As a mathematician I'm used now to the proofs of scientists, which can seem puzzlingly non rigorous at first. But the mathematics of Planetary Protection are particularly eccentric, even bizarre. They are based on out dated science from the 1970s, and arbitrary "magic numbers" and assumptions made by Carl Sagan and others based on those assumptions. There have been many suggestions that this needs to change but so far nobody has come up with a clear way ahead for an alternative.

Meanwhile the planetary protection officers and the COSPAR scientists continue to apply these regulations based on out dated ideas and legacy decisions. As far as we know, this adhoc method has worked, and they have been successful so far, at protecting Mars and other parts of the solar system from contamination by our microbes. But we don't know for sure if it worked, and if it did, this is as much due to luck and coincidence as to the details of the calculations used by the scientists.  

I  have no wish to criticize those involved at all. Carl Sagan and the others who did the original calculations did the best job they could with our limited understanding and knowledge of the day. The COSPAR teams of scientists then apply the numbers with as much rigour as they can. Then mission scientists and the planetary protection officers take great care to make sure that missions are sterilized to the standards set.
None of that is an issue, but the standards themselves are really quite strange when you look at them closely. We still have no ground truth from Mars to guide the regulations. Also our underlying assumptions are out of date and again we don't have any clear guidance to follow to update them.


It's not easy to see what we can do about it. Any group of scientists or legislators tasked with making new regulations is bound to complain of "insufficient data" every step of the way.

Still, many of the scientists do think there is need for some change in the regulations and often say so in the official reports and studies. I thought I'd write this, simply to open the debate and encourage awareness of these issues. Because in the end, planetary protection is not just a task for scientists. 

The underlying assumptions and target probabilities need to be looked at by the general population also. As with biohazard laboratory regulations, we are all involved in this. What are our aims?  How important is it to protect other planets, and ourselves? How much budget should we give to the planetary protection measures? These are things that the scientists can't answer for us, not really, though the likes of Carl Sagan did their best, in a situation where there was little by way of public interest or discussion of such issues.

Some questions we could ask are:

•  How much does it matter if we contaminate Mars or Earth? Is a one in a million chance of contamination acceptable? Or should it be one in a trillion, or one in ten to the power twenty?

  It's not possible to absolutely prevent contamination if we send a spacecraft to other locations in the solar system or return materials to Earth, so at some point you have to make a decision like this.
   

• Do we need to worry about sample returns even from asteroids and comets, or only from Mars and other places considered highly likely to have life? There is no way to be certain that there is no life, say, on a comet or asteroid, or even on Io and other unlikely places. We can't totally rule out life almost anywhere, when we have no idea what the possibilities are for other forms of life different from our DNA based life, or its capabilities.

  Some places like the Moon seem highly improbable to have any life at all but we haven't examined the entire surface close up, locations like caves with icy deposits and perhaps trapped heated water covered in protective tarry deposits that life could inhabit. Do we need to take account even of remote possibilities like that, or only the most likely scenarios?
   

• How can anyone decide on an acceptable probability of an environmental disaster of some sort on Earth as a result of back contamination, given that it surely can't be made totally 100% safe, especially if we risk returning Gene Transfer Agents or XNA? 
   
• How can anyone decide on an acceptable probability of contaminating Mars or other places in the solar system, given that we have no idea at present what we might find in the future, so what we might lose by contaminating these places?
   
• How much are we willing to spend to get clearer information to guide the scientists? Should we commit to purpose designed missions to Mars and other locations just to get better ground truth about the success of the measures taken so far. If so how much should we spend on this?
   
• Should we require extremely low probabilities, and simply not permit the experiment to continue if these measures can't be attained? Or should we be more pragmatic, and require as much sterilization as is possible at a reasonable cost and let it go ahead if ideal targets for sterilization are too expensive to apply? How much is a contamination free solar system worth to us financially?

THE PRACTICAL MATHEMATICS OF BIOSAFETY REGULATIONS

The problem here is that sterilization is not an all or nothing thing. We have no practical method that absolutely removes all microbes and yet keeps sensitive electronics and delicate tools in an acceptable state for use. 

Of course we could just heat everything to a few hundred degrees C every time it's used, and take it to temperatures so high that no life can survive for more than a second or two. The problem is, that our sensitive equipment couldn't survive those conditions either. We could use extremely high levels of ionizing radiation also, but that again would destroy electronic equipment and so isn't practical on a modern spacecraft.

Since total sterilization is not practical, one method often used is a sequence of decimal  reductions. Each step is designed to reduce the chance of finding a viable microbe at least ten fold. Apply this enough times, and you end up with a small population of microbes, and an extremely low probability of contamination. 

Then for the decimal reductions themselves, you use gentle methods of sterilization such as heat sterilization at just above 100C, or low vapour hydrogen peroxide. You start with a reasonably clean surface using sterile wipes, get the numbers down to a few hundred thousand, and then apply a sequence of decimation steps to get the numbers right down to acceptable levels.

So you can't regulate for perfect sterilization, that's impossible. The regulations have to specify a probability of contamination. But someone still has to "pick a number" here.

So you get regulations like this

From an operational standpoint, a sterilization procedure cannot be categorically defined. Rather, the procedure is defined as a process, after which the probability of a microorganism surviving on an item subjected to treatment is less than one in one million (10-6). This is referred to as the “sterility assurance level"
(Appendix B of Biosafety in Microbial and Biomedical Laboratories)

This figure could as easily be a chance of one in ten million, or one in a hundred thousand, or if we counted in base seven, would be perhaps one in 823543 (7^7). There is no rigorous way to come up with a number here, but somebody had to make a decision, Whatever number they picked got approved passed into legislation. Now the scientists are simply tasked with applying that number.

The situation is exactly the same with planetary protection. Someone came up with a number, this passed into regulations, and groups of scientists examine individual targets in our solar system to make sure our missions achieve those target probabilities.


But many more assumptions needed to be made because the missions, by their nature, are to places in the solar system which we know little about. We also know nothing at all about alternative forms of biology, but can only speculate using analogies with Earth life and experiences developed from our study of Earth life.

THE NUMBERS USED FOR PLANETARY PROTECTION - IT'S ECCENTRIC LEGACY

First the background. The regulations were developed for Mars originally and were based on our ideas for exploration of Mars, and these have now been applied to all the celestial bodies in the solar system.

When the guidelines were first drawn up, then the prevailing idea was that we should explore Mars first for a few decades, and then we would colonize the planet. There was no idea at all that we might want to keep Mars free of Earth life in perpetuity - though since then some scientists such as Chris McKay notably, and ethicists, have suggested that that might be what we need to do, depending on what we find on Mars.

So the objective was simply to keep Mars pristine for the few decades needed for the original exploration phase. At that time the scientists were optimistic about human exploration of the solar system, immediately after the landings on the Moon, and expected this phase to be over soon. By the start of the twenty first century, they thought, we would have colonies on Mars already.


Before then, the aim was to explore Mars as quickly as possible, find out as much as we could about the planet - which it was thought might not be that much as it seems so hostile to life - and then colonize it.

Before Viking, they thought that Mars might have conditions suitable for life, and so drew up guidelines for sterlizing Viking. It was sterilized to an extraordinary high level.

The viking landers were first sterilized by hand wipes and so forth until the landers had less than 300,000 colony forming spores on them. That means perhaps a hundred times as many dormant cells as typically only 1% of the cells in a habitat can be cultivated in a medium.

They then applied heat sterilization for many hours to reduce the numbers down to a probability of at most 30 colony forming cells on the entire spacecraft. So that means, perhaps 3,000 dormant cells on the entire spacecraft, though there is no way to be sure about that number exactly.

Then these numbers were further reduced by ultra violet radiation and cosmic radiation on the journey to Mars and finally also reduced by the hostile conditions on the planet surface itself. All of this was allowed for in the original calculation that Carl Sagan and others made. 

Assuming that they were successful in this procedure, and there is no reason at all to suppose they failed, it's entirely possible that the Viking landers had no viable dormant microbes on them at all. The chance that it contaminated Mars must be extremely low.

The target probability they aimed for was a one chance in 10,000 of contaminating Mars with any single mission, and one chance in 1000 of contaminating Mars in the exploration phase. As with the biosafety regulations, these are simply numbers that someone had to choose, as for the biosafety margins. There is no way we can derive such a number from first principles and axioms. Then everyone else just follows those guidelines. 

Then Viking came back with the results that the life on Mars seemed extremely unlikely because of the harsh conditions there (though some disputed those results).


THE BIZARRE CHANGE OF POLICY POST VIKING

So then comes the really bizarre and eccentric step - at least to a pure mathematician, though scientists might find it more acceptable. The scientists involved decided, that since the conditions on Mars are so harsh, then they are roughly equivalent to the heat treatment step for Viking. So they decided to drop the heat sterilization requirement for future surface missions.

You still have to sterilize the spacecraft. It's not true what some say, that our spacecraft are not sterilized. But you only have to reduce the numbers to 300,000 colony forming spores for the entire spacecraft, so perhaps about 30 million dormant cells in total. That may seem a lot but it is in fact very clean by most standards. 


Then the assumption is that the harsh environment on the surface of Mars does the four decimations of the original Viking heat sterilization plus the extra four or more decimations in the original calculations for the journey out and harsh conditions on Mars. 

The thing is, this wasn't based on any detailed models. They just made this assumption that the harsh conditions there add an extra four decimations over the ones assumed in the original calculations. But we have no ground truth for this at all. Nobody has sent any equipment to Mars able to check to see if these decimations do in fact occur. Nobody has done any rigorous calculations either to try to prove that the conditions result in these extra decimations. And our knowledge of the conditions on Mars are incomplete, certainly not enough to do a rigorous simulation of conditions on Mars here in our laboratory. It is hard to do that right now, and there are many details we don't know. Back then it was next to impossible.

Indeed since that decision was made, we have discovered many extremophiles, some with extraordinary capabilities for survival in conditions of high levels of radiation, extremely saline conditions, extremely low temperatures, Mars equivalent atmospheric conditions and so on. Indeed further back in 1967, the scientists didn't even know that there was such a thing as an anaerobic halophile - they thought that all microbes that survive in extreme salt conditions must use oxygen. Carl Sagan voiced a word of caution there that this might just be an artefact of experimental procedures in his 1967 Contamination of mars article.


Now the anaerobic halophiles are a huge area of study, and many extremophile halophiles are known some of the best candidates for life from Earth able to survive on Mars. 


We've also found out that Mars may be more habitable than originally thought. The DLR experiments have shown that some cyanobacteria can actually survive on the surface of Mars using the UV light in partially shadowed conditions to metabolize and using the 100% night time humidity of Mars as their only source of water.


Then we have the discovery of deliquescing salts, and halophiles in the Atacama desert able to get water just through deliquescing salts.


This also was before the discovery of the black and white smokers, and of sulfate reducing bacteria on the ocean bed, and many of the other discoveries in biology which inform present day ideas about Mars.


So there is no doubt at all that the assumptions they used are now well out of date. But what we should replace it by, that's where questions arise.


One point of view is that on Mars anyway - that we've used this standard for dozens of spacecraft so far. So, either it worked or hasn't worked, and either way so long as we continue to use this criterion then we have the same chance of keeping Mars pristine.


But - that's not really right. Each time we send a new mission to Mars, then it goes to a new area of Mars usually - and may encounter conditions not met by any rover before. And even if the probability is the same for each mission, a 1 in 1000 or whatever the chance is probability can eventually lead to contamination of Mars, just through repeated trials, and not because of any issues with any individual mission to the planet.


We also hope to target areas of Mars that are of especial interest for present day life such as the warm seasonal flows and the deliquescing salts. So these raise new issues. A natural thing might be to say, that the Viking levels of probability are fine. But those are based on outdated assumptions about the Mars surface. Would a population of 3,000 dormant cells (approx) on our spacecraft really be reduced to none at all through the harsh conditions? Some cells are astonishingly resistant to cosmic radiation, and this was not known at the time that Carl Sagan did his calculations. They are also astonishingly resistant to UV radiation and again that wasn't known, not the extreme UV tolerance of some micro-organisms. 


It's okay to have another four or five decimal reductions. But if there is one species that's resistant to the sterilization technique, and also resistant to the decimal reductions then it would survive all of this. Maybe just 30 of those 30,000 are highly resistant - but then most of them are able to survive the journey to Mars and the UV radiation and just one of them is all that you need to survive on Mars.

IMPOSSIBILITY OF A PROOF OF VALIDITY FROM AXIOMS OF PURE MATHS

As a mathematician I'm used to the way that scientists prove results, using methods of proof that can seem bizarre at times to a pure mathematician, even ignoring infinite error terms in Quantum Mechanics. 

I totally understand that when designing biosafety regulations, at some point someone had to pick a figure for an acceptable probability of failure. We can't expect a proof of its validity from axioms of pure mathematics.


But we can get more data through scientific research


RESEARCH NEEDED


The obvious thing to do is to send a rover to Mars to look at the spacecraft already there, and see if any life survived. We do have extremely sensitive biosignature detectors now, far better than anything available at the time of Viking. We also have SETG able to do DNA sequencing which the scientists concerned say can be ready to fly for 2018.


I suggest an obvious target here is Phoenix. The spacecraft was only expected to last a few months because they knew it would be crushed by probably a meter or more of dry ice as the winter progressed. So parts of it have probably broken off and got buried in the soil. Sealed components might have got breached.


Phoenix also was sent to target an area of Mars that was thought totally inhospitable at the time - but now, it's thought that it might actually be habitable, not everywhere surely, but where the concentrations of salts are right, there may just possibly be conditions for life in patches of deliquescing salts a cm or so beneath the surface. 


So, if any of our landers have contaminated Mars, I think Phoenix might be one of the best bets. The missions that crashed on the surface are also a prime target. But with Phoenix also we have the advantage that the lander itself studied its own landing site extensively, making it easier to spot changes. And it is also a great scientific target in its own right.


I suggest that we can analyse to see if any life from Earth survives on Phoenix. Also to test to see if any micro-organisms have spread to the sub surface environment around Phoenix.


If Earth life has spread, and started to colonize Mars, well there is at least a chance that we could do something about it. It probably spreads very slowly, and if it is a cm or so below the surface, then may not have been transferred to anywhere else on Mars yet in the dust storms, which can only pick up tiny grains of dust on Mars. We may be able to send rovers to contain it and even remove the contamination at a later date.


And either way, we simply need to know the answer, have we contaminated Mars or not? Hopefully not, but we need to know. And if not, we need ground truth on whether any of the dormant spores have survived, and how many, to get ground truth on how many decimal reductions the harsh conditions on Mars really provide. Do they supply the extra 4 decimal reductions that the scientists hoped when they relaxed the regulations? Or more even? Or less?


Until we know that, then our planetary protection discussions are never going to be firmly grounded in data and ground truth from Mars.