Many of us care deeply about the possibility of tigers, lemurs and such like becoming extinct in the wild. I'd like to suggest that we care as much about the possibility of microbes on Mars and elsewhere in our solar system becoming extinct through human activities.

So what's so special about microbes on Mars? Well first, the uniqueness. Tigers are closely related to cats. They are wonderful and remarkable creatures - and would be a tremendous shame to lose them - but basically it's the familiar tabby in another guise.

For microbes on Mars though - the most interesting possibility is that they may be unrelated to any living organism on the Earth. Think not so much a microbial version of a tiger:

A young adult tigress from the Terai in India - Sumeet Moghe

HaloBacterium Salinarum - isolated from Lake Zuf in Wadi Natrun, Egypt - recently genetically sequenced - Microbes like this could exist on Mars. At first sight they may not seem as stunning and exciting as tigers
But think of them as Microbe ETs, and perhaps you may see them in a different light, minute emissaries, tiny beings with a potentially totally different biochemistry.

Microbes on Mars, in the more interesting case, would be more like a microbial version of ET than a tiger

Mars could be the closest we have to an exoplanet in our own solar system - for biologists.

You may know that we have discovered many exoplanets (planets orbiting other stars) in the last few years.

In this rather cool visualization, the exoplanets of all the solar systems discovered so far are superimposed on our sun and its planets for reference. Cold planets are blue, hot planets are red. Planets that could support liquid water are green.

It's animated - for the animated version, click through to: Exoplanets orbits

Perhaps many of these planets have life, not just the ones in the habitable zones, but also others with life in subsurface oceans, or life with different chemistry in conditions impossible for Earth life. Or perhaps only a few have life, or none. But one thing they all have in common - they are light years away from the Earth.

Mars is right here in our solar system. At its closest, it is 4 light minutes away, and we have the technology to send spacecraft there right now. This is probably the only opportunity we'll get in our lifetimes to meet microbial ETs up close on the surface of a planet that was like Earth in the early solar system. Also it's the only opportunity ever to do that on an Earth like planet born in the same solar system as Earth.

I'll look at what we might find there as a series of scenarios. But first, let's look at Earth life.


While you watch this video, ask yourself - how likely is it that an independently evolved microbe ET would have all the same microscopic structures as these, working in exactly the same way as they do on Earth?

this shows how DNA gets converted to protein - together with error correction. This whole process is common to all life that we know

For details of what's going on here, see Translation: DNA to mRNA to Protein

More animations by Dew Berry, talking about his animations here

All the life we have on Earth, as far as we know, follows this same complex dance to generate proteins from DNA. What's more, we don't know of any other way to create life.

And that's just one of numerous processes going on in every cell.

There are ideas for creating XNA based life - but the idea would be to keep all this complex structure essentially intact, and substitute XNA for DNA, and tweak the rest to make the XNA work. Basically all we can do is tinker with it a little. We are nowhere near the level of knowledge needed to "invent" something like this ourselves or to change how it works in any essential way.


Some researchers plotted the increase of complexity of non redundant nucleotides in DNA. As life increases in complexity, it follows a near straight line on this log plot, through many different changes of structure of organism, from the prokaryotes, to the Eukaryotes with nuclei, worms, fish, and mammals.

They traced the timeline back, expecting it to cross the zero line at the time of origin of life, and found that the zero line is nearly ten billion years ago. That's over twice the history of the Earth.

This diagram shows the complexity of the DNA as measured using the number of functional non redundant nucleotides.
This is a better measure of the genetic complexity of the organism than the total length of its DNA. Some microbes have more DNA than a human being - much of that used for other purposes rather than for genetic coding, the so called C Value Enigma. Measuring it this way deals with that issue.

Notice that the prokaryotes; the simplest primitive cell structures we know; are well over half way between the amino acids and ourselves.

So, either evolution started before the beginnings of our solar system (perhaps brought here by impacts on another solar system that passed through our one as it was forming) - or else - evolution was far more rapid in its early stages.

Either way, you'd expect that as many stages of evolution were needed to get from non living chemistry to the early archaea, as were needed to get from the early archaea to modern mammals.

The first primitive cells were surely far smaller and can't possibly have been as complex as modern life. Perhaps they used only a few different chemicals.


This is the most exciting scenario of all, if the life on Mars has an independent origin.

The whole dance of life on Mars may be based around different chemicals. It might not use DNA, or might use different bases. But also - none of the mechanisms would be the same either. It would have its own ways of creating proteins from XNA, perhaps using different messenger molecules, or some other method altogether. It would have its own error correction methods, and ways of generating energy, and structure for the cell walls, and methods of reproduction.

We'd have a different dance of life from Mars to compare with the dance followed by all Earth life.

If independent in origin, it would have its own versions of DNA, mRNA, ribosomes, RNA polymerase, mitochondria, cell walls, lipids, proteins, gogli apparatus, lysosomes, microtubules, and all the other things that make up the complexity of modern living cells.

RNA polymerase used to decode DNA to mRNA, present in all living cells.

Golgi apparatus - essential organelle in most Eukaryotes

Ribosome translating mRNA into a protein

Microtubules, strands that stretch through cells, a bit like the corals in a coral reef.
ET microbes, if independent in origin, would have a completely different "ecosystem" of these structures.

Imagine that you have been brought up in the African savannah - with its grasses and trees and elephants and antelopes. You've never seen a marsh or a forest, or a beach. All your life you've lived in a hut in the African Savannah, never traveled more than a few miles from your hut, and that's the only thing you've ever known.

View of Ngorongoro from Inside the Crater

Then one day someone takes you to the sea shore, with its fish, shellfish, seaweeds, and sea anemones, and perhaps they take you on a dive to see a coral reef.

A Blue Starfish (Linckia laevigata) resting on hard Acropora coral. Lighthouse, Ribbon Reefs, Great Barrier Reef. Photo by Richard Ling

Here I'm using the analogy, that the interior of a cell is so complex it resembles an entire ecosystem.

The "ecosystem" of the interior of an ET microbe could differ from the "ecosystem" of Earth life, as much as the ecosystem of this Austrailian coral reef differs from that of the African savannah.

Think how much that would expand your horizons!

That gives an idea of what it would be like to find a microbe on Mars with a different biochemistry from Earth life.


Our earliest ancestors must have been far smaller than the modern cells, less intricate, fewer chemicals.

Maybe the first cells were just a few tens of nanometers across, and used just a few hundred chemicals or less. And perhaps before then, there were more primitive forms that weren't quite life as we recognize them. Perhaps they reproduced, but imperfectly. We might get "life forms" on Mars that are in between non life and life processes in how they function.

TED talk by Martin Hanczyc: The line between life and non-life

For more on this, Oil droplets mimic early life Lack of genetic material no hindrance to life-like behaviour. (and paper).

There are numerous ideas here, about where the first life might have evolved, and how it began. But there is no way to settle these questions. All we have left are a few bubbles of gas in ancient zircons, which tell us a lot about the composition of the early Earth atmosphere, but not much more than that. For some of the many ideas see abiogenesis.


So in this case, the Mars life has a common ancestor with Earth life, but it's far older than the last common ancestor of all Earth life, split off at an earlier stage.

We would still share the same Ur organism (Darwin's term) - earliest common ancestor. But at some point Mars life evolved differently.

So for instance, it might still have DNA and RNA, and use all the same nucleotides, but may differ in details of how the DNA is turned into proteins, or the error correction methods, or the cell metabolism and so on. Or perhaps it might have an expanded vocabulary. Or maybe based only on RNA without any DNA, or based on PNA, a suggested precursor for DNA.

PNA. The earliest ancestors of all life might have used this instead of DNA because of its robustness and simplicity (one of many suggestions for our ur ancestors). If so, might there be ancient lifeforms still surviving on Mars, based on PNA instead of DNA?

This would be a tremendously exciting discovery for biology.


In this scenario, Mars life has the same "Last common ancestor" as Earth life. It's essentially Earth life but with differences.

This shows how all modern life is related to the "Last common ancestor" in the centre. It's based on species with sequenced genomes. For interactive version see Interactive Tree of Life. Details: Toward automatic reconstruction of a highly resolved tree of life.
One way it could differ is in the way it does photosynthesis. Most Earth life does photosynthesis using chlorophyl, or similar pigments, and a process of carbon capture and oxygen release. However, uniquely, halobacteria (or haloarchaea) use Bacteriorhodopsin and Halorhodopsin for photosynthesis in a similar process to the processes we use for vision.

Shows how halobacterium salinarum gets energy from sunlight using, bacteriorhodopsin - similar to the pigments we use for vision, as a source of energy, instead of chlorophyll.

The light deforms the molecule which then acts directly as a proton pump which is used, for instance, to synthesize ATP. It does this without generating oxygen or capturing CO2.

Ordinary photosynthesis also works as a proton pump, but only indirectly, after carbon fixation and release of oxygen.

So - what if, over billions of years of separation from Earth, Mars life has evolved a unique method of photosynthesis using some other pigment, and perhaps some new method also, not used anywhere on Earth?

Other differences between Mars and Earth include

  • Perchlorates. Instead of the chlorides we have on Earth, Mars salts are rich in highly oxygenated chlorates and perchlorates, as well as sulfates. We have some microbes on Earth able to cope with these conditions - Mars microbes likely to be more tolerant of them
  • Near vacuum, Mars microbes may be able to metabolize in near vacuum conditions. Some microbes on Earth can do this but most go dormant as the atmospheric pressure goes down.
  • Cold conditions. Though there may be liquid water at temperatures suitable for Earth life, some of the water at least may be at temperatures far too low for Earth life, and at lower temperatures than any normally encountered on Earth, in mixtures of salts and water. Some Earth life can continue to metabolize right down to -20C. Could Mars life have evolved to use liquid water at even lower temperatures?
  • Nitrogen fixing. The Mars atmosphere has hardly any nitrogen, but it does have some. Would any Mars microbes be able to fixate nitrogen at such low levels?
  • Absorbing water from the atmosphere. Mars has 100% humidity at night, but still in near vacuum conditions. In the DLR experiments, some Earth lichens and microbes can absorb this water vapour and use it to metabolize and photosynthesize. Mars microbes might be evolved to do this better than Earth life.
  • Ionizing radiation. Some Earth life has remarkable resistance to radiation, able to survive in reactor cooling ponds. But - high levels of radiation are rare on Earth, and they probably evolved this capability in order to deal with dessication.

    Some Russian scientists, though, headed by Pavlov, suggested back in 2006 that some of these radioresistant microbes might have originated from Mars.

    Whether they did or not, seems reasonable to suppose that surface life on Mars would have high levels of tolerance for ionizing radiation.

    These lifeforms have the ability to heal their DNA even after cut into numerous pieces by cosmic radiation (see for instance, Secrets of a Salty Survivor). Mars life may be able to do so also, and perhaps cope with higher levels of cosmic radiation than Earth life.


You might think this the most boring possibility of all.

I think many prospective colonists would think, if we have good reason to suppose that there is no life on Mars, then - we should just go ahead as there is nothing there to be harmed. You get the impression sometimes, that they want to search for life on Mars just to show that there isn't any life there, to get the go-ahead to colonize.

But - I don't think that "no life on Mars" should give colonization the green light at all! Not so long as it has potential habitats on the surface, as seems increasingly likely.

For the potential habitats, see my last article, Where To Search On Mars For Droplets,&Shallow Flows Of Liquid Water - Where Microbial Life May Flourish

If it has these present day habitats for life - and has nitrates, organics, salts, liquid water - there must be something going on in those habitats. There has to be some complex chemistry of some sort.

According to some ideas for origins of life, then parts of present day Mars might even be ideal for prebiotic chemistry, for instance Hauk Trinks idea that life originated in ice

Did life originate in ice?

There is no way we can simulate Mars on Earth in any detail. We are restricted to small chambers a few meters across at most.

So - then we have an entire planet - with micro habitats with potential pre-biotic chemistry not contaminated with Earth life. That's an amazingly precious opportunity to study what happens on a planet before life evolves - and to find out what does happen, if life doesn't evolve.

Do we find RNA there for instance? Some have suggested that RNA can evolve by itself on a planet without life. Or do we find things that look like cells, with cell walls, but not alive? Again some have suggested that you can get a metabolism and things that are not alive, but look like cells, on a planet without life.

Or do we find numerous nanobes, the tiny life-like forms you find so often on Earth? Nobody knows if this is life or some inorganic process, or perhaps something between the two.

But whatever they are, nanobes may also occur on Mars. Surely do as we've found them in a Mars meteorite.

Nanobes on a Mars meteorite. Is this life, or results of non life processes, or something in between?

If so, perhaps we can find out more about them on Earth by studying their counterparts on Mars. Are they like Earth nanobes, or in some way different? Are they a form of life, a form of pre-biotic chemistry, or not related to life at all?

from "New life form may be a great find of the century" (1999) The nanobes discovered on Earth are equally mysterious. Nobody knows if they are life, non life, or something in between.

If we introduce Earth life to Mars, we may never be able to study a pristine Earth like planet without Earth life on it, in the foreseeable future.

Okay we still have Europa and Encladus - but those are permanently ice covered oceans and never had an atmosphere, or high levels of gravity.

Mars is the only planet in our solar system that resembled Earth at all, in its first few hundred million years - apart from Venus. Venus, though a closer sibling than Earth originally - is so hostile to present day life that we may not learn too much from it. It may just possibly have life in its upper oceans, but can't have Earth like life on its surface, as it is just too hot for our type of life, and is thought to be lifeless, probably.


This is what some scientists suggested recently in the "Over Protection of Mars". They gave this as a reason why we should give up protecting Mars.

I see this in quite the opposite way, though, let me explain.

First, this would be an astonishing discovery if true, because the amount of contact between the planets is rather small. Tons of material get exchanged true, but from relatively few impacts. Most of it is sterilized by millions of years of travel time between the planets. See Does Earth Share Microbes With Mars Via Meteorites - Or Are They Interestingly Different For Life?

It would suggest convergent evolution, that such a small amount of contact between the two planets is enough to preserve identical lifeforms on both planets.

We would need to look really carefully at Mars, to try to understand this amazing convergence.

We'd have so many questions to ask.

  • Why - with only the opportunity to exchange vacuum tolerant and radioresistant extremophiles between the two planets, and only the opportunity to do that on occasional meteorites every few tens of millions of years - why are the other lifeforms also similar?

    (Here I'm assuming there are habitats on Mars for microbes that couldn't survive the cosmic radiation for a journey there on a meteorite. For an example, see Even Ordinary Microbes May Survive Radiation on Mars)
  • How similar are they?

    There would surely be at least some minute differences, no matter how similar the lifeforms are, for instance, we have many variant species of Chroococcidiopsis and of Halobacteria on the Earth with slightly different capabilities, e.g. better tolerance to ionizing radiation.

  • Are there minute differences we haven't spotted yet?
  • How did they come to resemble Earth life so closely? Did they get to Mars somehow in the past, perhaps buried deep within a rock tens of meters across - or is it an example of convergent evolution?

Any introduced Earth life would confuse the study hugely, because of horizontal gene transfer. GTAs make it possible for totally unrelated archaea to exchange DNA fragments, and even between unrelated multi-cellular life.

In one striking example, pea aphids create carotenoids using genes derived from fungi by horizontal gene transfer. And remarkably, use this capability to do photosynthesis. This makes these tiny aphids one of the few insects with the capability for photosynthesis.

Pea aphids, by borrowing genes from fungi through horizontal gene transfer - one of the few insects able to carry out photosynthesis.
As a result, the tree of life now looks much more like this:

from: Horizontal gene transfer: perspectives at a crossroads of scientific disciplines, this version from wikipedia who reproduce it with permission from Smets and Barkay

This is like the conservation problem of hybridization of canines and felines with domesticated pets.

For instance the Scottish wild cat
Scottish wild cat and kitten in captivity

is under threat of extinction in the wild through interbreeding with the domestic cat.

Now, Mars certainly doesn't have wild cats. And it probably doesn't have anything resembling aphids.

But it may have lichens

Lichen from Earth from locations with no water, able to metabolize and photosynthesize in Mars simulation chambers on Earth.
And may have microbes. And may have small multicellular creatures also, is not ruled out.

first discovery of multicellular life that doesn't need oxygen (in 2010) - microscopic image in visible light, stained with Rose Bengal.

So, if Mars life resembles Earth life, especial care is needed to make sure that we don't mix Earth DNA with Mars DNA if we want to have a clear understanding of life there, and how it relates to Earth life.


You might get the idea that these are mutually exclusive - either life on Mars is related to Earth life for instance, or is unrelated.

But it could as easily be a mix of many different types of life.

It could for instance have Chroococcidiopsis and haloarchaea, even radiodurans on the surface, for photosynthesis (I know that radiodurans is an aerobe, but could it use oxygen produced locally by chroococcidiopsis?).

Along with that, it could have other photosynthetic life using different pigments which haven't yet been able to transfer to Earth for one reason or another. Either just chance, or they live only in fragile habitats (say on boundaries of salt and ice) that almost never get ejected with escape velocity - or are more vulnerable to vacuum conditions or to cosmic radiation or both.

Along with that, it could have XNA based life, based on a different biochemistry. We have the idea on Earth of a "shadow biosphere" - so the same could occur on Mars also.

Then in other places - because the habitats are so spread out and rare, perhaps it has places with uninhabited habitats. These then might have elements of prebiotic chemistry - perhaps in deep down hot spots - or places on the surface with concentrations of organics and nitrates.


So far, we have taken reasonably good care to keep invasive species away from Mars. But with pressure mounting for human missions to Mars, there's some pressure to reduce the level of protection.

Planetary Protection is done on the basis of probabilities. We can't sterilize our spacecraft completely, so the aim is to reduce the chance of introducing life to vulnerable locations in the solar system as much as possible. The usually quoted figure is, to reduce the chance of irreversible contamination to less than 1 in 10,000 for each mission.

With humans on Mars though, especially with recent discoveries of possible habitats for life there - it is generally agreed that we can't possibly maintain the same level of planetary protection as we do with unmanned spacecraft.

I don't think anyone has attempted any figures here, and we don't have the information we need to do an accurate calculation. But suppose, just for purposes of illustration, that the overall risk of introducing invasive species of Earth life to Mars increases from 1 in 10000 to 1 in 100 per mission with human expeditions to the surface of Mars?

Reasoning behind this 1 in 100. guess:

Bearing in mind the high risk of a crash landing on Mars - I'd be surprised if first human missions have a better than 90% chance of landing on the surface safely.

If habitats are as prevalent on Mars as the "swimming pools for bacteria" - not too unlikely that it's a 1 in 10 chance of a hard landing on Mars contaminating the planet - not immediately - but as a result of debris from the crash site spreading in the Martian dust storms.

Also, bear in mind, that humans would need sources of ice for water, and a hard landing could melt that ice forming a temporary habitat for some of the microbes right at the crash site.
So in that example, a 1 in 100 chance of contamination per mission, then over say ten missions to the surface, that would be a 9% chance of contaminating Mars with Earth life. Far far higher than the overall target of 0.1% for unmanned missions to Mars for the "exploration period" envisioned by Sagan.

This is just a guess, no basis for it at all really. Just putting forward a figure for purposes of discussion.

We would need to have detailed studies of Mars - and - most important of all - we would need to have a reasonably detailed survey of the potential habitats there.

Also we would need to have detailed understanding of whether Earth microbes can survive on Mars, and how well they are protected by the dust if blown around over the surface. Much research would be needed before we can have a clear picture of what the level of risk is associated with a human mission crashing on Mars.

But one thing is clear - whatever the level of risk is, it is a greatly increased level of risk over unmanned rovers.

So, should we care more about microbes that may exist on Mars, or about the humans who have a keen wish to walk on Mars in person?

I know that prospective Mars colonists include at least one exobiologist, Penny Boston, cave biologist, and co founder of the Mars society, who would like to study the Mars microbes up close in person.

Penny Boston collecting specimens during Mars simulation in the Utah desert.

But I suggest, that we need to look closely, and balance this understandable wish to go there and do a field expedition in person, with the risks involved for the life we want to study.

It's not like a field expedition on Earth. Mars is not like any place we have on Earth.

Our experiences here are of only limited value in predicting what will happen if we introduce Earth life to Mars.

Would precautions that are adequate on Earth be adequate to protect Mars?

Michael Meltzer in his "When Biospheres Collide" - a history of NASA'S Planetary Protection Programs" put it like this:

"One of the most reliable ways to reduce the risk of forward contamination during visits to extraterrestrial bodies is to make those visits only with robotic spacecraft. Sending a person to Mars would be, for some observers, more exciting. But in the view of much of the space science community, robotic missions are the way to accomplish the maximum amount of scientific inquiry since valuable fuel and shipboard power do not have to be expended in transporting and operating the equipment to keep a human crew alive and healthy. And very important to planetary protection goals, robotic craft can be thoroughly sterilized, while humans cannot. Such a difference can be critical in protecting sensitive targets, such as the special regions of Mars, from forward contamination.

"Perhaps a change in the public's perspective as to just what today's robotic missions really are would be helpful in deciding what types of missions are important to implement. In the opinion of Terence Johnson, who has played a major role in many of NASA's robotic missions, including serving as the project scientist for the Galileo mission and the planned Europa Orbiter mission, the term "robotic exploration" misses the point. NASA is actually conducting human exploration on these projects. The mission crews that sit in the control panel at JPL, "as well as everyone else who can log on to the Internet" can observe in near real-time what is going on. The spacecraft instruments, in other words, are becoming more like collective sense organs for humankind. Thus, according to Johnson, when NASA conducts it's so-called robotic missions, people all around the world are really "all standing on the bridge of Starship Enterprise". The question must thus be asked, when, if ever, is it necessary for the good of humankind to send people rather than increasingly sophisticated robots to explore other worlds"


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.

Some think introducing Earth life doesn't matter, others think it does matter. Why give one or the other preference in the debate?

Well the thing is - who knows - maybe the optimists are right, and we wouldn't cause any harm to Mars by introducing Earth life to the planet. But - what if they are wrong? Then we have lost our chance to keep Mars pristine.

So the benefit of the doubt here has to go to those who want to keep Mars pristine - at least until we have a reasonably clear picture of the consequences of introducing Earth life to Mars.

No-one complains about the need to keep invasive species away from islands with unique lifeforms. We know that they are one of the major causes of extinction of species. In the case of birds, they are right at the top of the list.

Major threats contributing to extinction to species of birds since 1500
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.

If we introduce modern Earth life to Mars, accidentally, or on purpose, then the biological history of Mars would be divided into two parts - its history before the introduced modern 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.


What we have here is a conflict of freedoms. That happens often.

So, first, undeniably, we have the freedom of prospective colonists to go to Mars and have a go at building habitats and greenhouses etc. It doesn't matter if you are skeptical about their chances of setting up a self sustaining colony on Mars. Humans are free to make mistakes. or to do things that others think are mistakes at least. They are free to try to fulfill their dreams.

But on the other hand, we also have the freedom of scientists to study Mars in its pristine state. Also we have the freedom of humanity to enjoy the benefits coming from researches done on Mars.

Arguably also there's an ethical dimension also in terms of the intrinsic value of the microbes themselves - not as individuals so much - as their value as species and the value of the microhabitats they may live in. For instance, perhaps not so many of us greatly value microbes on Mars right now. But in the future we might value them greatly, or our descendants might.

Handing on a solar system to our descendants with no native Mars microbes, and no native Europa or Encladus microbes may be on a par to handing on an Earth with no whales or elephants or big cats in it.

So we have a conflict here. But it's an asymmetrical one. If we delay colonizing Mars - then in the future, we still have the option to colonize the planet at any time. But if we contaminate Mars with Earth life now, then in the future, we never have the opportunity to restore Pristine Mars again.


Pristine Mars is like a giant library of knowledge. Introducing life to another planet before we can study it is like setting fire to the great library of Alexandria of ancient Egypt.

It's knowledge that could fill in many gaps in our understanding of evolution of life on Earth and evolution around other stars.


I've heard this argument often in discussions with Mars colonization enthusiasts. Okay it's valuable in its pristine state, they agree, who could disagree?

But what about its value as a place to try out terraforming for the first time ever? Surely that balances out its value as a place to find out about origins of life?

Their reasoning is that terraforming is something that will help us right now. But - what kind of an experiment is this that they have in mind, just introducing whatever microbes hitch a lift with the first humans on the planet?

Especially when you don't study Mars in its pristine state first. Any experiment should start by understanding what the conditions are like before you do the experiment.

No matter what happens later on - if you begin with human "boots on Mars" then this experiment starts with an accidental introduction of life to Mars before we have a chance to study it at all in its original state. I'm not sure if that should be called an "experiment" at all, in the scientific sense of the word.


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.

They are also far more controllable. You can build a habitat with almost 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.

You could set up a habitat to mimic Mars, or early Earth, or early Mars, or early Venus, or some exoplanet that interests you. Climate, atmosphere, gravity, everything as desired.

That truly deserves the name "experiment" in the scientific sense.

Also, you can finish the job in a few decades. A Stanford Torus could be completed within a few decades.

Terraforming a planet, if it can be done at all, takes thousands of years in even the most optimistic projections - and that's with mega technology, far more expensive than a Stanford Torus, such as constellations of mirrors in orbit and greenhouse gas factories spread over the surface of Mars - kept operating for centuries.

Does anyone think 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 anyone possibly have answers to that?

When we have no idea what the life could do - it's like a giant uncontrolled "experiment" with the whole planet as a petri dish. How do we know what our descendants a few generations from now will want? How do we know what we want ourselves?

See also:
Imagined Colours Of Future Mars - What Happens If We Treat A Planet As A Giant Petri Dish?

In my view, presenting an opinion for discussion, and summing this all up, it's hard to think of any discoveries from Mars that could make it a clear cut case, that it is okay to contaminate it with Earth life and that the effect of Earth microbes on Mars doesn't matter.


It's easy to get the idea that this is just an academic pursuit. Exobiologists are keen to study life on Mars, of course, but what about more practical people?

Well - first - I think it is clear that most people are interested in pure science, at least to the extent of wondering, for instance, about origins of life, and whether there is life on other planets, and whether it is different from Earth life. And the likes of the search for the Higgs boson has captured the public imagination even with no practical value at all.

But in the case of life on Mars, well it could be of immense practical value also.

First, look around, how much of modern society is based on plastics, and wood, and other products derived from life? Plastics of course derive from oil which comes from life.

Then what about medicine? With most medicines, doctors have no idea how the medicines work. All they know is that they do work. What might we learn from study of a new form of life, with different biochemistry, about how life works at a fundamental level?

Also nanotechnology. Life is a form of nanotech. Well XNA based life on Mars would be nanotech also. Again, who knows what it could be useful for?

Of course can't predict anything. But seems its practical potential is somewhat more than the Higgs boson at least (British undestatement there).


I've read workshop reports, and watched videos of planetary protection discussions for human spaceflight. Generally everyone agrees on the value of planetary protection.

But, nearly always - the speakers and authors stress that the aim is not to prevent human exploration, but rather, to find a way to make it possible for humans to explore Mars responsibly.

They seldom discuss the possibility that humans should be prohibited from landing on Mars for planetary protection missions.

However - I'd argue myself - that no action here, not sending humans to Mars - must always be an option on our agenda.


This is a key principle in political decision making and law, the Precautionary principle, as described in the Wingspread conference

When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.

In this context the proponent of an activity, rather than the public, should bear the burden of proof.

The process of applying the Precautionary Principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives, including no action.

Here, we don't have a direct risk to human health or the environment. So, we can't apply the principle directly.

But I think a similar principle should apply here.

When discussing sending humans to Mars, then the full range of alternatives needs to be examined, including, the possibility of no action.

Otherwise, what's the point in planetary protection, if you are required to find a way to let humans land on Mars no matter what the protection issues are?

Also, as with the precautionary principle, the precautionary measures need to be taken, even if we don't know yet how Mars would be contaminated.

We may not have confirmation yet that habitats exist on Mars surface. And we may not yet know for sure if Earth microbes can survive and reproduce in them. In that situation the answer should be that we need more research, not to give it the green light because we don't yet have a detailed model to show how Earth microbes would contaminate Mars.


So far I've just been talking about the "period of exploration and discovery" when we don't know what Mars is like, and have no idea if there is life there or what it has in its place.

But at some point in the future, if we keep Mars pristine, then we may have a thorough understanding of it. We can explore it with telerobots from orbit, or with more and more autonomous robots controlled from the Earth, or a mixture of the two.

Then at some point we may need to make a decision.

Options include

  • "Mars-forming" - if interestingly different life is found, to recreate early Mars or some other environment that is conducive to Mars life (Chris McKay's idea).
  • No action. No humans land on Mars and we continue to study it in pristine state
  • We find a way to send humans to explore Mars on the surface, without contaminating it with Earth life, or in a biologically reversible way.
  • Terraforming. We start a process of ecopoesis. But in the early stages, first few centuries, no humans land on Mars. That's because the process would probably work far better if you have nothing on Mars except for cyanobacteria, no other microbes to interfere with the processes there
  • Send humans there and just let the planet develop whatever way it goes. Maybe using paraterraforming - cover it in greenhouses - perhaps we have experience from space habitats that suggest that no great harm will follow from this, and we understand Mars life well, and have done simulations to see what the effect would be of introducing Earth life to the planet, and decided that it will do no harm.
  • Preserve parts of Mars in its pristine state. Is hard to see that working right now, but maybe with future technology and ways of partitioning off parts of Mars so that it is kept separate from Earth microbes. Is that possible?

However, we are clearly far from the point where we have the knowledge needed to make such a decision.

The obvious solution right now is simply not to send humans to Mars.


It is unfortunate that humans inevitably carry microbes around with us, in our bodies, on our skin, in the food, soil and air. That's why human exploration has planetary protection issues.

NASA have plans to put plants in a miniature cubesat greenhouse on Mars and grow plants there for 15 days to watch them germinate. This has no planetary protection issues, as the seeds would be sterilised of all microbes, the result of several generations of sterilized seeds and the growing medium can also be sterile. But you can't do this to humans - they would die.

If only we could be sterilized like robots or like plants! If humans were like plants, if we didn't need to have trillions of microbes associated with us to survive, then there would be no problem at all with us exploring and colonizing Mars.

But since we aren't like plants, I think, surely, we need to take great care.