BECAUSE IT IS IRREVERSIBLE
Unlike other forms of contamination, life can reproduce, and if you introduce Earth life to a planet for the first time, and it starts to reproduce there, there is no way to reverse that.Right now, our main motive is because we want to understand the solar system better, and the origins of life.
If the planets start getting colonized with Earth life, that will get in the way of that search in many ways.
Later on we might decide it is okay to introduce life to other planets, or might have many reasons for keeping it away. But if we ever do make that decision, it is one we can never go back on.
We can never, ever, roll back to the present solar system if we introduce a new life-form on Mars. The biological history of Mars would be divided into two parts - its history before the introduced Earth life, and it's history after that.
So we need to be totally sure that this is exactly what we want to do. And right now our international policy, as defined in the Outer Space Treaty, and clarified in many workshops at COSPAR, is to keep the solar system free of Earth life until we find out more.
WHY DOES IT MATTER
Even though it is irreversible - you might wonder, so what?
Well - it's partly because we want to study these places and find out about past life.
It's like a giant library of knowledge, introducing life to another planet before we can study it is like setting fire to the Alexandra library of Egypt.
It's knowledge that could fill in many gaps in our understanding of evolution of life on Earth and evolution around other stars.
Then - when we have no idea what the life could do - it's also like a giant experiment with the whole planet as a petri dish. How do we knowwhat our descendants a few generations from now will want? How do we know what we want ourselves?
If anyone says they know what will happen to a planet when they introduce new lifeforms to it and watch them spread and evolve as they colonize the planet - unless they have access to knowledge a few centuries ahead of our time - how can they possibly have answers to that?
If we want to do grand experiments like that, we can try doing them in space habitats first. Larger and larger space habitats. They are self contained, can be sterilized, and if something goes wrong, the worst that happens is that you abandon it and start again with a new habitat.
The are also far more controllable, with any temperature range, air pressure, and even level of gravity (spinning for artificial gravity), humidity, water, etc, everything can be set up just as you like it - and you can finish the job in a few decades.
While terraforming a planet, if it can be done at all, takes thosuands of years in even the most optimistic projections - and that's with mega technology.
See also:
Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?
WHAT DOES EARTH LIFE NEED
So, the most important thing in planetary protection is to keep Earth life away from places in the solar system where it can reproduce.
We also need to keep any alien life away from Earth if we think it could reproduce here.
Earth life needs liquid water - or at least ice - to survive. That also rules out vacuum conditions unless it is contained in some way (say with a layer of rock, or tarry deposits or whatever it is)
Also Earth life needs a range of temperatures from about −20°CC to about 122 °C, depending on the life form.
It can stay in a resting state, dormant, at lower temperatures and in complete vacuum, but at some point has to be revived, or reproduce.
That's why we take especial care not to introduce Earth life to Mars, or to Europa or Encladus - all places where we think it's possible that Earth life would reproduce.
DON'T CARE ABOUT MOON
So - we don't care about the Moon, because it's surface is a vacuum. In the case of the Moon only requirement is to document what you do so that others know, e.g. that Apollo left human wastes on the Moon etc.
It's not going to spread though, Earth microbes aren't going to start colonizing the Moon, the organics on the Moon will just stay there, get dessicated, sterilized by cosmic radiation, and finally will have no viable life left. Perhaps after only decades, perhaps after many millennia, it doesn't matter much because there is nowhere for it to contaminate on the Moon, it's just sitting there in a dormant state until it finally dies completely.
OTHER PLACES THAT DON'T MATTER
Similar rules apply to Mercury, also to most of the smaller asteroids and comets. You document what you do but there are no other regulations
Sun of course doesn't matter at all.
Jupiter's moon Io is probably too hot, for Earth life.
Saturn's moon Titan is probably too cold, for Earth life - though that's a somewhat borderline case as it might have an underground ocean.
PLACES THAT ARE BORDERLINE
Other places are borderline, such as the larger asteroids, Ceres for instance, and the very largest comets. They may have liquid water inside somewhere, or temporarily on the surface. May seems unlikely but you only know for sure after you study them, andwe've had many surprises.
OTHER REASONS FOR PROTECTING FROM EARTH LIFE
Sometimes even dead life is a problem. For instance if you are searching for pre-biotic chemistry. So we may need to keep Earth life away from some places for that reason.
E.g. if you have scientists studying the prebiotic chemistry of ice in caves on the Moon, or in the permanently shadowed craters at its poles - you might want to keep astronauts well away to avoid contaminating it with Earth amino acids, DNA, etc.
So, as we explore the solar system, there may be other special cases like this.
But the main objective is to protect areas of the solar sysem where Earth life could reproduce.
Where that gets a bit tricky is, that we don't really know all the limits of what Earth life can do. We still find new extremophiles from time to time that handle conditions that we thought, before, were impossible for Earth life.
For more about the planetary protection policies, and categories of places we protect etc, see
Planetary protection
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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