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geology of Gale Crater and Mount Sharp
David Palmer
post Jun 24 2014, 10:43 PM
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QUOTE (mcaplinger @ Jun 24 2014, 01:56 PM) *
Moreover, as has been discussed at length on this forum, the green tint is largely if not completely an artifact of the Mastcam IR cut filter bandpass, and is not a real feature of the martian sky.



I DID state that the actual sky color of Mars is near-neutral, and thus that any liminitations in the instrumentation, or slopiness in the processing of the data (or its ultimate display on a monitor) will skew it towards either the red or green or blue....and it is clearly the case that the earlier (especially Viking) images over-did the red. This is confirmed by the fact that Mars doesn't look that red in a telescope....it is actually a slightly ruddy yellow.....similar to desert terrains on Earth.
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djellison
post Jun 24 2014, 10:51 PM
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QUOTE (David Palmer @ Jun 24 2014, 02:43 PM) *
I DID state that the actual sky color of Mars is near-neutral,


And this 'statement' is contrary to the two peer reviewed articles I cited above. Despite being from different scientists using instruments on different spacecraft, 7 years apart, on different parts of the planet - the Pathfinder and MER (both A and B ) results are in excellent agreement regarding the color of the sky on Mars.
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David Palmer
post Jun 24 2014, 11:05 PM
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QUOTE (djellison @ Jun 24 2014, 12:59 PM) *
Of the entire 4 page work - this is the part that I find most obviously factually incorrect....


From section 4 - "there are actually very few perspectives within Gale that show the sky directly above the horizon"

This is patently false. Every image you include after that comment has the sky in it. Every. single. one. There are hundreds - hundreds and hundreds that do - as well as the end-of-drive MAHLI postcards that get taken after each significant drive. Your claim is just totally false.



My reply:
This is NOT correct....because any perspective within Gale Crater is ringed by hills. I never did say that sky wasn't showing in the pics....just that the sky DIRECTLY above the horizon is cut off. So the above comment involves a misreading of my statements. Also, I explicitely stated at one point that the photo I was describing only created the illusion of sky, that there were hazy hills in the background (it almost looks like sky, but you can see the outline of hills if you look close)....so it is incorrect to say that every image I include has sky.

And in fact, most of the photos I use ARE NASA's.....taken directly from the "raw image" files at http://mars.jpl.nasa.gov/msl/. The Olivier de Goursac panoramas are put in as a bonus (to be pretty), and are not what I'm basing my primary conclusions on.

In any case, I AM doing additional work on giving credits for images, so that is a valid criticism of my essay....this has been a work in progress for one year now, so it's not surprising that it needs some additional tweaking.
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serpens
post Jun 25 2014, 12:30 AM
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I think perhaps everyone needs to take a deep breath and have a nice cup of tea. Colour by definition is a human perception of reflected frequencies and that perception can differ to a degree between individuals. So the human perception of the colours of Mars, after selective frequency transmission and tweaking should best be described as a very good approximation and really is not worthy of this type of argument.

The "artesian hydrant" scenario you refer to would correlate to a spring mound effect and this has been considered previously as a possible influence on the development of Mount Sharp, but is considered a very low probability. For instance Figure 2 of the 2013 paper by Kite et al "Growth and form of the mound in Gale Crater, Mars: Slope-wind enhanced erosion and transport" provides a good comparative illustration of the stratigraphy expected from various influence forming Mount Sharp, including spring mounds. The thing with Gale is that we are looking at an erosional end state and really do not have a clue as to the structure of the crater, or floor depth when some of the features you discuss were formed. But if we consider the lower (seemingly non Aeolian) layers of Mount Sharp in the light of Mr Steno's rules and the apparent structural strength of some of the sedimentary rock observed by Curiosity then at some time there was a lot of material above the current floor that Curiosity is traversing. Despite the exceptional capability of orbital imaging and Curiosity's ground truth , the scope of "hands on" analysis is very limited. This means that even after Curiosity investigates accessible parts of Mount Sharp there will probably be multiple hypotheses alive and well at the end of the day, including possibly some of yours.

You make some good observations and interpretations but perhaps should revisit potential alternative influences. In analysing Gale I think it is necessary to consider it in the context the surrounding region and the somewhat unique North South crater rim height difference due to the position on the dichotomy boundary. There is reasonable evidence that a Mars Ocean existed, possibly a couple of ocean recharges, and that would have implications for partial flooding of Gale from an aquifer.
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Astro0
post Jun 25 2014, 01:10 AM
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David Palmer
post Jun 25 2014, 01:22 AM
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reply to Serpens:
I am very aware of the spring mound hypothesis and discuss it in my essay, and state that it could have contributed to the core of Mt. Sharp, but Rossi et al. envision Mt. Sharp being built of evaporate deposits, and of such activity being confined to the early geothermal phase of Gale Crater, so it is a different hypothesis from mine, where I am claiming that the artesian hydrant is an ongoing phonomenon that was active again fairly recently. And while the northern ocean would indeed have filled Gale Crater, I am claiming (on the basis of the Mt. Sharp channels and their delta deposits) that a deep lake existed in Gale LONG after that ocean had dried up. And the natural interpretation would be that a still-active aquifer occasionally commutes to the surface when conditions are right, the water table of which can be calculated to be well above the half-way point of Mt. Sharp on the basis of the surrounding terrain and the geologically recent Cerberus Fossae outflows.

Dave
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vikingmars
post Jun 25 2014, 02:45 PM
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About the color of the Martian sky : we had already this debate loooong ago in 1982 while processing the Viking Lander 1 images from its Monitor mission at the IPL in JPL. The two scientific papers cited hereabove (Bell et Al.: "Chromaticity of the Martian Sky" and Maki et Al. : "The color of Mars: Spectrophotometric measurements at the Pathfinder landing site") truly depict the real colors of the Martian sky which could be referred globally as a "salmon pink". However, the saturation of its color depends a lot on the quantity of dust (aerosols) suspended into the atmosphere : as you know, it can vary during the seasons.

Here is a processing done over 999 Sols with images from the VL1 Monitor mission that show the variations of dust suspended in the atmosphere over its landing site. Although the sky may look "blue" to your eyes 30 over the horizon, this is an artifact done by the processing of your brain : a problem also identified long ago also by the astronomers watching the Red Planet through their telescopes. Now, if you sample carefully the colors of the sky 30 above the horizon, when the dust opacity is at its minimum (Sols 1335 and 1957), you will notice that the Martian sky never loses its salmon pink hue. Its only much less saturated than normal and only tends to be more "grayish". But, it always keeps its "salmon pink" color just above the horizon.
So the Martian sky is definitely NOT blue !

PS : There was a huge dust storm blowing over the VL1 lander on Sol 1742 : this explains why you see a reverse in the luminosity of the sky.

Attached Image
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serpens
post Jun 27 2014, 03:44 AM
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Dave, I am answering here rather than by email because I would be interested in the take of a few of the extremely knowledgeable posters on this forum. Note by the way that I said influenced by spring mound effects, not that this is a sole cause. When I suggested looking at a regional context I was considering the topography surrounding Gale, in particular the North and North West which indicates significant erosion and indicates that that Gale was possible once covered to a regional level above the crater rim and then exhumed. If so we have to consider that the channels originating on Mount Sharp could be the end remnants of drainage channels that originated well outside the crater when the crater was filled to a common level. Erosion and exhumation of the crater and Aeolian deposition of the higher section of Mount Sharp would have removed any evidence. The implication of course is that any lake corresponding to the Mt Sharp deltas would have been quite shallow. There are remnant channels all over the area and in generating hypotheses on Gale we have to consider every possibility with respect to the changing regional terrain over many billions of years and the fact that we do not really have a clue as to the actual environment when some of these features formed. Atmospheric pressure, temperature, wind speed or water content. My take is that for a reasonably long period it was one heck of a dynamic environment and the topography of Gale and surrounds was nothing like it is today. There are lots of potential hypothesis for gale and the evidence of a lacustrine environment and I throw the above in simply to point out that every possibility has to be considered.
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David Palmer
post Jun 27 2014, 04:07 AM
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reply to Serpens:
The basis of my argument for the artesian hydrant in Mt. Sharp, is that the deltas (in particular the delta deposit of what I have christened the "Northern Channel," which Curiosity is now driving towards) would have been long-since worn away unless they were very recent, given the measured wind erosion rate in the local environment (low crater retention ages as calculated from orbital imagery, and also in-situ measurements of wind erosion rates by Curiosity). So these channels had to flow, and their deltas be deposited, when the topography of Gale, Mt. Sharp, and the surrounding terrain was essentially as we see it today. If there had been more than several hundred vertical feet of wind erosion (which appears to be only several tens of millions of years worth), any trace of those deltas would be gone.
And another reason that these channels could not have been part of any regional drainage system, is that they are oriented radially on the sides of a more-or-less conically-shaped mountain, so the mountain had to be the source of water.

Dave
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David Palmer
post Jun 27 2014, 09:35 AM
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reply to vikingmars ("About the color of the Martian sky"):

I never did say that the Martian sky was blue, rather that under CLEAR conditions it is close to neutral or grey. And when I refer to the "sky," I am considering the full dome of the sky, not just the area several degrees above the horizon. Just as on Earth, if there is white or brown haze near the horizon, we still say that the sky is "blue," not that it is white or brown, because blue is still the dominant color in the dome of the sky. But when we have a lander that is predominately photographing the landscape, a bias tends to be created such that the color of the sky just above the horizon, is psychologically perceived to be the defining color of the sky (in this case pinkish beige), because that's the only part of the sky showing in most photos, whereas if we were to step out onto the Martian surface and gaze upwards, we would take the color occupying the greater part of the sky's dome to be the defining color.....which in the case of Mars, is a mix of beige and grey or blue-green, under clear conditions, and better yet, at a below-datum location such as Gale Crater, where there will be more Raleigh scattering of the blue end of the spectrum. Also at Gale, the pinkish beige area just above the horizon is usually cut off by the surrounding hills.....so the net result at Gale is a fairly bright sky (even at the zeneth) that is usually a cross between beige and blue-green. The above examples that you site from the Viking 1 lander, have a darker sky under clear conditions than we ever see in Gale, due to the higher elevation and less of a total gas column, but under clear conditions I would still rate the Viking 1 sky as grey, or even blue-grey (and this is not due to the chromatic contrast issue that you mentioned....when I take pieces of paper and hide the surface and lower atmosphere in the illustration you provide, the higher areas under clear conditions still look bluish-grey to me).

NASA now seems to have realized their earlier failing in making the Viking photos WAY too red, and they have recalibrating photos archived that are said to approximate what a person would see on Mars, and they can be found at http://nssdc.gsfc.nasa.gov/photo_gallery/p...llery-mars.html
You will note that in some of these Viking photos the sky is green and in others grey (NOT red or orange or pink as in the old releases).

Dave
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scalbers
post Jun 29 2014, 06:40 PM
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I can point here to my post in another thread, where the brightness and color of Martian sky is simulated considering both Rayleigh scattering from the air molecules and Mie scattering from typical amounts of dust. Here it does look salmon/gray when one is away from the horizon and the sun. I haven't actually looked on a color diagram with some of the pixel values yet though.

http://www.unmannedspaceflight.com/index.p...mp;#entry206891

A back of the envelope calculation suggests the scattering from the dust aerosols would be roughly 16 times as much as the scattering from the gas part of the atmosphere in the darkest parts of the sky. I'm assuming aerosol optical depth is 0.2 on my envelope.


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marsophile
post Jun 30 2014, 04:02 AM
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Given the wide variation in the amount of dust, perhaps one should be wary about drawing conclusions from a calculation for one specific ("typical") level of dust.
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mcaplinger
post Jun 30 2014, 05:16 AM
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QUOTE (marsophile @ Jun 29 2014, 09:02 PM) *
perhaps one should be wary about drawing conclusions from a calculation...

With all due respect to the modeling, I'd only draw conclusions from properly-calibrated images, which I've seen none of in this thread (maybe the Viking images, though Viking calibration is somewhat problematic.) The coverage of the sky at different elevations has been very limited from MSL so far, though there was one zenith image taken with MAHLI if I recall correctly.

Of course, considering the title of this thread, the whole discussion of sky color is wildly off-topic.


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Disclaimer: This post is based on public information only. Any opinions are my own.
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serpens
post Jun 30 2014, 05:49 AM
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QUOTE (mcaplinger @ Jun 30 2014, 06:16 AM) *
Of course, considering the title of this thread, the whole discussion of sky color is wildly off-topic.


Amen to that
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vikingmars
post Jun 30 2014, 02:49 PM
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QUOTE (David Palmer @ Jun 27 2014, 11:35 AM) *
reply to vikingmars ("About the color of the Martian sky"):

Thank you Dave for your kind comments.
The link you are referring to are the images processed in 1990 by Mary-Ann Dale-Bannister (Washington University, St. Louis) who, successfully was the first to try combining high-resolution black-and-white images (BB3 and BB4 diodes) with the 3 lower-resolution channels (RED, GRN and BLU) images from the Viking Landers.
When the images were issued, we just stared at them, much amazed and impressed by their quality and their good definition.
BUT, the color balance was not done EXCEPT for the mosaic (link http://nssdc.gsfc.nasa.gov/photo_gallery/c...nglander2-1.txt ) which shows colors close the real ones and that was the ony mosaic aknowledged to bear "Real Colors at VL-2" by Mary-Ann herself (but not for the other ones).
A good proof is that, within the same data set, are some other processings done by Mary-Ann in which you can found one which is close to my mosaic BUT which does not reflect the real colors unfortunately (see herebelow). Her goal was to give a lecture about the effects of winds on Mars but NOT about the colors of Mars...

And, I must agree with you on this specific point, it is true that, under very, very low atmospheric opacities, the Martian sky tends to turn a "grayish-pink" 30 above the horizon smile.gif
Attached Image
<== (Mary-Ann processing done here only to show the low sky luminosity under a dust storm, NOT for the real colors)
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