Tardigrades can survive vacuum, low temperatures, and moderate radiation for quite a while. They're multicelled organisms. How tough is the toughest hypothetically viable single-celled extremophile? How sure are you that we've found all of them on Earth already?

It's a matter of caution. Contamination of another planet (or moon) is likely to be irreversible, may very easily make it difficult or impossible to determine the origin of any lifeforms there, or might outright take over and drive hypothetical native organisms to extinction (either before or after discovery), so all precautions are taken to avoid that.

Tardigrades can survive vacuum, low temperatures, and moderate radiation for quite a while. They're multicelled organisms. How tough is the toughest hypothetically viable single-celled extremophile? How sure are you that we've found all of them on Earth already?

It's a matter of caution. Contamination of another planet (or moon) is likely to be irreversible, may very easily make it difficult or impossible to determine the origin of any lifeforms there, or might outright take over and drive hypothetical native organisms to extinction (either before or after discovery), so all precautions are taken to avoid that.

Tardigrades can survive vacuum, low temperatures, and moderate radiation for quite a while. They're multicelled organisms. How tough is the toughest hypothetically viable single-celled extremophile? How sure are you that we've found all of them on Earth already?

It's a matter of caution. Contamination of another planet (or moon) is likely to be irreversible, so all precautions are taken to avoid that.

Tardigrades can survive vacuum, low temperatures, and moderate radiation for quite a while. They're multicelled organisms. How tough is the toughest hypothetically viable single-celled extremophile? How sure are you that we've found all of them on Earth already?

It's a matter of caution. Contamination of another planet (or moon) is likely to be irreversible, may very easily make it difficult or impossible to determine the origin of any lifeforms there, or might outright take over and drive hypothetical native organisms to extinction (either before or after discovery), so all precautions are taken to avoid that.