What keeps the earth's atmosphere from dissipating?
Gravity and the magnetic field
Gravity is what maintains most of the atmosphere on the Earth, but we are losing a little bit of it all the time. Solar winds would carry off a lot more of the atmosphere if it weren't for our strong magnetic field that keeps us safe from them. In contrast we can look at Mars' atmosphere, it has a very thin atmosphere, not only because it has lower gravity, but because, once its core stopped producing a magnetic field the solar winds stripped most of the atmosphere off. What remains is purely the effect of gravity.