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May 23, 2007, HiRISE release
mchan
post May 24 2007, 03:20 AM
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MRO is in sun-synchronous orbit so the nadir point below is either early morning or late afternoon local time to provide decent shadowing for most imaging. The orbit would have to be substantially adjusted to get a fly over noon local time.

What is the range of illumination angles due to seasonal variations from axial tilt?
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CosmicRocker
post May 24 2007, 04:55 AM
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QUOTE (elakdawalla @ May 23 2007, 03:41 PM) *
... It doesn't look to me like that one's on the Cushing et al list (see my blog entry for that list and image). Or is it?
I don't see it in the seven that they show. I think antipode is correct, suggesting, "Who know how many of these things might turn up, and who knows how big they might be?"

I took a hint from your discovery and started panning over that region with HRSCview, and I thought I saw several very small skylights that were not very convincing. I also came across the one you posted an image of, and then finally another apparently "new" one that was large enough to be convincing. It's actually a pair, but the second one is rather small.
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...Tom
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AndyG
post May 24 2007, 07:26 AM
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Excellent. ~100m across? >200m deep? What a place to dome over and start building.

Andy
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ustrax
post May 24 2007, 09:36 AM
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Is it really impossible to dig out something from the black?

So this can only be a fantasy...

What I did was an attempt to enhance some features that were already visible in the image shown here on the right but that also come out from the original one.

Note: I know that it has already a name but Cernunnos, the Celtic God of Fertility, Life, Wealth and the Underworld sounded just perfect for the occasion... tongue.gif


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djellison
post May 24 2007, 10:22 AM
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Yes - it really is impossible. That's not details - that's noise.

Doug
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Guest_zoost_*
post May 24 2007, 11:25 AM
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QUOTE (stevo @ May 23 2007, 06:21 PM) *
.. that anything that dark and featureless has to be liquidish, sort-of. Or not.


Judging from the non-sense making overhangs visible in the picture, I vote for liquid.
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djellison
post May 24 2007, 12:12 PM
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What sort of liquid at .006 bar and -80degC? Would a liquid not have specular reflection etc. Orbital images of lakes and seas on earth are not black. Do not confuse the 'black lakes' of Cassini Radar imagery with the optical wavelengths of HiRISE.

There is nothing nonsensical in the concept of overhangs. We see them on Earth - they could be even more extreme in the 1/3rd G of Mars.

Doug
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climber
post May 24 2007, 12:33 PM
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I can't imagine anything else than a hole in a (lava) tube.
I don't remember the width of the path of MOLA. Somebody suggested that the depth can be find in the data. Is that possible?


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ngunn
post May 24 2007, 12:42 PM
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Regarding that overhang:

We are told that the sun was 38 degrees above the horizon, or 52 degrees from zenith. This means that if the angle of overhang was less than 52 degrees from the vertical the sun would be directly illuminating part of the overhang wall surface underneath the visible rim. This would add significantly to the level of downward illumination inside, so my guess is it's not happening. That makes the angle of overhang quite severe and the rim quite thin. The direct sunlight entering through the skylight would then be falling on the floor of the cavern some distance down-sun. The light scattered from that bright spot would then be shining upward, illuminating the interior roof but not the part of the floor in our line of sight.
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djellison
post May 24 2007, 12:42 PM
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^

"Oh, for those of you wanting to overlay MOLA data, it's already been tried, without any luck."

Doug
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Guest_zoost_*
post May 24 2007, 12:47 PM
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There must be some liquids (carbon based) that absorb most /all visible light (independent of the light frequency). Is there any evidence that this hole is a hollow tube / entrance to a cave / ? Why is there no light on one of the walls?
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djellison
post May 24 2007, 01:02 PM
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Because the ceiling is overhanging. It's a collapse to a lava tube. We've seen other hirise images that take this to the extreme just leaving a single piece over the top of the old lava tube - itself with overhanging edges (the 'bridge' picture). The mechanism behind a formation like this is certainly not that unusual and indeed terrestrial analogues are not uncommon either.

http://www.texasbeyondhistory.net/plateaus/images/ap17.html

We're looking almost straight down it - so in actual fact it wouldn't take that much of an overhang to mean we don't see the sides even if they're lit.

That opening is 11,000 sq m . If there were 11,000 sq m of an unusual liquid - CRISM would have found it and we'd know about it. It's 8 or so Crism pixels across.

Doug
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akuo
post May 24 2007, 01:16 PM
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QUOTE (ustrax @ May 24 2007, 09:36 AM) *
Is it really impossible to dig out something from the black?

So this can only be a fantasy...


It's mostly noise, but on the scale of the whole hole, the upper part of the circle is lighter than the lower part. This might be the floor showing.

They just need to increase the exposure time for the next pass.


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Antti Kuosmanen
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ustrax
post May 24 2007, 01:17 PM
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QUOTE (djellison @ May 24 2007, 11:22 AM) *
Yes - it really is impossible. That's not details - that's noise.


Thanks Doug, not even a tiny single chance? smile.gif

I've justaposed what I got to the original image and that damn noise has the ability to act as if following some features in the contour of the rim... rolleyes.gif
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djellison
post May 24 2007, 01:19 PM
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I'm not saying that you're not pulling out three different shades from an 8 bit greyscale image, for which some photons from the bottom my be responsible. I'm saying it's impossible to say "that's the bottom". Lest we forget, similar techniques turned a black dunefield into all sorts of things.

I'm not sure that HiRISE can take longer exposures - 4x4 binning might help but of course we can do that with the image on the ground already smile.gif

Doug
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