Lunar boreal halo

By Phil Plait | August 27, 2009 11:56 am

OMFSM. I love this picture. Love love love.


LRO images Erlanger crater at the Moon’s north pole

[Click to embiggen.]

Is it a ring of fire on Mercury? A look into the gateway of hell? A promo for Halo 3?

Nope. It’s far cooler: it’s the rim of the lunar crater Erlanger poking into the sunlight.

Erlanger sits almost at the Moon’s pole, lying at a latitude of 87° north. At that latitude, the Sun is forever near the horizon, only getting a little above or below the limb depending on the Moon’s orbit around the Earth. In this case most of the crater is not lit by the Sun, but the raised rim gets up just high enough to see some sunlight, illuminating it in this wonderful way. Note the small patch of lunar surface to the lower left of the picture that’s in sunlight as well. That’s another indicator that how much sunlight you get at the Moon’s poles depends a whole lot on the local topography of the terrain. Um. Selain? Lunain? Whatever.

Anyway, in this shot by the fantabulous Lunar Reconnaissance Orbiter, you can see that the rim of the crater has seen its share of action; it’s dotted with smaller craters from later, smaller impacts. That part of the Moon is lousy with craters; it’s an old region, meaning that the pelting it got from cosmic debris billions of years ago is still visible. On other areas of the Moon, like the dark patches you can see with your unaided eye, giant impacts flooded vast regions of the surface with lava, filling up and smoothing over any craters, wiping them off the face of the Moon. That never happened near the pole, so Erlanger still stands high and proud.

But it also runs deep: the bottom of the crater is low enough to rarely get sunlight. Scientists think that ice from comet impacts over the eons may accumulate in such deep craters, so Erlanger is a prime target for LRO’s bistatic radar, which can penetrate into the lunar surface and reflect back to the probe. The type of reflection can indicate what materials lie beneath… including frozen water. LRO and the Indian probe Chandrayaan-1 just targeted Erlanger last week for such an observation.

Will they find water? We don’t know yet. But we’re still looking, and in fact the other half of the LRO mission is LCROSS (Lunar Crater Observation and Sensing Satellite; follow it on Twitter!), which will impact the lunar surface near the pole on October 9th of this year. The hope is that any water will be flung up into space where it will be more easily detectable. This has been tried before, but so far we’ve seen no water on the Moon.

But maybe this next time we’ll strike transparent gold. And if we do, I wonder… is there a child already alive on Earth today who may one day drink Erlanger water as they sit on Armstrong Base, admiring the view of the lunar landscape out the mess hall window?

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (33)

  1. Joe Bogus
  2. Christine P.

    Hey, I thought this blog was about skepticism, not astronomy.

  3. Zyggy

    Very Cool.

    ….and I love “OMFSM” 😉 At first I tried to put a bad word into it, then realized it doesn’t even need one.

  4. Bill

    Dear Mr. Plait –

    Please find enclosed an invoice in the amount of $243.56, intended to cover half of the cost of the re-calibration of the upper end of my Cool Meter ™. While I would normally absorb the cost of this calibration myself, I find that the pictures such as the one above that you regularly post on your blog are the primary reason that I must have the service performed. It therefore seems only fair to me to share the expense. Please note that I’m only charging you half of the total cost. Since I decide whether to look at the image you post or not, I’m perfectly willing to accept my share of the responsibility, and thus the expense.

    – Bill

    P.S. Now who do I talk to about sharing the medical cost of a sprained tongue, from jabbing it so firmly into my cheek?

  5. Christine P. (#2) : Yeah, I don’t see what any of this has to do with vaccinating creationists against Fox News.

  6. Keith

    You’re spoiling us Phil. Two awesome astronomy pics in one day.

  7. Cheyenne

    I hope that Astrologer from the San Francisco Examiner speaks out against LCROSS. Her article criticizing the Kaguya impactor generated the funniest blog comments in the history of the internet.

    And wow that photo is cool. Kind of eerie though. It’s like looking into the abyss.

  8. Chip

    Imagine walking along the sunlit very top rim of that crater, then stepping a bit down the slope into the jet black shade, pausing and lifting the mirror part of your visor and glimpsing up at the Milky Way – then returning up into the narrow sunlit path. On your way to the little lunar research station and (maybe) water treatment plant.

  9. Timothy from Boulder

    “Erlanger Water.” That’s a nice catchy name for a brand of bottled water.

    Regarding LCROSS: If you’re up for a laugh, crackpot-style, go visit the self-styled “researcher” at Mars Anomaly Research (Google It) for his “Don’t Bomb The Moon Activism” campaign.

  10. @Cheyenne,

    Did you read her article “What is Life?” where she tries to respond to the “The Moon isn’t alive” commenters? She goes from “things that are breathing are alive” (not true, of course) to matter vibrates and since vibrating is sort of like breathing, the Moon is alive!

    It just occurred to me that my cup of tea is alive by her definition. I guess I should ask for its permission before I drink out of it any more.

    But just to appease her, and to paraphrase Homer Simpson: “Moon, If you want us to continue smashing things into you, please give us no sign whatsoever… thy bidding will be done *KER-SMASH!*”

  11. Simon

    A bistatic radar uses a receiver antenna physically separated from the transmitter antenna, unlike a “normal” radar which uses the same antenna for both.

    Where is the receiver antenna on LRO? The info on NASA’s site about the Mini-RF ( doesn’t really say.

    Is it perhaps on LCROSS or Chandrayaan-1? If so, that would be an impressive technical feat, using two separate spacecraft in different orbits as the two halves of a bistatic radar.

    I suspect it’s rather more mundane – just two different antennae on board LRO. Someone tell me I’m wrong!!

  12. BILL7718

    Are all of us that looked at that pic going to get a phone call that we will die in 7 days and then have some slimy young ghost girl crawl out of our computer screen and…


  13. I'd rather be fishin'

    An amazing photo. I don’t have time to search for all the neat stuff on the web. I’m glad you’re doing it for me.

  14. DUDE!

    I will never tire of looking at such photographs. Technology is crazy! And the amazing things we get to see thanks to leaps in technology advancement (thanks largely to NASA and people foolishly want to cut NASA!)

  15. For you Space 1999 fans – squint your eyes and shake your head back and forth and up and down real hard and it kinda looks like the Meta Signal. Don’t operate heavy machinery for a few minutes if you do this.

    Serious questions – are they banking on the Sun having not hit the ice in those craters? Have they found any evidence of ongoing sublimination on the Moon? Or could there be another process at work that might be leading to the negative results of previous tests?

  16. I like “selain”. Selene just doesn’t get enough love these days. Plus I prefer Greek over Latin.

  17. PJE

    Wouldn’t any water ice at the bottom of a crater sublimate away? Especially after a long time?


  18. @Simon (and @BA): Check out:

    Apparently LRO and Chandrayaan-1 can, in fact, operate as a bistatic radar, however, it might be slightly inaccurate to refer to either one of them as _having_ a bistatic radar.

  19. Simon

    @Chris – ah-hah, that’s it!

    Then it is indeed an impressive feat! Thanks :-)

  20. Gary Ansorge

    18. PJE:

    As long as the temp the bottom of the crater experiences stays below 152 Kelvins, it won’t sublimate. See below:

    “The sublimation temperature of water ice is 198 K under Martian atmospheric conditions.
    The sublimation temperature of ice in a vacuum is 152 K (seems too small)” and here is the url for that quote:

    Gary 7

  21. See that little hillock just to the left there, below the tiny crater inside the rim? That is the official location for the (future) Kuhnigget Moon Base and Spa.

    Reservations now being accepted. Equal opportunity employer, tho we give preferential treatment to those with purple hair, go-go boots and skin tight clothing.

  22. KC

    That is teh Cool!

  23. That image is *strikingly beautiful*, BTW. Somebody give those astonomers a prize!

  24. 23. kuhnigget Says:
    Kuhnigget Moon Base and Spa.

    Reservations now being accepted. Equal opportunity employer, tho we give preferential treatment to those with purple hair, go-go boots and skin tight clothing.

    I would assume that’s the females. How about males with pure white hair?


  25. That depends, J/P=?….how do you look in muscle shirts?

  26. Bill Nettles

    Is Riddick living in the basin of that crater?

    But, really, Phil, this is not a picture of the moon. It’s a CT cross section of an eyeball.

  27. 27. kuhnigget Says:

    That depends, J/P=?….how do you look in muscle shirts?

    Oh, welll………. :(



    (Actually, I have relatively few white hairs, considering I just (June) passed ‘double nickel’ in age.. and still have only a ‘high forehead’… should find my 8th grade class photo and compare to current appearance)

  28. LPOD says “terrain”.

    Then you can blame your use of English on someone else!

  29. Charles J. Slavis, Jr.

    Of course there is life on the moon. How else did we find those petrified wood rocks?


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