A lunar crater is graben the spotlight

By Phil Plait | September 11, 2012 7:00 am

I am endlessly fascinated by the Moon. There may be an inherent bias there because it is, after all, the closest astronomical object in the sky. Still, it has an amazingly varied surface with lots of really odd features.

One of my favorite types of things to look at are overlapping features. It can produce a very complicated terrain, difficult to understand. Or can also create a lovely tableau that cleanly separates the two features, like this very pretty shot from Lunar Reconnaissance Orbiter (LRO) showing a fresh crater near a graben:

[Click to enlunenate.]

A graben is a crack or fracture. They form on the Moon when the crust is stretched, splitting the surface. They look like long, relatively straight and narrow valleys with steep sides. You can only see a part of it on the right side of the image above; the Sun is shining from the right and illuminating the left-hand side of the graben. The picture below is zoomed out and should help you see the situation.

The crater is clearly younger than the graben feature. The radial streaks around the crater are called rays, and are formed when plumes of material ejected from the impact fall back down to the ground. They’re common around young craters; solar wind, later impacts, and even thermal compression and expansion of rocks over the Moon’s day-night cycle eventually erode them away.

You can see the rays extended over and into the graben, so the crater must be younger. It’s hard to say just how much younger, but even relative ages can help geologists understand the lunar surface better. And detailed images like this – you can see individual blocks of rock inside the crater itself – are crucial for study. Someday, I think, human geologists will be investigating places like this in person, and mapping missions like LRO will make that possible.

Image credit: NASA/GSFC/Arizona State University

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CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
MORE ABOUT: crater, graben, LRO, Moon, rays

Comments (16)

  1. Jake

    To this day I can’t read anything about the moon without feeling immensely irritated that we haven’t been back there in 40 years.

  2. thetentman

    Clever headline, Phil. Thanks.

  3. Crater overlapping a graben? Oh, rilley?


  4. Just saw “The Wall” for the first time in many years. Easy to spot just after 1st quarter.
    152 degrees East isn’t on the visible side, is it. Should have bought an x-ray telescope.

  5. Uite

    I’m curious, what’s the scale of this picture?

  6. Arthur Maruyama

    @ Uite:

    The LRO site says that the close-up image is 2.5 km square, so–converting to miles–that would make the crater roughly a quarter-mile in diameter.

  7. Randy A.

    A better definition for a graben would be a valley formed between two normal faults. The valley is formed when the land between the faults is dropped down. If the land is pushed up by faulting, it’s called a horst.

    Here on Earth, the basin and range province in North America (Nevada, plus parts of the surrounding states) has many grabens, separated by horsts. Perhaps the most famous graben is Death Valley.

    Normal faults are a type of dip-slip fault caused by tension. As Phil mentioned (and the linked article discusses), normal faulting on the moon implies that something stretched the moon’s surface — or at least that part of it.

  8. Cynipoo
  9. Anyone else have trouble seeing the graben as a depression? My brain keeps trying to “assign” illumination from top left, making it look like a ridge, and the crater like a dome.
    Stupid brain.

  10. shunt1

    Amazing image!

    Are we sure that it is a graben instead of a rill? Obviously the linear depression can be traced beyond the larger image provided. It just reminded me so much of hadley rill from Apollo 15.

    It really does not matter, since the crater if much younger and the rays tell an important story. Anyone notice how the impact was almost vertical?

    @Bipedal Tetrapod:

    “Anyone else have trouble seeing the graben as a depression?”

    Funny, but I have never had that problem.

    Perhaps because I grew up with images of the moon since the 1960’s and simply look for light direction first?

    I have always found it rather interesting how so many people are almost 3D blind when presented an Anaglyph. Could this be related?

  11. Nigel Depledge

    The BA said:

    . . . it is, after all, the closest astronomical object in the sky.

    [Emphasis mine]

    As opposed, presumably, to those much closer astronomical objects under the sea . . . ?


  12. Nigel Depledge

    The BA said:

    They’re common around young craters; solar wind, later impacts, and even thermal compression and expansion of rocks over the Moon’s day-night cycle eventually erode them away.

    Don’t forget the constant micrometeorite bombardment.

    Or do you include that within “later impacts”?

  13. A.J.Rimmer

    @Bipedal Tetrapod:
    ‚ÄúAnyone else have trouble seeing the graben as a depression?‚ÄĚ

    Right click on image -> rotate image 180 degrees.

    You’re welcome.

  14. Another Phil

    @9. Bipedal Tetrapod . Don’t worry, my brain is doubly stupid as I see it as a crater and a ridge, regardless of the differing illumination. I can see it as a graben but not as a first impression.
    I suspect you are used to looking at images of fossils or similar as scientific convention is to illuminate from top left.

  15. @A.J.Rimmer
    Doesn’t even take 180 degrees. As soon as I tilt it so the graben passes vertical – maybe 25 degrees, then it clicks. And I can see it after that when returned to normal. Just an interesting neurological phenomenon.
    I’m used to looking at the moon under all kinds of illumination, but somehow looking through an eyepiece doesn’t give me the same “processing error” as looking at a photograph.

  16. James Kottenstette

    I am not convinced that the evidence cited proves that the crater is younger than the graben. Look at the “small craters” along the sunlight terminus near the bottom of the graben: I think you will recognize that these craters are really boulders! boulders that reflect sunlight where craters would be totally dark. Whether this is true or not, I can find no reason to believe that the walls of the graben did not slowly form later as the (graben) surface showing the rays simply sagged when the graben subsided. You can see a swam of what must be smaller boulders clinging to the bright side of the graben (between 1 and 2 o’clock.) I would argue the the boulders simply collected at the bottom as the graben deepened; hence the crater is older.


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