Case for liquid water on Mars

MARS is in the news quite a bit at the moment. This week it has been closer to us than at any time since 2003, so it’s looking very bright in our night sky.

But that is not all. The big news is that recently scientists announced for the first time that they have strong observational evidence for liquid water on the planet.

This result does not in itself answer the question as to whether there is water on Mars, because we already knew that the answer to that question was “yes”, and we have known about it for quite some time.

Firstly, spacecraft photographs taken from orbit around the planet have shown us remarkable evidence that there was flowing water in the past. There are places where it may have been responsible for the formation of gullies, and we can clearly see erosion around obstacles. Orbiting spacecraft have also remotely detected water ice just underneath the surface.

Secondly, craft that have landed on the surface have returned evidence of past flowing water based on observations of the Martian surface material. Not only that, but perhaps most importantly, NASA’s Phoenix lander in 2008 touched down in a polar region of Mars and dug into the surface, finding water ice directly.

So we have known about water being there in the form of ice, and have seen past evidence that it used to flow, but this latest result is different: liquid water seems to be there now. It is important not to start imagining rivers, lakes or oceans, because unfortunately the water that is likely to have been found is about one and a half kilometres beneath the surface, and it would hardly be drinkable.

The detection was made with an instrument aboard the European Space Agency’s Mars Express orbiter, which arrived at the Red Planet in late 2003. This instrument is called MARSIS, which stands for Mars Advanced Radar for Subsurface and Ionosphere Sounding.

Essentially, what it has been doing is to record radar reflections from Mars, which can penetrate surface material to a depth of a few kilometres. The particular reflections from a region about 20km across in the region of the south pole of Mars, which were studied by MARSIS between May 2012 and December 2015, have been thoroughly analysed, and indicate the likely presence of a subsurface “lake” underneath the polar ice cap.

It may seem odd that this water, assuming that it really is there, is not frozen, because the temperature at that point would be around -68C, which is 68C below the normal freezing point of water. However, under certain conditions it could, even at that low temperature, be in liquid form. This is especially because of the presence of a large amount of salts, which lower its freezing point.

Seawater here on Earth is a good, but far less dramatic, example of this: it freezes at around -2C. More significantly, we know that there exist lakes of liquid water below the Antarctic ice that are at a temperature of about -13C. Again, the presence of salts lowers the freezing point.

This discovery of what is most probably a lake on Mars still needs more observational evidence to back it up, but scientists consider a salty lake to be by far the most likely explanation for the radar reflections. Of course, scientists rarely, if ever, “prove” things: they gain evidence to support a theory. As this evidence becomes stronger and stronger, they become more confident that they are correct.

What will be needed is a set of observations at much higher resolution, or sharpness. This will reveal more about the lake, including its shape, and about the possible presence of more of them. The resolution of the MARSIS instrument is only about five kilometres, so you could think of this as roughly the equivalent of a bright spot in a picture taken with your camera being only a few pixels across.

Although we need to learn more, I am very excited about this discovery. It is what we had been hoping to find, and it adds more to Mars being the most “earthlike” of the sun’s planets.