensation of light compounds where it was cold.
But close to the protosun these elements would
remain in the gas state.
SO... you can think
of a snow line at the orbit of Jupiter... closer
in to the sun it was too HOT to make the gas
condense... Only the silicates (rocks) and metals
(iron) could condense...Hence the Terrestrial
planets .... Thats why the terrestrial planets ate
based on rocks and metals...
however, where things were cooler, the gases could
condense into ice particles. This is why the
Jovians are gas rich and the terrestrial are not.
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