Header image: channels on Mars, up to several hundred metres long. Image credit NASA/JPL/University of Arizona.
As mentioned earlier, today marked the date of NASA’s press conference announcing a ‘Mars Mystery Solved’. Speculations abounded as to what this ‘mystery’ was, with most commentators suggesting it would relate to the long-debated question of whether or not liquid water could be found on the surface of Mars. The existence of liquid water could have implications for not only future manned missions to Mars, since water sources would provide a much-needed foothold to establish a permanent base, but also the possibility of life having existed, or even existing still, on the planet’s surface.
And this afternoon, those rumours were confirmed. Accompanying the announcement was a Nature Geoscience paper, which details the data supporting NASA’s conclusion. The evidence centres around formations known as Recurring Slope Linae (RSL) observed on the Martian surface. These channel-like features appear during warm seasons, resembling narrow dark streaks that grow steadily downwards as the warm period continues. (Note: ‘warm season’ by Martian standards actually means a temperature of around 250-300 Kelvin, which can translate to as low as a cosy -23 °C.) Even before this discovery was made, it looked like these channels were formed by some kind of liquid flow, and water’s hydrogen-bonded fingerprints were all over it.
Image showing RSL (dark streaks indicated by arrows) on Mars. Source, credit University of Arizona/Georgia Institute of Technology/United States Geological Survey/Institute of Geological Sciences (Poland).
Experimental confirmation comes courtesy of CRISM, the spectrometer on board the Mars Reconnaissance Orbiter. This instrument picks up the visible light and infra-red radiation reflected from the surface, and analyses the individual frequencies present to identify the chemical composition of the surface material. This allows CRISM to search for evidence of specific minerals – in particular, hydrated salts, which, as their name suggests, are salts which have incorporated water into their structure. If these salts existed, the water inside them would have to have come from somewhere, pointing strongly towards flowing water on Mars. And hydrated salts it has found.
‘But water freezes at 0 °C,’ I hear you cry. ‘How can it possibly stick around in liquid form on Mars if it gets so damn cold?’ The key is the presence of salts dissolved in the water. When water freezes, the water molecules, now neatly arranged in a solid lattice, push out any impurities – in this case, the salts – originally dissolved in the liquid. This means the impurity molecules now have less room to arrange themselves, meaning a reduced variety of arrangements they can take up, and hence a reduction in their entropy.
Structure of ice – water molecules joined together by hydrogen bonds (green). Image from here, credit Oregon State University.
Entropy, essentially a measure of the disorder of a system, is the backbone of the Second Law of Thermodynamics: the universe does its damnedest to increase entropy whenever it can. When pure water freezes, the jiggling water molecules of the liquid state take up the neat regular arrangement of the solid state; their entropy has decreased. When impure water freezes, the combined entropy decrease from both the water and the salt has the universe feeling very uncomfortable indeed. To compensate, and allow the freezing to actually go ahead, the temperature of the system must be much lower. (When something freezes, the surroundings get a temperature rise, increasing their entropy: the lower the ambient temperature was originally, the greater this entropy boost.) In other words, liquid water is able to exist at much lower temperatures than it would be if it was pure.
So, salty water is likely responsible for these channel formations. One mystery that remains is the source of the water itself. The regions examined in the paper were around the planet’s equator, and most of the known water on Mars is stored as ice, near the poles or deep underground. It’s been suggested that the salts themselves could begin the whole process, absorbing water from the atmosphere to form a briny film. However, a steady flow of water would require a steady supply of moisture from the air , and it is currently unknown whether the Martian atmosphere is humid enough to allow this.
The Curiosity rover, which took measurements showing that it was possible for salty water to exist as a liquid on the Martian surface. Image from here, credit NASA/JPL.
Others have proposed that underground sources of water could be breaking through to the surface, although this is complicated by the fact that many of the streaks appear to originate from higher ground. Maybe a mixture of the two factors is responsible. Perhaps it’s something else altogether. Either way, water on Mars: probably a yes. Break out the party hats.
Header image source: here