explain why the jovian planet atmospheres are called primitive atmospheres while terrestrial planets are called evolved atmospheres. Solution Jupiter\'s atmosphere is composed mostly of hydrogen and helium, which are the same gases that dominate the Sun\'s and other stars\' composition. Likewise the early Earth atmosphere was most likely H2 and He. But the Earth\'s gravity is too weak to retain these gases for a long time. So contrary to the Jovian atmosphere the Earth\'s atmosphere changed and evolved. Hydrogen and helium were quickly replaced by an atmosphere produced by volcanic out-gassing. It contained gases like H2O, CO2, SO2, CO, S2, Cl2, N2, H2 and NH3 and CH4. But no free O2 as this is not found in volcanic gases. A lot more water came from outer space brought by comets bombarding the proto-Earth. As the Earth cooled, H2O rained out and the oceans formed. It didn\'t stop there, as oxygen started to buildup as soon as the Archean era, more than 2.5 billion years ago, by means of photochemical dissociation. That is breakup of water and ammonia molecules by ultraviolet. The O2 level rose to 1-2% of the current level. At these levels O3 (Ozone) can form to shield the Earth surface from ultraviolet light. Once the Earth was properly shielded, photosynthesis by bacteria and plants could start: CO2 + H2O + sunlight = organic compounds + O2, increasing the oxygen level to today\'s levels..

1. explain why the jovian planet atmospheres are called primitive atmospheres while terrestrial
planets are called evolved atmospheres.
Solution
Jupiter's atmosphere is composed mostly of hydrogen and helium, which are the same gases that
dominate the Sun's and other stars' composition. Likewise the early Earth atmosphere was most
likely H2 and He.
But the Earth's gravity is too weak to retain these gases for a long time. So contrary to the
Jovian atmosphere the Earth's atmosphere changed and evolved. Hydrogen and helium were
quickly replaced by an atmosphere produced by volcanic out-gassing. It contained gases like
H2O, CO2, SO2, CO, S2, Cl2, N2, H2 and NH3 and CH4. But no free O2 as this is not found in
volcanic gases. A lot more water came from outer space brought by comets bombarding the
proto-Earth.
As the Earth cooled, H2O rained out and the oceans formed.
It didn't stop there, as oxygen started to buildup as soon as the Archean era, more than 2.5
billion years ago, by means of photochemical dissociation. That is breakup of water and
ammonia molecules by ultraviolet. The O2 level rose to 1-2% of the current level.
At these levels O3 (Ozone) can form to shield the Earth surface from ultraviolet light.
Once the Earth was properly shielded, photosynthesis by bacteria and plants could start: CO2 +
H2O + sunlight = organic compounds + O2, increasing the oxygen level to today's levels.