I thought I'd post this because there are many who haven't followed the latest findings, who still think that present day life on the surface of Mars is absolutely impossible because of UV light, ionizing radiation, and perchlorates, and because the atmosphere is in almost perfect chemical equilibrium. 

That is indeed what most scientists believed, prior to about 2008. But it is now generally agreed in the field that if there do turn out to be nutritious warm and wet habitats on the surface of Mars, they will be habitable. 

Nowadays the debate is focused on whether these habitats exist - and whether there is anything living in them, if they do exist. So what has changed, you might wonder? After all - on the face of it - the conditions there do seem pretty lethal...

It's particularly topical right now because of this news story that has broken recently, about a discovery of a new way of creating habitats with liquid water on Mars on the interface between salts and ice 

The professor says 

""Based on the results of our experiment, we expect this soft ice that can liquify perhaps a few days per year, perhaps a few hours a day, almost anywhere on Mars. So going from mid lattitudes all the way to the polar regions. This is a small amount of liquid water. But for a bacteria, that would be a huge swimming pool - a little droplet of water is a huge amount of water for a bacteria. So, a small amount of water is enough for you to be able to create conditions for Mars to be habitable today'. And we believe this is possible in the shallow subsurface, and even the surface of the Mars polar region for a few hours per day during the spring." (transcript from 2 minutes into the video onwards)"

That's Nilton Renno, who lead the team of researchers. See also Martian salts must touch ice to make liquid water, study shows

Nilton Renno is a mainstream researcher in the field - a distinguished professor of atmospheric, oceanic and space sciences at Michigan University

For instance, amongst many honours, he received the 2013 NASA Group Achievement Award as member of the Curiosity Rover " for exceptional achievement defining the REMS scientific goals and requirements, developing the instrument suite and investigation, and operating REMS successfully on Mars" and has written many papers on topics such as possible habitats on the present day Mars surface.

And this sparked headlines in many papers such as:

So, especially if you read the many articles on this subject from about six years ago, you might wonder, why didn't he add "But there can't be any life there because of the UV, ionizing radiation, atmosphere in equilibrium and perchlorates"?

Go to the Life on Mars article on wikipedia, and you read:

"Although Mars soils are likely not to be overtly toxic to terrestrial microorganisms, life on the surface of Mars is extremely unlikely because it is bathed in radiation and it is completely frozen..."

Hasn't the professor read wikipedia? Why does he think there could be life on the surface of Mars?

Read Encyclopedia Brittanica, and it is a bit more up-beat about life on the surface

 It could be argued that the best strategy is to look for fossil remains from the early period in Mars’s history when conditions were more Earth-like. But the Martian meteorite debate and disagreements about early terrestrial life point to the difficulty of finding compelling evidence of microbial fossil life. Alternatively, it could be argued that the best strategy is to look for present-day life in niches, such as warm volcanic regions or the intermittent flows of what may be briny water, in the hope that life, if it ever started on Mars, would survive where conditions were hospitable."

From The question of life on Mars (Encyclopedia Brittanica)

So, I thought, it might help to explain why we no longer think that this rules out indigenous life on the Mars surface. I'll look at each in turn - except for the atmosphere in equilibrium. That needs a longer answer so I'll do that in a separate article.

IONIZING RADIATION

You can find many pre 2008 articles saying just that, some of them cited in the wikipedia article. 

But the difference is, that before 2008, they were looking at the possibilities for dormant life, on the surface. The idea they had then - was that the surface is currently totally lifeless - because, it was thought, the near vacuum air pressure would cause any liquid water there to evaporate rapidly. So should be no water left.

So - pre 2008 - the idea was that if there are any dormant spores on the surface, they have been there, dormant all the time, since the last time Mars had a slightly thicker atmosphere. I.e. they would have been dormant for millions of years.

Ionizing radiation could easily destroy even the most radioresistant microbes in a million years.

However that doesn't mean that Mars is laced with instantly lethal radiation.

Europa is. Jupiter has such strong ionizing radiation that humans would only last hours in vicinity of Europa before they die of radiation poisoning. And even highly radioresistant microbes wouldn't last long.

But the surface of Mars - though it gets far more radiation than the surface of Earth - gets roughly the same amount of radiation every year as the interior of the ISS.

The surface of Mars, with its thin atmosphere to shield from radiation...

Receives about the same amount of cosmic radiation per year as the interior of the ISS (above the Earth's atmosphere but shielded by our ionosphere )

Humans would find it hazardous long term - increases your risk of getting cancer. And not much to do to protect from it except stay buried under meters of radiation shielding - spacesuits and rovers would be no protection.

But microbes don't get cancer. Their DNA gets damaged, yes, but the most radioresistant microbes have remarkable abilities to heal the damage to their own DNA. They can do so in real time, no need to reproduce.

The way it works is that these microbes have structures that keep the DNA fragments in proximity to each other when they are damaged. When they are able to metabolize again, the cell joins those fragments together to repair the DNA.

As an example, take Chroococcidiopsis, one of the microbes we have on Earth best able to survive in Mars surface conditions.

Experimenters have found that it can repair, 2.5 kGy of damage  within 3 hours given the opportunity to wake up for a few hours and metabolize.

Here a kGy is a thousand Grays, and a mGy is a thousandth of a gray. So 2.5 kGy corresponds to 2500/0.076 or over 32,000 years of radiation on the surface of Mars

So if it wakes up for a few hours every year, it will have no trouble at all keeping its DNA repaired.

And - that's nowhere near a lethal dose. If able to wake up for 24 hours, it can repair 64,000 years worth of damage.

Also this microbe, though amongst the most radioresistant of Earth microbes, has not evolved in the presence of high levels of cosmic radiation - at least as far as we know. The generally accepted explanation seems to be that this is a side effect of its evolution of mechanisms to resist dessication damage to its DNA. So Mars microbes may well be far more radioresistant even than this.

The most radioresistant microbe currently known is Thermococcus gammatolerans. 

Thermococcus gammatolerans - an obligate aerobe from hydrothermal vents, the most radioresistant organism known, able to withstand 30 kGy of gamma radiation, and still reproduce. That's about 400 thousand years (30,000/0.076) worth of surface radiation on Mars at the radiation levels detected by Curiosity during the current solar maximum - possibly it could survive surface radiation for longer than that when you include periods of solar minimum.

It's not a likely candidate for the Mars surface as it lives in deep hydrothermal vents at the bottom of the sea and requires high temperatures. But - its radioresistance seems to be a side effect, it certainly doesn't encounter cosmic radiation down there. So a Mars microbe, adapted through billions of years of evolution to the ionizing conditions on the surface of Mars may well be as radioresistant as this, or more so.

ULTRA VIOLET RADIATION

The levels of UV on the surface of Mars are also very high and would destroy most microbes within seconds. 

However, first of all UV light is easily shielded. It differs in that way from cosmic radiation, which goes through meters of rock without noticing it. UV light is like ordinary light - it can be blocked by just about anything that casts a shadow.

A mm or so of soil will block it. Also if a microbe is in the shadow of a rock, or pebble, it is shielded. Even if it is in a tiny microscopic crevice in a grain of Martian dust, it is shielded, especially since the Martian dust contains iron oxide, which is rather effective at shielding out UV light.

Then, it also turns out that our pal  Chroococcidiopsis has remarkable abilities to shield itself from UV light. In cold deserts and high mountains, then you do get significant amounts of UV light. Nowhere near the Mars levels but enough so that cyanobacteria have evolved some protection from it. Lichens also have developed UV resistance, with various specialized pigments to block it out.

This UV resistance is so good, that when the German aerospace company DLR (sort of their equivalent of NASA) researched into this - they found that some lichens, and our friend Chroococcidiopsis, could survive conditions on the surface of Mars in partial shade. Not just survive - in their Mars simulation chambers, they found that they could metabolize and photosynthesize, slowly, on the simulated Mars surface. These are still early stage experiments - but it looks promising that you might get photosynthesizing lifeforms actually on the surface of Mars using the sun's light for energy. Either in partial shade, or shaded by a thin layer of dust - or protected by transparent rock such as quartz. 

PERCHLORATES

The levels of perchlorates on Mars are hazardous to humans - yes. We've found high levels of perchlorates in the dust. But some microbes actually eat perchlorates. It is nutritious food for them. So it doesn't by any means rule out microbial life on Mars.

THIS DOESN'T MEAN THERE HAS TO BE LIFE ON MARS

Nowadays we think there is at least a chance of living, metabolizing life on the surface, that wakes up for hours every year.

Now - I'm not saying that there has to be life on Mars. Professor Renno is one of the scientists most optimistic about discovery of present day life on Mars nowadays. He is one of several who think this way.

However, there are also respected researchers who think it unlikely that these habitats exist. Others they may think they exist as habitats - but that they are likely to be uninhabited, with no life in them (could life have survived for billions of years with just these small droplets of water on the surface that remain habitable - or has it perhaps gone extinct, at least on the surface - does it perhaps only now survive in deep subsurface habitats?).

But, what is outdated is the idea that it is absolutely impossible for life to exist on Mars.

• Back in the 1970s then most scientists thought there is no life on Mars and never has been. 
• Up to about 2008 most scientists thought there might well have been life on Mars in the past, and it may be there deep down, but is no longer there on the surface.
• Since 2008, though there are many still think that life on the surface is unlikely, there are also many who are optimistic that we will find present day life on the Mars surface (or a few mms or cms below it) - and you would struggle to find a single scientist, today, who says it is totally impossible.

So, I thought I'd explain this in some detail. 

I'M A TROLL FOR SAYING THIS - ACCORDING TO THE CURRENT WIKIPEDIA EDITORS OF "LIFE ON MARS"

Part of the reason for writing this is because of a discussion I've had on the talk page of the Life on Mars page on wikipedia with an editor who is convinced that life on the surface of Mars is impossible because of ionizing radiation. He labels me a troll for saying anything different and gave me this rather fetching icon:

Other editors of the page are in agreement with him there. At least, nobody else has stepped in to support my proposed edits to the page.

And all my posts to the talk page now get hidden as soon as I post them. Including a post I just did to the talk page a few hours ago to tell them I've just written this article about our discussion there. They remain convinced that present day life on the surface of Mars is absolutely impossible because of ionizing radiation.

Mind you, those are posts to the page where editors discuss what to put into the article. I used to be an editor of the article myself (in a very minor way, mainly adding citations) but haven't attempted to edit it since last year when its entire section on the present day habitability of Mars was deleted and rewritten to say that present day life on Mars is impossible. Now, they regard even posts on this topic to the talk page "troll like activity".

So this ionization argument can be very persuasive, it seems. 

I hope this short article can help clear things up a bit. Not sure if it will make any difference to the Life on Mars article in wikipedia. I'm not sure that anything will convince the current editors of that article short of actual discovery of life on the surface of Mars.

But hopefully a few who have been convinced by the pre 2008 ionization articles and papers, may get a better, more up to date perspective on it as a result of writing this.

WIKIPEDIA AS A SECONDARY SOURCE

Oh, and I'd just like to add,  I use the encyclopedia all the time and frequently link to it as well, have done so in this article itself indeed, for the page on Chroococcidiopsis. 

It has some tremendously good articles. Also many sections are bang up to date, even updating on the very day that news breaks about some new discovery. Also, in the other topic areas I contribute to there, such as maths and music - I've had only the most respectful and interesting conversations on the talk pages and it's been a pleasure working on the articles.

But it is a secondary source, not a primary reference. It's always wise to follow up citations and read the source material for yourself if unsure. Since this experience - I also often take a look at talk pages for other articles. They can give you an idea of editorial bias of the article, or assumptions of its contributors, if any, and may have different conclusions or approaches not in the main article. 

SOME OF THE POSSIBLE HABITATS ON MARS

This is a good recent (2013) overview of the research on possible water habitats on Mars which looks at just about all the possibilities to date,, likely or unlikely. It doesn't mention this ice / salt interface idea however as that is new leading edge research.

See Water and Brines on Mars: Current Evidence and Implications for MSL by Renno (the professor in the video) and Martinez.

There are other features on Mars that may be caused by liquid water as well but they have alternative explanations such as dry ice effects.

The three ones most likely to be liquid water, as surveyed in that paper - probably salty brines are

• warm seasonal flows, which have no other good explanation at present - they form on sun facing slopes, only when temperatures are above 0 C so temperatures far too high for dry ice, and are not at all correlated with winds and dust storms. They form in spring, every year, gradually extend during the summer, and then fade away in autumn.
• deliquescing salts supported mainly by the droplets on Phoenix's legs and the observation of suitable salts to deliquesce on Mars - this is the idea they tested - but found was not fast enough to explain the Phoenix leg droplets. It still could create habitats on Mars - just not as rapidly as the new ice / salt interface idea.
• subsurface melts caused by the solid state greenhouse effect
  
  This should happen on theoretical grounds, that the sunlight should create melt water below sheets of transparent or translucent ice, as it does here on Earth - but no feature on Mars has been conclusively identified as caused by it as yet. The "dune dark spots" could be caused by this process - but they have alternative explanations.

SEE COMMENTS SECTION BELOW

As usual, any thoughts, ideas, corrections, etc be sure to say.

I'm going to follow this up with an article on James Lovelock's 1970s remarkable prediction that Viking would find no life on the surface of Mars - just from studying the composition of the atmosphere - and about recent developments that mean that his argument does not rule out life there in the sparse populations we now expect to find.