Keep Mars human free!

It is not impossible that Mars has its own life.
If humans land there then they will contaminate it.
Looking at the science,
including recent discoveries the past three years,
Jonathan Cowie argues for a manned Mars orbit only mission


Mars by Viking. © NASA.


It is not impossible, indeed arguably quite likely, that there is life on Mars.

Some have argued that the initial rise of life is a hard evolutionary step.  Yet, the evidence from our own planet clearly contradicts this, for if the rise of life was a hard step then we would not expect it to happen quickly.  But there is clear evidence that life rose on Earth almost as soon as it could have, if not sooner!

First a bit of science. Carbon comes in two stable isotopes: the common carbon-12 (atomic mass 12) and rare carbon-13 (approximate atomic mass 13) with the extra mass coming from a neutron in the atom’s nucleus. When life takes up carbon (such as through various photosynthetic mechanisms) it preferentially takes up carbon-12: life had no need to develop a mechanism to absorb the very rare carbon-13 when nearly all the carbon was in carbon-12 form. (It’s an atomic vibration thing: carbon-13 vibrates slower at a given temperature.)

What this means is that the proportion of carbon-13 to carbon-12 in all living things (such as you) and things made of things that once lived (such as coal) have less carbon-13 to carbon-12 than non-biological carbon (such as that in the carbonate that furs your kettle).

In short, depleted carbon-13 in a carbon sample (especially compared to that in surrounding inorganic carbon strata) is a sign of life.

Among the oldest carbon-13 depleted carbon in grains within geological strata is that from Akilia island off West Greenland. It is thought that these grains could be old as 3.85 billion years.(1) Further 13C depleted carbon has been also been found in 3.7 billion year old rocks from Usua, again West Greenland.(2)

There is some – some might say speculative – evidence for life from even earlier still.(3) However, this was before the Late Heavy Bombardment (LHB) of asteroids: a period of time in the early Solar System thought to be caused by gas giants adjusting their orbits into a more stable configuration and disturbing asteroids some of which hurled into the inner Solar System.  If so, you might think that such pre-LHB life would be wiped out by the bombardment. Yet some calculations suggest that it could have survived through this time.(4)

There is other (non-carbon) evidence too, but the above should suffice to make the point.

In short, life got going on the Earth almost as soon as it could have.  But what were things like on Mars?

Today, Mars has lost much of its atmosphere due to it being smaller with a lower gravitational field allowing slow atmospheric escape, and also that it lacks a core dynamo generating a protective magnetic field that prevents Solar wind ablation of its upper atmosphere.  But matters in Mars’ early days were very different.

Early in the life of Mars, there was a much denser atmosphere before it became lost.  There was also a core dynamo generating a magnetic field and initiating plate tectonics.  This is evidenced in areas of ancient Martian crust by magnetic reversal lines (similar to those by the mid-Atlantic spreading plate zone on Earth) detected by The Mars Global Surveyor probe’s magnetometer experiment in 1999.(5, 6)

Today, water is limited on Mars but it is present: it exists as ice (mixed with carbon dioxide) at its poles and also as a liquid beneath the said poles’ ice as well as ice in permafrosts and subsurface liquid brine in some areas.(7, 8)  Briny water has also been observed escaping permafrost on some slopes on Mars.(9)  Further its geology, as revealed by lander probes, shows that some considerable amount of water is bound to minerals.(10)  It has been calculated that that in excess of 9% by volume of the Martian mantle may contain hydrous (water containing) mineral species.(11)

Given there is water on Mars today, albeit much of it locked up, what of the distant past before water escape, along with atmosphere loss, took its toll, and back when Mars had some protection with a magnetosphere from Solar wind atmospheric ablation? As it turns out there is considerable evidence that the primordial Mars did have an ocean including evidence of ancient shorelines and river deltas.(12)  It is thought that there could have been an ocean covering much of Mars’ northern, with a small sea in its southern, hemisphere. These were likely formed before 4 billion years ago (bya) and possibly lasted to after 3.5 bya but not as long as to 3 bya.(13)

So, assuming a similar co-evolution of life and planet timeline as on Earth, then life on Mars could have been established by about 3.8 bya.  This was shortly (in the geological sense) after the planet’s formation ~4.5 bya and if so any earlier life would have to survive the late heavy bombardment 4.1 to 3.8 bya.  If there was an ocean on Mars before 4 bya then there would have been a habitat for life. In short, if a similar pattern of co-evolution of life and planet narrative took place on Mars as it had on Earth, there could well have been life on the very early Mars in the form of something analogous to simple celled (sort of bacteria like) life on Earth.

This begs the question that if there was life in a long-gone Martian ocean, could there still be life on Mars today?  Mars’ atmosphere may offer some clues.

In 2003 the ability of Mars’ probes to detect lower levels of gases in Mars’ atmosphere revealed that there was methane at a concentration of around 7 parts per billion by volume (ppbv) or 0.007 x 10-6%. For comparison this concentration is two orders of magnitude lower than on our living planet Earth at 1,800 ppbv (or 1.8 x 10-6%).  A year later, in 2004, the Mars Express Orbiter and ground-based observations suggested the presence of methane in the atmosphere at a level of about 10 ppbv (0.01 x 10-6%).(14)  In 2018 the Curiosity rover showed that Martian methane pulsed with the seasons with release peaking in the summer.(15)

The source of this methane is currently a mystery.  It could very well be produced by life.  However, there are also geological (non-biological) mechanisms that generate methane.(16)

So, if seasonal methane on Mars is not conclusive of life, is there any other evidence?  Here, the discovery of the seasonal variation in the Martian atmosphere’s oxygen concentration is interesting.  About 0.1% of Mars’ atmosphere is molecular oxygen compared to nearly 21% of Earth’s. And, from measurements in the Gale crater, this figure itself varies seasonally by about 30%.  It is very hard to account for this oxygen in a non-biological way.(17)

Yet, one of the main arguments against life on Mars life today is that its surface is toxic to much of life as we know it.  A product of Mars losing its ocean are chemicals called perchlorates. The breakdown of these result in chemicals (including hypochlorite which is the active constituent of household bleach) on the Martian surface that are toxic to many forms of life as we know it.(18)

On Earth, some simple, single-celled species (prokaryotes) can evolve to live in extremely hostile environments (cf. antibiotic resistant bacteria and extremophiles). Such evolution happens quite easily and quickly: antibiotic resistance arises within the scale of human lifetime.  So adapting to the slower change from an ocean Mars to a drier one over many millions of years, is not beyond the realms of possibility. But, even so, could life survive such conditions?

The Atacama Desert in Chile is the driest place on Earth where the annual average rainfall of less than 20 mm reduces weathering rates and leaching losses to below the accumulation rates of atmospheric delivered salts and dust. This means that the amount and concentration of dust and salts increases so resulting in an extremely dry and high salt concentrations.  Its altitude also means that it has a high ultra-violet (UV) irradiation which, though not as great as that on Mars due to Earth’s thicker atmosphere, is still considerable as the Earth is closer to the Sun.  Consequently, as on Mars, UV light does enable photolysis of chlorates to perchlorates. In this sense, the Atacama Desert is fairly analogous to Mars.  Having said all that, in the Atacama Desert rainfall does occur roughly once a decade and also occasionally water condenses directly onto surfaces (as garden dew does).  Here, this water could be considered to serve the same function as subsurface water or condensed water vapour on Mars.  Researchers have found that prokaryotes survive even in the driest areas of the Atacama Desert.  Species found included those that belonged to the Geodermatophilaceae.  They are known to colonise hyperarid habitats and tolerate high levels of oxidative stress, desiccation, salts, and metals. They are also somewhat UV resistant.  Additionally, traces of some fungi were found belonging to the Ascomycota and Basidiomycota but mostly at a depth 20-30 cm below the surface.  All this suggests that any ancient, putative Martian life could have possibly survived the transition from Mars’ early aquatic stage, through increasing drier cycles, to today and perhaps they may even found a briny subsurface niche beneath today’s severely dry surface. (19)

Today on Earth, we do find prokaryotic cyanobacteria beneath thin, translucent stones in seemingly dry deserts. Morning dew condensing on the stones, though quickly evaporating, sees enough to moisten the ground beneath them and so the microbes live.  Meanwhile, sufficient light penetrates the stones for them to photosynthesise.   Here, the key thing about cyanobacteria is that they photosynthesise in a way that releases oxygen, just like (non-prokaryotic) advanced plants do.  Could this be an explanation to the seasonal oxygen found in Mars’ atmosphere? Could there be some Martian analogue to cyanobacteria on Mars?

And here’s the rub.  If there is life on Mars, do we want to contaminate the planet with Earth’s life?

Since the 1960s, there has been continued interest in exploring Mars.  Indeed, right now (autumn 2020), there are three missions to Mars (from the US, China and Saudi Arabia).  There are also proposals for crewed missions to land on Mars.

Impression of Perseverance lander deploying. Perseverance is currently (autumn 2020)
on its way to Mars. © NASA

If humans did land on Mars then they would have to be careful not to contaminate the planet.  Not long after landing, the astronauts will want to pee, and within a day take a dump.  The thing is that faeces have an extremely high microbial load.  So if humans did land on Mars then they really should return to Earth with their waste.  Even so, there would still be some risk in contaminating the planet.

It is probably not lost on SF fans the irony of Earth microbes contaminating any putative Martian ones. This was something I mentioned in a science journal(20) near the occasion of the 120th anniversary of H. G. Wells’ novel The War of the Worlds.(21) when NASA’s Curiosity rover – which was not germ free – was diverted away from an area of Mars that could possibly harbour putative Martian life.  (You may recall that in Wells’ novel, the Martians were eventually, accidentally defeated by exposure to Earth’s extant microbial diseases.)

One alternative, to humans landing, would be to have a crewed mission only to Martian orbit.  There would still be benefits to such a mission.  Mars’ two moons could be explored.  Also, direct remote controlling of a lander would be possible without that pesky time delay due to the speed of light limitation and the huge Mars-Earth distance.  Geological samples, fired from a remote-controlled lander back to Martian orbit, could also be collected and brought back to Earth.  Finally, if proof of concept was a goal, it would demonstrate the feasibility of interplanetary human transportation.  Actually having humans landing on Mars would confer little added value other than political prestige, and I’m not sure any politician is worth that.

So, I implore your good selves, do start a debate.  Talk about the issue on relevant social media platforms.  Have it as the topic for panel discussions at SF conventions among whatever else you can do.  Remember, once we contaminate Mars it will be near impossible to return it to its pristine state.  We are already making a mess of one world.  Let’s not screw up a second.

To paraphrase (forgive me) Arthur C. Clarke: ‘All these worlds are yours – except Europa and Mars. Attempt no human landings there’.(22)

Jonathan Cowie


Jonathan Cowie, is one of SF² Concatenation’s founding editors and an SF fan.  In real life he is an environmental scientist who has had a career with British learned scientific (mainly biological) societies.  He has worked on a range of mostly human ecology (in the broadest sense) related topics.  The past decade or so he has developed an interest in Earth system science, biosphere change and evolution.
          He can be found at Concatenation Science Com.



1.  Mojzsis, S. J., Arrhenius, G., Mc Keegan, K. D. et al (1996) Evidence for life on Earth before 3,800 million years ago. Nature, vol. 384, p55-59,  and  McKeegan, K. D., Kudryavtsev, A. B. & Schopf, J. W. Raman (2007) Ion microscopic imagery of graphitic inclusions in apatite from older than 3,830 Ma Akilia supracrustal rocks, west Greenland. Geology, vol. 35, p591–594.

2.  Hassenkam, T., Andersson, M. P., Dalby, K. N., Mackenzie, D. M. A. & Rosing, M. T. (2017) Elements of Eoarchean life trapped in mineral inclusions. Nature, vol. 548, p78-81.

3.  Dodd, M. S., Papineau, D., Grenne, T., Slack, J. F., Rittner, M., Pirajno, F., O’Neil, J. & Little, C. T. S. (2017) Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature, vol. 543, p60-64.

4.  Abramov, O. & Mojzsis, S. J. (2009) Microbial habitability of the Hadean Earth during the Late Heavy Bombardment. Nature, vol. 459, p419-422.

5.  Connerney, J. E. P., Acuña, M. H., P. J. Wasilewski, P. J., et al. (1999) Magnetic lineations in the ancient crust of Mars. Science, vol. 284, p794-8.

6.  Stevenson, D. J. (2001) Mars core and magnetism. Nature, vol. 412, p214-9.

7.  Orosei, R., SLauro, S. E., Pettinelli, E., et al. (2018) Radar evidence of subglacial liquid water on Mars. Science, vol. 361, p490-3.

8.  Usui, T., Alexander, C. M. O. D., Wang, J., Simon, J. I. & Jones, J. H. (2015) Meteoritic evidence for a previously unrecognized hydrogen reservoir on Mars. Earth Planetary Science Letters, vol. 410, p140-151.

9.  Ojha, L, Wilhelm, M. B., Scott L., Murchie, S. L., et al. (2015) Spectral evidence for hydrated salts in recurring slope lineae on Mars. Nature Geoscience, vol. 8 (11), p829-832.

10.  Carter, J., Poulet, F., Bibring, J. P., Mangold, N. & Murchie, S. (2013) Hydrous minerals on Mars as seen by the CRISM and OMEGA imaging spectrometers: updated global view. J. Geophys. Res. Planets, vol.118, p831–858.

11.  Wade, J., Dyck, B., Palin, R. M., Moore J. D. P. & Smye, A. J. (2017) The divergent fates of primitive hydrospheric water on Earth and Mars. Nature, vol. 552, p391-4.

12.  Di Achille, G. & Hynek, B. M. (2010) Ancient ocean on Mars supported by global distribution of deltas and valleys. Nature Geoscience, vol. 3, p459–463.

13.  Citron, R. I., Manga, M. & Hemingway, D. J. (2018) Timing of oceans on Mars from shoreline deformation. Nature, Vol. 555, p643-6.

14.  Formisano, V., Atreya, S., Encrenaz, T. et al. (2004). Detection of Methane in the atmosphere of Mars. Science, vol. 306, 1,758–1,761.

15.  Webster, C. R., Mahaffy, P. R., Atreya, S. K., Moores, J. E., et al. (2018) Background levels of methane in Mars’ atmosphere show strong seasonal variations. Science, vol. 360, p1,093–1,096.

16.  Oze, C. & Sharma, M. (2005). Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars. Geophysics Research Letters, vol. 32 (10), L10203.

17.  Trainer, M. G., Wong, M. H., McConnochie, T. H., et al. (2019). Seasonal variations in atmospheric composition as measured in Gale Crater, Mars. Journal of Geophysical Research: Planets, vol. 124, p3,000–3,024.

18.  Wadsworth, J. & Cockell, C. S. (2017) Perchlorates on Mars enhance the bacteriocidal effects of UV light. Scientific Reports, vol. 7, 4,662.

19.  Schulze-Makuch, D., Wagner, D., Samuel P. Kounaves, S. P. et al. (2018) Transitory microbial habitat in the hyperarid Atacama Desert. Proc. Natl. Acad. Sci., vol. 15 (1), 2,670-2,675.

20.  Cowie, J. (2016) Correspondence: Martian dance of fiction and fact. Nature, vol. 538, p317.

21.  Wells, H. G. (1898) The War of the Worlds. William Heinemann: London, Great Britain.

22.  Clarke, A. C. (1982) 2010: Odyssey Two. Granada: London, Great Britain.


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