Planetary Protection

Planetary protection is about preventing contamination of life between planets.  This for two reasons: first, so alien planets are kept in their natural state for study, and second, so Earth is not contaminated by possible harmful extraterrestrial life.  Mostly, this consists of attempting to sterilize spacecraft to different degrees depending on what category of planetary protection mission it is, which you can read about here.  The current best method of decontamination is cooking spacecraft at high heat for a couple of days.

However, the topic of this post is what humans should do when manned spacecraft travel to Mars.  It will not be possible to keep a manned spacecraft at the same level of sterilization, just because of the humans.

There are a couple of basic options that exist.

The science option:
Life on other planets should be preserved until it has been completely studied, without any confusion over whether said life has accidentally been moved from Earth.  In theory, you could record everything about a life form, and then it would not matter what happened to it, but in practice methods for studying things will always be improving, and so you will not learn everything about it.

The survival option:
Anything that could be dangerous to humanity without providing an equal or greater benefit should be destroyed.  This is maybe a reasonable, if callous, option for microbial life, but what if the life is more complex?

The preservation option:
Nothing must be destroyed or changed.  This sounds good, however, any exploration or travel to other planets changes them, and leaving planets alone will eventually result in a disaster destroying or changing things there.

So what's the best option?  None of them.
A good analogy for this things is colors.

These are squares are completely red, green and blue.

They are unarguably colors that would be a very bad choice for, say, a living room.  And a compelling argument for any color is how bad the other two would look.  If you imagined your living room painted that green, you would think that maybe that blue wasn't so bad.  The same with the options of planetary protection.  All options, when taken to their extreme, sound bad, and the strongest argument for any one option is how terrible the other two would be taken to their extreme.

Unfortunately, none of the options are the best one.  We must proceed with a mix of these, based on what kind of life we find of Mars, if any.