The discovery of water on Mars has been claimed so often that I’d forgive anyone for being skeptical about the latest announcement. Frozen water, ice, has been proven on Mars in many places, there are lots of ancient canyons hundreds of kilometres long that must have been carved by rivers, and much smaller gullies that are evidently much younger (including a few where changes have been seen).
In 2008 the Phoenix lander actually saw water on Mars. When it scraped away at the dirt, it found water-ice a few centimetres down, but more excitingly droplets that could hardly be anything other than water were seen to form on the lander’s legs.
It was suggested that the water had condensed around wind-blown grains of calcium perchlorate, a salt mineral whose properties enable it to scavenge water from the air and then dissolve it. Moreover, whereas pure water would freeze at the local temperature at that site (between -10°C and -80°C), water containing enough dissolved salts could stay liquid.
We’ve still to find anywhere that you could throw a stone and make a splash today, but it’s now pretty certain that there are places where liquid water trickles downhill. It happens on sunward-facing slopes in spring or summer, making dark streaks called ‘Recurring Slope Lineae’, or RSLs. They’re a metre or so wide and about a hundred metres long, and they lengthen and then fade away seasonally. They are smaller than gullies, from which they also differ in lacking signs of erosion even at their upper ends. The favoured explanation of RSLs has been that trickles of liquid water appear when the temperature gets warm enough, with the damp ground looking darker than the dry ground to either side. The water supply expires in summer, and the dark streaks fade away.
The crucial new evidence comes from the CRISM spectrometer on Mars Reconnaissance Orbiter, which shows hydrated salts such as magnesium and sodium perchlorate on the RSL surfaces, but not on the ground to either side. So it looks like we’ve had very salty water flowing downhill. The salts would slow down the evaporation of the water despite Mars’s very low atmospheric pressure, AND depress the freezing point to help keep it liquid even at sub-zero temperatures.
What we still don’t know is whether the water oozes out from storage below the ground or has been sucked out of the atmosphere by the salts themselves. That happens in Earth’s Atacama desert, and provides a habitat for microbes in an otherwise barren environment. If microbes can make a living like that on Earth, there seems no reason why they should not be capable of doing the same on Mars. So the association of salts and (ephemerally) liquid water on Mars provides a viable habitat. The big unknown is whether Mars has any microbes at all. I think it almost certainly does, not necessarily descended from life that began on Mars independently of life on Earth, but because we now realise that viable microbes can be carried between the two planets inside lumps of rock blasted off by large meteorite impacts.
Featured image credit: ‘Horsehead Nebula’, by WikiImages. Public domain via Pixabay.