Take a look at this ‘open eye’ crater on Mars

This image of Aonia Terra, an upland region in the southern highlands of Mars shows a 30 kilimetre-wide which is nestled within a landscape of winding channels. Resembling veins running through a human eyeball, these channels are likely to have carried liquid water across the surface of the red planet around 3.5-4 billion years ago. The image was taken European Space Agency’s (ESA) Mars Express on April 25.
These channels appear to be partly filled with some kind of dark material and seem to actually be raised above the surrounding land in some places. ESA proposes many possible explanations for this. It could be possible that erosion-resant sediment settled at the bottom of the channels when water flowed through them. It could even be that the channels were filled in with lava later on in Mars’s hory.

The image from ESA’s Mars Express reveals many different colours on the surface around the crater. This suggests that this region of Mars is made up of a variety of materials. The surface is a warm red, melting into a darker brownish-grey closer to the crater on its south (which is on the left side of the image above). In that region, many buttes are visible These flat-topped towers of rock are created when land is gradually worn away water, wind or ice.
A dark dune field rests on a lighter surface inside the crater. Closer inspection revealed that the crater is apparently filled with more buttes and cone-shaped hills. These can be seen as evidence of many materials accumulating inside the crater in the past.Best of Express PremiumPremiumPremiumPremiumPremium
The surface is lighter and smoother on the north of the crater (which is on the right side of the image.) The main crater’s rims and channels appear less defined on this side. To the far right of the image, the surface becomes even smoother.