You are mixing two items: 1. Planetary protection (https://en.wikipedia.org/wiki/Planetary_protection https://en.wikipedia.org/wiki/Planetary_protection): life

You are mixing two items: 1. Planetary protection (https://en.wikipedia.org/wiki/Planetary_protection https://en.wikipedia.org/wiki/Planetary_protection): life on Earth might be dangerous for that of another planet, if we land on it. That's why for example the Mars rovers are prohibited from driving near areas where we think there might be/have_been life. And that's why Cassini was sent to burn into Jupiter's atmosphere: so that we would not contaminate any of its moons by mistake. 2. Clean-room precautions for satellites are mostly to prevent leaving anything on them that would freeze and burn depending on Sun exposition.