system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: NASA is destroying the Cassini probe in Saturn's atmosphere to prevent from contaminating Titan and Enceladus (which could harbour life). So why aren't they worried about the Huygens lander that's already on Titan doing the same thing?
gpt
Probes that are *meant* to land on other worlds are meticulously sterilised before launch.   
Since this is an expensive and time-consuming procedure, this step is foregone in other probes, and they are safely destroyed by crashing them into gas giants.