Hey, I learned this in science a few weeks ago!!
The least dense layer is the exosphere which is the outermost layer of the atmosphere.
In my science book it says, "it is least dense because it is
farthest from the earth, whose gravitation force decreases in accordance
with the inverse-square law. Since molecules and ions in this region
have a better-than-even chance of escaping from the gravity of the
earth, many of them do escape, to be replace by infalling matter from
space itself, or by atmospheric matter from the layers below."
Hope this helps!!!!!!!!!
