SUMMARY: This image, taken by ESA's Mars Express spacecraft, shows an unusually shaped impact crater Hesperia Planum region of Mars. The crater is approximately 11 x 24 km (7 x 15 miles) across and has ejecta surrounding it where molten rock splashed around when a meteor carved it out. While most impact craters are circular, this elongated shape means that the space rock hit Mars at an extremely low angle (less than 10 degrees). Similar craters have been seen on the Moon.

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Why would it have similar high walls on either end if it was a low angle strike?

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It looks to me that it could be the result of a large meteor that broke into three pieces just prior to impact. The center one impacting first.
Jack

That's what I was thinking too, it looks too symetrical lengthwise, more like multiple overlapping craters than a low angle impact.

I'm glad I'm not the only one who sees the image differently then the experts. I agree, I see three impacts, not one from a low angle. I think they need to re-examine the photo.

Maybe the ejecta are the most telling feature, I see two almost symmetrical "fans" on either side of the elongated crater, and almost no ejecta on both "ends". I would expect little ejecta at the first point of impact (according to the caption the South-end), but a humongous amount of ejecta spreading from the other end, which is conspicuously absent.

Cheers.

If it were a triplet, wouldn't the last impact have left a rim all the way around?

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Hi,
This may be the result of a simultaneous impact of a very close group of objects, or that of an oblong object. In either case, this should create an oblong crater.

Regards,

Günther

I'm not sure it does, but with this reasoning, shouldn't the ejecta show what happened (see my previous post)?

Cheers.

http://www.hq.nasa.gov/office/pao/Hi...62/hrp119b.jpg

Butterfly ejecta patterns are seen around moon craters, too, like this one from APOLLO OVER THE MOON: A VIEW FROM ORBIT (NASA SP-362) [Figure 113]



It may not be an intuitive result, but it sounds like those with the thrilling job of smashing high-speed objects into large bodies at low angle are familiar with the effect.

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Maybe the was not roughly round or had an irregular shape.

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If it was a single low angle impact, wouldn't the crater rims at the opposite ends be non-symmetrical? Seems to me like they would.

I'm of the opinion that this was more likely three particles that impacted at almost the same time. That is why the one at the lower right is almost circular. There may have been walls separating the individual impacts at one time, but the flat crater floor indicates there has been some flooding or infilling that may have covered up the lower walls.

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"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

From what I've read it takes an extremely low angle to get an ellipsoidal crater. Moderate angles still produce round craters, so the meteor's direction of travel doesn't seem to affect the morphology all that much. Most of those round craters you are familiar with were not produced by meteors striking normal to the sirface, by simple probabilty.

The shape of the volume and rim of the crater are mostly the result of melting and vaporization -- an explosion rather than a kinetic bulldozing. I would imagine for low- to high-angle impacts, the material that is vaporized is roughly (hemi-)spherical and results in a round crater. At extreme lower angles, I'd expect the volume vaporized would become more stretched and the explosion would yield an elongated, but still fairly symmetric, crater shape and rim.

A Brief Introduction to Hydrocode Modeling of Impact Cratering (PDF 2.8 megabyte) is mostly about modeling but has a figure showing the shape (unfortunately only in 2D) of shock pressurizations for a varety of impact angles. Down to 30 degree angles, I'd describe the pressures as roughly spherical, maybe even lower, but at 15 degrees, you can see the stretching effect. I suspect the shape of the part that goes boom reflects those pressures.

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On first viewing my interpetation of the impact was EXACTLY as Jack has explained it... multiple impacts much like the collision Comet Shoemaker-Levy 9 had with Jupiter in '94. The impact angle of 10% makes no sense to me, however, I would like to hear how they arrived at their finding?

Curt
American ex-Patriot residing in Sweden

Which means this would have had to have been an extremely low angle if it was a single impact, which I think reinforces the point that the crater walls on the ends are too symmetrical.

I think the consensus here is that the body was broken into multiple objects just prior to impact.

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"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

To me, it looks a little bit like this:
http://www.grisda.org/origins/images/09087-02.gif

Do you have a cite for a study or experiment that shows oblique single-body impacts always leaving asymmetric rim ends? Thanks.

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Isn't that what they see in laboratory tests?

Well, Barringer's studies indicated that they usually result in a round crater.

http://www.barringercrater.com/science/main.htm

http://www.barringercrater.com/science/

So it shouldn't have created the long trench that we see, and that the author's posit came from a single impact.

Here:

http://www.apl.ucl.ac.uk/lectures/3c11/impacts2.pdf

is a discussion of impact cratering, and if I read it right it says that at high velocity, oblique impacts result in circular craters (see page 3-4.)

It is hard to imagine how a single impact could have created an elongated crater like the one in the OP.

I was struggling to internally see how that could have happened, and assumed (without really believing it) that it would have to be a very low angle, almost a graze, and that would have left other evidence such as differences in the slopes of the crater walls.

I really don't think it was a single impact, as I don't see how a single impact could have left the crater... Maybe if it was really low velocity...

__________________
"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

Thanks for the beautifully clear example(s), they show that impact cratering has it's problems, you call it "counterintuitive", I call it incredible; the shape of the crater is clearly not in the direction of the ejecta, just like the scar on Mars. I have never seen any ejecta pattern from impact studies that showed this strange pattern. I would welcome any examples.

Cheers.

Yep, but with a few exceptions, of which I'm one.

Cheers.