I don't think it's illegal, but it is enforced by COSPAR's recommendations on Planetary Protection. Missions to Mars actually have a much higher threshold for a

I don't think it's illegal, but it is enforced by COSPAR's recommendations on Planetary Protection. Missions to Mars actually have a much higher threshold for avoiding contamination than, say, missions to the moon. https://en.wikipedia.org/wiki/Planetary_protection https://en.wikipedia.org/wiki/Planetary_protection