I had thought about these issues also.

The thing that bothers me is how do we know we have not already contaminated Mars.
We know that some microbial life can live in extreme environments. What if we go back 10 years from now and we see bacteria on many rocks. Were they there all the time or was it because we brought them there by accident on a previous expedition.

Of course the real danger is what we bring back here. We have to be very very careful with samples. If something gets out that we cant handle, there is some chance that it can literally kill all life on earth if we cant stop it.

A corollary to that is we dont even know for sure that our current techniques for isolating species is adequate. We assume whatever we bring back is not too different from what we know about life so far. What if it is so different that it can escape what we believe is adequate for keeping the entity contained.

I have to wonder also if there could be a virus on Mars. We could bring something back that really causes humanity a lot of problems.

We end up having to trust the scientists that are working on it.
Some say they already screwed up by sending information about us into deep space. The intention is that we are inviting friends. But in nature we see that different species dont always get along and when they dont they fight and kill each other. So we may be inviting a new enemy.
We never got to vote on that either it was decided for us without us.

certainly back in the first missions, the "sterilising" of our crafts to Mars was less than we do now,
the advantage in this case,
is we know the sort of bugs we have on earth, and can test for them and "know" how to eliminate them,
as best we can,
Its difficult without killing the spacecraft, but..

stuff from mars,
we have no idea what could be ,

Think Aids / HIV, How long was that ragging before we could identify what it was .

How can we honestly test for Mars contaminants ?

isolate a bunch of humans, and expose them to the stuff, and leave for a few decades to see any long term effects ?
if they catch something, what do we do with them ?

Very difficult decisions ahead.

 

IMO, predatory and parasitic life, be it bacteria, viruses or microbes, develops its ability to affect its prey through a long interactive evolutionary process. The chances of Martian life, if there's any, affecting earthly organisms should be practically zero. This because life on earth has had more than 1/3 of a billion years to adapt to ever changing hostile conditions.

 

certainly back in the first missions, the "sterilising" of our crafts to Mars was less than we do now,
the advantage in this case,
is we know the sort of bugs we have on earth, and can test for them and "know" how to eliminate them,
as best we can,
Its difficult without killing the spacecraft, but..

stuff from mars,
we have no idea what could be ,

Think Aids / HIV, How long was that ragging before we could identify what it was .

How can we honestly test for Mars contaminants ?

isolate a bunch of humans, and expose them to the stuff, and leave for a few decades to see any long term effects ?
if they catch something, what do we do with them ?

Very difficult decisions ahead.

Yeah and these days it is believed that life as we know it here developed partly because of viruses. But we dont know what kind of viruses can be out there, or can mutate into something uncontrollable.

What about a life form that is part virus part acid. It can eat thought metals over time and then infect animals.

I guess this is just part of mankinds quest to know the unknown and in the process maybe that is what kills all civilizations in the end. Maybe that is why we dont see any aliens yet. They all went where no man has gone before and couldnt get back.