A penetrating, double-ringed crater on Mars

By Phil Plait | September 27, 2012 6:55 am

Mars is weird. Right? I mean, it’s a whole other planet. So you expect it to be weird.

But then I see pictures like this one from the Mars Reconnaissance Orbiter’s HiRISE camera, and I am reminded just how weird it is:

[Click to chicxulubenate.]

Most craters you see are pretty simple: something impacts the ground at high speed, BOOM!, and you get a crater like a dish tossed into soft sand. But this one has two rings, one inside the other. That can happen with huge impacts producing craters hundreds of kilometers across, but this one is small, only 230 meters from side to side – an American football stadium would just fit inside this crater.

The most likely explanation for the double ring is that the Martian landscape here is layered. There’s rock and sand on the surface, but underneath that is a layer of ice. The big rim is from the displaced rock, and the inner, smaller ring is from the impactor plowing through the ice. Each layer has a different strength – rock is harder than ice – so it’s as if two craters were formed, one inside the other. Radar observations of Mars from orbit have indicated there’s ice under the surface in this region, so that fits.

Similar double-ringed craters have been seen on Mars – though the structure and history is by no means well understood! – and some have been found on the Earth’s Moon as well. Those tend to be big, as I mentioned, though they don’t have to be.

By the way, the image above is color enhanced to show details. The blue may be from carbon dioxide frost, which can be seen in similar color-enhanced HiRISE images. The ripples in the center are sand dunes, sculpted into parallel waves by the ceaseless Martian wind.

Craters this small on Earth are extremely unlikely to form; the impactor would be maybe 20 meters or so across, and objects that size tend to break up when they ram through our thick atmosphere at high speed. Mars has much thinner air, so rocks that size can hit intact. Studying craters on Mars is a chance to see what these hypervelocity impacts are like under very different conditions, which helps us understand them. The physics of extremely high-speed collisions is hard to study experimentally – accelerating large objects to that kind of speed is both difficult and more than slightly dangerous – so it’s nice to have a lab like Mars where we can observe these effects.

Tip o’ the lens cap to HiRISE on Twitter. Image credit: NASA/JPL/University of Arizona.


Related Posts:

- Mars craters are sublime
- Desktop Project Part 1: A weird Moon crater
- The artwork of the Martian landscape
- A fresh Martian impact

CATEGORIZED UNDER: Cool stuff, Pretty pictures

Comments (21)

  1. Eleri

    No matter what I do, I can not convince my brain that the crater goes *down*. It insists on looking like a hill with a weird peak. Sorta like cupcake frosting :)

  2. PsyberDave

    Me, too. Stupid brain. I tell it and I tell it, and it keeps telling me the crater is a hill.

    In other news, I’m watching Phil on How the Universe Works talking about gravity and galactic collisions and megaflares.

  3. dessy

    Yeah – its a funny illusion!

    “Everywhere I move, its eye keeps following me…”

  4. Cindy

    I even turned my laptop upside down to try to make it appear as a crater and not as a hill.

    The problem is that the other little dots look like craters and not rocks casting shadows.

  5. oldebabe

    Me, too. And I thought it was just me that couldn’t see that these were craters…

  6. Corey

    Eleri, your intuition is right. If you notice the incident angle of the light, it will concur with what you’re intuitively perceiving as coming from right of the frame. Traveling from outside the crater toward the inside, you dip down into the crater, then rise back up a hill-like object. then down again into a second dip, and finally back up to the cap.

    I don’t think the author of the article either is conveying this, perhaps because they didn’t catch onto that, or because they just didn’t explain it in a very good way. Alas, when you’re dealing with images of objects you’re not used to viewing it can be very easy to get confused.

  7. Chip

    I must have a Martian brain – I see it going in, and rising at the ridges. But I am familiar with the illusion of craters looking like bumps, usually if lit from the side.

    I also see what might be a fragmentary third ring between the small interior ring and and the large outer ring. Curving around the upper right and the lower left.

  8. Keith Hearn

    I was having a terrible time seeing it as a hole instead of a hill. What finally worked for me was to close my eyes and mentally picture a crater lighted from the same angle (I used Ubehebe Crater, because I’ve seen it in person so it’s easier to recall, but just imagining a generic crater should work). After visualizing it for 30 seconds or so, I opened my eyes and voila! it was a crater. We see more hills than craters, so the mind tends to go with what it expects. You just have to get your mind ready to expect to see a crater.

  9. Bandsaw

    I concur with the other commentors on the challenge of viewing crater images. I’ve found the only way I can get them to show to my brain as craters is to download and rotate the image so that the light is coming in from the top of the frame. Phil, for those of us with limited visual cortices, could you post these more often with the light coming from the top? Or are you trying to help train us to think more three dimensionally?

  10. Corey

    The “crater illusion” is more prominent when you’re viewing from a counter-intuitive angle. It is emphasized when the image does not provide proper context. We are quite used to the bottom of a photo being closer to the camera than the top of it, but if we are viewing the bottom (as we perceive it) of a sphere-like object in space, the top of the image is closer. This is why flipping the image over works.

    Light does play a role in many illusions, but even in cases where these photographs are lit from a more common angle for us, they throw us off.

  11. Matt B.

    I had to put my hand in the way of the image in Paint before I got it rotated 90 degrees to the right. Looking at it while I rotated it 90 degrees at a time didn’t work, even when the colors were inverted.

  12. I love it how my eyes still kepp seeing a mount and not a crater.
    And I like how “we” (as a species) discover all this.
    “Simply” by launching something. The wonders of the universe are right around some corner (depending were ypu do your slingshot).
    Just as I like the show the Red Arrows gave here yesterday. Physics right in front of you!
    *sigh*

  13. Clive DuPort

    I eventually saw this as a crater after shrinking the picture to about an inch wide. When that small I couldn’t get it to look like a hill at all. Now I can’t see the full size one as a hill. Isn’t perception a weird thing?

  14. puppygod

    Funny. When I look at it, the crater indeed looks like a hill. But when I tell my brain “it’s crater” it kinda pops in and then I see it as a crater, as it is. At least as long as I’m looking at it. As soon as I look somewhere else and look back at it – it’s the hill again! I guess that’s the default setting of my visual cortex.

  15. @Clyve DuPort

    Like when you were a kid, in bed… shadows and forms in the curtain. So your mind went *eekz* and you saw a face. Then after you turned the light on, it was gone, light out again and it was not there anymore. I used to try if I could get the face back, sometimes you could and sometimes it was gone forever.
    Eyes and brains are a weird mix sometimes. And fun to play with.

  16. Stan9fromouterspace

    The size of a stadium? I say put a dome over that sucker and throw the first interplanetary monster-truck-Mars-mudbog rally! The ticket sales alone would fund several years of deep space research. This Sunday-Sunday-SUNNDAAYYY!

  17. pumpkinpie

    It can go from crater to hill or hill to crater as I watch it, no blinking or looking away. It’s not necessarily when I “tell” my brain to change it. It just happens on its own. But usually it only remains a crater for a second or two.

  18. That´s cool.
    Yesterday I was watching “Pavlov” a dutch science series about us and our sometimes (weird) brains. Why some people are better in some things than others and what they experience when doing their thing. Yesterday was about people who can taste sounds (a musician), or see colors when they hear/see a word (a DJ) etc. It´s about how our brains work. Facinating stuff. They shove ´m MRI´s and do ECG´s etc and you see were their brains light up when presented with a word/sound/image, not only in the word/etc part of their brain but also in their taste sensation part or whatever they sense.
    So I´m not surprised you can see both at will and others not.

  19. reidh

    must be seen from more oblique angle ( for me ) to be appreciated.

  20. Andrei

    I understand the layered terrain (well, how would it be called on Mars…) but how can you rule out a double impact? I mean a smaller crater created in the middle of larger one?

  21. HPR

    I tried rotating the picture every which way, and even covering the brightly lit area, and could not for the life of me get it to look like a crater. I finally Photoshopped the hell out of it, darkening the highlights, lightening the shadows, and adjusting the midtones. That made it looks mostly ike a crater, but the left side of the blue ring still looks very very tall, and the bright spot on the right still seems to go up instead of down.

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