Planets Made of Gas

Why are the inner planets are made of rock and free of gas and the outer planets have trapped gas as part of their structure?
- Kat (age 14)
Elsie Allen Highschool, Santa Rosa

The planets Venus, Earth, and Mars actually do have thin (as compared to the size of the entire planet) atmospheres. Mercury has a very low-pressure atmosphere. Most of the reason for this is that small planets do not have a gravitational pull strong enough to hold lighter gases in, and they escape to space. The most common elements in the universe are hydrogen and helium, and a strong enough gravitaitonal field will make these light elements stay together. Both hydrogen and helium escape from Earth’s, Venus’s, and Mars’s atomospheres, leaving heavier molecules behind, and there aren’t as many of those. What’s left is the rocky and metallic core. It isn’t outlandish to believe that the outer planets also have metallic and rocky components at their cores too, but since they are so massive, they are shrouded in the thick layers of hydrogen and helium which prevent us from seeing inside. There are of course many other components to the atmospheres of the gas giants, but they are mostly hydrogen and helium.

Pluto again has very little atmosphere, and it is mainly made up of nitrogen gas -- pluto is very small and its weak gravity won’t hold lighter elements in. In fact, the nitrogen gas liquefies for part of Pluto’s year because it gets so cold when Pluto’s orbit takes it beyond the orbit of Neptune.

Tom

So what we’ve explained is why the big planets still have hydrogen and helium which have escaped from the small planets. We haven’t explained why the big planets tend to be farther away from the Sun. We can try to find out about that too if you’re interested. It might be partly due to higher temperatures speeding up evaporation of the inner planet atmospheres. /Mike

(published on 10/22/2007)