Spilling a few tardigrades on the Moon isn't a big deal, because there is almost certainly no life there.
But imagine if, in the year 2120, we discover thriving colonies of live bacteria 40 meters below the surface of Mars. Life on another world! It would change everything! But suppose upon further inspection the bacteria turn out to be a species from Earth, stowed away stuck to the treads of the Perseverance rover [1].
Was there life on Mars before we put it there? Was there a unique kind of Martian life, with DNA completely different from ours, that has been forever eaten away by our accidental contamination? We would never know.
For this reason, NASA and the Soviets agreed in 1967 to pursue a policy of Planetary Protection [2] ensuring that no probe or lander contaminates another planet.
But imagine if, in the year 2120, we discover thriving colonies of live bacteria 40 meters below the surface of Mars. Life on another world! It would change everything! But suppose upon further inspection the bacteria turn out to be a species from Earth, stowed away stuck to the treads of the Perseverance rover [1].
Was there life on Mars before we put it there? Was there a unique kind of Martian life, with DNA completely different from ours, that has been forever eaten away by our accidental contamination? We would never know.
For this reason, NASA and the Soviets agreed in 1967 to pursue a policy of Planetary Protection [2] ensuring that no probe or lander contaminates another planet.
[1] https://www.jpl.nasa.gov/news/press_kits/mars_2020/launch/mi...
[2] https://en.wikipedia.org/wiki/Planetary_protection