It's a probability thing. You simply can't sterilize any machine 100% with current technology and end up with a machine that is still functional and able to fly to Mars and land there and do experiments there. So they do it based on probabilities.
The way it is done is a bit arbitrary. It's based on the Coleman Sagan equation Planetary protection - Coleman Sagan (Wikipedia)
originally. That's an equation for working out the chance of contaminating another celestial body per mission, based on the number of spores on it on launch, and various factors that reduce those numbers as the mission progresses.
Anyway it was used for Viking, but not since then, since the conditions Viking found were so inhospitable for life, they made a rather arbitrary decision that if they reduce the number of microbes to 500,000 per spacecraft on launch, then the hostile environment on Mars does the rest of the reduction for you.
The aim in either case is to have a 1 in 10,000 chance of contaminating Mars per mission and a 1 in 1,000 chance of contaminating Mars during the preliminary exploration period. Where that 1 in 1,000 figure was just a figure chosen pretty much arbitrarily by Sagan and Coleman because they needed to have a figure to devise a policy. If you attempted certainty, you simply couldn't fly to Mars.
You could well argue that those probabilities are not strict enough.
And the 500,000 spores figure, really that's not based on any detailed scientific justification at all, it's just based on the idea that, roughly, the surface is so inhospitable that it approximately corresponds to the dry heat sterilization phase for Viking before launch. The 500,000 spores is the figure used for the number of spores on Viking before heat sterilization phase, so they got it just by taking the Viking protocol and removing the dry heat sterilization phase from it.
And there have also been several failures of the protocols as well.
So the upshot is that we can't assign a definite probability of contaminating Mars, but it's not zero.
So, we might have contaminated Mars. Nobody can say for sure we haven't. But the conditions are so harsh there that most people would say it is unlikely that we have yet, especially as the spacecraft are quite a bit sterilized, if not as much as Viking, down to that 500,000 spores per spacecraft on launch. Which may seem a lot, but it gets reduced further during the trip out there due to cosmic radiation and UV radiation - and the vacuum conditions. Yet, undoubtedly there are viable spores on our spacecraft on Mars but most think they are dormant, not doing anything.
It's all based on the idea of an "exploration period". If you think that we might need to keep Mars clean in perpetuity, then at some point we would have to go to Mars and sterilize all the spacecraft or remove them from the planet. Presumably using some future technology that lets us do that without introducing more contamination to Mars in the process.
If there is life on Mars from Earth, then we should take even more care n the future, study what happened, and do our best to reduce the impact. For instance, suppose it is in just some small patch, say around the Phoenix lander or something, started to reproduce, perhaps there is a chance of doing something about it before it spreads.
I think myself that a good mission to do, once we can sterilize our spacecraft better than we can now, is to send a lander to the Phoenix location and see if any life has spread from the lander. Because it's one of the most vulnerable I think. Not sterilized for a "special region" as nobody thought it would be suitable for life back then. And it was crushed by some meters of thickness of dry ice (expected) and nowadays there are several ideas for possible ways that habitats for life could exist at the Phoenix location. Which doesn't mean that there are habitats for life there, they are just hypotheses. But - it can't be ruled out either at present.
The main ones I know of relevant to the Phoenix location are
(I'd land it some distance from Phoenix and study the hopefully pristine area first, with biosignature and life detection, and then travel up to Phoenix and study the area around Phoenix and the spacecraft itself and see if there is any sign of local contamination and if any spores survive on the spacecraft itself).
And if we have contaminated Mars, and if we find we can't reverse the effect, it's spread too far (once it spreads in the dust storms for instance, how can you stop it?) - we then will have to look and see what the impact is. It would be like finding out that we have introduced rabbits to a remote pristine island. So then you look at what impact they have, and see what you can do about it, if anything. But the very last thing you do in that situation is to introduce rats, dogs, cane toads etc etc. Maybe if it is just a few species, we can learn to identify them and distinguish them from native Mars life, if it exists and maybe we can work around it in some way, and maybe if we are lucky, they are not especially well adapted to Mars, and impact on native life, if it exists is not going to be so huge.
But most would say, that there is at least a pretty high chance we haven't contaminated Mars yet. We can't say for sure I think until we have some ground truth from Mars on the topic.
This describes some of the methods currently used to protect Mars from Earth microbes: NASA Office of Planetary Protection
The way it is done is a bit arbitrary. It's based on the Coleman Sagan equation Planetary protection - Coleman Sagan (Wikipedia)
originally. That's an equation for working out the chance of contaminating another celestial body per mission, based on the number of spores on it on launch, and various factors that reduce those numbers as the mission progresses.
Anyway it was used for Viking, but not since then, since the conditions Viking found were so inhospitable for life, they made a rather arbitrary decision that if they reduce the number of microbes to 500,000 per spacecraft on launch, then the hostile environment on Mars does the rest of the reduction for you.
The aim in either case is to have a 1 in 10,000 chance of contaminating Mars per mission and a 1 in 1,000 chance of contaminating Mars during the preliminary exploration period. Where that 1 in 1,000 figure was just a figure chosen pretty much arbitrarily by Sagan and Coleman because they needed to have a figure to devise a policy. If you attempted certainty, you simply couldn't fly to Mars.
You could well argue that those probabilities are not strict enough.
And the 500,000 spores figure, really that's not based on any detailed scientific justification at all, it's just based on the idea that, roughly, the surface is so inhospitable that it approximately corresponds to the dry heat sterilization phase for Viking before launch. The 500,000 spores is the figure used for the number of spores on Viking before heat sterilization phase, so they got it just by taking the Viking protocol and removing the dry heat sterilization phase from it.
And there have also been several failures of the protocols as well.
So the upshot is that we can't assign a definite probability of contaminating Mars, but it's not zero.
So, we might have contaminated Mars. Nobody can say for sure we haven't. But the conditions are so harsh there that most people would say it is unlikely that we have yet, especially as the spacecraft are quite a bit sterilized, if not as much as Viking, down to that 500,000 spores per spacecraft on launch. Which may seem a lot, but it gets reduced further during the trip out there due to cosmic radiation and UV radiation - and the vacuum conditions. Yet, undoubtedly there are viable spores on our spacecraft on Mars but most think they are dormant, not doing anything.
It's all based on the idea of an "exploration period". If you think that we might need to keep Mars clean in perpetuity, then at some point we would have to go to Mars and sterilize all the spacecraft or remove them from the planet. Presumably using some future technology that lets us do that without introducing more contamination to Mars in the process.
If there is life on Mars from Earth, then we should take even more care n the future, study what happened, and do our best to reduce the impact. For instance, suppose it is in just some small patch, say around the Phoenix lander or something, started to reproduce, perhaps there is a chance of doing something about it before it spreads.
I think myself that a good mission to do, once we can sterilize our spacecraft better than we can now, is to send a lander to the Phoenix location and see if any life has spread from the lander. Because it's one of the most vulnerable I think. Not sterilized for a "special region" as nobody thought it would be suitable for life back then. And it was crushed by some meters of thickness of dry ice (expected) and nowadays there are several ideas for possible ways that habitats for life could exist at the Phoenix location. Which doesn't mean that there are habitats for life there, they are just hypotheses. But - it can't be ruled out either at present.
The main ones I know of relevant to the Phoenix location are
- Deliquescent salts - they may be too cold and too salty for life, but there are many ways they could be mixed together with different chemical composition (perchlorates, sulfates, chlorates, chlorides, and magnesium, potassium, sodium etc) and with dust from the soil and perhaps some combination gives the conditions for life there.
- Liquid water on salt / ice interfaces. This is a new idea these form in just minutes in experiments on the Earth
- Solid state greenhouse effect - melting of a layer of liquid water below a covering of translucent dry ice.
(I'd land it some distance from Phoenix and study the hopefully pristine area first, with biosignature and life detection, and then travel up to Phoenix and study the area around Phoenix and the spacecraft itself and see if there is any sign of local contamination and if any spores survive on the spacecraft itself).
And if we have contaminated Mars, and if we find we can't reverse the effect, it's spread too far (once it spreads in the dust storms for instance, how can you stop it?) - we then will have to look and see what the impact is. It would be like finding out that we have introduced rabbits to a remote pristine island. So then you look at what impact they have, and see what you can do about it, if anything. But the very last thing you do in that situation is to introduce rats, dogs, cane toads etc etc. Maybe if it is just a few species, we can learn to identify them and distinguish them from native Mars life, if it exists and maybe we can work around it in some way, and maybe if we are lucky, they are not especially well adapted to Mars, and impact on native life, if it exists is not going to be so huge.
But most would say, that there is at least a pretty high chance we haven't contaminated Mars yet. We can't say for sure I think until we have some ground truth from Mars on the topic.
This describes some of the methods currently used to protect Mars from Earth microbes: NASA Office of 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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