Optical delusion

If I had seen this picture without a caption, I’m not sure what I would have guessed it was. My first thought was of chocolate pudding with whipped cream and chocolate flakes mixed in. Or some very muddy water with pond scum swirling about. Maybe something microscopic? But on closer examination I can see that my initial impression of a wet surface is incorrect, because in places the surface has the look of blown sand.

It is blown sand, but the colors don’t match any landscape I’ve ever seen. Of course, the earth has such varied terrain, there are no doubt many amazing landscapes I’ve never seen, even in a photograph. But this one isn’t any place on earth. This photo was taken in space, near the north pole of Mars.

It doesn’t look at all like photos I’ve seen previously of the surface of Mars. They tend to be monochrome and – to my eye – fairly monotonous. What are those things sticking up in lines, as though they’re growing out of the surface? And what is the white that looks almost like snow or ice?

The white stuff is ice, apparently – but it’s not frozen H2O, it’s frozen CO(carbon dioxide). And those things sticking up? Well, they’re not actually sticking up, they’re lines of dark sand cascading down a slope made mostly of lighter colored sand.

My eyes don’t want to believe this. The lines are clearly sticking up out of the ground. I remind myself that our eyes are easily fooled, as many books of optical illusions can easily illustrate. Moreover, there is no reference point here, no familiar feature that lets us estimate the scale of the picture. Those apparent hills and valleys of sand could be the sort you make with your feet when you walk on loose sand at the beach, and the dark lines sticking up could be blades of something like grass. Or the hills could be mountains, and the apparent trees the size of California redwoods.

Of course, I know there aren’t trees or grass on Mars, and the astronomer who wrote this caption has studied detailed photos of Mars, and those really are drifts of dark sand. But I have trouble getting my eyes to see that.

I pick one batch of dark lines near the top, and try to imagine what it would look like if there were a sort of a cliff there, with the dark lines coming down the face of the cliff. The top of the cliff doesn’t look as it should – I guess the light sand is blowing over the edge too, so that no actual line  is formed to show the edge. I am beginning to see the dark streaks of sand now, instead of trees.

I pick another spot, near the bottom of the picture, and do the same thing. But when I look away, then look back, my eyes again turn the unfamiliar patterns into an earth-like landscape dotted with trees.

I suppose it’s just as well. I’ll never have to navigate Martian terrain, so my eyes don’t need to adapt to its peculiarities. They do just fine – most of the time – making sense of terrestrial landscapes.

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