All our missions to Mars since Viking have been to places where scientists at the time thought that there was no possibility of any habitats for life. This is something that gets re-evaluated from time to time. So for instance, after discovery of the warm seasonal flows, or Recurrent Slope Lineae, then those also need to be treated as potential habitats that could be contaminated until or unless we discover otherwise.
For those missions, the guideline is that we sterilize the spacecraft to have a maximum of 300,000 cultivable spores per spacecraft. This undercounts the number of dormant bacteria (many may not be cultivable, typically one in a hundred is cultivable in most habitats). So there could be millions of dormant microbes on the spacecraft at launch.
This gets reduced by the vacuum conditions, cosmic radiation, ultra violet radiation, conditions on Mars when you get there and so on. Still it is thought that there are many dormant microbes that survive all of that.
So, where does that magic number 300,000 come from?
Well there is no totally compelling reason for it, there can't be. It is the pre-sterilization figure for Viking. Which was then heat treated to have 30 spores total by heating it through four decimal reductions (reduction to a tenth).
You could sterilize completely of course by bringing it up to some very high temperature in a sealed container. But that would ruin your spacecraft.
So, in practice, you can't sterilize completely, but only reduce the number as much as possible - with current methods anyway.
So - Viking then discovered that conditions on Mars were far more hostile to life than expected. So an ethical decision was made, that the sterile conditions on Mars are probably roughly equivalent to those four decimal reductions of Viking, so therefore only the 300,000 pre-sterilization step needs to be done.
That 30 target spores for Viking in turn was calculated based on the idea that a 1 in 10,000 chance of contaminating Mars per mission is acceptable. Which was in turn based on the idea that it is okay to have a chance of 1 in 1000 of contaminating Mars during the exploration phase - which at that time was thought to be likely to consist of - -they guessed - 60 missions to Mars, of which 54 would be successful, and 30 flybys or orbiters, before Mars exobiology is thoroughly understood.
So the numbers were chosen by Sagan and Coleman to achieve at least a 99.9% chance of keeping Mars contamination free during those 60 missions to the surface and 30 flybys or orbiters.
That of course is an ethical choice also. There is no experiment you can do that comes up with those numbers. They had the choice at that point, of not sending any missions to Mars, or sending missions there but accepting a probability of contamination. If you do that, someone, at some point has to make an ethical decision of what level of contamination is acceptable. And - Sagan and Coleman made that decision - with no other authority to use as a basis as they were first in the field - they just chose the figure 99 .9% as their ethical choice.
Which led to the 1 in 10,000 per mission when combined with their assumptions of numbers of missions.
Now we no longer think in those terms at all, of an exploration phase of a fixed number of missions. Rather it is open ended. But still - there is no better reasoning available, so they just go by those figures, pretty much, due to historical precedent.
This may change if we get data from Mars at some point about how many dormant spores there are on our spacecraft, but still at some point someone has to make an ethical decision for an acceptable probability level.
The main options seem to be:
So - with the equatorial regions, we have precedence and custom, will probably continue to use that 300,000 spores unless someone comes up with a good argument for some different figure or approach.
With the "special regions" however, which now includes the RSLs and all the higher latitudes - the policy is less clear because we have never sent any missions to such places except in error (not knowing that they were special at the time, or spacecraft crash).
Now even the "special regions" on Mars are not very habitable. Not known if they have habitats at all and if so, quite possibly the most habitable spots on the entire Mars surface would be less habitable than the arid core of the Atacama desert or the McMurdo dry valleys in Antarctica.
Still- if there is possibility of liquid water there - it would seem that our rovers could bring Earth life to them. And not clear what is an acceptable level of sterilization. Are the 30 spores of Viking enough? Microbiology has moved on a lot since the 1970s, and so also has our understanding of Mars, so is the reasoning of Sagan and Coleman still valid today?
It's not easy to see how this is going to play out. Unless we find a way to totally sterilize our rovers, which may be possible.
One thing we can say. Just about everyone involved in planetary protection agrees that we have to protect special regions from Earth life. It would be a tragic anticlimax of all our searches on Mars to find life there that we brought ourselves and which reproduced on Mars, originated on Earth.
I think there is almost complete agreement there apart from a couple of astrobiologists who have argued on not very strong grounds that they think that Mars is already inhabited by Earth life brought there on meteorites, so they think we are doing nothing more than what happens already with meteorites.
That seems a reasonable policy - to base it on duplicating what happens naturally - that if it is happening anyway we are doing no harm.
But the issue is we have no data there. Though meteorites can we think transfer life between planets - there is, so far, no proof of that happening for real, and there are many microbes could get to Mars on a spacecraft that could not survive transfer on a meteorite.
Their paper in nature was immediately rebutted by another paper in nature by the present and past planetary protection officers for the USA.
So this view, though it got a lot more publicity at the time than its rebuttal - has definitely not been accepted, indeed probably not many outside of the couple of authors of the paper would agree that we can stop protecting Mars from Earth life.
Case of watch this space. But I think one thing to be aware of is that at some point someone has to make ethical decisions, which impact on everyone. I think fuller discussion of these decisions is a good idea, so that they can get as wide a range of participation in the decisions as possible. Scientists particularly may not always be the best people to make these decisions, which they may have to make because nobody else is interested in the choice, or understands it, or feels capable of having an opinion on it. But I think it is better if more people are involved, the more the better. For that to be possible, then people have to understand the issue and the choices that need to be made.
This is something sometimes mentioned in the articles on this topic - the need to engage the general public in not just understanding the issues, but also participating in the decision making - most often that's in context of sample return but I think it is also important in the forward direction as well.
For those missions, the guideline is that we sterilize the spacecraft to have a maximum of 300,000 cultivable spores per spacecraft. This undercounts the number of dormant bacteria (many may not be cultivable, typically one in a hundred is cultivable in most habitats). So there could be millions of dormant microbes on the spacecraft at launch.
This gets reduced by the vacuum conditions, cosmic radiation, ultra violet radiation, conditions on Mars when you get there and so on. Still it is thought that there are many dormant microbes that survive all of that.
So, where does that magic number 300,000 come from?
Well there is no totally compelling reason for it, there can't be. It is the pre-sterilization figure for Viking. Which was then heat treated to have 30 spores total by heating it through four decimal reductions (reduction to a tenth).
You could sterilize completely of course by bringing it up to some very high temperature in a sealed container. But that would ruin your spacecraft.
So, in practice, you can't sterilize completely, but only reduce the number as much as possible - with current methods anyway.
So - Viking then discovered that conditions on Mars were far more hostile to life than expected. So an ethical decision was made, that the sterile conditions on Mars are probably roughly equivalent to those four decimal reductions of Viking, so therefore only the 300,000 pre-sterilization step needs to be done.
That 30 target spores for Viking in turn was calculated based on the idea that a 1 in 10,000 chance of contaminating Mars per mission is acceptable. Which was in turn based on the idea that it is okay to have a chance of 1 in 1000 of contaminating Mars during the exploration phase - which at that time was thought to be likely to consist of - -they guessed - 60 missions to Mars, of which 54 would be successful, and 30 flybys or orbiters, before Mars exobiology is thoroughly understood.
So the numbers were chosen by Sagan and Coleman to achieve at least a 99.9% chance of keeping Mars contamination free during those 60 missions to the surface and 30 flybys or orbiters.
That of course is an ethical choice also. There is no experiment you can do that comes up with those numbers. They had the choice at that point, of not sending any missions to Mars, or sending missions there but accepting a probability of contamination. If you do that, someone, at some point has to make an ethical decision of what level of contamination is acceptable. And - Sagan and Coleman made that decision - with no other authority to use as a basis as they were first in the field - they just chose the figure 99 .9% as their ethical choice.
Which led to the 1 in 10,000 per mission when combined with their assumptions of numbers of missions.
Now we no longer think in those terms at all, of an exploration phase of a fixed number of missions. Rather it is open ended. But still - there is no better reasoning available, so they just go by those figures, pretty much, due to historical precedent.
This may change if we get data from Mars at some point about how many dormant spores there are on our spacecraft, but still at some point someone has to make an ethical decision for an acceptable probability level.
The main options seem to be:
- We find a way to sterilize our spacecraft completely, with no microbes at all
- We choose an acceptable probability of contaminating the planet or moon we are studying
- We base our policies on historical precedents, like the 300,000 spores and recognize that we can't actually calculate a probability of contamination, but based on an intuition that this level of sterilization is "about right" - and we need to have some numbers to make decisions and in the absence of data, well this is as good a number as any - that is the current policy.
- We stop protecting Mars - unlikely that anyone will choose this in the near future because of the importance of the search for alternative origins of life etc
- We simply stop all robotic exploration until we find a way to sterilize perfectly or at least, somewhat better than today. This is the situation with some Antarctic subglacial lakes - the Russians stopped for a fair while at Lake Vostok for instance and there are many subglacial lakes not studied because of risk of contaminating them. Easy to send robotic explorers down to explore them, but not thought to be biologically acceptable to do that.
So - with the equatorial regions, we have precedence and custom, will probably continue to use that 300,000 spores unless someone comes up with a good argument for some different figure or approach.
With the "special regions" however, which now includes the RSLs and all the higher latitudes - the policy is less clear because we have never sent any missions to such places except in error (not knowing that they were special at the time, or spacecraft crash).
Now even the "special regions" on Mars are not very habitable. Not known if they have habitats at all and if so, quite possibly the most habitable spots on the entire Mars surface would be less habitable than the arid core of the Atacama desert or the McMurdo dry valleys in Antarctica.
Still- if there is possibility of liquid water there - it would seem that our rovers could bring Earth life to them. And not clear what is an acceptable level of sterilization. Are the 30 spores of Viking enough? Microbiology has moved on a lot since the 1970s, and so also has our understanding of Mars, so is the reasoning of Sagan and Coleman still valid today?
It's not easy to see how this is going to play out. Unless we find a way to totally sterilize our rovers, which may be possible.
One thing we can say. Just about everyone involved in planetary protection agrees that we have to protect special regions from Earth life. It would be a tragic anticlimax of all our searches on Mars to find life there that we brought ourselves and which reproduced on Mars, originated on Earth.
I think there is almost complete agreement there apart from a couple of astrobiologists who have argued on not very strong grounds that they think that Mars is already inhabited by Earth life brought there on meteorites, so they think we are doing nothing more than what happens already with meteorites.
That seems a reasonable policy - to base it on duplicating what happens naturally - that if it is happening anyway we are doing no harm.
But the issue is we have no data there. Though meteorites can we think transfer life between planets - there is, so far, no proof of that happening for real, and there are many microbes could get to Mars on a spacecraft that could not survive transfer on a meteorite.
Their paper in nature was immediately rebutted by another paper in nature by the present and past planetary protection officers for the USA.
So this view, though it got a lot more publicity at the time than its rebuttal - has definitely not been accepted, indeed probably not many outside of the couple of authors of the paper would agree that we can stop protecting Mars from Earth life.
Case of watch this space. But I think one thing to be aware of is that at some point someone has to make ethical decisions, which impact on everyone. I think fuller discussion of these decisions is a good idea, so that they can get as wide a range of participation in the decisions as possible. Scientists particularly may not always be the best people to make these decisions, which they may have to make because nobody else is interested in the choice, or understands it, or feels capable of having an opinion on it. But I think it is better if more people are involved, the more the better. For that to be possible, then people have to understand the issue and the choices that need to be made.
This is something sometimes mentioned in the articles on this topic - the need to engage the general public in not just understanding the issues, but also participating in the decision making - most often that's in context of sample return but I think it is also important in the forward direction as well.
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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