Bad Astronomy: Misconceptions and Misuses Revealed, from ...

194 ARTIFICIAL INTELLIGENCE
moons have more ice in their composition, for example, perfectly
consistent with a disk that had a distribution of materials along it.
To be generous, this argument is at best disingenuous on the part
of the ICR. If the collapsed disk theory hadn’t jibed with that most
basic observation, it would have been thrown out before it ever
got proposed.
Since about 98% of the sun is hydrogen or helium, then Earth,
Mars, Venus, and Mercury should have similar compositions. Instead,
much less than 1% of these planets is hydrogen or helium.
When they formed, the inner planets probably did have a much
higher amount of these gases. However, the gases are very lightweight.
Imagine flicking your finger on a small pebble. It goes flying!
Now try that on a station wagon. The car won’t move noticeably,
and you may actually damage your finger. The same sort of
process is going on in the Earth’s atmosphere. When a molecule of
nitrogen, say, smacks into a much smaller hydrogen atom, the
hydrogen gets flicked pretty hard, like the pebble. It can actually
pick up enough speed to get flung completely off the Earth and out
into space. When the nitrogen molecule hits something heavier, like
another nitrogen molecule, the second molecule picks up less speed,
like the station wagon in our example. It pretty much stays put.
After a long time, the lighter atoms and molecules suffer this same
fate; they all get flung away from the Earth. Over the lifetime of
the Earth, all of the hydrogen and helium in the atmospheres of
Earth, Venus, and Mars have basically leaked away, leaving the
heavier molecules behind.
Jupiter and the other outer planets retained their lighter elements
for two reasons: they are colder, and they are bigger. A colder
atmosphere means the collisions occur at slower speeds, so the lighter
elements don’t get lost to space. A bigger planet also has more
gravity, which means the planet can hold on more tightly to its
atmosphere. A small hot planet like Earth loses its hydrogen; a big
cold one like Jupiter does not.
So the collapsing cloud theory predicts that initially the planets
may have had a lot of hydrogen and helium in their air, but