Why do some planets or moons have atmospheres and some do not?

Answer 1

Planetary formation is not as well understood as some would have you believe. However, the prevailing theory is that planets form around new stars that have an accretion disk of available material.

The heavier, rocky material is closer to the sun, and the lighter, gassy material is further away, with a cloud of icy material at the fringes of the planetary system.

The predominant theory about atmosphere formation is that 'outgassing' is responsible for maintaining an atmosphere. This is a process that relies on a geologically active planet, with a molten core, that heats up the interior of the planetary body to a point where gasses can emerge. Secondly, the planetary body needs to be big enough to hold the atmosphere down through the force of gravity. Third, the planet needs to be far enough away from the star or large body so that any emitted radiation doesn't strip the atmosphere from the planet or moon.

Answer 2

Commonsense had the answer - if the body does not have enough gravity, the atmosphere can escape into space - the mechanics of this is that the molecules on the fringe of the atmosphere reach escape velocity when excited by sun's energy.

(It is nothing to do with spin, as the previous respondant said)

Answer 3

Larger spinning spheres can maintain an atmosphere, but smaller spinning objects do not create enough gravity to maintain an atmosphere.

Take care,
Troy

Answer 4

Some do not have enough gravity to hold onto the atmosphere.

Answer 5

Mercury is so close to the sun that it can not have any atmosphere becasue the sun would blast it away

Answer 6

Because thats how god wanted to be ,, and he made them like that