A Mars colony will be tough as I am not even sure we can get people to Mars without too much DNA damage from cosmic rays. However we are learning mroe about Mars including evidence for a lake and more about moons and other planets including evidence of liquid water father from the sun than I would have imagined. There must be many working directly with NASA results and other efforts here on extremophiles that have ideas about what we have learned in the last 10 years that change our views on life and it's possible existence outside the Earth.

If there is life out there, we can be sure the extremophiles will exploit the environment wherever possible. It seems to me that the more we know about extremophiles the more likely it seems that life can exist in the most extra-ordinary circumstances and we haven't to limit out search too much by assuming extraterrestrial life will mimic the general trend on Earth. It seems more likely that life 'out there' will mimic the extremes of life 'down here'.

That's a bit like asking how did life evolve in the first place and if we knew that, we'd solve many an argument. If the panspermia theory is assumed, then life can establish wherever the seed organism finds suitable (or near suitable) conditions.

On Earth, some extremophiles are clearly relictual communities (insects in dry valleys in Antarctica for example) left behind when conditions changed and gradually (through survival fitness) evolved to be more suited to the extreme conditions they found themselves in. Or organisms that have ended up in limited and harsh conditions, such as the mountain top flies of Africa, relicts of much more widespread populations from a cooler Africa post the ice age.

I don't know much about bacterial extremophiles, but I assume some utilize the same evolutionary pathways, while others evolve into extremophiles on the fringes of extreme conditions, gradually invading and colonizing the resource.

I appreciate that Kenneth, but I think it is important not to confuse the notions 'simple morphology' and 'primitive'. The prokaryotes (or Noemura as may be preferred) we observe today are the product of 850 million years of evolution and this is by no means primitive. Archaea (thermophiles and other extremophiles) are thought to have separated from the other micro-organisms as far back as the Palaeoproteroiz as a response to survival in harsh conditions.

Of course if the panspermia theory is accepted, numerous seeding events may have lead to extremophilic and nonextremophilic lineages evolving as independent events, but examining that theory more specifically, we might assume that the 'seed' organisms would require to be extremophilic in order to withstand the damaging radiation and extreme cold of outer space, suggesting that extremophiles colonized the Earth and latterly expanded their range to less extreme environments.

As for where amino acids came from, this is still debated, but various concepts are in play ranging from panspermia to primitve-soup theories . The most plausible is the clay-replication theory based on observations that clay bodies replicate and 'invade' adjacent soils. It is suggetsed that lterations in the inorganic replication permitted organic structures to be incorporated, leading to organic replication ... ultimately leading to organic structures such as proteins.

It's really outside my field Kenneth, taken from bits and pieces I've read following Alexander Graham Cairns-Smith. For example see: Clay Minerals and the Origin of Life Hardcover by Alexander Graham Cairns-Smith & H. Hartman (Editors).

With respect to the above discussion between Ken and Andrew (which is 3 years old), I suggest the reading of: "Origins of Life: New Model May Explain Emergence of Self-Replication on Early Earth - Template-assisted replication, which helps polymers grow longer while passing on sequences from generation to generation, could have enabled jump from monomers to self-replicating polymers"

https://publishing.aip.org/publishing/journal-highlights/origins-life-new-model-may-explain-emergence-self-replication-early

As a first step towards self replicating structures (still far from RNA...)

Then to go further down the road of primordial life look at:

Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions Matthew W. Powner, Beatrice Gerland & John D. Sutherland. Nature, Vol. 460, May 13, 2009

Systems chemistry on early Earth." By Jack W. Szostak. Nature, Vol. 460, May 13, 2009

and

Kensuke Kurihara et al., “A Recursive Vesicle-Based Model Protocell with a Primitive Model Cell Cycle,” Nature Communications 6 (September 2015), doi:10.1038/ncomms9352.

and From Self-Assembled Vesicles to Protocells:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2890201/

The Origins of Cellular Life

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2926753/

John,

With respect to the original question, I would say that the story of the 1976 life experiments is still not closed and one should mention the study "Reanalysis of the Viking results..." by Navarro-Gonzalez et al. published in JGR, the "Comments..." by Biemann and again the "Reply to comment..." by Biemann and Bada by avarro-Gonzalez et al. I'll attach at the end the three papers for those here wishing to make their own mind. Of course for those wishing to go back to the original thoughts behind the "LR experiment" there is no better source than Gilbert Levin site: http://www.gillevin.com/mars.htm

Honestly, 31 years later, I am still no at ease with the way the matter was handled by NASA. The main reason to dismiss Levin's LR results was stated as twofold: the absence of free water and the absence of organic molecules (as discussed in the aforementioned papers). Since then, free water has been discovered (though not much and in particular conditions, i.e. MRO's spectrometer / image ESP_030570_1440 in 09/2015) and organic molecules as well (Curiosity's SAM / Sol 290). This does not by itself make Levin's results conclusive but at least this does not explain why no other attempt to detect directly life on Mars has ever been made (e.g. chiral preferences in the utilization of enantiomers of racemic substrates, etc.). http://www.gillevin.com/Mars/JoC_12-29.pdf

Since Viking, no life detection experiment has been sent to Mars !

The key question now is simply: WHY ?

Could it be that to preserve NASA budgets and Mars exploration, the mystery had NOT to be solved so that things could be kept going for 35 more years ?

Or is it simply, given what Mars is today that the best chances now are to look for very ancient fossil life in sedimentary rocks deposited at the time Mars had oceans, billion years ago ?

In any case, back to the question, the increasing knowledge gained these last decades of the psychrophiles (extremophiles adapted to cold conditions and surviving to more than -20 C) gives us hopes that chances are high that some existed in the martian geological times, and may be still DO exist today.

Kenneth,

I agree with the major reservation you make, with respect to the absence - yet - of organic carbon. It seems that we're not as much in dire straights with respect to carbonate rocks and clays, nor are we with the outcome of the UV exposure.

https://www.researchgate.net/publication/23676900_Orbital_Identification_of_Carbonate-Bearing_Rocks_on_Mars

https://www.researchgate.net/publication/222540208_The_Ultraviolet_Environment_of_Mars_Biological_Implications_Past_Present_and_Future

I concur that it's hard to find life - even extinct - without organic carbon !, but we will not necessarily find an "isotopic signature of oxygenic photosynthesis" as these organisms might have been (are ?) extremophiles of various (unknown ?) kinds, anoxics, lithophiles, etc.

Finding life somewhere remains the (philosophical) game changer of mankind.

Article Orbital Identification of Carbonate-Bearing Rocks on Mars

Article The Ultraviolet Environment of Mars: Biological Implications...

Ken, what's interesting in the right graph, is that with just 10-1 you get a very decent protection (you can even go down to 10-2). If I get the graph correctly, it means that with atm=1b and just 2% O2, conditions would be OK with respect to UV exposition. In fact even with 0,2% O2 it's still OK to get enough 03 ?

Thanks Kenneth for both documents. I've downloaded them and will read them carefully asap.

Best.

Thank you all for the great comments and insights- I am an interested amature in this area. Regarding UV life on earth developed a UV sensor for protection in order to leave the oceans - see  Plant UVR8 photoreceptor senses UV-B by tryptophan-mediated disruption of cross-dimer salt bridges. Christie JM, Arvai AS, Baxter KJ, Heilmann M, Pratt AJ, O'Hara A, Kelly SM, Hothorn M, Smith BO, Hitomi K, Jenkins GI, Getzoff ED.Science. 2012 Mar 23;335(6075):1492-6. doi: 10.1126/science.

Regarding energy srouces and oxygen- we are interested in the use of perchlorate - which may have generated the first oxygen on earth  - see Perchlorate Reductase Is Distinguished by Active Site Aromatic Gate Residues. Youngblut MD, Tsai CL, Clark IC, Carlson HK, Maglaqui AP, Gau-Pan PS, Redford SA, Wong A, Tainer JA, Coates JD.J Biol Chem. 2016 Apr 22;291(17):9190-202. doi: 10.1074/jbc.M116.714618.

the UV photoreceptor is for UV not visible light. Light originated in water that UV does not penetrate well. There is a chemical pathway that generate O2 from perchlorate which is plentiful on paces like Mars