Option 1 states that the atmosphere has definite upper limits but gradually thins until it becomes imperceptible. This statement is incorrect. The atmosphere does not have definite upper limits. It extends far beyond Earth`s surface and gradually becomes thinner as altitude increases. However, it does not become imperceptible completely.

Option 2 states that the atmosphere has no definite upper limits but gradually thins until it becomes imperceptible. This statement is correct. The atmosphere does not have a clear boundary or upper limit. It extends into space and becomes thinner with increasing altitude, eventually blending into the vacuum of space.

Option 3 states that the atmosphere has definite upper limits but gradually thickens until it becomes imperceptible. This statement is incorrect. The atmosphere does not have a clear upper limit, and it does not thicken as altitude increases. It becomes thinner.

Option 4 states that the atmosphere has no definite upper limits but gradually thickens until it becomes imperceptible. This statement is incorrect. The atmosphere does not have a clear upper limit, but it does not thicken as altitude increases. It becomes thinner.

Therefore, option 2 is the correct statement about the atmosphere.