Now we know, it's the second of those "Three mysteries" which they solved - by detection of water in the RSLs.  However, they didn't directly detect flowing water. Instead, they found hydrated salts. So let's look at this a bit more closely - why are they so confident this is evidence for flowing water? And what next - is there any way to follow it up, and what about the other mysteries?

 So, first things first, why they are so confident this means flowing water? Well it's partly because the slopes only flow in spring through to autumn, when the temperatures are high enough, like this

They found the hydrated salts only towards the end of this cycle when the streaks were at their broadest. In the early spring when the streaks were short and narrow, they were unable to detect hydrated salts - either they weren't present or there in quantities too small for them to detect.

So something is hydrating the salts in these slopes seasonally at the same time as the streaks reach their broadest extent.


Now, our observations of Mars are rather limited. Although it is currently surely the most studied planet, outside of Earth, even more thoroughly scrutinized than our Moon, still it has nothing like the constellations of satellites continually observing Earth.

They did the measurements using CRISM - this is the slide Lujendra Ojha used in his presentation

CRISM is one of the instruments on Mars Reconnaissance Orbiter. It lets us do spectroscopic mapping of Mars.

But - it's not got the same resolution as HiRISE, the optical imager. 

HiRISE spots the streaks at widths of 5 meters widest, right down to 30 centimeters - the optical resolution limit. It benefits from Mars's thin atmosphere - because it is so thin, we can spot fine details on the surface rather more easily than on Earth, with comparatively small satellites also.

CRISM however has a best resolution of 18 meters per pixel. So there is no way you can use it to distinguish the composition of the streaks from the composition of the slopes around them. And they didn't do that.

It's got another limitation as well. It can only look at the streaks at around 3 pm Mars time. That is, unfortunately, the time of day when they are likely to be at their driest. We'd dearly like to be able to observe them in the mornings, which is probably when they are actually flowing. Perhaps we might even get the spectral signature of water in that case. But sadly it's not possible. MRO is in a slowly precessing sun synchronous orbit inclined at 93 degrees (orbital period 1 hr 52 minutes). Each time it crosses the Mars equator on the sunny side, South to North, the time is 3.00 pm, in the local solar time on the surface, and that is true all the year round. (See page 8 of Mars Reconnaissance Orbiter Communications).

What they did was to focus it on the streaks at times of the year when the streaks were very broad. At those times - it saw hydrated salts. At other times of the year it didn't see them.


They did not actually see flowing water. But the only models for these RSLs involve liquid water flowing down the streaks. It is hard to see how the streaks could grow in spring, and then fade away in autumn, otherwise.

There are many other streaks on Mars, and the most usual explanations involve dry ice or winds and dust storms. But these form at temperatures far too high for dry ice, temperatures from -23 to 0 Centigrade - dry ice evaporates at -78.5 C. So there is no way it can be anything to do with dry ice, and it is nothing to do with the storms.

So, we are in this situation where the only models that make much sense involve liquid water flowing. Flowing water would hydrate the salts. And we spot hydrated salts. It seems very convincing evidence, even though they haven't spotted flowing water directly.


They mentioned another idea here also. This is to do with how the streaks form. They found evidence of hydrated perchlorate salts. Now these don't need to be hydrated by flowing water. On Mars they can also take up water from the atmosphere, the process of deliquescence. Like this, another of the slides in the presentation:

This is one of the three main hypotheses for the formation of the streaks. However they were careful to say, they haven't proved that the perchlorates cause the streaks.

The perchlorates could just be picked up from the soil by the flowing water as it flows down the slope. From the shape of the streaks, and the way they spread, they seem to come from a source at the top of the slopes. That source could be liquid from deliquescing salts. But there are two other main hypotheses

  • The deliquescing salts. It's a bit of a challenge to get huge quantities of water forming in this way enough to feed the slopes, but it seems to be possible.
  • Reservoirs at the top of the streaks - this is hard to explain though as some of the streaks start from near the peaks of mountains, and the reservoirs would soon be exhausted. Still, it hasn't been ruled out, the reservoirs could be filled when the Mars atmosphere was thicker, so more precipitation - and it's orbit is continually changing in eccentricity and its axis changing and there is evidence of flowing water in gullies as recently as 500,000 years ago.
  • They come from deep aquifers. To be liquid below the surface, you would have to have geothermal heating. Some of the models here have flows of water feeding those aquifers in turn from very deep down, the kilometers deep hydrosphere where Mars is warm enough, and pressures are high enough for liquid water - there may be a liquid layer on Mars at great depth (this depends on how much water is left from Early Mars - if a lot is left there should be an extensive deep hydrosphere, which could in turn feed aquifers closer to the surface, if not, then perhaps there is no water down there or very little). 
So - they all have their good points, and all are also challenging. This discovery doesn't settle the question. The presence of the salts makes the deliquescing salts more plausible, but it hasn't disproved any of the other ideas, not yet. I'm not sure it even favours the deliquescing salts. It's also possible that different RSLs have different sources of water.

So - we know about the hydrated salts now. We have a correlation with the RSLs. Surely there is some connection, maybe one causes the other or both have a common cause? It would seem reasonable to suppose that the hydrated salts occur in the visibly dark patches, since they observed them only when the dark patches were very broad. But they can't actually prove that, since the resolution of CRISM isn't good enough to prove that the salts are in the streaks rather than the ground around them.

But put all together with everything else, it is a convincing case for flowing water.


And the amounts of water are quite large. Alfred McEwan, project leader for HiRISE, says they made a rough estimate, for the amount of water in the streaks only in the Valles Marineres region.

They assumed only 5% water, thickness of 10 mm which is about what you need for the material to flow at all - and they came up with 100,000 cubic meters of water flowing. That's a hundred thousand metric tons, so quite a lot of water. But only in the form of wet soil.

It sounds a lot. It is a lot of habitat if it were habitable. But it's not a lot of water by Earth standards.

Suppose you have a stream, flowing at one meter per second, a slowish walking speed. Then if it has a cross sectional area of half a square meter - that's a tiny stream, say average 25 cm deep and 2 meters wide - deep enough so you need to get your wellies on to cross it then you get 100,000 cubic meters of water flow past in 200,000 seconds. Or in just over two days. And that would be all the water in all the RSLs in the entire Valles Marineres region for a year flow through your little stream in two days.


So it's not a lot. But for a microbe of course, it is a huge amount of water. Leading to the question, could it be habitable?

Well, we don't know. The salts can be liquid right down to -40 °C or depending on the mixture, down to -60 °C. The water starts to flow when the surface temperature is between -23 °C to 0 °C. But that doesn't mean the water is at those temperatures. Could be the surface heats up, and the water is deeper down and flows at lower temperatures. Mars has a very steep temperature gradient in the top couple of cms of soil. Even when it is 20 °C at midday, then a couple of centimeters down, you'd hardly notice anything has happened.

You can read the paper in Nature if you follow up the link from the BBC story here, as Nature have an arrangement by which they make their papers available to anyone to read for major news stories like this, so long as you get to their site from one of the big name journalist news outlets.

In the conclusion they say

"These results strongly support the hypothesis that seasonal warm slopes are forming liquid water on contemporary Mars. The spectral identification of perchlorate in association with RSL, also suggests that the water is briny rather than pure. Terrestrially, in the hyper-arid core of the Atacama Desert, deliquescence of hygroscopic salts offers the only known refuge for active microbial communities and halophylic prokaryotes. If RSL are indeed formed as a result of deliquescence of perchlorate salts, they might provide transiently wet conditions near surface on Mars, although the water activity in perchlorate solutions may be too low to support known terrestrial life. The detection described here warrants further astrobiological characterization and exploration of these unique regions on Mars. This enhanced evidence for water flow also provides new clues as to the nature of the current Martian hydrological cycle"

So - these observations don't settle anything about the long standing question of whether the RSLs are habitable. If the water comes from deliquescing salts, it depends on the salt mixture and its temperature, and how salty it is. When they refer to "water activity" there, they mean, how salty it is. If very salty the water is not available even to microbes adapted to salty conditions - there is just too little water for them to be able to take it up even though the solution is liquid.

Here is another image from the presentation.

As you can see, the perchlorates extend the range of stability in both directions, very cold and warm. Though at the warm temperatures there it would evaporate quickly. You shouldn't deduce from this that a pool of perchlorate brine at 24 C would necessarily last for long on the surface before drying up, any more than a pool of liquid water just above 0 C - but it is stable there. It might though be very cold, or very salty, or quite likely, both.

Still it could also be habitable. We just don't know at this stage. It's not certain even that the water comes from deliquescence, it could be generated by some other process at the top of the slope.

And - this is just one of many possible habitats that have been suggested for life on Mars. It  is perhaps the habitat that is most easily observed from orbit.


Well, none of our rovers are able to study them. They said in the presentation that some dark streaks have been found on Mount Sharp, with a remote chance they could be RSLs. I'd missed that news - does anyone know more - or is it as yet unpublished research? - do say in the comments.

Anyway - if there were RSL on Mount Sharp - well sadly, Curiosity probably couldn't study them. It is not adequately sterilized to go close to a habitable region on Mars. This is something they knew right from the start.

They mention in the presentation that our rovers get sterilized by UV radiation. But UV radiation is just light. It is very easily blocked, just a mm or two of soil, or a thin sheet of paper or metal will block it, or in the shadow of the rover. So, there is no way, actually, that you could count on the high levels of UV on Mars to sterilize your rover. At most you could give it some credit, for reducing the microbial burden on your rover - but any spore that happens to be in a crack or crevice shielded from UV light will be completely unaffected by it. Or a spore that is somehow brushed off and falls into a shadow.

So - if there are RSLs on Mount Sharp, then Curiosity could observe them from a distance - for instance see if they extend seasonally. But sadly, it couldn't possibly be cleared for planetary protection to go up to them and observe them close up. 


The RSLs are also on very steep slopes. Typically the slope is 33 degrees, i.e. a 1 in 1.54 slope,  and a concave slope with the runoff a slightly gentler 27 degrees, or 1 in 1.9626 or about 1 in 2. (see section 5.2 of this paper, "HiRISE Observations of Recurring Slope Lineae (RSL) during Southern Summer on Mars").

That would be a steep slope if you were hill walking. You could walk up it, but

That is not an easy slope for a rover to traverse. Or indeed a vehicle. It is probably not far off this slope:

Humans on foot could climb it but surely not in spacesuits (major risk of falling over and damaging your suit which would of course risk killing you).

As an example of how hard humans can find it to climb steep slopes in spacesuits - Alan Shephard and Ed Mitchell tried to climb a 300 foot slope (91 meters) to the edge of a small crater called Cone Crater, to look for rocks that could have flown up there from the lunar bedrock. But they found it far harder than they expected. They had to carry the cart and on the steep slope, had to rest often. Eventually they ran out of time and had to turn back (or they risked running out of oxygen). Later the geologists figured out they missed the rim by about 100 feet.

This is a 1 in 2 slope, used for the annual Gloucester Cheese Rolling contest

But how could you go there anyway and study them in person, if there is a chance of habitability, without a greatly increased risk of introducing Earth life and finding what you brought with you?

Humans it seems could only arrive on the scene after you have proved that Mars is not habitable, if we ever do. Of course the great hope is that there are some habitats still on Mars where early Mars life may survive, even if just microbes or lichens. As I've said in many of my other posts here, surely we need to keep humans away from the Mars surface at least in this preliminary exploration phase where the search is to try to find out if Mars is habitable and has life on it. They seem to have no great benefit on the surface, given that they can't go anywhere near anywhere interesting. 

And also, the global dust storms can carry Earth life - a microbe spore perhaps imbedded in a grain of sand, protected from UV light by the iron oxides in the dust, anywhere on Mars - an observation Carl Sagan made decades ago.

And how can you protect Mars in the case of an accidental hard landing of a human occupied spacecraft, perhaps in the higher latitudes (like Mars Climate Orbiter). So, well I don't think therefore that humans on the surface will pass planetary protection any time soon - though in orbit, or the exciting missions to Phobos or Deimos, then they very probably could with a carefully designed mission, and could if so be of great benefit operating experiments and driving rovers on the surface through telepresence and telerobotics. See my To Explore Mars With Likes Of Occulus Rift&Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos

What about our rovers?


Well this we could do. Even very steep slopes, could be explored by a rover on a cable. Such as this balloon wheel rover.

AXEL Rover - Mars Cliffbots

But then we have the problem of planetary protection. The COSPAR workshops have explored the concept of a rover that does not need to be totally sterilized, just the part that needs to make contact with a habitable environment.

But in practice, how could you do that? Your rover on the slope, exploring the RSLs - well you don't yet know where the habitable environments are for one thing - in the RSLs? At the peak? Maybe to one side of them? And - how are you going to explore a hillside with numerous RSLs without driving across them? And - they are seasonal, and next year, maybe the place your rover drove over is then a place where an RSL forms next year? And what about spores carried by the dust storms, even just a few meters?

Surely our rovers would need to be thoroughly sterilized, wheels, and all. To such an extent there isn't any chance of a spore landing in the RSLs and contaminating them?

I'm not sure we have that capability. Would Viking be sterilized enough? It was amazingly well sterilized, but still, thought it has some microbes on it.

There are promising developments here, especially the supercritical CO2 sterilization being explored for planetary protection right now.

But I think we are at present in a similar situation to the Russians when they first drilled down nearly all the way to Lake Vostok in Antarctica. They would have dearly loved to drill all the way - but they didn't know the lake was there originally and simply weren't set up to explore it in a way that would prevent contamination by surface life. And the technology wasn't yet ready to do so anyway.

It is not just contamination of the sample directly - that we might while taking the sample, introduce Earth life to it. The problem also is contamination of the habitat the sample comes from - if it turns out that there are habitats on Mars that extremophile Earth life could inhabit.

I'm sure the COSPAR astrobiologists who will have to sign off any missions to Mars will take extreme care  here, because it would be their professional responsibility to make sure that Mars is not contaminated with Earth life.

It's natural to wish we could send a mission to these places right now. But - we also have to do it right. We absolutely must not bring Earth life to these habitats as we explore them.

For the time being, it is certainly safe to explore them from orbit. We can do a lot with the instruments already there, and in future more sensitive instruments could find out a lot more.

And - we need to work on future rovers that will be sufficiently sterile to be able to explore these regions of Mars. Sadly, our present day rovers can't yet be sterilized sufficiently well for this.


Of the three mysteries I mentioned in NASA Says Mars Mystery Solved - What Is It? - Three Mysteries About Recursive Slope Lineae, this partially answers the second one.

  1. Where does the water come from - how do the reservoirs get replenished
  2. Can we confirm that it is water, and if so how does it flow?
  3. What is special about these sites?

Yes, now we are pretty sure that it is water and flowing, though not sure how much, or how it flows and sadly we can't currently watch Mars with CRISM in the morning which is when the models suggests the flows happen.

But we still have no idea how the reservoirs get replenished - or if it comes form the atmosphere. And we don't really know what is special about these sites and why they have RSLs while others don't. Okay they spotted hydrated perchlorates - maybe that is a partial solution, but it seems that perchlorates are fairly common on Mars.


The RSLs are the best known of the possible habitats for life on Mars but that's mainly because they are amongst the easiest to study from orbit.

There are many others. For instance, the "flow like features" associated with the Mars geysers in Richardson crater.

These are very interesting, though they form at much lower surface temperatures than the RSLs. It's not such an obvious case as the RSLs perhaps.

Nevertheless, all the models for these features - for the ones in the Southern Hemisphere anyway - so far involve liquid water. And in one of the favoured models, one that seems plausibe, the liquid is fresh water at just above 0 °C! Remarkable, for Mars..

That might seem impossible on Mars, but it works, because in that model, the water forms in clear ice (like the blue ice of Antarctica) at a depth of perhaps 10 or 20 cms below the surface.

This happens in Antarctica, for instance in the Ross Ice Shelf, you get shallow pools with thin layers of ice above them that are believed to form due to this solid state greenhouse effect. The ice acts much like the carbon dioxide or water vapour in our atmosphere, trapping heat - but in a solid rather than a gaseous form.

And on Mars, it would trap the water as well. So, it wouldn't evaporate so could stay liquid like water in a pressure cooker, even above its normal boiling poin for watert on Mars. A very similar "solid state greenhouse effect" process, this time involving dry ice as the solid state greenhouse, is thought to cause trapped dry ice below the surface to turn to gas, which then builds up pressure and explodes, leading to the Martian Geysers.

Ice though would form a liquid, not a gas, so flow, not explode, and may be the explanation of these features. The upshot is that these features mark the site of the only hypothesized fresh liquid water on Mars currently. Though there is another explanation involving a form of liquid water that occurs in very thin films in between rocks and ice. In this alternative model, the water is still fresh water, but it is at well below 0 °C. In both models the fresh water doesn't stay fresh. As it escapes from the ice, it picks up salts, and then flows down the slope somewhat like the RSLs from that point onwards.

So, we undoubtedly have many surprising discoveries still to come with these Mars surface habitats - or non habitats - whatever they are. For more about this see my Are There Habitats For Life On Mars? - Salty Seeps, Clear Ice Greenhouses, Ice Fumaroles, Dune Bioreactors,...


Any questions about this? Any thoughts about the topics covered? Do say in the comments.

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And other articles on my Science blog here.


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Why Mars Surface Life May Leave No Traces In Its Atmosphere: Our rovers may need to go up close to see it (Amazon)

Rhythms from Martian Sands: Did the Viking Landers find life in 1976 - or what did they find? - astonishingly, we still don't know (Amazon)

Are there Habitats for Life on Mars (Amazon)