Deimos!

By Phil Plait | March 9, 2009 1:45 pm

The HiRISE camera onboard MRO just took this phenomenal picture of one of the moons of Mars, Deimos:

HiRISE view of Deimos

That is so cool! Deimos is the smaller of the two moons, a lumpy ball 15 x 12 x 10 km in size (Phobos is 27 x 22 x 18). Deimos has incredibly weak gravity; its escape velocity is only about 20 km/hr, so you could throw a baseball right off into space, and biking without taking an unintentional EVA would be difficult.

There is also another picture showing Deimos from two slightly different angles (though the angle to the Sun had changed enough to provide a different lighting angle). The resolution is such that you can see features as small as 60 meters across, smaller than a football field.

You can see some small impact craters on its surface, and it also has lots of smooth areas. Those latter are probably covered in regolith, rocks ground into fine ground powder from eons of micrometeorite impacts (much like the surface of the our own Moon). As the HiRISE page points out, you can see subtle color differences, with areas around craters being lighter, and smoother areas redder in color. As material like that is exposed to ultraviolet light from the Sun, it undergoes photolysis — having its chemical structure changed — and it becomes redder. So you can get an idea of relative ages of the surface just by looking at the color! As you might expect, regions around craters are younger, and tend to be lighter in color.

The one exception is the crater to the right of the picture; it’s possible that the regolith has flowed "downhill" even in the low gravity, covering the crater.

Pictures like these remind me that the solar system is not just a collection of lights in the sky; it’s composed of worlds to explore. And the more we reach out and look, the more fascinating, beautiful, and exciting it becomes.

CATEGORIZED UNDER: Cool stuff, NASA, Pretty pictures

Comments (37)

  1. Z-man

    It looks oddly smooth. Even the craters look like they’ve been subjected to some sandpaper to remove the rough edges. Cool pics.

  2. Brian Schlosser, Lurker

    Speaking of the Martian moons, I noticed an error in the new “Watchmen” film… Not giving too much away, but at one point a character is on Mars, and in the background, over the horizon, are Deimos and Phobos… but they are WAY too big and round, like two Lunas up in the salmon sky. A minor quibble, but one I spotted right away, thanks to my vigilant reading of Bad Astronomy!

  3. “That’s no moon!”

    (Someone had to.)

  4. Mchl

    The 2km mark gave an idea. How about superimposing one of those pics onto satellite picture of Earth surface? If done to scale, we could compare the size of the features on Deimos, to things we all know.

  5. Brian, I saw that as well. I may write up a review of Watchmen — there wasn’t a whole lot of science in it — and that will be in there.

  6. @Z-man:
    It is so smooth probably because it is covered in regolith – fine grit and powder from all these impacts have half buried the older craters in a cover of dust.

  7. Z-man

    Marco,

    Thanks for the explanation. I knew there was a reason for it, I just thought it looked a little odd on first glance that there was a smoothness to the features.

  8. MadScientist

    It’s obviously a large pyramid with the brilliant white casing stones still largely intact but pitted and rounded by passing space rocks. More proof that aliens built the pyramids.

    Although 20km/h is the escape velocity from that moon, what additional velocity is needed to escape Mars?

    Since people are mentioning the ‘watchmen’, I recently read one of those weird-a$$ articles that defends comic book physics. Allegedly a physicist was consulted and one suggestion from the physicist was “X will need a lot of power, so you might want to put in a particle accelerator”. Wow. I didn’t know particle accelerators were used to generate power these days; in my universe particle accelerators are power hogs and consume more energy than a small town. I guess there are physicists and there are Acme Physicists.

  9. Brian beat me to it. (As I said in my review, having two moons is not the same thing as having two Moons!) And, yes, I am eagerly awaiting your review!

    Here’s a non-spoiler Physics question from the movie:

    If, say, a person were thrown into and through a window in a high-rise with sufficient force to shatter the window, would the body be likely to land on the sidewalk directly outside the building, or would its forward movement carry it furthter away? Would the trip through the window result in a sufficient slow-down of the horizontal movement of the body so it would fall more-or-less straight down?

    Follow-up question: if, at the same time, a small, lightweight, circular metallic object (say, the size of a smiley-face button) were to go through the window with the same trajectory, would it land in roughly the same place as the body?

  10. No science in Watchmen? http://www.sciam.com/article.cfm?id=how-accurate-is-watchmen Well, maybe not a lot of Astronomy, but there was science. :)

  11. Wes

    This might be a really dumb question, but it’s something I’ve wondered about. When it comes to a very small object with very low gravity like Deimos, how is it that objects strike it with enough force to create a crater? Wouldn’t Deimos’ low gravity mean that even a large meteorite would only weigh, maybe, a few ounces at the surface?

    This answer to this might be really obvious, and I’m just missing it for some reason.

  12. Wes

    Oops, made a typo. “This” in the last sentence above should be “The”.

  13. Torbjörn Larsson, OM

    A pretty Deimostration of HiRISE capability.

    over the horizon, are Deimos and Phobos… but they are WAY too big and round, like two Lunas up in the salmon sky.

    Um, haven’t seen the movie yet, but I believe that is compatible with the Watchmen universe.

    AFAIU Dr Manhattan could change mass and form of any objects, and teleport them to any orbit to boot. (Possible spoiler alert: And I believe he was redecorating some of the landscape, so it would even be likely if there was any noticeable differences.)

    Not that I would let up on pointing out any Bad Astronomy underlying the movie.

    [The button misadventures must be in the movie, not the comic where the fallen body is never shown IIRC.]

  14. Torbjörn Larsson, OM

    Wes, there is plenty of kinetic energy to go around even with low gravity.

  15. Wes, there is plenty of kinetic energy to go around even with low gravity.

    Which is a polite way of saying, Smack into something at a few kilometers a second and see how big a hole you make.

  16. The Other Ian

    Hurray! I’ve got one of the HiRISE photos of Phobos as my wallpaper. I’ve just been waiting for a good photo of Deimos that I can put up on the other screen.

  17. Wes, I think the point is, the objects that caused these craters weren’t necessarily gravitationally attracted towards Deimos, they just happened to be zipping through the same volume of space that Deimos was occupying at the time, with a heck of a lot of kinetic energy and no atmosphere to slow them down. I believe the same is true of most collisions in space. Most things that were puttering along at speeds slow enough to be gravitationally captured by the various planets and moons (and/or fall onto them) were captured or accelerated by the various planets and moons billions of years ago.

  18. Wes

    # Torbjörn Larsson, OM Says:
    March 9th, 2009 at 3:21 pm

    Wes, there is plenty of kinetic energy to go around even with low gravity.

    # kuhnigget Says:
    March 9th, 2009 at 3:28 pm

    Wes, there is plenty of kinetic energy to go around even with low gravity.

    Which is a polite way of saying, Smack into something at a few kilometers a second and see how big a hole you make.

    # Harold Says:
    March 9th, 2009 at 3:50 pm

    Wes, I think the point is, the objects that caused these craters weren’t necessarily gravitationally attracted towards Deimos, they just happened to be zipping through the same volume of space that Deimos was occupying at the time, with a heck of a lot of kinetic energy and no atmosphere to slow them down. I believe the same is true of most collisions in space. Most things that were puttering along at speeds slow enough to be gravitationally captured by the various planets and moons (and/or fall onto them) were captured or accelerated by the various planets and moons billions of years ago.

    A ha! That was my mistake. I was only thinking in terms of Deimos capturing things with its gravity. For whatever reason, the fact that these objects might already be cruising along at a high rate of speed relative to Deimos didn’t occur to me. *forehead slap*

    Thanks guys! :D

  19. Peter F

    I have a dumb question — how did the MRO happen to capture this shot of the moon? Was it just a chance close encounter in high Martian orbit, coincidentally a year after the amazing shots of Phobos?

  20. Michelle

    Wow… that thing looks so smooth. If it wasn’t an humongous space rock I’d pet it.

    …It’s a retarded thing to say uh?

  21. RE: Watchmen

    The graphic novel gets some bits wrong, too – like how long it takes light to get to Pluto (I loaned mine out so I can’t verify the exact quote)

  22. J. Clevy

    May I tweak your definition of regolith just a bit? Regolith isn’t limited to fine particles. There could be brecciated chunks and coarse gravels along with the sand and clay-sized particles.

    Think of regolith as soil without the organic component.

  23. @Peter F: I don’t think this opportunity was particularly different from any other time when Deimos and MRO happen to be over the night side of Mars. Planning imaging of Deimos (and Phobos for that matter) requires quite a bit more planning than normal HiRISE imaging because they are having to turn the spacecraft significantly off-nadir in order to pull it off. So I think it is more of a matter of setting aside the time to plan these sequences in order to do them. Considering that last year they took the images of Phobos, my bet is this is simply additional work that you don’t want to do too often or you will strain the sequence planners, though the results are spectacular.

  24. C.A. Mel

    As just interested bystander, maybe you can answer a question for me. How large, or massive, does an object have to be in order to have enough gravity to form a sphere?

  25. Joe Meils

    I’m glad I’m not the only one who seems to have his interest piqued by the little moon’s smoothness. I always assumed that both of the Martian moons would appear very much like some of the asteroids we’ve encountered: darker and pockmarked to the point of looking like they were fugatives from a BBQ grill.

    I assume the reddish color is due to the surface picking up dust that has been thrown up into it’s orbit from the planet below… but the lack of major cratering just facinates me.

    BTW, I caught the Watchmen flub too… my question about the film is: when did we first know about the “smiley face” crater on Mars? Was this something in the graphic novel, or was it something the filmmakers exploited when those photos were in the popular press a few months back, while they were still in production?

  26. Valdis Kletnieks

    C.A. Mel: To collapse to spherical under gravity, it’s gotta be several hundred miles in diameter (exact number depends on the density of the material composing the object, and the structural strength). Ceres just barely makes it.

  27. Gary Ansorge

    C.A. Mel:

    Ceres is about 950 km in diameter and is a sphere.

    I think that’s about the lower limit for an object to gravitationally collapse into a sphere.

    Gary 7

  28. Corey

    I declare martian law! Sir Phobos! Sir Deimos!

    Yes…this is dated, but I loved that show.

  29. Vorn

    Joe: the happy face crater, officially named “Galle”, was found by the Viking 1 Orbiter, sometime between 19 june 1976 and 17 august 1980, the time when the orbiter was active. It is 230km across, so the movie got the scale more right than the comic did, as far as the debris field from the crystal fortress goes.

    http://en.wikipedia.org/wiki/Galle_(Martian_crater)

  30. Gary Ansorge

    AH, so much construction material, just sitting there not being used,,,

    As a very rough estimate, one could probably hollow it out and build a habitat housing several hundred thousand people and other critters.

    Ah, the dreams,,,

    GAry 7

  31. Beagledad

    Phobos and Deimos are so small, do they really deserve to be called “moons”? Is there any chance that Neil deGrasse Tyson will downgrade them to “dwarf moons” or some such? Mini-moons, maybe?

  32. Torbjörn Larsson, OM

    @ kuhnigget:

    Which is a polite way of saying, Smack into something at a few kilometers a second and see how big a hole you make.

    Heh! Polite, but not so funny!

  33. bhrobards

    How could any regolith be left with a 20km/sec escape velocity?

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