Like asteroid, like moon

By Phil Plait | October 5, 2011 9:00 am

The other day I posted a great picture of Saturn and its rings taken by Cassini. While digging around in my archives looking for other posts about the rings, I found one from earlier this year that had a picture of the icy moon Enceladus with the rings in the background. When I saw the picture, I got a jolt: there was a crater chain on the surface that looks just like the one on the asteroid Vesta!

Here’s a side-by-side comparison:

Enceladus is on the left, Vesta on the right (click those links for higher-res shots). Pretty cool, huh? You can see both have two big overlapping craters of roughly the same size, and a smaller third one roughly aligned on top. The set on Vesta is nicknamed — for obvious reasons — "Snowman".

Craters like this form when the impacting object is not a single body; for example, many asteroids are known to be binaries, with both objects about the same size. Getting hit by that would leave two craters either very close together or overlapping, depending on the sizes, distances, and velocities of the impacting bodies.

Sometimes, too, there are long chains of many craters, sometimes dozens. We see those on the Moon and Mercury, for example, and they may be from comets that have disintegrated into many pieces before they hit, like the comet Shoemaker-Levy 9 did before it whacked Jupiter over and over again in 1994.

The impacts on Vesta and Enceladus look remarkably similar. But I wonder. The two big craters on Vesta both have lots of shared characteristics: size, sharp rims, and so on. They’re the two biggest craters on Vesta, so it would be very unlikely to get them so close together unless they were from the same event. But the third crater has a softer rim (implying greater age due to erosive forces like the solar wind and smaller impacts over eons), is smaller, and doesn’t quite line up with the other two. There are also several craters that size on the surface. It’s possible it’s unrelated to the other two, and coincidentally nearby.

Enceladus, though, looks like all three are related. Even though one is smaller, it lines up pretty well with the other two and has similar features. Maybe this really was a triple-system that hit. The asteroid Kleopatra, for example, has two moons (though I must note Kleopatra is big, which helps it hold on to two moons; an impact from something like that would come close to shattering a moon like Enceldaus).

I have no real scientific conclusion to draw here, except that multiple-body asteroids and comets are certainly more common than we might have thought 20 years ago. It’s amazing that the evidence for their existence was literally carved into the surfaces of other big bodies out there. With all of this new and marvelous imagery we’re getting from our robots plying the solar system, I wonder what other things we’ll learn as we build up this huge database of pictures?


Related posts:

Icy moon and distant rings
Vesta’s double whammy
kaBLAMBLAMBLAM
WHAM! Bulls-eye!

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (26)

  1. Charlie

    Crop rings? Proof that aliens are out there.

  2. Chris

    The remains of frosty the snowman

  3. Lupine

    Or a big footprint.

  4. Sharkey

    Why would a comet disintegrate before hitting a low-atmosphere, low-gravity body like the Moon or Mercury? Would tidal forces be strong enough, given the relatively-small size? Or would the tidal forces of nearby bodies (the Earth or the Sun, respectively) cause the break-up?

  5. I have no real scientific conclusion to draw here

    It’s COOL! What other conclusion is required? :D

  6. @Sharkey it wouldn’t have to be the low-atmosphere, low-gravity body that did the breaking up, or they may never have been a solid object to begin with.

    Or it means Frosty is real, and he loves us, or at least moons.

  7. CraterJoe

    I think the Aorounga Craters are much more interesting and closer to home.

  8. Robin Byron

    I hope that’s not an ownership mark from some civilization gone mad with galactic property rights.

  9. CR

    Cue the “NASA is just re-using setpieces and models over again” conspiracy crowd in 3… 2… 1…

    Seriously, although these crater chains are similar, it’s interesting to notice just how different they are. And that’s apparent just by looking at the pics a little closely, as Phil has described in this post.

    Speaking of craters, I still like the picture of–oh, which Saturnian moon was it? Dione, I think–that shows a crater & its central peak practically sliced in half by a long rift. (Sorry I don’t have a link, but I had seen it at the Cassini image archive a couple years ago.) So much astronomy and geology in one pic… I wish we could go there in person, but I’m glad we at least have these tantalizing robotic views.

  10. SkyGazer

    “so it would be very unlikely to get them so close together unless they were from…”
    You really know how to tickle the alufoilhats don´t you.

  11. CR

    Just tried a quick look through the CICLOPS image diaries, but I couldn’t locate the photo I was thinking of. I tried many of the rocky/icy moons: Dione, Enceladus, Tethys, Rhea, Mimas… weird. If I find it, I’ll post a link later.

  12. There’s another such feature on Iapetus, near the east end of the dark oval.

  13. Tom Callahan

    Shouldn’t the one on Enceladus be “Snowman”, given that it’s icy? The one on Vesta can be “Pig-Pen’s Snowman”.

  14. BigBob

    I dug out this reference to a chain of craters on the Moon (our moon, the Moon). It’s another triple, pretty much in the middle of the disk as we see it.
    http://www.lunarrepublic.com/atlas/sections/e4.shtml
    Bob

  15. Crux Australis

    Proof that NASA is using the same stock photos for their hoaxes!!11!!eleven!!

  16. Crux Australis

    Doh! CR beat me to it. I’ll show myself out.

  17. Sawdust Sam

    It’s clearly a universal copyright mark – there’s one on every lump of rock if you look in the right place.

  18. Chris Spratt

    There is an old double crater in Quebec Canada.

  19. Levi in NY

    Maybe it wasn’t an object with two moons, but a rubble pile that was torn apart by Saturn’s gravity?

  20. @^ Levi in NY : You beat me to that suggestion. Many asteroids (eg. Itokawa & Mathilde*) and some small moons (eg. Phobos) are thought to be loosely bound, low density, “fluffy” agglomerations of stuff rather than entirely solid bodies.

    I also wonder whether it could be co-incidence or secondary impacts – a fragment of debris kicked out lunar ray~style by the initial impact then orbiting briefly before spiralling back to make a new crater (or series of craters) in line with the original one?

    @ 19. Chris Spratt : “There is an old double crater in Quebec Canada.”

    Indeed there is – the Clearwater Lakes :

    http://en.wikipedia.org/wiki/Clearwater_Lakes

    Plus I’m pretty sure there’s a few other known examples from elsewhere too.

    @14. Tom Callahan : “Shouldn’t the one on Enceladus be “Snowman”, given that it’s icy? The one on Vesta can be “Pig-Pen’s Snowman”.”

    Yep. I was thinking the same thing. The “Snowman” on icy Enceladus, the “Stone man” on Vesta perhaps?

    ———————-

    * See :

    http://en.wikipedia.org/wiki/25143_Itokawa#Description

    &

    http://en.wikipedia.org/wiki/253_Mathilde#Description

    for more.

  21. CR

    Finally found the pic I was talking about, from Cassini’s October 2005 flyby of Dione. (I knew it was Dione… I just missed the pic in my rush to find it!) http://www.ciclops.org//view_media.php?id=8265
    It’s a very big pic, and you’ll have to scroll down & around to see the whole of the landscape. The bisected crater I mentioned is on the right limb of the crescent, near the lower right corner of the pic. (There are actually two bisected craters in that area, close to each other, but the leftmost one has a better defined central peak.)

    …Should I have said “follow link to Dionenate”? ;)

  22. Nigel Depledge

    The BA said:

    The set on Vesta is nicknamed — for obvious reasons — “Snowman”.

    Isn’t there a Snowman crater near one of the Apollo landing sites on our moon?

  23. Nigel Depledge

    Oooh! Can I do the joke?

    Wait! That’s no moon . . .

  24. Tribeca Mike

    John Frederick Coots and Haven Gillespie predicted this is in their 1934 song “Santa Claus is Coming To Town.”

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