I don't think we will ever. Or at least not in the foreseeable near future. It's a bit like asking "When will we colonize Antarctica". or "When will we colonize the sea bed". But much harder to colonize Mars than either of those.

COMPARISON OF COLONIES ON MARS WITH COLONIES ON SEA BED OR IN ANTARCTICA

Mars at the equator is the same temperature as Antarctica on Earth, but actually gets far colder than Antarctica at night, gets so cold that it's below the freezing point for dry ice.

It has the same problem of no oxygen to breathe, as the sea bed. If living on the sea bed, then breathable air is available just a few tens or hundreds of meters above. Far easier than generating it in a closed system habitat. Yet we find that too much trouble for it to be worth living on the sea bed, for most people.

You can't just use an oxygen breather like a jet fighter pilot or climbers on mount Everest because the air is too thin. Your lungs wouldn't be able to take in any oxygen at that low partial pressure, and what's more the water on the linings of your lungs and mouth would boil.

You need a full pressure spacesuit as for the ISS EVAs.

ADDITIONAL ISSUES FOR MARS

In addition to those problems, there's the cosmic radiation, which you can't protect against in spacesuits, so you would have limits on how much EVA you can do, similarly to the limits on how much time an astronaut can spend on the ISS, but if living on Mars that limit would be over your entire lifetime. So inhabitants would have to spend most of their time indoors in their habitats unless they want an increased risk of cancer likely to strike when you are young - and who would choose to do that given the option?

Then you have to build your habitats to hold in the pressure of the air inside, which is ten tons outwards pressure per square meter - and then cover them with a couple of meters of regolith. No way that can compete with house building on Earth. Where you  have no air pressure to contain, no need for radiation shielding, and you can go outdoors without a spacesuit to build or repair houses.

Also - minor point in a way because it is an unknown - but it's not known yet if Mars gravity is suitable for human long term health. Particularly it's not known whether it would be safe for either child or mother to give birth in Mars gravity. and whether children would grow up normally on Mars (would you have to live in spinning habitats on the surface)?  Our bodies are fine adjusted to the Earth gravity and are complex, different systems respond in different ways to zero or lower gravity, so it's not possible to just work out by interpolation or modeling, You need experiments with humans in low gravity to see how our bodies respond to it.

And then of course you've got the expense and difficulty of sending equipment to Mars. Far easier to supply equipment to Antarctica or the sea bed, but we don't think of colonizing those places.

TERRAFORMING IDEAS ARE SKETCHY

And the ideas of terraforming Mars are very sketchy, and make lots of assumptions e.g. about the amount of CO2 on Mars, which may not be valid (we only know of enough CO2 to increase the pressure to 2% of Earth's pressure) and is based on a timeline of 1000 years (Mars society estimate) to get to the point where trees can grow there, so long as you keep pumping greenhouse gases into the atmosphere to keep it warm - or use large planet sized thin film mirrors in space to keep it warm. That atmosphere would be poisonous to humans (CO2 above 1% concentration in the air kills us even with plenty of oxygen) so you'd need aqualung type closed system air supply with buffer gas such as nitrogen to survive. They optimistically assume that within a few more thousand years you'd have an oxygen atmosphere with nitrogen - but that would involve removing nearly all the carbon from the atmosphere and replacing it with nitrogen (from where? Nitrates on Mars maybe?) - if done biologically that might more likely take a hundred thousand years. And then you need to step up on the greenhouse gas production / area of mirrors because an Earth type atmosphere is even less warming than a CO2 atmosphere.

All that might be possible but there is so much to go wrong and as it is for our descendants a thousand years from now, who knows if this is what they will want? And how likely is it that any government on Earth will sustain such a mega project for a thousand years at costs of billions of dollars every year? And given the harsh conditions, it's not realistic to suppose that colonists on Mars do this building of 200 nuclear power stations and mining 11 cubic kilometers of fluorite ore every century just to warm up the planet. Or build planet sized thin film mirrors, millions of square kilometers, in orbit around Mars. That level of mega technology would have to be supported from Earth.

ANTARCTIC SETTLEMENTS AND SUBMARINES

We do have settlements in Antarctica and research ships / habitats where researchers can live permanently below the surface of the sea for as long as they want. And submersibles that can dive deep below the sea. And submarines that let you live deep below the sea surface for months on end without resupply from above.

So - it would be possible I think to send humans to Mars to explore, following a similar model.

BUT ON MARS WE ARE SEARCHING FOR LIFE

Then the best way to explore Mars if we do send humans there, is from orbit. Because first of all we are searching for independently evolved life on Mars, past and present day. The very last thing we want to do is to just discover life we brought there ourselves.

That's the same here on Earth for some places. We want to find out if there is life in the subsurface Lake Vostok in Antarctica. Though it would be possible to melt down to it and put a submersible into it, robotic controlled - they haven't done that yet because of the need to make sure it is not contaminated with surface Earth life, so we can be sure that we study the life that is there already. They are careful just about drilling indeed there was some controversy when the Russians finally drilled through, after years of waiting to develop the technology. Not thought they have introduced surface life to Lake Vostok, because the water was captured under high pressure surging from below. But their samples could have been contaminated by surface life so putting some question on the results.

Similarly, our robots on Mars are carefully sterilized. And if we send any to "special regions" where life may be present they will have to be even more carefully sterilized. Indeed we haven't yet sent any spacecraft sterilized to Mars to that level, not since Viking in the 1970s.

CAN'T MAKE HUMAN HABITATS INTO INVERTED BIOSAFETY LABS ON MARS

No way human occupied spacecraft can be sterilized in the same way.

Every time you exit your spacecraft, there's a leak of air from the airlock, even with the suitport (airlock attached to the back of a spacesuit which you crawl through to get into the spacesuit). Bound to need to vent air from time to time if problematical gases build up. Almost certainly going to vent human wastes and waste biproducts in some form, even just things like packaging from supply vessels.

It's not really possible to imagine a human habitat on Mars as designed like an inverted biosafety facility with the microbes that accompany humans as the hazard to be kept away from Mars.

And dust storms can spread microbe spores anywhere on Mars imbedded in a dust grain.

IN THE EVENT OF A HARD LANDING OF A HUMAN OCCUPIED SPACECRAFT

But whatever your thoughts about that - some think it may be possible because of the harsh Mars environment, that the numbers of escaping microbes could be reduced enough to not be an issue. But how can you possibly make it safe in event of a hard landing? Almost certainly one of the human missions would crash - and a crashed spacecraft, with bodies spread out over Mars, and food, air, water etc - after that and a few dust storms later, then any biosignatures of life you found on Mars, your first assumption would be that it came from that crashed spacecraft.

So, robots really do do it better on the Mars surface. Because they can be sterilized.

BROADBAND COMMUNICATIONS WITH MARS

The obvious thing then is to have humans in orbit to operate the rovers via telepresence, or else to control mars surface rovers from Earth.

Control from Earth would be much easier with increased bandwidth - the main problem at the moment is bandwidth more than it is the light delay. Indeed typically they download data just once a day and upload new instructions for the next day. Our rovers could be further away from Earth than Pluto and we'd hardly see any difference in the way we operate them.

So the light speed delay from Earth is not really the main factor at present. So first we need broadband connection to Mars - whether we send humans there or not. And that would immediately hugely speed up the progress of robotic exploration on Mars. They would be able to drive much further each day - instead of mapping out 100 meters of travel for the next day, because you won't hear back again until the next day - you could map out 100 meters of travel for the next half hour. It would probably be twenty times faster just because of that. And there are various ways it can be speeded up even further, using artificial real time, a technique from computer games, if we had broadband communications with Mars. This involves things such as putting communications satellites in orbit around Mars dedicated as their primary mission to relay data back to Earth, obviously probably observing Mars as well but with big radio dishes and their primary mission is to achieve broadband communications. Or it might well be that they use laser communication.

We have to do that anyway for human missions. Or at least it would be much harder without and as part of the expense of a human mission, this would be a minor item on the budget sheet. It would not make a lot of sense to send humans to Mars long term with just the existing capabilities for communications back to Earth. Do that first I think.

ADVANTAGES OF HUMANS IN ORBIT OVER HUMANS ON THE SURFACE

Then, humans in orbit would control robots on the surface. This is

• Far safer for planetary protection. I'd also send them to Mars on a ballistic transfer orbit so they don't have the risky injection burn but instead the spaceship is captured by Mars without any burn at all into a distant orbit, and then gradually spirals down to lower orbits using ion thrusters or similar. This is safer for the humans and safer for Mars. And has the advantage you can do this kind of transfer almost any time, not just once every two years with Hohmann transfer.
• Or first mission could be a free return double Athena mission (Robert Zubrin's idea) - does two flybys of Mars, with several hours of low latency telepresence - and in between parallels Mars spending months in fairly close proximity, close enough for useful advantages over controlling robots from Earth. This also is a rather safe orbit, with no need for an insertion burn, because it is "free return". As soon as you are on course for Mars, as you leave Earth, you are already on an orbit that will take you back to Earth 700 days later, with only minor course corrections needed to make sure you keep on the same course. Theoretically, ideally, you never need to fire the rocket again until you get back to Earth.
• Less technology to develop. No need to develop a lander for humans on Mars, which is a big technological challenge, or a return vehicle from the surface which is even more of a challenge
• Safer for humans. You explore Mars while in a shirt sleeves environment, in your spaceship. If there is an accident on the surface, you lose an expensive robot but not a human life. And robots can potentially be repaired. And don't run out of oxygen, don't have to keep going back to a base to get food and oxygen. Can just sit in one spot for years if you don't need them to move, with no resupply at all.
• Not likely to get stranded on Mars. Indeed with the double Athena, you'll come back to Earth automatically after 700 days, no matter what.
• More immersive - to human eyes the Mars surface is just muddy browns and greys. The images you see are almost always image enhanced to make it easier for human geologists to recognize rocks on Mars. Via telepresence you can experience an image enhanced Mars even with blue skies like on Earth if you want. And everything is automatically streamed to you from the surface as that is how you see. So us folk back home can experience it just as you do. Which would be stereo vision, enhanced vision, binaural sound could be, easy to put a Newman head on your rover, haptic feedback also - you could feel the touch as the astronaut picks up a rock on Mars.
• Get more work done. The humans never need to suit up which takes ages. You don't have to travel from your base to the place of interest. Instead your robot just stays where it is, even many tens or hundreds of kilometers from the landing site, patiently waiting for the next instruction. Or it does things it can do automatically, drilling, maybe driving, some experiments, until you wake up and start controlling it again.
• You aren't limited to a single area but can control robots anywhere on the surface of Mars. The only comparison study I know of comparing exploration from orbit via telepresence with a surface mission said you'd do as much work from orbit as three similar teams of humans could do on the surface in the same time. I think with advances in robotic autonomy and telerobotics, that factor might be higher now.
• The orbit is a spectacular one - the HERRO one for instance comes in low over the Mars surface, skimming close to the polar ice caps, and then over the equatorial regions, then the other pole, and does that twice a day, every day, always approaching the sunny side, for as long as the mission lasts. It's a sun synchronous Molniya orbit. From orbit Mars would look much more hospitable than it does from the surface, continually varying landscape as you skim over it, sometimes clouds, dust storms etc, a bit like in the ISS.

For these reasons I think the first human expedition to Mars will involve studying it telerobotically from orbit.

SHOULD WE EXPLORE PHOBOS AND DEIMOS WITH HUMANS ON THE SURFACE? - THE PROBLEM OF JUNK AND WASTE

Then we might perhaps explore Phobos and Deimos next. They are good sites for long term bases to study Mars - assuming it is okay for planetary protection which would involve studying those moons first. There's a remote chance of life from Mars on Phobos in spore form brought there by impacts on Mars - I think enough to be beset to study first before sending humans there, but it's a low risk situation. Then also there's the effect of a human habitat on the moons. We can get ideas of how that works with humans on our Moon.

Do human habitats inevitably get surrounded by piles of junk, tons discarded every few months, and human waste, similarly to what you'd get if you kept all the progress missions that dump waste from the ISS? If so then you might give it some thought before sending humans even to places like Phobos and Deimos because of the impact of our habitats on the tiny moons we want to study. Phobos is only 27 × 22 × 18 km and the outer moon Deimos is even tinier, 15 × 12.2 × 11 km.

But if humans can live in quite a clean closed system way on the Moon then it makes sense to send them to Phobos / Deimos. If not, can explore those moons telerobotically just like the Mars surface.

SO GRADUAL DEVELOPMENT LIKE ANTARCTICA

So that's how I see it developing. Exploring. Settlements eventually. But no great rush.

When the first explorers arrived in Antarctica, they did set up temporary bases. Eventually they set up long term bases which are now manned every year 24/7. I expect a similar progression in space missions.

ECONOMIES OF SCALE FOR LARGE SPACE HABITATS

As for humans living in space permanently - I think it's possible but would need economics of scale. If you had a large habitat - say a crater covered biome or a Stanford torus, depends how easy it is to maintain. One reason it might be easy to maintain is that the ratio of surface area to volume goes down a lot as the habitat gets larger and you only have to maintain the outer skin. And if living in a cave on the Moon, even more so, especially if there are lava tube caves several kilometers in diameter as some think is possible on the Moon.

So, if it is really easy to maintain such large habitats, it might possibly be economic to maintain a colony of say 10,000 in orbit around Earth or on a lunar crater or in a lunar cave, where it would not be possible for a much smaller colony of a dozen or so. A really large colony might possibly get to the point where it is almost as easy to live there as on Earth.

But if so I see this as a fair way into the future most likely. And more likely to happen first in Earth orbit or close by. And using materials from Near Earth Asteroids and the Moon. They have all the resources you have on Mars in a form much easier of access. And far easier to support from Earth.

FOR MARS

I don't think this will happen for Mars at least for many decades for planetary protection reasons, because we have already found many potential life habitats there and there is a lot to explore and study even if we don't find life in the early expeditions there. And I simply can't see  an international COSPAR workshop of astrobiologists, who would have to approve a human mission to the surface, having enough information to be able to say with confidence that it is safe to send a human mission tot he surface in the near future. Especially given the need to account for the possibility of a human ship hard landing on Mars in one of the special regions there. Hard to imagine they would conclude that a hard landing like that would be safe for planetary protection with present day technology.

But we have exciting possibilities with the telerobotic exploration from orbit. And there is also much to find out about the Moon, our closest neighbour yet so little explored.

LONG TERM - IF WE FIND INTERESTINGLY DIFFERENT LIFE ON MARS

Long term, if we find life on Mars and it is interestingly different from Earth life, some astrobiologists such as Chris McKay suggest that we should leave Mars to the martians, even if they are only microbes. Carefully remove all traces of life from the planet - sterilize our landers with future tech or remove them - and then follow what happens from above. If we do transform the planet, the aim then would be to transform it to be more habitable to the Martians even if they are just microbes and lichen or some such, to try to roll back to the more hospitable early Mars.

There is plenty of habitat area in our solar system for humans. Enough material in the asteroid belt alone to build habitats for a thousand times the land area of the Earth. So we don't need Mars for habitats with any decent future space tech.

That could also be one reason why our future descendants a thousand years from now, far from welcoming our clumsy first attempts at terraforming Mars, might instead say to themselves "If only they hadn't tried to do that!". They are great ideas to explore on paper, can give us insights into Earth, exoplanets, past history of Mars etc. But I think we are very far from knowing enough about Mars or about how planets work to make an informed decision about whether to attempt it or what the effects are likely to be or whether our descendants would welcome our attempts.

See also To Explore Mars With Likes Of Occulus Rift&Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos

Asteroid Resources Could Create Space Habs For Trillions; Land Area Of A Thousand Earths

and my recent quora answers: Robert Walker's answer to Is Mars capable of supporting life?

and Robert Walker's answer to Mark Watney introduced life forms originating from Earth to the Martian environment.  Did he act ethically by colonizing Mars to survive?  Should he have done anything differently?

and many other answers here on quora in this topic area.

See also what astronaut Chris Hadfield has to say - comparing the Moon to Antarctica.

""I expect we'll treat the Moon, for the next 100 years, like we've treated Antartica for the last 100 years. With an initial outpost briefly and then longer stays and people staying through the entire winter, and eventually we will have permanent habitation on the Moon." Then, he says, humans will go as far as Mars. "

Chris Hadfield: ISS commander on colonising the Moon (Wired UK)

Should we explore or colonize space?

Explore, definitely, though that's speaking as a space enthusiast. I think one is bound to have enthusiasm for something that one is especially interested in. And I think myself that it is right that we decide this collectively, so everyone is involved in the decision, not just the enthusiasts.

Colonizing - less sure about that. Because first, there is nowhere anywhere near to Earth that is anything like as hospitable as Earth. So that's a bit like asking, should we colonize Antarctica? With current technology, and given that we have plenty of space on Earth that's far more habitable than anywhere we know of in space, I'd say no, there's no point in that. Any colony also would surely be bound to fizzle out once people realize how hard it is to live there.

Settlements, though, yes. Like in Antarctica, scientific bases. I don't think we need a "push" to colonize space. I think that as we explore it, then settlements will arise naturally. And if we do find a way to make large scale closed systems in space, we may colonize it also. But there's no great benefit in rushing into that.

Indeed a future with space colonies might be less stable than one without. Especially a big push that lead to colonies with millions of residents.

There's no way space colonies can be as easy to build as houses on Earth in the near future. So the only way this can happen is if Earth subsidizes the colonies. For instance they could be built by wealthy organizations that earn trillions from selling metal to Earth. That could lead to a future where it is possible to build space colonies quite easily. The companies that mine the asteroids would be earning plenty to pay for space colonies maybe for thousands, even millions of people.

But they would be doing that by taking resources from Earth in exchange for the metals. The amount of work into the project. Electronics manufactured on Earth. Spaceships manufactured on Earth.

Could be a future where the people on Earth are in poverty subsidizing the people in space. That I think would not be a good future.

What's more, space colonies would have high technology, with fast spaceships. And a spaceship crashing into a space colony at kilometers per second - the colonies would be very fragile.

So a future with space colonies has to be a future which is also very peaceful. If you had the likes of ISIS in a space colony, with all the technology they have - they could destroy other space colonies easily, with the only defense, to bury your colony deep underground. I think a future with battling space colonies living in deep underground shelters in the Moon etc would be a dystopia I'd not like to see happen.

It would not be possible to survive by escaping into the woods or caves or whatever when attacked either, because there is nowhere out there with breathable air. If your house is destroyed on the Earth, you may well have got away in time and can build again. If your habitat is destroyed in a space colony, there would probably be no survivors, not from a deliberate attack even just with a deliberately crashed spaceship hitting the habitat. Even if you were out doing an EVA at the time in a spacesuit or lunar rover or whatever, you'd just survive as long as your oxygen lasted and then you've had it.

So - we need to be peaceful first, at least in space, before we get to the point where we have millions of people in space. Otherwise space colonies will destroy each other through wars similar to those on Earth.

We do have peaceful co-operation in space at present with the Outer Space Treaty. But if we had millions in space, quickly, then that agreement might erode in various ways and I think we don't want to rush into such a future.

And unless our technology advances hugely, I also think that a future where there is a reasonable balance between Earth and space colonies would have many more people on Earth than in space - reflecting how much easier it is to live on Earth. Nothing we can do can make Earth less habitable than anywhere in space, and ideas of terraforming other planets are not practical over centuries and many doubts and questions about whether they would work over longer timescales.

I think it is possible, but would happen slowly, and that slow is good here. Done slowly, we can adapt and find out how to live in space safely, and then how to have even large numbers of people in space safely, living together in enough harmony not to destroy each other.

And as for going into space to escape Earth- we'd bring all our problems with us. If we go into space as warlike people, then it actually endangers the Earth because space colonies could easily gang up and drop asteroids on Earth if there was some conflict - or threaten to do so. E.g. in that dystopia future scenario of wealthy and selfish space colonists trying to get poor Earth inhabitants to do things they don't want to do.

And I think the same applies even more so to the galaxy as a whole. We should not even think about colonizing another star, until we can see right into the future, what the implications would be of a spreading wave of self replicating humans through the galaxy. Self replicating robots can be programmed, have controls to stop them going out of hand. Self replicating humans though, if spread through a galaxy beyond light speed distance - there is no way we can put that genie back in the bottle, and the results could be disastrous to the galaxy.

Exploring just fine. Small settlements, great. Robotic exploration, great. With self replicating technology we could have robotic explorers around every single star in the galaxy reporting back to us eventually for future generations. But - colonizing - I think we must step slowly there and not get too carried away.

See also Self Replicating Robots - Safer For Galaxy (and Earth) Than Human Colonists - Is This Why ETs Didn't Colonize Earth?

which originated as my answer to a couple of questions on quora:

• Robert Walker's answer to Are we technologically close to a feasible "von Neumann probe" for exploration?
• Robert Walker's answer to What should be humanity's approach to exploring space and discovering alien races once we're able to travel to other planets?

CAN THERE BE LIFE ON MARS?

Yes definitely. About ten years ago nearly everyone thought it was impossible. Apart from a few such as Gilbert Levin who has maintained all along that he thinks Viking discovered present day life already with his experiment - and some who thought there could be life in caves.

But that all changed with Phoenix in 2008. And with this sequence of photos:

It might not look like much. But it seems to show droplets of some material forming on the legs of Phoenix. then they grow, sometimes two droplets will merge, and then suddenly they disappear, presumably fallen off.

Phoenix had no way to examine those droplets. But the leading hypothesis is that they are droplets of salty briny water that formed on the salt thrown up on its legs when it landed. Probably taking in water from the atmosphere (deliquescing). It's hard to think of anything else that could be liquid in those conditions and the droplets certainly look like some form of liquid.

Pheonix also made isotope measurements of the Mars atmosphere which showed that the carbon in the carbon dioxide was replenished from volcanoes, and the oxygen must have exchanged chemically with something else, presumably water on the surface. That's rather a lot to discover just by measuring the ratio of isotopes for the two elements that make up the main constituent of the Mars atmosphere :).

They couldn't show whether the water is there all the time or only there sporadically. But over periods of millions of years, there is water present on Mars, liquid water, not ice. Could be thin films of liquid but is some there. Could be formed sometimes from volcanic activity or impacts, or could be it is always there.

So that got everyone thinking about it afresh. Because if there is liquid water on Mars, that suggests there might be a chance of life there.

The ionizing radiation is not a problem for present day life if there is liquid water there right now. It is for Europa - the flux of radiation for the Europa is so extreme that any surface life wouldn't have much of a chance not if it is right on the very surface.

But on Mars the level of ionizing radiation is the same as is experienced inside the ISS. The ISS has a problem with microbes like all space stations - they have to be cleared out from the atmosphere, indeed the atmosphere is kept very dry to discourage microbial growth. And that's just ordinary microbes. Radiation hardy microbes wouldn't even notice the levels of radiation on the Mars surface. Harmful to humans - humans are limited to a few years on the ISS because of rules of maximum lifetime radiation dose. But many microbes can even repair DNA damage within a few hours while alive and won't even notice Mars surface levels of radiation.

The reason ionizing radiation was thought to be a problem for life on Mars prior to 2008 was because they thought there was no possibility of water on or near the surface, and so, that any life there would have to have survived in a dormant state for millions of years. A very gentle dose of radiation each year can build up to be deadly for microbes over millions of years. After all that's a million times the dose you get in a single year. And what's more, it works exponentially. If ten million years of radiation is enough to reduce your population of microbes to a tenth, say, of the original, then twenty million years of radiation will reduce it to a hundredth, thirty million years to a thousandth, and so on.

Over billions of years the Mars surface radiation is so damaging that over three billion years it could reduce entire layers of organics meters thick to just water and gases and not much else. That is why we probably have to dig at least several meters, and ideally 10 meters to have a good chance of detecting early life on Mars, dig down deep enough to find deposits protected from surface conditions (or if very lucky, find newly exposed deposits that in the very recent past were ten meters underground)..

But over a period of a year or a century even, this radiation has negligible effects on many microbes. So, as soon as there is a possibility of water there, there is a possibility of life also.

The next discovery was this

Warm Season Flows on Slope in Newton Crater (animated)

This shows a cycle through spring, summer, then finally winter. As you see these streaks form in spring, extend down the slope, get wider, then fade away.

They are not thought to be damp patches as such. But the timing is not correlated in any way with the winds on Mars. It's too warm, far too warm for dry ice. The streaks start to form when the surface temperatures get above 0C on sun facing slopes.

All the models for this involve water in some form. Until recently there was no direct detection of water there. And we still don't have it. The problem is that the spacecraft that takes these photos can only take the photos in the early afternoon, the very worst time of day to spot water on Mars as that is when it is driest. Early morning would be a great time but because of its orbit, which takes it closer to Mars on the sun facing side at the time every day - it can't take those photos we'd like it to take to help solve the mystery. The streaks are also too narrow for the resolution of spectroscopic mapping of Mars.

So, thought we just wouldn't know for a long time. But then recently with careful work, they didn't find water, but they found the next best thing, strong evidence of hydrated salts on Mars in these RSLs.

Now, there are lots of streaks of many types on Mars. Most are caused by things such as land slip, wind erosion, even blocks of dry ice rolling down slopes, and dry ice also forms gullies The RSLs are extremely rare, only a few spots known on Mars where they occur. And other apparently identical slopes don't have them.

Still this very rare phenomenon on Mars does seem, best, even only theories so far, to be caused by water.

Another development, Nilton Renno who is in charge of the REMS surface weather station on Curiosity - a notable scientist with many honours - he and a team of other scientists did experiments involving putting salt next to ice in Mars surface conditions. He found that little millimeter scale droplets of water formed very readily in those circumstances.

"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. --- This is a small amount of liquid water. But for a bacteria, that would be a huge swimming pool ... 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, from Nilton Renno video (youtube)

Curiosity also found evidence of water beneath the sand dunes on Mars, which was quite a surprise. Just a cm or so below the surface and it was indirect evidence - increased humidity as it drove over them. If so, this water dries out in the daytime and is probably very salty - but on the other hand life does have a way sometimes of creating its own micro-environments. Nilton Renno thinks there is a chance it could be habitable though most think it would be too salty and too cold.

Then there's the work of teams of scientists at DLR, german space agency. They have experimented with various lichens and green algae from cold dry places on Earth. They found that some of them, like this lichen.

Pleopsidium chlorophanum collected at an altitude of 1492 m above sea level at "Black Ridge" in North Victoria Land, Antarctica. This lichen lives at altitudes of up to 2000 meters in Antarctica.

They found that this lichen could survive in Mars surface conditions without any water at all. The way it does it is that it is able to collect the moisture from the atmosphere. In the daytime the Mars atmosphere is very dry indeed, goes down to 0% relative humidity. At night though, it cools down so much, that the tiny amount of water in the atmosphere becomes 100% humidity. In the morning you get these frosts that form briefly before the air gets too hot and warm and it evaporates again. Huge temperature extremes from day to night drive this cycle. At night it can sometimes get so cold that dry ice - solid CO2 - would be stable even at the equator. In the daytime it gets well above OC.

Well as it transitions from night to day, then plants like this lichen could get just enough moisture from the 100% humidity of the air to keep going.And they have pigments that protect them from UV light. If they were on the rocks, at an angle to the sun so rather indirect sunlight, the lichens could photosynthesize and metabolize. The fungal component of the lichen, though it needs oxygen, is able to survive too, getting enough supplied by the algae component to keep going.

So - this is quite borderline stuff and lichens are slow growing and the experiments haven't been that long. But the take home message so far seems to be that even in equatorial regions of Mars there is at least a slight possibility that some forms of lichens and cyanobacteria could survive. Bear in mind that none of the microbes and plants tested on Earth evolved on Mars or are specifically adapted to it. Yet some seem to have that capability as indirect result of other adaptations.

Gilbet Levin also thinks that life could be able to take advantage of the early morning frosts in some way, though he doesn't go into a lot of detail as to how that could happen.

His ideas were revisited recently when, looking over the old Viking data, a scientist discovered evidence of circadian rhythms in the experiment. What's more rhythms not synchronized with the temperature cycles induced from outside but offset a little. This seems a quite strong indication of life to some people. If this is right, then Viking already discovered life in the equatorial regions in the 1970s.

This is possible as the viking labelled release experiment is very sensitive. We haven't sent any life detection experiment as sensitive as that to Mars since then. Sadly it was not conclusive because of the unusual and unexpected chemistry of the Mars soil. But it still remains the only experiment sent to Mars that had any chance of detecting present day life at typical concentrations found, for instance, in the Atacama desert core. Curiosity would not be able to find life at those tiny concentrations as any kind of recognizable organics signal because it is just too sparse.

Anyway so where there is water or humidity, there's a possiblity of life. But it depends what form the water takes. Skeptics say that yes, there is water on Mars (we can't deny that after Phoenix's atmospheric measurements and now Curiosity as well) - but that it is all too cold or too salty for life. It could be liquid right down to well below -20C, usually accepted limit for life to reproduce, if it is also sufficiently salty. So liquid water doesn't prove it's habitable.

So - nobody knows. But on the other hand there are many potential habitats on Mars. And even if many of them are inhospitable to life, too cold, too salty or whatever, it just needs one of those habitats to pan out to make Mars habitable to life.

Some skeptics will tell you that the Mars soil is too poisonous for life because of the perchlorates. But perchlorates are most poisonous at higher temperatures than on Mars. Poisonous to humans certainly. But some microbes actually use perchlorates as food. So it could even be an energy source for Martian life. The perchlorates don't seem to be an issue, and may indeed be an extra habitability factor for hardy mocrobes.

For more details about these potential habitats see my Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,...

For the new ideas about the Viking labelled release experiment and the possibility that they may have detected life back in the 1970s after all, see Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know

For the hydrated salts discoveries for the RSLs see Why Are Hydrated Salts A Slam Dunk Case For Flowing Water On Mars? And What Next?

For more about why cosmic radiation is not lethal for life on Mars on the surface if these liquid water habitats exist, see UV&Cosmic Radiation On Mars - Why They Aren't Lethal For The "Swimming Pools For Bacteria"

And I have many other articles up about related topics as well as quora answers.

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Robert Walker's posts on Science20

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