Unusually Shaped Craters

[CalliArcale]

Hello, all!<br /><br />During the thread about Iapetus possibly being artificial, one poster raised the observation that some of Iapetus' craters aren't perfectly round. This lead to a fair amount of debate over what shape to call them. In any case, the overall impression among those puzzled by the crater seemed to be that impact craters are normally circular. I happen to know of one that is very definitely a perfect circle, so I thought I'd go looking. Even poking around largely at random, I found quite a few. I thought I'd post some of the results of that search, and invite discussion on the topic.<br /><br />I'd particularly be interested from hearing from JonClarke. He remarked that geologic structures within the planet/moon/etc prior to impact can influence the shape of an impact crater. That makes sense to me, but it's intriguing to me because I'd previously mainly thought of them as pretty random, and influenced mainly by the angle of attack.<br /><br />Anyway, here's the first funky crater I thought of. It's probably the most famous crater on Earth, even more famous than Chixulub, because this one was the first to be identified as an actual impact crater. It's called either Barringer Crater or Meteor Crater, and it's in Arizona. It's roughly rectangular, with beveled corners that make it look a lot like a giant football stadium. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[CalliArcale]

Here's one on Mars, in the ever-popular Cydonia Mensae. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[CalliArcale]

Here's the edge of a huge crater on Iapetus, in Cassini Regio. It got mentioned in that other thread. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[CalliArcale]

This one's just plain pretty, and I couldn't pass it by. <img src="/images/icons/wink.gif" /> It's very regular and very circular, but it's got a double ring and a very prominent central peak. It's from Mars' Isidis Basin. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[CalliArcale]

And here's a relatively cratery part of Enceladus. Several of these are not all that circular, including the large one towards the top of the image, which appears to have been eroded a bit by other impacts. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[CalliArcale]

Funky!<br /><br />That *had* to have been an oblique impact. Pretty cool. <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[JonClarke]

Hi Calli<br /><br />The following is off the top of my head. I can check some specific references if people need them.<br /><br />Under ideal circumstances into a uniform sunstrate, a hypervelocity impact will make a circular crater. this is because the impactor essentially expodes, and the expanding spherical shock wave excavates a hemispherical cavity. hence the very strong similarity between impact craters and other natural (phereatic and phreatomagmatic) and artificial (bomb and shell) craters.<br /><br />In the real world things get more complicated. Even as the crater forms, the sides collapse and the bottom fills in, resulting in the classic shallow bowl shape. If there is a very strong fabric in the substrate, such as strongly layered or fractured rocks, these can control the degree of slumping. I understand this is why Meteor crater is "square", because of the orthogonal joint pattern in the underlying limestone.<br /><br />If an impact is small and occurs on a steep slope the crater will be often ellipitical and assymetric, with the long axis down slope. Some of the craters in the Columbia Hills are like this and look quite odd.<br /><br />Very low angle impacts are also elliptical, oriented in the direction of travel of the impactor. I believe there is a critical angle where this occurs. At very low angles you get paired impacts - the primary ellipical impact and a secondary impact which is often quite irregular where the excavated material gets thrown out in a mass and then digs a second crater. there is a crater in Argentia, I understand that is like this. But the best example is Messier on the moon. If you look at crazzieeddie's photo, the right hand crater in the primary and the very irregular left hand crater is the secondary. The very low angle impact also explains why the rays from Messier are almost unidirectional.<br /><br />Crater walls are very steep and fractured and therefore prone to slumping. Slumps generally carve out curving indentati <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[vogon13]

Sorry I can't reference a picture, but were those big 'gouges' in Argentina ever proven to be the result of an oblique meteor strike? <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[JonClarke]

I think you mean the Rio Cuarto cluster. http://www.unb.ca/passc/ImpactDatabase/images/rio-cuarto.htm<br /><br />I understand there is some debate still, bot most workers think they are very low angle impacts.<br /><br />Jon<br /><br /> <div class="Discussion_UserSignature"> <p><em>Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars</em>  Arthur Clarke</p> </div>

 

[vogon13]

That be them. <br /><br />Thanx! <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>