The water could then flow out onto the surface when it reaches a break, like a crater wall.

Lateral explosions excavated a notch in the southeast crater wall.

The crater wall and interior have been heavily eroded by many subsequent impacts, leaving the features rounded and irregular.

Inside the crater walls, they saw a 200m-thick stack of layered rock that could be identified as being formed by running water.

It has come via a technique known as 'nearinfrared reflectance spectroscopy', used to analyse the wavelength of sunlight scattered off crater walls and other features of the moonscape.