The rover Curiosity recently failed to detect methane on Mars. What might this result mean for Mars’ present or past habitability?

Mars


This is my final year dissertation in full. It was submitted on 21 March 2014 and scored 100 per cent. Minor edits were made, but little more than small changes in phrasing. References have been included at the end.


Section 1. Abstract
1.1 Abstract

Section 2. A Brief History of Methane Detection
2.1 Initial Findings
2.2 Methane, and the Martian Atmosphere
2.3 Potential Biotic Sources of Methane
2.4 Potential Abiotic Sources of Methane

Section 3. Methane Detection, or lack thereof, via the Curiosity Rover
3.1 Disappointing Results
3.2 The Tunable Laser Spectrometer (TLS)
3.3 Margin for Error, and Conflicting Conclusions
3.4 Implications on the Possible Habitability of Mars

Section 4. The Habitability of Mars
4.1 Present Day Conditions
4.2 Deliquescing Salts
4.3 The Gale Crater
4.4 The Atacama Desert on Earth
4.5 Radiation

Section 5. Theories Alluding to Life on Mars
5.1 Extremophilic Bacteria
5.2 Nanobes
5.3 Lichens

Section 6. Conclusions
6.1 Conclusions

Section 7. References
7.1 References


Section 1. Abstract

1.1 Abstract

Methane presence has been identified as a tracer for organic life on other planets, given that 90 per cent of methane on Earth is produced by living systems. After detecting in 2003, and confirming in 2004, that methane was present on Mars, astrobiologists were keen to use the Curiosity rover to analyse the Martian atmosphere from its surface. But a lack of detection using the Tunable Laser Spectrometer (TLS) in 2012 and 2013 led to a revision of the estimated amount of the gas on Mars. Ultimately, the lack of any methane casts doubts over whether or not the red planet may be inhabited by microbial life at all, as we hoped. 

The implications as to the present or past habitability were naturally brought into question. Methane, though a reliable tracer, is not a definitive marker as to whether or not life may be present on Mars. Conditions, hyper-arid and intolerable, would play host to some extremophilic bacterial species for instance, known to survive in similar conditions in the Atacama desert on Earth. 

Additionally, such lifeforms as lichens have been tested under Mars-like conditions, and not only survived but thrived. Exploring potential prehistoric forms of life, such as nanobes found in meteorites, serves as a reminder that life may have existed as different forms in the distant past. 

The prospect of there being life on Mars, at least in the form of methanogenic bacteria, has been damaged considerably. But Mars’ surface is certainly habitable by known life forms, and its conditions are mirrored on Earth in certain regions known to be teeming with microbial life. Whether or not life currently inhabits these supposedly habitable regions remains to be seen, as research is not being conducted in directly searching for the presence of life on Mars. 


Section 2. A brief history of methane detection

2.1 Initial Findings

Researchers, on three separate teams, first encountered the possibility of the presence of methane in the Martian atmosphere in 2003 (Mumma et al 2009). They were using high-dispersion infrared spectrometers to detect methane and water vapour at three ground-based telescopes, measuring over longitudinal intervals in Northern early and late summer 2003. The methane detection was verified in 2004 by the Mars Express spacecraft. The search in the Northern Summer of 2003 covered an estimated 90 per cent of the planet’s surface, and spanned three Martian years. 

(NASA 2009) The detected methane release from the Northern Summer in 2003. Up to 30 parts per billion were found in some areas.

Occurring in extended plumes and the maxima of latitudinal profiles, the methane was thought to have been released from discrete regions. One plumage detected was supposedly comparable with a massive hydrocarbon seep at Coal Oil Point. Spatial variability, in addition, suggested that localised subsurface sources were present, as well as seasonal variations. The level of methane found even led to an estimation that Mars would have had to be producing 270 tons per year (Krasnapolski 2006), and would have had to be produced from an active source, given that methane would be relatively unstable. 

Since 2003 the three groups reported spectral data from Mars Express, which contained five unidentified spectral features between wavelength 3000 and 3030 cm-1, which coincides with the expected position for methane (Mumma et al 2009). The previous searches for methane on Mars were negative prior to 2003, although in 1999 the Mars Global Surveyor was tasked with charting the evolution of methane on the planet, and the results were eventually used (following ten years of work) to support the claims that methane is present on Mars.

2.2 Methane and the Martian Atmosphere

After the detection of what was thought to be significant quantities of methane in the Martian atmosphere, devising theories as to its source became the main task at hand, thought the definitive origin of this atmospheric methane, whether it is biotic or abiotic, remains uncertain.

The atmosphere of Mars is strongly oxidised (Mumma et al 2009), and is composed of carbon dioxide (95.3 per cent), nitrogen (2.7 per cent), carbon monoxide (0.07 per cent), oxygen (0.13 per cent), and water vapour (0 to 300 parts per million). Reduced gases such as methane and other species are rare. When speculating about the origin of the methane that was detected on Mars in the early 2000s, scientists involved in the field have looked to Earth for an indicator.

2.3 Potential Biotic Sources of Methane

Methane may have been produced by water-rock interactions in the Martian interior, or in volcanic hot spots (Formisano et al 2004). The source is also suggested to have been via thermal decomposition, comet and metior impacts, or via serpentization (various). But, as has been thought by the wider astrobiological community, methane is an important biological tracer, and its presence may allude to the presence of microbial life.

The suggestion arose from observations of the methane composition in the atmosphere on Earth. 90 per cent of the atmospheric methane on our planet is known to be produced by living systems (Krasapolski et al 2006), so naturally any observation of methane on Mars would alert researchers to the potential existence of any such biological systems. Thus, the presence of methane is generally believed to be a tracer for extraterrestrial microbial activity. If microscopic lifeforms would be methane producers, it has been suggested they would perhaps dwell below the surface – where it may be warm enough for liquid water to still run freely (NASA 2009). However, biotic origins of methane was not the only potential surface for the detected gas that was discussed by researchers.

2.4 Potential Abiotic Sources of Methane

Abiotic sources of this detected methane were the subject of discussion among many teams following the initial confirmation in 2004. Oze and Sharma (2005) suggested that olivine hydration in Martian regolith and crust may be a major methane source, which would have contributed to the warming of early Mars in a significant fashion. They continued to outline how similar abiotic sources may include hydrothermal alteration of basalt, or be somehow related to the hydration of olivine and pyroxenes, i.e. serpentinization, close to the surface. However, counter research has cast doubt on these methods due to a lack of current volcanic and hydrothermal activity or hot spots (Hauber et al 2011).

Studying abiotic methods of methane production on Earth also allowed researchers to propose other methods. Methane has been discovered, for instance, in vent fluids issued from peridotite-hosted hydrothermal systems. Formation of fluids containing carbon dioxide, and methane, have been reported in a number of experiments involving hydrothermal reactions (Oze and Sharma 2005).

It was also suggested that methane was produced by way of chemical reactions in meteorites upon entry, driven by the intense heat, would lead to methane production. Although research has since indicated that this has been ruled out, and an alternative source may be that organic compounds on meteorites are converted to methane by ultraviolet radiation (Keppler et al 2012).


 

(NASA 2014) The Tunable Laser Spectrometer used by the Curiosity rover to detect methane

(NASA 2014) The Tunable Laser Spectrometer used by the Curiosity rover to detect methane


Section 3. Methane Detection, or lack thereof, via the Curiosity Rover

3.1 Disappointing Results

Following the successful results during the previous decade, NASA would send the Curiosity rover to Mars with the hope that similar measurements would be acquired as had been done from the vantage point of Earth. However, after several attempts to discover methane in the Gale crater between August 2012 and September 2013, the rover failed to discover the levels of methane that had been predicted.

Although it was expected that methane would be found at levels as high as 45 parts per billion (Webster and Brown 2013). The first measurement at the landing site indicated there was less than 5 parts per billion of methane, at the point of measurement. And in July 2013, NASA published results of new atmospheric analysis, reporting a lack of methane around the landing site. In September 2013, NASA once again reported that the methane detection was very low, and that this would perhaps correspond to an upper limit of only 1.3 parts per billion. This was one sixth of the previous estimates (Webster et al 2013). For reference, the atmospheric methane presence on Earth is thought to be roughly 1700 parts per billion.

3.2 The Tunable Laser Spectrometer (TLS)

The instrument used in the Curiosity rover was the atmospheric analyser, the Tunable Laser Spectrometer (TLS). The TLS would perform precision measurements of oxygen, and carbon isotope ratios in carbon dioxide and in methane. This would be conducted in the Martian atmosphere, in order so as to distinguish between geochemical or biological origin. After four analyses, it was concluded with 95 per cent confidence that there was between 0 and 5 parts per billion of methane. The results led to a revision of the upper limit of Martian methane.

3.3 Margin for Error, and Conflicting Conclusions

There are a number of reasons that have been suggested as to why, in general, the results have led to the confusion and the conflict of results between the ten years of the initial discovery and the landing on Mars. For instance, it was estimated that methane would be found to levels of 45 parts per billion. However one possibility is that some of the methane in Earth’s atmosphere may have distorted potential ground-based measurements from telescopes.

Another element of the mystery is that if methane was present, it would last for hundreds of years in the atmosphere of Mars, and would not have escaped quickly after detection. Without a definite method to remove the methane from the atmosphere, the new measurements indicate that methane would not be produced by any mechanism, including geological, biological, or by ultraviolet degradation of organics delivered by the fall of meteorites or dust particles.

3.4 Implications on the Possible Habitability of Mars

The lack of any discovery was assessed as a “killer blow” for the potential of finding life on Mars by some news sources covering the lack of a discovery. In reality, the assessment of the lack of methane on Mars, which has revised previous estimations, is highly primitive in nature, as extensive searches of the surface of the planet have not yet been conducted. Although it may seemingly rule out the presence of methanogenic bacteria currently living on the planet, some conflicts arise with previous results from Earth which, although conducted remotely, were definitive in the idea that methane on Mars was not only a real possibility, but that it had sources, and was mapped in a highly specific manner.

But, again, the lack of methane as a ‘tracer’ would only be indicative of a lack of methanogenic bacteria currently present on Mars. This has little implications as to any notion that methanogenic bacteria was around in the past (perhaps distant), and does not necessarily make any indication as to whether or not other tracers of life would be present. Furthermore, the lack of methane on the surface does not rule out the potential for subsurface methane deposits, or methane production (biotic or abiotic).

The conditions on Mars are such that, at present, there are many known species of microscopic lifeforms on Earth that would be able to thrive. Extremophile bacteria, for instance, are known to tolerate the intolerable, and research into lichens as well as nanobes may indicate there are alternatives to the traditional concept of methanogenic microbial life, which has been (to this point) all but ruled out.


Mars landscape


Section 4. The Habitability of Mars

4.1 Present Day Conditions

As discussed earlier, the Martian atmosphere comprises carbon dioxide (95.3 per cent), nitrogen (2.7 per cent), carbon monoxide (0.07 per cent), oxygen (0.13 per cent), and water vapour (0 to 300 parts per million). Reduced gases such as methane and other species are rate (Mumma et al 2009). But  the physical conditions of the planet are mostly known to be dry, arid and utterly inhospitable (Walker 2014). A reduced atmospheric pressure on Mars also means the boiling point of water in the Gale Crater is closer to 5 degrees Celsius, rahterr than 100 as it is Earth. Water would boil shortly after it melts. The axial title of Mars, in addition, means the poles can be warmer than the equator at times, which would to snowfall at the equator, and the formation of ice sheets. But these do not remain for any extended duration, and the planet tilts back over some time.

4.2 Deliquescing Salts

Although frosts in equatorial regions occur, there is hardly any water vapour, and absolute humidity is low. Yet, at night, it may become cold enough as to reach 100 per cent for relative humidity (Walker 2014). This ‘relative humidity’ would lead to the formation of comulus clouds. Though there is no more water vapour in the air than before, the colder temperature of the air means it cannot carry as much water, and so condenses as the droplets of water of a cloud – which leads to such phenomena as morning dew, ground fog, and frosts.

And as the temperature falls, the absolute humidity does not change. There is no more water vapour than before, but due to a reduced capacity for the air to carry vapour, the relative humidity would rise until it reaches 100 per cet, and leads to the formation of the previously mentioned dew, or fogs, etc. On Mars, vapour forms frosts, or ice crystal fogs – which can be compared with Earth’s cirrus clouds. Conditions are too cold and the air is far too thin for ‘water vapour’ clouds, generally speaking, and so when the frosts melt, it is thought that – though they evaporate too quickly to be useful for potential life – an effect of a sort of temporary high-humidity at soil level is created, which can be exploited.

4.3 The Gale Crater

After analysing the conditions at the Martian Gale Crater, it has become apparent that there are a number of indicators that present day life may even be a possibility, let alone the conditions being adequate (Fisk et al 2013). Although it must be noted that this is speculative, and just based on visual evidence that has been acquired. The notion is based on the discovery of deliquescing salts, and water droplets. Dark drops of water are known to form gradually, merge occasionally, and drop off, upon which time then do not form at that same spot again.

Ratios of isotopes in the atmosphere also show that carbon dioxide must have come recently from volcanic eruptions, and that oxygen has subsequently changed atoms with some materials on the surface’ meaning that there must have been some sort of dissolving in water. This, only marginal, may be considered evidence for the abundance of water in the form of either thin layers or occasional melting. The idea of a permanent liquid layer is the most promising for the habitability of the planet, however.

It has also been suggested (Fisk et al 2013) that melting salts could create a humidity; which would be captured by the salts in the Gale Crater, potentially via magnesium perchlorate and calcium perchlorate activity. Ferrous and ferric iron, or ferrous iron and perchlorates would act as the ‘energy source’ for any potential activity. Moving sand dunes would also mix the reducing lower parts of soil with oxidising surface layers, and resurface nutrients from below.

4.4 The Atacama Desert on Earth

The Atacama Desert has been observed as a possible parallel environment to that on Mars in general, due to its similar dryness, and lack of rainfall. Despite hostile conditions, certain bacterial species are yet known to flourish via the process of deliquescence; microbial life using slight damp in which to survive. Colonisation of halite from the Atacama Desert was studied by Raman spectroscopy (Betancourt et al 2004), and life was shown to exist among the salt crystals from the ‘hyper-arid’ core.

Originally it was thought that the conditions in this region were so harsh and intolerable, that it proved to be a sort of ‘dry limit’ for life on Earth. But in 2002, life was found in this region via a process of DNA sequencing (Walker 2014); previously it was believed that only dormant life which relied on rare rains could exist, thought it is now known that rain is not in fact required at all.

Work carried out (Parro et al 2011) proved that life which relies on deliquescing salts had been found two or more metres below the surface of the Atacama desert. This microbial life uses a mixture of halite, perchlorates for the process of deliquesence, and sulphates, perchlorates, and nitrates alongside acids such as acetate or formate as sources of energy. These nutrient conditions are, in essence, Mars-like.

4.5 Radiation

One touted possibility against the argument that life may be present on Mars would be the massive exposure to cosmic radiation led on by the occasional axial tilt, and the fact that this would destroy life near the surface. But now it is understood that some forms of life are only expected to survive for a few months in the winter, and some would be able to survive for centuries and millennia, namely the Thermococcus gammatolerans, exposed to the full force of this type of radiation (Jolivet et al 2003).

Research has shown (Jolivet et al 2003) that the radioresistant microorganisms Thermococcus gammatolerans, that are obligate anaerobes sourced from hydrothermal vents, are able to withstand kGy of gamma radiation, and still be able to reproduce. For reference, this is the equivalent of 400,000 years’ worth of surface radiation on Mars. The microorganism, based on Earth, perhaps did not evolve in an environment exposed to high levels of radiation, but has developed a resistance as a possible side effect or as an effect to counter damage that could be done to its genetic material. On a speculative basis, microbes that may have evolved in an environment which is often exposed to high levels of radiation may be adapted specifically to that environment, and may be even more resistant than Thermococcus gammatolerans.


MARS AL

Section 5. Theories alluding to Life on Mars

5.1 Extremophilic Bacteria

When considering the conditions on Mars, known for their utmost hostility to most life on Earth as would know it, researchers’ attentions were turned to examining known species of bacteria that would be able to tolerate these conditions, and flourish. Species of extremophile are known to withstand and thrive in conditions under which most life would be killed without question. The first extremophile discovered was Thermus aquaticus, which withstood temperatures of above 70 degrees Celsius (Shostack 2005).

But more extremophilic bacteria soon discovered included the hyperthermophile Pyroblobus fumarii which would be able to withstand temperatures of above 112 degrees Celsius. Others included the Psychrophiles which could survive in condtions below the point of freezing, and Pizeophiles & Xerophiles, varieties that would be able to tolerate nuclear radiation, and live in aviation fuel. Meanwhile, Acidophiles and Alkaliphiles would tolerate highly acidic or highly basic conditions, and Halophiles would be considered to survive ‘heavy-duty brines’.

Strategies used to survive such conditions include either erecting a barrier against the harsh environmental factors, or changing the metabolism (Shostack 2005). Some halophiles protect themselves from a saline environment by increasing the concentration of salts within. With uniform salinity, the bacterial species need not fear that osmosis will drain the water from within to the outside.

Similarly, Psychrophiles are equipped with proteins which enable them to adapt to the cold. Some of these acts as ‘antifreeze’ to lower the freezing point of water, preventing congealing, expanding and sundering of the cell. Other proteins and enzymes are also specifically formulated to ensure that chemistry continues when the temperature dips below conventional comfort. The species known as Deinococcus radiodurans is able to use a highly complex DNA repair system, located within its cell walls, to recover from exposure to huge bursts of radiation. Such properties in known bacterial species allude to the fact that at the very least, Mars would prove habitable for their growth and survival.

5.2 Nanobes

One highly-contested theory as to the presence of life on Mars arises in the ‘nanobe’ which, since its discovery some years ago in the late 90s, has been subject to debate as to whether or not it would constitute life. These nanobes may or may not be living – this is still under debate – but it is assumed (Walker 2014) that these represent an earlier prehistoric stage of life and evolution, when evolution to the size and the complexity of life as we know it by today had not yet occurred.

Upon discovery (Urwin et al 1998), nanobes pushed the preconceived limits as to what was required for independent life forms. These were tiny, seemingly living creatures 20 to 150 nm in length; roughly the same size as viruses. Colonies were observed to engage in rapid growth; to the point that colonies were visible to the naked eye, forming a dense network of tendrils when observed under microscopic conditions.

Laboratory analysis by the Australian team led by Urwin showed that the nanobes seemed to be able to reproduce quickly, and even signs of DNA were found. It was established that they had biological elements such as carbon, oxygen, and nitrogen, and were shown to have two distinct layers when cut in half, as well as a possible nuclear area which holds the genetic material.

Structures resembling these nanobes were found in the Mars meteorite ALH84001 (Walker 2014), although it was unclear whether these structures were ‘fossils’ of prehistoric life forms, or something inherently inorganic. IT is thought that the machinery required for a normal cell to survive would not fit into the nanobe, but a marked lack of research into these potential life forms has not been carried out to a satisfactory level from which conclusions can be drawn. It is believed, however, that they represent evolutionary steps between amino acids and the modern cell, much in the same way that there are several intermediatory steps between primitive cells and complex organisms.

5.3 Lichens

Other potential manifestations of life that would flourish under Mars’ habitability are lichens, which are known to survive and thrive in tundras and cold, high, dry mountainous conditions; surviving and growing without the presence of water (de Verra et al 2010). These are protected from ultraviolet light, as well as drought, in conditions in which they are exposed to higher-than-normal ultraviolet light. The lichens would survive by taking in water vapour without the need for salts.

Lichens were tested under Mars-like conditions (de Verra et al 2010) and were found to be well-resistant to the perceived harshness, able to grow normally, photosynthesise, and metabolise. Mars simulation chambers reproduced the atmosphere, the levels of ultraviolet radiation, and the day-to-night relative humidity variations. Carbon dioxide concentrations at normal Earth atmosphere would reduce the photosynthetic activity, it was found, although when the atmospheric pressure was lowered to Mars-levels, a compensatory effect took place upon the photosynthesis, and it is returned to a normal rate.

Five lichen species were identified for their extremophilic characteristics, (Sancho et al 2007), and were used in recent astrobiological studies which tested how well they would survive and metabolise in Martian conditions. These lichens were shown to be able to survive in almost any conditions replicating those at the surface of Mars, provided there is a consistent source of light, the carbon dioxide level remains steady in the atmosphere, there is a moisture in the air, and some sources of nitrogen, again, thought to already exist in the form of nitrates on Mars.


Section 6. Conclusion

6.1 Conclusion

The presence of methane on Mars is not the definitive tracer to there being life on the planet in the present day, nor does it amount to indicating anything whatsoever on Mars’ habitability, either present or past. Mars is known for its conditions which are hostile to life; the fact that its hyper-arid, and exposed to such huge amounts of radiation, for instance. However, after analysing the conditions on Mars, it is clear that its ‘habitability’ is almost irrelevant to the notion of life existing on its surface, as not only are there known species of bacteria for instance that thrive under such conditions, but that lifeforms have been known to survive and reproduce under existing conditions on Earth, and in Mars-like conditions replicated on Earth.

Mars is, with little doubt, habitable by some forms of life, which has been determined by the swathes of studies that have been conducted by several teams over the previous few years. More encouragingly, conditions on Mars were known to be less harsh in the distant past. Whether or not life exists on Mars in the present day, however, is another matter. Research is not entirely conclusive, and is inadequate. The Curiosity rover will continue to search for methane with its retuned TLR, and the definitive results of establishing methane presence on Mars are awaited.


Section 7. References

7.1 References

Atkinson N, 2010, ‘Mars methane gets even more mysterious’, University Today, accessed online on 20/03/14: http://www.universetoday.com/74091/mars-methane-gets-even-more-mysterious/

Betancourt J, Meier R, Quade J, 2004  ‘Microbial life in the Atacama Desert’, Science, 306:1280

Bruce D, 2014, ‘Mars deltas key to red planet’s potential past life’, Forbes, accessed online on 20/03/14: http://www.forbes.com/sites/brucedorminey/2014/01/31/martian-deltas-key-to-red-planets-potential-past-life/

Christner B, 2013, ‘Double-strand DNA break repair at -15 decrees C’, Applied and Environmental Microbiology, 02845-13

De Verra J-P, Mohlmann D, Butina F, Lorek A, Wernecke R, Ott S, 2010, ‘Survival potential and photosynthetic activity of lichens under Mars-like conditions: a laboratory study’, Astrobiology, 10(2); 215-227

Fisk M, Pop R, Renno N, Mischna M, Moores J, Wiens R, 2013, ‘Habitability of transgressing Mars dunes’, 44th Lunar and Planetary Science Conference, accessed online on 20/03/14: http://www.lpi.usra.edu/meetings/lpsc2013/pdf/1434.pdf

Formisano V, Atreya S, Encrenaz T, Ignatiev N, Giuranna M, 2004, ‘Detection of methane in the atmosphere of Mars’, Science, 306

Hauber E, Broz P, Jagert F, Jodlowski P, Platz T, ‘Very recent and wide-spread basaltic volcanism on Mars’, Geophysical Research Letters, 38(10)

Howell E, 2013, ‘Curiosity rover finds no methane on Mars. What’s happening?’ Universe Today, accessed online on 20/03/14: http://www.universetoday.com/104930/curiosity-rover-finds-no-methane-on-mars-whats-happening/

Jolivet E, L’Haridon S, Corre E, Forterre P, Prieur D, 2003, ‘Thermococcus gammatolerans sp. nov., a hyperthermophilic archaeon from a deep-sea hydrothermal vent that resists ionizing radiation’, International Jouurnal of Systematic and Evolutionary Microbiology, 53(3)’ 847-851

Keppler F, Vigano I, McLeod A, Ott U, Frunchtl M, Rockmann T, 2012, ‘Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere’, Nature, 468(7401); 93-96

Krasnopolsky, 2006, ‘Some problems relating to the origin of methane on Mars’, Icarus, 180(2); 359-367

Mumma MJ, Villanueva GL, Novak JE, Hewagama T, Boney BP, DiSanti MA, Mandell AM, Smith MD, 2009, ‘Strong release of methane on Mars in Northern Summer 2003’, Science, 1165243

Oze C, Sharma M, 2005, ‘Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars’, Geophysical Research Letters, 32

Parro V, de Diego-Castilla G, …, Gomez-Elvira J, 2011, ‘A microbial oasis in the hypersaline Atacama subsurface discovered by a life detector chip: Implications for the search for life on Mars,’ 11(10); 969-996

Sancho LG, de la Torre R, Homeck G, Ascaso C, de Los Rios A, Pintado A, Wierzchos J, Schuster M, 2007, ‘Lichens survive in space: results from the 2005 LICHENS experiment’, Astrobiology, 7(3), 354-443

Shostak S, 2005, ‘Extremophiles: Not so extreme?’ Space, accessed online on 20/03/14: http://www.space.com/175-extremophiles-extreme.html

Staff writer, 2013, ‘Subsurface microbes alive millions of years may exist on mars – DNA repair mechanism make it possible’. The Daily Galaxy, accessed online on 20/03/14: http://www.dailygalaxy.com/my_weblog/2013/10/subsurface-microbes-frozen-millions-of-years-may-exist-on-mars-dna-repair-mechanism-make-it-possible.html

Steigerwald B, 2009, ‘Martian methane reveals the red planet is not a dead planet’, NASA, accessed online on 20/03/14:http://www.nasa.gov/mission_pages/mars/news/marsmethane.html

Than K, 2006, ‘Martian life could have evaded detection by Viking landers’, Space, accessed online on 20/03/14: http://www.space.com/3038-martian-life-evaded-detection-viking-landers.html

Urwin PJR, Webb RI, Taylor P, 1998, ‘Novel nano-organisms from Australian sandstones’, American Mineralogist, 83; 1541-1550

Walker R, 2014, ‘Rhythms from Martian sands – Did Viking find life in 1976? Perhaps it;s time we found out’, Science 2.0, accessed online on 20/03/14: http://www.science20.com/robert_inventor/blog/daily_rhythms_martian_sands_did_viking_spacecraft_find_life_way_back_1976-131815

Webster CR, Mahaffy PR, Atreya SK, Flesch GJ, Farley KA, 2013, ‘Low upper limit to methane abundance on Mars’, Science, 342(6156); 355-357

Webster G, Brown D, 2013, ‘NASA Curiosity rover detects no methane on Mars’, NASA Jet Propulsion Laboratory, accessed online on 20/03/14: http://www.jpl.nasa.gov/news/news.php?release=2013-285

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