When you look at the Moon, you see a surface covered in craters. Yet the Earth, which is bigger and has more gravity — and therefore, you’d think, be hit more often than the Moon — hardly has any craters!
The difference, of course, is that the Earth has weather and tectonic activity. Craters erode, and over time go away*. But not all of them do. Some are so big they take hundreds of millions of years to erode, while others are in dry climates where erosion is limited… like, say, in Algeria. Where the Tin Bider crater lies!
This picture, from the Earth Observatory-1, shows the roughly 6 km (4 mile) wide crater, located in the high desert of northern Africa. It has a complicated terraced structure, indicating that the rock inside may have slumped after impact — a common feature in larger craters. It has undergone some erosion, too… not surprising, given its age of about 70 million years!
There’s an interesting thing about this crater. North is up in the picture, so the sunlight is coming from the south, from the bottom of the picture. Regular readers of this blog know that this induces the well-known crater/dome illusion (another example can be found here). Our brains expect light to come from above, so when it comes from below the shadows send mixed signals to our brains, and we interpret craters as domes and vice-versa.