Well, when the planets formed 4.5 billion
years 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. Temperature was HIGH
close to the protosun and temperature 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 .... That's why the terrestrial planets
ate based on rocks and metals...
Further away, 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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