Vesta's double whammy

By Phil Plait | August 16, 2011 6:02 am

The Dawn spacecraft entered orbit around the main belt asteroid Vesta just a few weeks ago, and images are coming back in dribs and drabs. NASA just released this fantastic one, taken by Dawn’s wide-angle camera:

[Click to protoplanetate.]

Pretty cool, eh? Vesta is about 500 km (300 miles) across, so you’re seeing about half the rock from top to bottom here. The most obvious thing are the two ginormous craters. Note the scale bar; the bottom crater is about 70 km across, and the top one about 50. The fact that they nearly overlap, and are clearly the two biggest features for a big area around them, makes me think it was a double impact. Many asteroids are binary, so two objects a few kilometers in size and orbiting each other 50 or more kilometers apart would do the trick*. [Update: Emily Lakdawalla agrees.]

The bottom crater is weird; the bottom right edge looks like it’s collapsed a bit, marring the near perfect circle of the rim. That feature itself looks like an arc of a circle; might there have been a third rock that hit? That seems unlikely, and I have a hard time believing even a piece of the rim of a previously existing crater would’ve survived the impact!

Also, look around the two craters. See how far away from them, the surface is saturated with smaller impact craters? Near the big two, though, there are fewer. It’s a sure bet the impacts threw out a lot of debris which blanketed the area. The escape velocity of Vesta is a meager 350 meters/sec (about 750 mph); a lot of the stuff blown out on impact would’ve been moving faster than that! So some would’ve escaped the asteroid entirely, but some would’ve settled down over hundreds of square kilometers of area around the site.

Both crater floors have a filled-in appearance. No doubt a lot of energy of the impact went into melting the surface, which flowed inward. Some of that might be slumped material from the crater edges, too. My knowledge of how craters form is limited, especially on asteroids. But I’d love to see high-resolution images of this! That would answer a lot of these questions straight away.

And of course, we’ll be seeing those soon. Dawn is slowly moving down toward Vesta, gradually lowering its height until it settles into its final orbit. At that point, we’ll be getting really high-res shots, and maybe a few enigmas will be solved… only to be replaced, no doubt, by ten times as many. Fun!

[Edited to add: there is that biggish third crater at the top, of course, but I'm not sure it's related to the other two. It's much smaller, for one thing, and there are several other craters that size nearby that appear unrelated. It has a softer rim, implying greater age due to erosion (meteorite impacts), and isn't aligned with them -- though the sharp curvature of the asteroid makes that difficult to verify; in some images they look more colinear (that is, aligned).]

Image credit: Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA


* At first the size of the impactors was a guess, but then I poked around and found this crater diameter calculator. I put in values for Vesta, and found that the impactors would’ve been roughly 5 km across. My instincts, sometimes, are good.


Related posts:

- Vesta in breathtaking detail
- When asteroids collide
- KaBLAMBLAMBLAM!
- Hubble captures picture of asteroid collision

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (21)

  1. William R. Dickson

    Maybe I’m looking at this wrong, but what’s with the shadowing? The left rim of the top crater seems to be in shadow (expected if light is coming in from the left), but chunks of the right rim of the lower crater also seem to be in shadow, giving it an almost convex appearance. It looks almost as if someone mashed some Fun-Tac onto the surface of Vesta. What’s going on there?

  2. Pareidolia! My first thought was, “That looks like a snowman!” My second thought was, “Is that the Vestal Virgin?”

    AWESOME pic!

  3. Andy

    I’m not an astronomer nor a photographer… But it seems to me that an image taken top dead center over a crater would give these appearances of shadow. The the east edges you see shadow inside the rim, while at the west edges, you see shadow outside the rim. So while the top view can give the appearance of being convex, I don’t think so.

    As to the bottom-right “collapse”… Is it possible that the asteroid that struck that position was odly shaped with maybe a big crater of its own at that position? Something like a “donut-hole” with a fair-sized bite taken out of it? It would seem that could possibly explain a softer rim at that location… The bite point wouldn’t have as much energy to impart into that area, would it?

  4. Hevach

    @1. The shadows on the left seem to be outside the crater, with the ones on the right being inside. I think you’re just seething the classic crater/dome illusion, made more confusing because the curvature making light different across the image (the craters at the bottom right have almost no shadow as if they’re close to noon, but the ones at the left have long shadows as if they’re close to the terminator, which I think you can even see at the bottom left).

  5. hhEb09'1

    @William, I don’t see anything that isn’t consistent with light from the right?

    Phil’s comment, in his edit, about the softer rim seems to apply to the large two–the lower right one seems younger. It also seems to overlap and overlay the left upper one. So, not a double impact, but two separate impacts, seems to me. The latest seems to have “flowed” to the right a bit too.

  6. Chris

    Speaking of impacts… In previous posts you didn’t mention the new theory that our moon may have been the product of a collision of two protomoons. We’d love to hear your thoughts if it’s plausible or the authors have been huffing too much moon dust.

  7. Oh boy! I get to play protoplanetologist, too!

    I’m willing to bet the area to the bottom right of the larger crater looks the way it does because of variations in the terrain (vestain?) that existed before the impact. Look at the way this area lines up with the “grooved” striations running in a parallel row down and to the right. Notice how the linear ridge, marked by shadow, within the crater lines up almost exactly with those broad features. Coinkidink? Prolly some looser material within the grooves went kablooie when the rock hit, while the ridge of more solid material stayed relatively intact.

    That’s my story and I’m sticking with it. Until something better is suggested, at which time I will deny all knowledge of it and claim my computer was hijacked by a bitter nerd from Anonymous.

  8. Pete Jackson

    When we consider main belt asteroids like Vesta, a lot of collisions are going to come in at not much more than Vesta’s escape velocity, so that the rebounding material will have quite a bit less than Vesta’s escape velocity and won’t go all that far. Hence, we’ll see mounds of ejecta near the crators, but not the extensive ray-chains indicative of high speed collisions.

  9. Keith Bowden

    “protoplanetate” – Good one!

  10. Pretty cool, eh?

    Yup. :-)

    What would Vesta’s surface temperature be again – well below freezing I’d expect? ;-)

    Great picture. Thanks BA & thanks to the Dawn team too. :-)

    I’m really looking to more from Dawn as well – especially from Ceres. Can’t wait for that.

    ***

    “I consider it quite conceivable that the day may come when Ceres will be the astronomical centre of the solar system.”
    - Page 66, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by Isaac Asimov, Mercury Press, 1973.

    Once thought to be rocky, we now believe Ceres may contain 200 million cubic kilometres of water in its mantle. This is more than the amount of fresh water on the Earth.
    - Page 10, “Ceres may be a failed miniplanet” by Jeff Foust in Astronomy Now magazine, November, 2005.

    “… he had left out a planet. It was not his fault; everyone leaves it out. I leave it out myself when I list the nine planets, because it is the four-and-a-halfth planet. I’m referring to Ceres; a small but respectable world that doesn’t deserve the neglect it receives.”
    - Page 63, chapter 5 “The World Ceres” in ‘The Tragedy of the Moon’ by Isaac Asimov, Mercury Press, 1973.

  11. Michael Swanson

    Try as I might, I can’t see craters. Those look distinctly convex to me. I had to copy the image on to my computer and then rotate it…

    Ooh! Craters!

  12. Phil, you said: “That seems unlikely, and I have a hard time believing even a piece of the rim of a previously existing crater would’ve survived the impact!”

    You’d be surprised. Very frequently, especially in heavily cratered regions of Mars, this is the case where you can have a 10s-km crater overlapping a still-visible sliver of a smaller crater. I’m not saying that’s definitely the case here, but you should not dismiss it so quickly via your own argument from personal incredulity. ;)

  13. DigitalAxis

    @11: Me too, actually. It was only afterwards I realized looking at the craters in the top left would solve it too.

  14. Navneeth

    Phil, I’m surprised that you haven’t mentioned the resemblance between the appearance of these three craters to that that of the ‘snowman’ on the moon which is close the ‘Straight Wall’.

    http://darkerview.com/darkview/uploads/Astronomy/Astrophoto/CD22-64-09358-DC.jpg

    (A bit to the top and right of Rupus Recta.)

  15. Mary A.

    Zooming in, Those two hills or small craters…whatever they are called they are on the left side across the center and the bottom craters, look like heads.. One, a side view of a scull and the other a facing forward mans face with a beard… creepy!

  16. Anchor

    “…there is that biggish third crater at the top, of course, but I’m not sure it’s related to the other two. It’s much smaller, for one thing, and there are several other craters that size nearby that appear unrelated. It has a softer rim, implying greater age due to erosion (meteorite impacts), and isn’t aligned with them — though the sharp curvature of the asteroid makes that difficult to verify; in some images they look more colinear (that is, aligned).”

    I agree that it is unlikely unrelated to the other two (it’s obviously significantly older than the two bigger ones, although there is a very small possibility that a smaller moon struck first and got swamped by the other two bigger strikes in the same event).

    But it’s size and alignment has nothing whatever to do with determining whether it was associated with the other two. If it was another smaller moon in a 3-body system it could have had any position with respect to the other two at the time of impact. Alignment isn’t a necessary condition for a hypothetical pre-impact association.

    And why should its smaller size not simply indicate a smaller moon?

    In any case, Vesta’s gravity is too feeble, and typical relative velocities in the asteroid belt are too swift, for a single body loosely held together to have been separated by Vesta’s weak tidal gradient in the brief time of approach before impact. The multiple crater chains we see on the outer Galilean Satellites of Jupiter, for example, are thought to be caused by bodies that were stretched apart into a number of fragments by the tidal influence posed by their close passage by Jupiter before impact.

    Think Comet Shoemaker-Levy hitting one of them in quick proximal succession in a near-single event: blam-blam-blam! like machine-gun fire before they separated very far from one another…instead of winding around and getting increasingly widely separated over many months and falling back to strike Jupiter as a long train of fragments over a period of days. Vesta just can’t exert the necessary tidal stress for long enough to do that to an impactor of typical composition just before it strikes. If it DID, the body would have to be composed of something like fluff, and fluffy fragments aren’t going to create such emphatic craters as these which were obviously produced by pretty energetic and nominally rock-dense impactors. They appear to be very much the same age, and I agree that the impactors responsible were a double asteroid.

  17. Clive DuPort

    I got the “convex crater” illusion for a while until I turned my laptop on its side so the light was from the top, now I can only see the craters as concave. Isn’t the human internal image processor a funny thing?

  18. Brian S

    Am I correct in my assumption that the double impact is a relatively recent event? I would say so because of the clean nature of the debris field. I can only spot three crisp impact sights within the field (two in the upper left and one in the lower right). The rest of the craters are covered with debris and softened.

  19. jess tauber

    Vesta Schmesta….gimme Ceres. No, I mean, give me Ceres. I’ll take good care of her, erect giant mirrors and lenses to warm her waters, and after suitable cleaning, filtration and conditioning transport earth sea life there (since the rest of you are eating and polluting your way through the phylogenetic system). Eventually I shall retire to my yacht there to enjoy the balmy breezes and low gravity, and Flipper.

  20. Matt B.

    May I suggest a name for a rock band composed of astronomers? –Asteroid Grooves–

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