As Newton figured out, the force of gravity between two objects is
given by F = G*M*m/(R*R), where G is a constant, M and m are the masses
of the two objects, and R is the distance between the centers of the
objects. You can use this formula to figure out the force on an object
on the surface of any planet. If you plug in the objects mass for m,
the planets mass for M and the radius of the planet for R, you will get
the force. A common way to re-write this is F = mg, where g =
G*M/(R*R), and happens to also be the acceleration of an object dropped
near the planets surface (sometimes called the surface gravity).
If you go to a different planet, you will get a different answer
value for "g". If the planet is bigger it will usually work out that g
(which depends on the mass and the radius of the planet) will be bigger
too. In other words, the gravity on the surface of a bigger planet is
usually bigger than the gravity on the surface of a smaller planet.
Now, what about oxygen. This depends on two things. first of all
there must be some way for oxygen to form in the first place (like
plants, for example, or certain other chemical reactions). Second, the
gravity of the planet has to be big enough to keep the oxygen atoms
from just flying away from the surface of the planet into space. This
may sound odd, but just think of oxygen (or any other gas) atoms as
little baseballs. If you throw a baseball on the moon, it can go a lot
farther than if you throw it on the earth, This is because the gravity
on the surface of the moon is not as string as the gravity on the
surface of the earth. If the moon were really small, you would be able
to throw a baseball fast enough that it would never come back down.
(This is called escape velocity). Its the same with gas atoms like
oxygen. They all move pretty fast. If they are on a planet where the
gravity is really weak, they would all eventually escape and there
would be none left.
(published on 10/22/2007)