The internet is buzzing with the announcement from NASA that they are going to announce the solution of a longstanding Mars mystery on Monday. at 11.30 am EST. That's 4.30 pm BST. Then, as many of the news stories have pointed out, the list of speakers includes Lujendra Ojha, a Nepal born researcher from Katmandu who discovered the "Warm Seasonal Flows" as an undergraduate back in 2011. It would be surprising to include him unless the solved mystery is something to do with these streaks. now often called Recursive Slope Lineae. So, what are they, and what are the main mysteries about them?

Note, this was written before the announcement, when everyone was trying to guess what the announcement would be about. As it turns out, it was to do with the second of these three mysteries about the Recurrent Slope Lineae (RSLs): "Can we confirm that it is water - does water really flow on Mars seasonally, and if so, how does it stay liquid as it flows?".  

Their results confirm that it is water, great news - everyone suspected this - but it's great to have that finally confirmed. As to how it stays liquid as it flows, that's still to be figured out, also the other two mysteries I describe here remain a mystery. 

Read on if you want to know more about the RSLs and the three main mysteries about them. Or, to find out more about the announcement itself, and where we go next, see my post: Why Are Hydrated Salts A Slam Dunk Case For Flowing Water On Mars? And What Next?

First, here is Lujendra Ojha, back in 2011 when he discovered the warm seasonal flows.

And this is what he discovered - (later sets of images of them)

Warm-season flows on a slope in Newton Crater spread over the slopes as the year progresses
They may not look like much. Mars is now known to have many dark streaks that form and disappear. Most are created by the wind, avalanches, or they are the result of dry ice subliming.

The dark slope streaks

Not to be confused with the RSLs - these "dark slope streaks"are probably caused by avalanches of dry sand. One of many changes on Mars spotted from orbit, which to a non expert eye may resemble the RSLs, but they are thought to be completely different phenomena.

Amongst all these changes you get on Mars, these ones are different. They only form on sun facing slopes when local temperatures rise above 0 C. This rules out dry ice. They are seasonal, and not correlated in any way with winds or dust storms, which rules out wind effects. So, the only possibility left seems to be liquid water in some form.

They have also been found even in the very dry equatorial regions, on the slopes of the Valles Marineres. Each asterisk on this map shows one of the sites

Sixteen sites in total marked, over entire surface of Mars. See Water seems to flow freely on Mars

They are exceedingly rare on Mars. The total surface area of Mars is about the same as the land area of the Earth. Yet there are only sixteen sites known for these RSLs, over the entire planet. There aren't many phenomena on Earth as rare as that.

So, that's one mystery - why do some sites have them, when other apparently similar slopes, don't?

Now, naively you might think that the dark streaks are water, making the sand dark. On Mars water boils as soon as it melts, unless it is very salty, because the air is so thin (opposite effect of a pressure cooker which works by raising the boiling point of water).

Indeed, ice, over most of Mars, fresh water sublimes directly into the atmosphere just as dry ice does on Earth. Salt water melts at a much lower temperature, but still, in equatorial regions any salt water near the surface is close to its boiling point and would soon evaporate, without some protection from the warm temperatures on the surface, which at midday can go as high as 20 C in direct sunlight.

So, the water probably flows beneath the surface, slightly, probably just a cm or so below at most (or it would be so cold it freezes solid), and if so, the dark streaks must be some indirect effect of the motion of the water just below the surface.

The main mysteries about them are 

  1. Where does the water come from - all the models assume water, but differ in ideas of where it comes from.
  2. Can we confirm that it is water - does water really flow on Mars seasonally, and if so, how does it stay liquid as it flows?
  3. What is special about these sites - why do they have the RSLs when other similar sites do not?


Here, the main thing to explain is how the streaks replenish themselves. 

They happen every year in the same places, year after year - so without a way to refill the reservoirs that supply them with water, soon all the water would end up at the bottom of the slope or evaporate into the atmosphere.

There is a lot published on this, but to summarize, the main theories are, 

  • The water comes from the air, through special "salts" - such as calcium perchlorate, that take up water from the atmosphere ("deliquesce")
  • The water comes from deep underground via geothermal heating and transport over long periods of time - maybe even from kilometers below the surface originally - very slowly
  • The water comes from ice deposits at the top of the slopes that somehow got there at an earlier stage of Mars history, when its atmosphere was thicker and perhaps had rain or snow to fill them.el as the RSLs - but in that case it is hard to understand how they can keep going, eventually all the water would end at the bottom with no way to circulate it around.

The idea it comes from deep underground is plausible because the sites are so rare, and many similar sites don't have these RSLs. Maybe the ones with the RSLs are the ones with geothermal heating.

But the other ones are also plausible too, these sites might perhaps have specially suitable concentrations or mixtures of salts for the deliquescing salts, and they could also have special conditions that help preserve the ice deposits for the idea of ancient precipitation replenishing them hundreds of thousands of years ago..

The idea it comes from the air is plausible as some mixtures of salts will take up water from the atmosphere in Mars conditions rather easily despite the near vacuum conditions. In that case, there is no need for any ice to be present there at all, just the salts.

Note, chemists, when they talk about salts, mean any ionic compound from the reaction of an acid with a base. So not just sodium chloride, table salt (which is common on Earth) but also sulfates, chlorates, perchlorates etc. On Mars nearly all the "salts" are in these highly oxidised states. Also the salts can involve calcium, potassium, magnesium etc as well as sodium.

He is presenting a paper on the 28th on the theory that the water comes from the air, but that by itself, if it is just an alternative theory, doesn't seem like a major announcement as there have been several ideas published on different theories, including the idea of deliquescing salts - that idea by itself is not new. 

The abstract reads:

"Recurring Slope Lineae (RSL) are seasonal flows or seeps on warm Martian slopes. Observed gradual or incremental growth, fading, and yearly recurrence can be explained by seasonal seeps of water, which is probably salty. The origin of the water is not understood, but several observations indicate a key role for atmospheric processes. If sufficient deliquescent salts are present at these locations, the water could be entirely of atmospheric origin."

More details here

However - if his research is really conclusive that would be a major step forward - so far all the models for them have involved liquid water but none of the models are conclusive


Curiosity has already confirmed that this deliquescence happens in the sand dunes in Gale crater. As Curiosity drives over the sand dunes, it has detected lower readings of water vapour, then higher levels again as it drives off them. It has also found calcium perchlorate. 

Curiosity on Mars - it's readings of levels of water vapour as it drives over different terrain ,and its discovery of perchlorates, together give clear evidence of the presence of water, at night only, about 10 cms below the surface of the sand in the sand dunes.

These two readings together are thought to pretty much confirm that there is liquid water there just below the surface even at Gale crater, which was quite a surprise. Models suggest it is about ten centimeters below the surface, possibly a little deeper, in thin layers,, and forms at night.

You can read the original paper if you follow through the link from the BBC article here - Nature has an arrangement by which major papers are made available for free if you get to them from journalist reports from some of the main news sites.

Also (if these readings are correct), there is enough water there at night for habitability except that it is too cold for Earth life to metabolize and reproduce. 

In the daytime it is warm enough for life to metabolize and even reproduce, but it is too dry. 

So at Gale crater it seems we have both those conditions for life separately but not together. Nilton Renno however has suggested - not as a paper just in informal remarks - that perhaps even at Gale Crater, that life could survive using biofilms which create local microclimates within the biofilm. 

This is another good summary - and includes Nilton Renno's remarks about the biofilms..

It might also be that life on Mars finds ways around these problems that we haven't predicted.

One idea, I don't know how likely it is on Mars, but to show how life sometimes is able to survive in places that at first sight seem impossible - in the dry high Atacama desert, some microbes survive in salt pillars of salt, which has been suggested as an analogue for Mars. They were found in conditiona where the humidity was too low for life. As it turns out, tiny pockets of brine are found in the salt at relative humidities down to 30%, far lower than the deliquescence point of salt of 75%. This happens through spontaneous capiliary condensation in these micropores and the microbes use it to survive. This wasn't predicted by anyone in advance. They just found life there, and then figured out how it was surviving. See Novel water source for endolithic life in the hyperarid core of the Atacama Desert. Could life on Mars also find extra humidity in micropores in salt deposits?

Certainly, if not at Gale crater, many researchers think present day life on Mars is a possibility for habitable regions in the polar regions of Mars. For this reason the entire upper latitudes of Mars are designated "Special regions" for planetary protection as you see in the map of the RSLs above - because there is at least a remote possibility of life there.

Gilbert Levin of course, for many years, has been saying that Viking already detected life in the Equatorial regions - and recently some other scientists have been coming round to his views, though it is very much a minority view at present, see my Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know .

These recursive slope lineae occur in the equatorial regions, as well as polar regions, another big surprise, like Curiosity's discovery. Depending on the mixture of salts, and how and when they form, perhaps they could be habitable? 


The other thing is that, though the conditions don't seem compatible with anything else apart from liquid water - they appear on sun facing slopes, not correlated with winds, only when temperatures are above 0C - still, nobody has yet actually detected water there.

The observation is difficult - the water is probably underground, a few mms or cms, may only flow in the morning and the observations are in the middle of the day, and the spectroscopic observations have resolutions of order of meters, not the 30 cms of optical measurements.

And, it's not yet clear how water is able to flow there. Best guess is maybe it is very salty and flows a little below the surface.

Could they have found a way to detect water there from orbit? And find out how it is able to flow?


We should find out soon what it is. And - since it is new research, we can't predict in advance what the solution of the mystery is. But if you ask me to guess on what the mystery is, well - guess would be some progress on one or other of the current mysteries about this not very well understood phenomenon on Mars, such as:

  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?

That's just because they say in the announcement:"major mystery solved" and those would seem to be the main mysteries to do with RSLs.

We will just have to wait and see what it is they have found. Look forward to it :).

Incidentally, if the RSLs are habitable, that would be an example of a type of habitat that could have almost no effect on the Mars atmosphere, even if populated with a high density of microbial life, since the habitats themselves occupy so little area and are so rare on Mars. For more on this, see Why Mars Surface Life May Leave No Traces In Its Atmosphere: Our Rovers May Need To Go Up Close To See It.

It's an interesting week for habitability of Mars. We also get the results in for the first year of operation of the Indian MOM spacecraft. See also my India's Mars Orbiter Spacecraft - Results Of First Year Of Methane Search To Be Announced On Thursday


And other articles on my Science blog here.


I have made some of my blog posts into booklets for kindle, so you can download them and read off-line, formatted as books, with table of contents etc.

Check out my kindle bookshelf.


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)