Mars craters are sublime

By Phil Plait | May 24, 2012 7:00 am

Someday, Mars will stop surprising me.

Today is not that day.

The image below was taken by the HiRISE camera on the Mars Reconnaissance Orbiter, which has been taking devastatingly high-res pictures of the Red Planet for many years. While passing over the edge of the Tharsis Shield — a huge uplifted region of Mars home to its four gigantic volcanoes –it saw this bizarre fieldof craters:

[Click to hephaestenate.]

First, you may think these are mounds and not craters, but that’s an illusion. Our brain uses illumination to gauge up and down in pictures like these, and assumes the sunlight is coming from above. However, these really are craters, but the illumination is coming from below — north is roughly toward the top of the picture and the crater field is at a northern latitude of about 50°. Flip the picture over if it helps (I’ll be honest, even doing that makes it hard for me to see these as other than mounds; confounded brain!). You can see more examples of this illusion here, here, and here.

But that’s not the weirdest thing about these craters. What’s really odd is they aren’t circular! Impacts are generally round unless 1) the impact is at a very shallow angle, b) the terrain suddenly goes from one kind of material to another, creating a discontinuity, or γ) something happened after the crater was formed to distort it.

A shallow-angle impact is almost certainly not the case here, since there are so many craters spread out over the region that an incoming object would’ve had to break up into a gazillion pieces, all of which came in at that angle. Not impossible, but it seems unlikely.

The changing terrain idea doesn’t work, since again the craters are spread out over the area. You might see one crater with a sudden break in its rim or change in shape, but dozens? Spread out in all directions? Nope.

That leaves after effects, and in this case we have two more clues. One is that all the craters have their long axes aligned; in other words they all point in the same direction. The other is that every crater has roughly the same features: a small crater in the lower left hand side, and the shallow floor reaching up and to the right. Another clue? The illumination angle again: the Sun is shining so that it illuminates the upper right side of the crater wall.

Why is that last part important? Because sunlight on Mars turns ice into water vapor.

Here’s the idea. At this latitude on Mars, water ice has been seen just below the surface. In fact, one way it’s seen is when small meteorite impacts carve out craters, exposing it. These craters are all small (the biggest one in this shot is about 75 meters across), and judging from their appearances are all about the same age. That implies they’re secondary craters, caused when ejected debris from an impact rains back down to the surface.

These secondary impacts plopped down all over this terrain, blasting through the top layer of the Martian surface and exposing the ice underneath. The Sun then started to melt the ice (or more properly sublimate it; with the very thin atmosphere there ice goes directly from a solid to a gas), and as it did so the terrain around it collapsed a bit. The Sun shines from the south to the north, so the northern rims of the craters receive the most heat, and expand in that direction.

Then, some time later, what you have left are small circular craters – the original impacts – off to one side of an elongated crater that extends in a northward direction, and that extended part has a flat bottom due to the disappearing layer of ice that used to be there.

Tadaaa! Now, this is just an idea and may not turn out to be correct (note: this isn’t my idea either; I’m paraphrasing and expanding on the description on the HiRISE site). But it does neatly explain everything we see in this picture.

I’m fascinated by geology, whether it’s here on Earth or on other worlds. Like astronomy, events leave lasting evidence, and sometimes when you look at a landscape — or a galaxy or a nebula or a star — you see the clues from many events that happened after one another, and it’s not always easy to disentangle them.

It’s a puzzle, but you know what? Puzzles are fun. Getting to the answer is where the joy is, and then having the answer is where satisfaction lies. And once you have that, why look over there! Another puzzle! Yay!

Science is a Universe full of never-ending puzzles, and I don’t know a single scientist who would have it any other way.

Image credit: NASA/JPL/University of Arizona


Comments (37)

  1. I’m fascinated by geology

    We know that! :) If you weren’t an astronomer, I figure you’d be a vulcanologist judging by pictures you seem to like.

    Science is a Universe full of never-ending puzzles, and I don’t know a single scientist who would have it any other way.

    Which always reminds me of a great Carl Sagan quote that highlights a differece between scientists searching for the most correct answer, versus many folks who would rather just be told any answer, no matter what truth lies in it.

    Who is more humble? The scientist who looks at the universe with an open mind and accepts whatever it has to teach us, or somebody who says everything in this book must be considered the literal truth and never mind the fallibility of all the human beings involved?

  2. Powerdroid

    Uh, professor?
    Doesn’t the angle of illumination change constantly? Why would one direction be preferred? I must be missing something.

  3. George Rockingham

    I was able to trick my mind into seeing the actual craters by pretending each was a Sarlacc pit, with the dark spots full of writhing tentacles.

  4. Jay

    Powerdroid: “the crater field is at a northern latitude of about 50°”. So from that location the Sun always appears more-or-less to the south.

  5. Powerdroid

    Thanks, Jay. That’s what I was missing. I’m still having a hard time picturing the geometry, though. Even at the exact north pole, for example, where every direction is South, the sun would still appear to circle, because the planet is rotating, right?

  6. Erik N

    @Powerdroid: Shaded relief maps (even of norther regions) almost always are drawn with the light coming from the upper left because people get confused otherwise. It may be that people’s expectation for shading is because of that map convention or that the convention may be necessary because there is some other reason that people expect light to originate in the upper left.

  7. kevbp

    @Powerdroid: Mars’ angle of inclination is around 25 degrees, which is close to Earth’s (23). So at 50 degrees latitude, the sun makes an arc across the sky similar to what it does here (I’m up in Canada).

    The solar flux (I think that’s the right word) is strongest when the sun is most directly overhead (around ‘noon’) – but at 50 degrees latitude, that still places the sun to the south. In fact, that is when it is most directly south, hence the preferred “direction of sublimation”. (which would also make a great name for my next band)

  8. Sam H

    Anyone notice what looks a hell of a lot like sinkholes in the pits of the largest craters? I can count six of them, at least (possibly as many as 8), though it could be another illumination illusion – I wouldn’t say that though, as they all have their shadows lined up the same way as the crater rims.

    Perhaps the impacts are over an old lava tube that formed afterward – this is Tharsis, after all, and the effect has been seen before in this region. My knowledge of geology is miniscule though compared to astronomy – thots, anyone? :)

  9. Powerdroid

    Thanks, Erik, but that’s not what I’m hung up on. According to the explanation given, for an observer at that spot on Mars, the sun rises and sets in pretty much the same place on the horizon. I’m ready to believe it, but I’m just having trouble explaining it to myself.

    And thanks, kevbp, for the relevant details. That sounds like what I was missing.

    Anyway, cool picture. I’ll shut up now.

  10. Chris A.

    We humans, having evolved in an environment where light usually emanates from above us, are wired to assume that light comes from the top of an image. (That’s why lighting your face from below with a flashlight makes it look spooky and weird.)

    For the life of me, I can’t see these blasted things as pits, no matter how I turn the image. “So” I thought, “if they look like mounds in all orientations, then they should look like pits if I invert the color.” When I did that, they STILL LOOK LIKE MOUNDS IN ALL ORIENTATIONS!


  11. Chuck Newman

    (New commenter. Total amateur. That said …) I can’t keep from thinking you have something wrong here. I think it’s more likely your eyes have not deceived you and the direction of the sun is somehow incorrect. There are 2 clues. One is the shadows. In each of the 3 illusion examples, you can see a soft edge side and a hard edge side to the shadow. The soft edge side is toward the light source. You can see those same shadow features in a couple of these mounds (formerly called craters).

    The second clue is the oblong shape. The mounds look like drips of honey on a tilted surface. I think the stuff (sand?, liquid?) came up from underneath through a hole just underneath those bellybuttons. When the flow stopped, sometimes it stopped with an innie and sometimes it stopped with an outie. The bigger mounds got outies and some of the smaller ones got innies. It’s possible the holes were cause by the stippling of an impact out of frame. If this is on some kind of volcanic cinder cone, maybe these are inactive vents.

  12. Chip

    The elongated craters, as noted originally smaller impacts but later expanded, appear to be now behaving like ‘sink holes’ on Earth, and basically collapsing.

  13. Don’t Mars will stop surprising us any time soon. Earth still has plenty of surprises and we live here.

  14. Keith Bowden

    The brain is a marvelous organ, but funny. Usually when Phil posts a picture like this and says “these are craters, not mounds”, I actually see them as craters and have to shift my perception to see the mound illusion. On this one, though, I do see them as mounds and have to work really hard to see them as craters.

    @11 Chuck Newman
    I’m guessing that, while this image is taken from directly above, we also have shots of the area from angles which reveal that they are indeed craters. And there are probably other methods they have of determining the topography which I’m sure someone else can expand on. :)

  15. I’m an alien. While these craters looked like bumps initially, i automatically pushed my visual system to see them as craters too. Not just craters vs bumps, but both at the same time. Not impossible. Perhaps i spent too much time with video games and watching rotating tesseracts. After getting a handle on a 4-D cube, most things are easier.

  16. Menyambal

    I got it look right by focussing on the big crater toward lower left. I got the original small crater to work in my mind, then looked out at the sublimation part and could see it as a pit.

    Phil, you could have rotated your post of the photo to make the light seem right for the rest of us. That might have helped. Maybe.

    The orientation as it is shows the most melting is occurring on the sunny side during the afternoon. Which is as it should be.

    I think it is hard to see properly at any rotation, because what we are seeing is literally unearthly. There isn’t anything like those craters, except maybe sinkholes, here on Earth, and few sinkholes look like that.

  17. Dave

    My mind instantly thought, “nipples”.

  18. Funny, I had to spin the image around a few times before my brain finally switched to seeing them as depressions.

    Weird craters. The sharp light at the edge is like a crust and the dark spot in the middle is a deeper hole.


  19. Okay… here is a version where I’ve managed to rotate it–no standard 90 degree rotations work. Mind you, the brain is overloading because there are so many things going on. Not only are there craters here, buth there are craters with sinkholes inside, craters with upthrusts inside, and craters with craters inside.

  20. CR

    Just a thought for everyone, one which might not help: try sculpting/modelling some craters in sand and/or clay and take some pictures of them. See if you see the mound effect in your own photos, or if KNOWING that they are craters helps your brain interpret them as such. Our brains are easily tricked by what we think we see, but maybe knowing what you’re looking at might help sort things out.
    An added benefit to making models: you can change the lighting angle from directly overhead to extremely shallow, and study the various effects the light & shadows make. Doing this a few times from different angles might also help your brain be able to sort things out in future viewings. (I suppose one could do the same for model mounds, too. Try it out!)

  21. heng

    i was enjoying the craters before reading the text… after reading “mound” it took me minutes to see the craters again… and they’re constantly flipping….
    TMI indeed ^^

  22. Daniel J. Andrews

    This time I too saw them as craters and can’t see them as mounds. Usually it is the other way around.

    At first glance the craters remind me of ones I’ve seen formed on the edge of lakes in the mud/sand in the spring where the ice under the mud melts away.

  23. rob

    shouldn’t that be areology and not geology?

  24. Another Phil

    The overlap of the 2 in the middle is interesting as I’d expect a sharp edge where one pit erodes into another. That adds to the illusion of mounds.

    I was only able to see them as pits by zooming right in to the left of the overlappers and the one 1/3 up left of centre, they’ve got several small mounds in them and once I got them as lower-left illuminated the crater edges fell into place.

  25. Crudely Wrott

    If there is a 3-D fly-by video of this area I’d bet that the true spatial properties would become obvious instantly. Perhaps one of the talented people here can find or make one.

    Until then I’ll just have to ignore my errant visual interpretation and accept my intellectual certainty that what looks like up is actually down.

  26. When I look at window frames on a wall, the sun always shines from above. Rotate right 90 degrees and you might find it easier to view as a crater.

  27. James

    I can only see them as craters. With as Phil says, the sunlight eroding the northern walls. But then, I’ve always been weird…

  28. James Church

    Has anyone else noticed what look to be holes. 1 on the left and a few on the right center frame. Any explanation as to what these may be?

  29. lunchstealer

    Those elliptical craters look a lot like the elliptical Carolina Bays, which are odd depressions found along the coastal plain of North and South Carolina.

  30. Buggar

    I just came here to say I see craters and cannot see them as mounds.

  31. Gunnar

    @Larian LeQuella #1

    I like that Carl Sagan quote a lot too! It aggravates me no end how religious fundamentalists who are so unshakeable in their conviction that the scriptures are inerrant, despite any amount of evidence to the contrary, regard themselves as humble, while regarding scientists who submit their convictions to the demands of the best available evidence and are willing to modify them when evidence warrants it as arrogant.

    BTW: I love your website! :)

  32. Gus

    I inverted the image in Photoshop and I can see the craters easily – see

  33. tommy dublin

    Great picture. Tho’ no matter what I do with it it still looks like wax drips that have been stabbed with a nail, apart from the 5 very obvious big nipple illusions. It’s a mind bender no doubt.

  34. Messier Tidy Upper

    @14. Keith Bowden :

    @11 Chuck Newman – I’m guessing that, while this image is taken from directly above, we also have shots of the area from angles which reveal that they are indeed craters. And there are probably other methods they have of determining the topography which I’m sure someone else can expand on.

    Well there’s laser altimetry as used aboard the Mars Global Surveyor (MGS) mission – see :

    Laser altimeter measurements have produced an unprecedented global topographic map of Mars. The instrument revealed a multitude of highly eroded or buried craters too subtle for previous observation, and mapped canyons within the polar ice caps.

    Then there’s also familar RADAR* methods such as those used by the Mars Reconnaissance Orbiter (MRO) as animated on youtube here :

    Curiously enough, I can’t find any reference to altitude determining / topography mapping instruments aboard the Mars Odyssey spaceprobe :

    that took this image here – see link in the OP. Nothing listed in its primary instruments anyhow – maybe they’re under secondary or something. I would presume the Mars Odyssey has something like that though unless it does just rely on previous maps.

    * No, I’m not “shouting”, RADAR is an acronymn durnnit! 😉

  35. Matt B.

    The only way I could get them to look like craters for more than half a second was to rotate 90 degrees to the left and invert the colors. I even tried pretending I’m holding a flashlight (a.k.a. torch) in my left hand. Part of the problem is that this picture was actually taken at an angle, so rotating right gives an incomprehensible perspective for land.

    @17 Dave: So this would be a close-up of Chris Griffin?

    @23 Rob: The “geo-” in “geology” means “earth”, not “Earth”.

  36. Tribeca Mike

    After looking at this image a hundred times, I have to admit the optical illusion is perfect. Now, please, can I see it the “right” way now?

  37. EdR

    Marvellous illusion but the light appears to be coming from the right-hand side of the photo. I’m sure when I “hepaestenate” the picture even further I can see three boulders on the surface with shadows cast straight to the left. Am I seeing things correctly?


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