What is resolved first, an card when-player-summon effect or a summoned card effect?
Chain activation 1: Opponent's monster summoned, and tries to destroy your monster. SS1 (SS2 if quick effect)

Chain Activation 2: Planet Pollutant Planet activates SS2

Where is Alien Mars? Because it is a SS1, it cannot be activated in this chain

Chain Resolution 2: An A-Counter is put on your opponent's monster.

Chain Resolution 1: Your opponent destroys your Alien Mars

Therefore, your opponent's monster destroys your monster, but you managed to put an A-Counter on it with your Planet Pollutant Virus.