Why do these martian craters look odd?

In this high res picture of the surface of Mars, http://esamultimedia.esa.int/images/marsexpress/269-2803066co-ApollinarisP_H.jpg I presume all the round markings on the surface are meteorite impacts. However, instead of looking like hollow craters, to my eye they look like perhaps circular canals (though I know the eye is easily fooled). I suppose I was wondering what the profile of these craters is. The floor of the crater looks flat - I wouldnt necessarily expect an impact to give me a flat surface

WAG: Could it be something to do with the thin atmosphere, distorting the path of the meteorite or something?

actually the large crater in the very bottom right looks how I would expect an impact to make - the smaller craters on the other hand look too shallow and flat

Remember that unlike the Moon,mars has an atmosphere. But it has no water.

The result is you’re not looking at impact craters; you’re looking at the residue of an impact crater after 10,000 or 100,000 or 1,000,000 years of being scoured by the wind and having dust & sand settle in it’s basin.

What the heck is he talking about was my first impression. These all look like ordinary craters. Then all of a sudden my perception switched and I saw them all as circular canals. Presuming that’s what you were thinking of it’s all about optical illusions and what angle your mind decides the light is coming from.

When my mind isn’t messing with me, the craters look perfectly fine to me. But then I’m not expecting them not to be flat.

As long as I stare and as hard as I try, I cannot see a circular canal in the photo. In fact, I have no idea what a circular canal should look like.

Shouldn’t it be nothing more than a lake—in this case a long dried out lake?

And if it is, then it’s not a canal. Or is it?

I am buffaloed.

One of the problems I have is of course I am not used to looking at such photos with the shadowing and making sense of it. It took me a while to work out what was bugging me. Looking at the top left of the photo there is a very large crater. the light is coming about 1 oclock. Entering the crater from 1 oclock is a big shadow from the rim, then the middle, and then light reflected from the other wall of the crater. However then there comes another dark shadow, which presumeably is another shadow from a raised rim.

Keeping going straight then we come to a pea sized crater. The shadow appears to go right round which I presume is comes from a profile like this (this probably wont format properly with different fonts)


It was the shadow all around that made me wonder, but it makes sense now

Just my WAG but what I believe you are seeing is, in fact, a raised disk of terrain surrounding the impact crater. When the meteor hit, it likely compressed the ground around the impact as well as creating the crater. The packed area around the crater would be more resistant to the scouring effects of the wind and the result would be the area around it errodes away leaving a disk.

Or it could just be an optical illusion.

The picture is “upside down” with respect to lighting conditions.

Save the image and use a viewer than can rotate it 180 degrees. It will look a whole lot different.

This is standard for many astronomy pictures.

Well, I can’t get the image to display, but try turning it upside-down, either by saving it and flipping it in some graphics program (almost anything, including MS Paint and Mac Preview, should be able to do this), or by turning your head upside-down to look at it. This will often make the true relief jump out at you.