(a) If we consider only the superball itself as a system, momentum is not conserved. During its collision with the Earth, the impulsive force exerted on it by the Earth is considered as an external force. Momentum is not conserved if there is external force acting on the system. The momentum of the ball is in the downward direction before the collision, and it will become upward after the collision. There is momentum change during the collision and this change in momentum is caused by the impulsive force from the Earth (an external force).

(b) If we consider the superball and the Earth as a system, then the total momentum (momentum of ball + momentum of Earth) is ALWAYS conserved. The interacting forces between the ball and the Earth are considered as internal. There is no external force acting on the system. Momentum is conserved if there is no external force. As the ball is falling, the Earth is "rising" very slowly to maintain a constant total momentum. Both the ball and the Earth will rebound in opposite directions during the collision so that the total momentum is always the same.

(c) It will not change our argument in parts (a) and (b) if the superball is replaced by a piece of putty that will stick to the Earth's surface after the collision. However, in the case of superball, the collision is "near elastic", and the energy loss during the collision is very small. The putty will loose most of its kinetic energy during the collision because the collision is completely inelastic.