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)
