APOD: A Hole in Mars (2014 Mar 09)

Comments and questions about the APOD on the main view screen.
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by geckzilla » Sun Mar 09, 2014 9:30 pm

Guest wrote:Yes, I assumed the other variant too, but if this is a depression, the light is consistent, but the semicircular shadow looks kind of weird. A better resolution maybe will clear away my confusion.
The photo is abundantly clear to me. I don't think I could understand it much better without sliding down the sides and falling into the thing. One thing I have wondered about this is whether there is a conical pile of soil at the bottom of the hole to match all of the displaced material above. That could be one reason why the light inside the hole looks a bit darker. Not only is there less light but it could be the shaded side of a pile of soil.
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by neufer » Sun Mar 09, 2014 9:59 pm

Click to play embedded YouTube video.
Geckzilla sliding down the sides and falling into the thing
geckzilla wrote:
The photo is abundantly clear to me. I don't think I could understand it much better without sliding down the sides and falling into the thing.

One thing I have wondered about this is whether there is a conical pile of soil at the bottom of the hole to match all of the displaced material above. That could be one reason why the light inside the hole looks a bit darker. Not only is there less light but it could be the shaded side of a pile of soil.
It's just Corvettes all the way down:
http://www.usatoday.com/story/news/nati ... e/5417171/
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by Beyond » Sun Mar 09, 2014 10:14 pm

The caption refers to it as a "cone", not a depression. A cone is what i see. It would seem that anything landing inside of the cone, would end up sliding down through the hole at the bottom, just as on earth, anything entering an Antlion's cone trap, would find itself heading downward towards the Antlion that's waiting down in the center of the cone.
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by Chris Peterson » Sun Mar 09, 2014 10:20 pm

Beyond wrote:The caption refers to it as a "cone", not a depression. A cone is what i see. It would seem that anything landing inside of the cone, would end up sliding down through the hole at the bottom, just as on earth, anything entering an Antlion's cone trap, would find itself heading downward towards the Antlion that's waiting down in the center of the cone.
I don't see any reference in the caption to cones. It's called a hole surrounded by a crater. Craters of this size have somewhat conical interiors. The important point is that the feature is concave- it is a surface depression. The earlier reference to "cone" appeared to be suggesting the feature is actually convex- a hole on the apex of a hill.
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by Beyond » Sun Mar 09, 2014 10:46 pm

Chris Peterson wrote:
Beyond wrote:The caption refers to it as a "cone", not a depression. A cone is what i see. It would seem that anything landing inside of the cone, would end up sliding down through the hole at the bottom, just as on earth, anything entering an Antlion's cone trap, would find itself heading downward towards the Antlion that's waiting down in the center of the cone.
I don't see any reference in the caption to cones. It's called a hole surrounded by a crater. Craters of this size have somewhat conical interiors. The important point is that the feature is concave- it is a surface depression. The earlier reference to "cone" appeared to be suggesting the feature is actually convex- a hole on the apex of a hill.
Dang!! I must be older than i think i am. After i saw you use the word "depression", i went back through the caption and couldn't find "depression" anywhere, but did see the word "cone".
Now that you've replied with not being able to find the word "cone" anywhere in the caption, i just went back through it and can't find it either and also still can't find "depression".
I figure this tired old brain did a short-cut and translated 'crater' to 'cone' and neglected to tell me. :lol2:
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by zaqs » Mon Mar 10, 2014 12:38 am

Chris Peterson wrote:
Guest wrote:I do not understand the illumination of this picture.

The external shadow of the cone is on the same direction as the illuminated bottom of the hole. It is as there are two sources of light.
I think you're being fooled by the famous concave/convex optical illusion. There is no cone here. The hole is at the bottom of a depression.
I'm agree with you, the shades are not coherent for a cone (and i'm not a troll :roll: )

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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by Nitpicker » Mon Mar 10, 2014 2:05 am

If the depression is volcanic and caused by subsidence, would it be more correct to refer to it as a caldera and not a crater?

Note that the depression is ~300 times smaller than the 47km diameter, 5km deep caldera at the peak of Pavonis Mons. Also note that the side slopes of the volcano are rather mild, 7% on average (4° incline). Whilst I'm not sure I'd like to live/exist/barely survive that close to a massive shield volcano -- however lacking in recent activity -- I suppose it might be warmer than some parts of Mars.

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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by rstevenson » Mon Mar 10, 2014 2:21 am

People keep talking about this as if a hole just appeared in the roof of a cavern and loose material fell in, making a crater. But that does not explain the rim around the crater. The only way you get a rim is when a rock of some sort crashes into the planet. In this case it happened to hit the side of a volcano where there are lava tubes, and it punched a hole in one of the tubes. And yes, probably some material then slid down the sides of the crater into the hole. But it's still, I'm sure, an impact crater.

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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by geckzilla » Mon Mar 10, 2014 2:27 am

Rob, I would guess that the rim could be caused by prevailing winds. It is slight enough. Something like a dune formation.
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by rstevenson » Mon Mar 10, 2014 2:39 am

Even if an impact is a simpler explanation? It gives us the rim and the hole too.

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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by Nitpicker » Mon Mar 10, 2014 2:51 am

From what I can tell, I think this depression is on the NE slope of the volcano and the image is oriented with North up. (Someone please correct me if I am wrong.) So, the volcano's slope is falling from bottom-left to top-right in the image. The top-right of the edge of the depression is therefore a slight crest, and may be a natural place for a dust dune to form.

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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by geckzilla » Mon Mar 10, 2014 2:58 am

rstevenson wrote:Even if an impact is a simpler explanation? It gives us the rim and the hole too.

Rob
It could be a crater, sure, but the rim to me looks wind blown and the surrounding ground texture also looks to me something like some kind of soil fine enough to be blown. So visual evidence of a crater could be obliterated by erosion. I find it difficult to determine if it is just a place where a lava tunnel was weak in the ceiling or if an impact was required to form it. It seems too clean to be a crater.
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by Nitpicker » Mon Mar 10, 2014 3:06 am

It seems unlikely (to me) that any impact capable of forming this depression, did not completely obliterate the cavern below. And the impact is unlikely to have occurred before the creation of the lava tube, since the entire mountain is formed by lava flow (if I understand my volcanology well enough).

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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by neufer » Mon Mar 10, 2014 3:36 am

geckzilla wrote:
rstevenson wrote:
Even if an impact is a simpler explanation? It gives us the rim and the hole too.
It could be a crater, sure, but the rim to me looks wind blown and the surrounding ground texture also looks to me something like some kind of soil fine enough to be blown. So visual evidence of a crater could be obliterated by erosion. I find it difficult to determine if it is just a place where a lava tunnel was weak in the ceiling or if an impact was required to form it. It seems too clean to be a crater.

Assuming that the dunes on the left of the crater are longitudinal Seif dunes the prevailing winds probably blow from left to right. Hence, the right crater rim will end up being pelted by a turbulent sheet of light soil/sand flying off the left crater rim. This turbulent sheet would probably deposit a horizontal dune along the right crater rim followed by horizontal dune ripples.
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by geckzilla » Mon Mar 10, 2014 3:48 am

Pits likely formed as a result of multiple collapsed sections of lava tubes on the moon:
http://www.nasa.gov/mission_pages/LRO/m ... chain.html

A similar argument about the possibility of impacts is discussed. This example is much more clearly along some linear geological (selenological?) feature.
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Re: APOD: A Hole in Mars (2014 Mar 09)

Post by Thor127 » Wed Mar 12, 2014 1:39 pm

It appears to be a sink hole. The lack of ejecta seems to rule out an impact crater.