Why Jovian Planests Thicker Atmos. Then Terrestial Planets? ?
Temperature and composition Basically the nebula from which the planets formed would have been 75% hydrogen. But to retain an atmosphere, the gas around a planet must not achieve escape velocity. Since the terrestrial planets are closer to the sun they have higher temperatures, which means atoms move faster. But how fast the atoms moves depends on their mass. Hydrogen (H2) is very low in mass, so at a given temperature it moves much faster than, say N2 or O2 or CO2 (which are much heavier). Consequently, the temperature on the terrestrial planets and their escape velocities means that they can;t retain H2 as an atmosphere – but the outer planets are cooler, so they can retain H2, and since H is more abundant than O or C or N – there is more for it to hold into and the atmosphere gets big. ADDED: I want to comment on the last answer – its not mass – the mass of the solid part of the planet does dictate the escape velocity – but the mass of the solid part of Jupiter is not much bigger th
