I wrote an explainer summing up (almost) all we know about the recent NASA announcement of finding water on Mars for the Mumbai Mirror. An excerpt:
Some time in its past, a fifth of the Martian surface was thought to be covered in oceans, kilometres deep, before something happened for all that liquid to disappear. In time, what was also thought to be a thicker atmosphere dissipated, supposedly leaking away into space through a series of chemical reactions, leaving Mars to be the desolate land it is today. These are two of the more important mysteries that scientists want to understand for signs of whether something similar could happen on Earth as well as to make sense of our immediate planetary neighbourhood.
Because with large oceans of liquid water and an atmosphere rich in gases like oxygen, Mars could’ve harboured life — at least life that resembled the life that exists on Earth. Imagine how exciting that would be, to find out that at some point, there was someone next door. For this, many of the world’s space-faring nations have spent billions building, launching and operating orbiters, satellites that get into orbit around Mars and study the atmosphere and surface properties; landers that drop down on the surface; rovers, the little cars loaded with science instruments moving around, drilling into rocks, probing the dust.
On September 28, NASA announced that it had found evidence of liquid water flowing on Mars. This is a deceptively ambiguous statement for many reasons. Foremost is that NASA has been announcing similar news since the 2000s because there are many ways to infer the signs of liquid water, but the only thing that will tell us for sure if there’s liquid water on Mars is if we spot liquid water itself. This hasn’t happened yet.