Pit Chain On Mars

[VanderL]

http://images.spaceref.com/news/2004/2004....21.R1400543.gif

Here we see an image of a "chain of pits". If one would see only one "pit", it would immediately have been labelled impact crater. Am I the only one who is not convinced by a process that supposes collapsed lava chambers where somehow the material "magically" disappeared to leave this chain?
So, are all pits craters, or all craters pits, or are they neither?

Cheers.

[antoniseb]

are all pits craters, or all craters pits, or are they neither?

I can't tell by looking. I imagine that it could have been a loosely assembled small asteroid that tumbled apart in the Martian atmosphere causing the chain, but your thought that it could be a series of sink-holes into an empty cave of some sort isn't out of the question either. No matter what it turns out to be, the picture is very striking. Thanks for posting it.

[Sp1ke]

Are they pits? They look to me like they rise above the surface rather than being craters. Presumably if the light source is at top right, they are bumps and if it's at bottom left, they're craters. Do we know for sure which it is?

[VanderL]

I linked to the enlarged image, here is the smaller image including the explanation (just scroll a bit down the page)
http://mars.astrobio.net/news/modules.php?...article&sid=390

Sp1ke, are you still seeing them as elevated? Maybe when you turn the picture (or yourself) upside down it becomes clear. It is an illusion because the direction of light is unfamiliar, you have no reference, but once you see it, they are obiously depressions. I wonder though why we must call them pits when they clearly look like craters.

Cheers.

[Sp1ke]

Knowing that the sun is coming from the lower left makes it clear that they are pits. But I don't know if it's just me but even hanging upside down over my PC, they still look like elevations, not depressions. It must be an optical illusion or just my perception. I'll have to take a trip over to Olympus Mons to see for myself

My guess is that they're called pits rather than craters since they're created by a surface collapse, rather than being impact craters.

[VanderL]

Well, thanks Sp1ke, the explanation is that they must be pits, because they are near a volcano and they are in a chain, but otherwise they look exactly like craters.
Imagine one of thos pits, solitary, in a plain. No one it his right mind would call it a pit then it would be an impact crater. Indistinguishable from a pit, with exactly the same features and dimensions, so how can these things that look exactly the same (to my untrained eye at least) be formed by completely different processes. It just doesn't make sense.

Cheers.

[Duane]

It is not just because of the location, size and orientation of this pit chain that the image science team felt that it likely arose from something within the volcano, as opposed to something striking the surface.

There is no evidence of ejecta, however faint, around the pits. This suggests that nothing was thrown out, so something had to have caused the area to depress. In the absence of material around the depression, material had to have sunk away from underneath it.


It would be the same with a pit in the middle of a plain. In the absence of an ejecta sheet, there would have to be another explanation other than an impact crater. This is especially true if it is a large crater. Such explanations could include volcanic origins, "sinkhole" type depressions, and probably others I can't think of right now In each case, there are clues to look for, and that is what the imaging teams do--look carefully at the object in question and try to locate clues to it's formation.

Great picture by the way, you continually amaze me with the images you locate

[VanderL]

There is no evidence of ejecta, however faint, around the pits. This suggests that nothing was thrown out, so something had to have caused the area to depress. In the absence of material around the depression, material had to have sunk away from underneath it.

Thanks Duane, at least that's an answer that would explain some of the features, although it is of course difficult to see from this image how the material has "sunk away" to leave the pits in this fashion. On Earth there is much more (water) erosion that can explain how material can sink away. Is this chain an exact expression of how chambers below the surface are arranged?

Cheers.

[Duane]

Intuitively VanderL I would say probably not, but the honest answer is I don't know.

Similar chambers on Earth (ie sinkholes) are often larger than the actual pit that forms, so on that alone I would suggest that the actual chambers may be different.

[VanderL]

Could there be a series of big caves undernath these sinkholes? Maybe the LGM are living there?

Cheers.

[Duane]

Ya, carving faces for sale to tourists that they expect to arrive in the near future