I haven't written much about my favorite extraterrestrial planet Mars in quite a while -- primarily because there's not been much to say about it for quite a number of years. The problem -- at least in my estimation -- is the real reluctance of the planetary science community to go against established "knowledge" as promulgated by the Big Men of the Field. This has been true for decades, and one can assume it will be true for the foreseeable future.
But in advance of the release of the new Ridley Scott film, "The Martian," word comes that "flowing water" has been found and confirmed on the surface of Mars. Uhhh. Well....
Sure. Why not?
The streaks that have given rise to the current findings have been known and noted for nearly 20 years, and the "discovery" that they are presently active has been recognized for almost as long. The problem has been that their cause is mystifying. Because of a long-time consensus among researchers that the surface of Mars is too cold and the atmosphere is too thin for liquid water to exist at all, research tended to discount the notion that the streaks (known as linea) were the result of flowing water, or if they were that the water which made them had flowed recently -- which could be any time within the last few million years.
This tendency to discount what seems so obvious has been strong among Mars researchers for many a long year, and it continues today.
The idea that water can -- and does -- currently flow at the surface of Mars, at least intermittently, is still regarded with some suspicion. The idea that other substances may be liquid and flow at current Martian surface temperatures and pressures is generally dismissed entirely.
Myself, I was once a fairly strong advocate of current surface water on Mars as it seemed to me that the visual evidence* of what appeared to be geysers, springs, flows and so on was too strong to be ignored, yet it was ignored. Or the evidence was interpreted so as to preclude water or any other flowing liquid.
(* link is to a site which hosts thousands of images of the Martian surface acquired by the Mars Global Surveyor between 1997 and 2003, images that I and many others scoured intensely as they were posted for various interesting features, including streaks and other signs of current flows.)
After a time, I began to think about other volatiles on the surface that could mimic water -- look just like water but not be water and act almost identically under the current surface conditions.... what substance could do that?
It occurred to me that an aqueous solution of sulfuric acid -- which is stable in liquid form down to around -63C and .05mb atmospheric pressure -- would be an ideal candidate. It would appear to be liquid water, and it would act just like liquid water (apart from its solvent qualities), but it would not be liquid water. It would be battery acid. There would be no easy way to tell from a distance (such as an orbiting satellite or from Earth) just what the substance was. The only chemical difference between sulfuric acid and water is the presence of a sulfur atom and three additional oxygen atoms.
But at the time I was proposing sulfuric acid as a potential surface fluid on Mars, the consensus was that the likeliest fluid -- if there were current fluid flows -- would be water, probably water heavy with salt. Well, it would have to be because only brines can sustain a liquid phase under current Martian surface conditions. Even then, the period of liquid phase would tend to be brief unless the salt content was high.
When the Phoenix Lander exposed a patch of ice directly under the craft, the water scenario brightened considerably. Ice would indicate fresh water (although a brine could freeze at low enough temperatures). But there was a puzzlement: droplets of some fluid appeared -- and stayed -- on the landing struts for quite some time. It was at first assumed that these droplets were water liberated from the ice below by the heat of the retro rockets as the craft landed. But how could it be? The droplets were in shadow, and the temperature in shadow was well below the freezing point of even brines. These droplets appeared to remain liquid for... days, weeks, months. The atmospheric pressure was also low enough that when patches of ice were exposed during digging efforts they sublimated directly into gas once exposed to the air and sunlight. There was no observable liquid phase -- to water ice.
Some other fluid must have been seen on those landing struts, but what? It almost certainly wasn't water.
As analysis continued, salts were observed in the soil, one of the most interesting being perchlorate, used as a cleaning solvent on Earth. It was speculated that perchlorate in solution with water would produce a brine which could withstand surface conditions and remain liquid, and potentially, it could harbor microbial life.
This was actually a quite radical departure from previous "knowledge" about the surface of Mars. After the ambiguous life-detection results of the Viking Landers (c. 1976) a theory got going that suggested that the surface was sterile, and it was sterilized by a combination of some sort of hyper-oxide in the soil (substances which have never been observed) and the ultraviolet flux from the sun which is unimpeded by the Martian atmosphere. This theory was maintained for many years, and proposed that not only was the surface sterile, but it would be lethal to any living thing, including you and me. Simply breathing Mars dust might well be enough to kill you.
I never believed it, but it was the standard model of Mars for many years.
Just as a blood-red surface and brilliant orange-red sky were standard for so long. Though they were false renderings made for publicity, they remained the most frequently seen images of the Martian surface for decades despite repeated calls from the public for more accurate renderings.
The streaks that have caused all the buzz were first seen in the Mars Global Surveyor images in the late 1990s, though there were hints of them long before that. While much of the surface of Mars appears to be static -- except for dust storms and occasional small impacts -- there are areas that demonstrate considerable activity, including dust devils, some features that may be fluid geysers, active sand geysers, snow fall at and near the poles, possible glaciers, apparently explosive deflation of hillsides, land slides, and these streaks. They're found mostly in mid latitudes and equatorial areas mostly along the rims of craters and cliff faces. They tend to be a good deal darker than the surrounding terrain, but in some cases, they are quite a bit lighter. They have been observed to grow quite long and then they stop. Typically they fade away after a few months, and frequently they reappear in the same general areas once the surface temperatures warm up. Usually, they are seen when surface temperatures are between 250K and 300K -- in other words, somewhat cooler than to well above the melting point of pure water ice.
While it seemed possible to those with open minds that these streaks were the result of some kind of water flows, it was very difficult, indeed nearly impossible, to prove. Liquid water could not flow or persist at the Martian surface for any length of time because the temperature and pressure were in most places too low for too long. Thus even if there had been flowing water at the surface in the past, it would have been temporary unless the temperatures and atmospheric pressures were a good deal higher than they are today.
I'm one of those who tends to think that Mars is currently in a warm period, perhaps the warmest it's experienced in the planet's history. I doubt there was ever a warmer and wetter past, and I doubt there was ever an ocean as we would know one. There may have been a relatively heavy ice cover in some -- or many -- areas and there may have been intermittent lakes in many areas. But such water (or other fluid) which appeared at the surface for however long it may have done so came from below in almost every case.
Including those streaks. One of the problems with understanding them is that in most cases they begin at or very near the tops of crater rims and cliff faces. In many cases, their origin is a point source, really undetectable at the resolution of orbiting cameras, so it's impossible to tell just what the source of these streaks is. Somehow the fluid, whatever fluid it is, has to get up to the highest surface level before flowing downslope, sometimes for a kilometer or more.
I've often wondered why the streaks begin at the tops of cliff faces or crater rims rather than taking the easy route and forming at the bottoms where the fluid -- whatever it is -- might pool and be detected as a lake or pond even if ice covered for much of the time.
Actually, there are indications that that happens in various places at various times, but those who study the streaks don't seem to pay much attention to the indications of current ponds and lakes... ah, the division of labor...
Still, there is no explanation for the origin of the streaks at the tops of crater rims and cliff faces rather than their bottoms. The mechanism is thought to be hydrologic pressure from below which could be due to almost anything, Exactly why it manifests where and how it does remains a mystery.
The notion is that the streaks are underlain with a layer of permanent ice and that the flowing water which makes the streaks visible runs over the ice when temperatures at the surface are high enough. Interestingly, atmospheric pressure doesn't seem to play much of a role in the presence or absence of streaks, it's all about latitude and temperature.
There have been indications for many years that much of the surface of Mars is underlain with ice, not solely near the poles where it was expected but all over the planet. In some areas, this ice is very near the surface, essentially at the surface. In others it may be a few meters below. The indications come from hydrogen detection by orbiting spacecraft and the detection of widespread hydrated minerals. The belief is that only water (and ice) could cause these spectral signatures. And of course ice was found essentially at the surface at the Phoenix landing site.
So, there's no absence of ice on Mars, nor is there an indication that this ice is not water ice. I would imagine that it is water ice, and depending on where it is found, it is probably in a highly pure state. Meltwater from these ice deposits might well be potable. Whoo-hoo! Mission to Mars, here we come!
Well, that is unless it is filled with nasty microbes... Is that possible? Sure. There's no sign -- yet -- that such is the case, but it's worth noting that no orbiter or lander sent to Mars since the Vikings in 1976 have carried biology-detection instruments, not even biology inferring instruments. It's worth asking why not, but answers probably won't be forthcoming. Not only have there been no direct biology detection instruments, there have been no instruments which could analyse the soil for the hypothesized hyper-oxides which supposedly sterilized the soil at the Viking landing sites of any organic matter and thus of any biology. There weren't even instruments to measure the ultraviolet flux that was supposed to be a contributing factor in the complete sterilization of the Martian surface.
It seemed as if the hypotheses advanced to explain the failure of the Vikings to detect biology for certain in 1976 were being accepted on faith without even an attempt at verification. To me, as an interested observer, it was bizarre.
It occurred to me, though, that quite possibly the hypotheses of a sterile Martian surface were never accepted (13 pg pdf), and that was why there was no apparent effort at verification. Why verify something you know isn't true? On the other hand, it might have helped to falsify those sterility hypotheses, no?
What has been learned is a sideways falsification, I guess. There are organic materials in the soil, for example, materials which, for whatever reason, the Viking instruments did not detect. There are no signs of hyper-oxides sterilizing the soil. Instead, there are widespread peroxides and perchlorates and salts and significant regions of near-surface ice, all of which indicate the presence of water both historically and currently. The notion that the surface of Mars is dry and dead and sterile is slowly -- painfully slowly -- yielding to a more complete understanding...
The surface of Mars may somewhat resemble a terrestrial desert, but it's not much like the surface of the Earth at all. It mimics... but it isn't the same.
The surface has many signs of flowing and pooling liquid -- both historic and recent/current -- but just what that liquid is or was isn't certain. It is assumed to be water, probably quite briny, but again, it's not certain. It may have been water in some places, whereas something else -- such as sulfuric acid -- may have been the surface volatile in others. There are indications of sulfates and sulfur in various places on Mars, which in combination with hydrogen and oxygen -- both of which are in fair abundance -- can produce various strengths of sulfuric acid. As an aqueous solution, it would behave almost like water -- except that it would remain liquid at very low temperatures and pressures, though eventually it would sublimate away like water.
So. The breathless announcement last week that "flowing water" had been "discovered" -- or rather confirmed -- on Mars has to come with a caveat or two. Those streaks have long been known and have long been speculated to be caused -- somehow -- by water flowing downhill. But how or why the water involved would be pumped to the tops of cliffs and crater rims in order to be released to flow down the cliffs and into craters is still a mystery. It doesn't really make hydrological sense, but it seems to be what happens.
Hydrated minerals/perchlorate salts have been spectrally found in association with the streaks but only when they are dark, not when they are faded or light. I have a problem with that in that hydrated minerals and perchlorates appear to be commonplace on the surface, found nearly everywhere. If the fading or lightening of these streaks is due to evaporation/sublimation, then the presence of salts should be stronger not weaker when the streaks have faded or lightened. But not what's found.
There has apparently been no spectral signature of water found in association with these streaks, and that's interesting all by itself. Water is simply assumed. Not necessarily wise given the way Mars deceives and mimics.
I suspect that we will not actually know about the surface of Mars until and unless people go there to measure and test and explore, and somehow survive the encounter. That adventure has been long-delayed, but what with all these new findings of "flowing water" -- and the blockbuster movie opening tomorrow -- you never know.