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From: Martin Shough <parcellular.nul> Date: Thu, 14 Jun 2007 19:24:45 +0100 Fwd Date: Thu, 14 Jun 2007 16:14:00 -0400 Subject: Martian Colours [was: Mars Rover Finds >From: Nick Balaskas <Nikolaos.nul> >To: UFO UpDates - Toronto <ufoupdates.nul> >Date: Mon, 11 Jun 2007 12:36:14 -0400 (Eastern Daylight Time) >Subject: Re: Mars Rover Finds 'Puddles' On The Planet's Surface >>From: Martin Shough <parcellular.nul> >>To: <ufoupdates.nul> >>Date: Sun, 10 Jun 2007 22:47:47 +0100 >>Subject: Re: Mars Rover Finds 'Puddles' On The Planet's Surface >>>From: Nick Balaskas <Nikolaos.nul> >>>To: UFO UpDates - Toronto <ufoupdates.nul> >>>Date: Sun, 10 Jun 2007 15:06:26 -0400 (Eastern Daylight Time) >>>Subject: Re: Mars Rover Finds 'Puddles' On The Planet's >>>Surface ><snip> >>>For further evidence as to why I believe that the original >>>colour Viking images were the correct ones you simply have to >>>look at the red, white and blue U.S. flag printed on the two >>>Viking Mars landers. The U.S. flag's colours look much closer >>>to the true colours when seen in the original images than in >>>those corrected reddish tinted ones. >>Doesn't the colour depend on the ambient illumination, which >>involves the scattering properties of the Martian atmosphere >>and the reflectance at different frequencies of the surfaces >>of the local landscape? >Yes, the apparent colour of the Martian rocks and other surface >features will depend, in part, on the ambient illumination but >is this ambient illumination on Mars really red? It is not. The >reflected light off the Martian surfaces does not contribute to >the colour of the Martian sky anymore than the green fields of >Saskatchewan make the prairie skies look green. Of course I wasn't suggesting that the sky colour is caused by reflection from the land, that's just silly. The question I asked was directed specifically at your argument based on the colours of the flag, because you seemed not to be alive to the issues of callibration. You said that we "simply" have to look at which Mars images show the American flag with the "true" colours of a flag on Earth to see which are genuine. But the comparison is far from simple, it seems to me. It is not a question of the camera passively 'just looking'. Digital data from separate colour channels gets rendered into images by software that needs to take account of sensor responses and filter spectra that aren't flat, and you can't do this at all without making guesses about what the natural colour balance 'should' be. The whole process is obviously artificial, just like when you dye a flag in the first place this is an artificial process adapted to the light you are doing it in, and you need to know if there is some reason to expect the colour balance of the local daylight to be different on Mars. And in this sense the issue is connected to the colour of the sky. What you seem to be alleging is that the sky of Mars seen by an average human eye _should_ be what you call "a blue Earth-like sky". But the pressure, gas composition and general cleanliness of the Earth's atmosphere and the sun brightness are (surprise, surpise!) 'just right' for producing a bright blue Earth-like sky (Google a rather famous paper by Bohren and Fraser 1985), whereas Mars is probably not. What Rayleigh scattering there is on Mars will be blue, but unfortunately the pressure is 1/100 that of Earth and the scattered brightness of the blue light will be proportionately weak, and the sunlight reaching Mars is about 1/2 as bright to start with. So even though you will naturally adjust the equivalent exposure of your camera to give you a nice bright image this doesn't mean the naked-eye Martian sky would 'really' be bright, it wouldn't, it will be dark compared to Earth. The blue scattering is a large-angle scattering that favours higher elevations towards the Martian zenith, where the atmosphere is thinnest, and here it will a be a tint in a dark sky unlike the bright blue of earth (maybe think of the zenith above a very high altitude aircraft on Earth). In addition impurities in the atmosphere will tend to overwhelm the Rayleigh effect, especially where your cameras are going to be looking. At lower elevations as the sky brightens towards the horizon the saturation of the zenithal blue will pale and fade and here the colour could well be dominated by Mie scattering from hazes of fine mineral dust which remain a long time in suspension on Mars and will preferentially absorb the blue, reddening the light. This is the sky that will appears in your landscape panoramas and horizon shots. Another factor that might be involved is that oxygen molecules are virtually absent, and not only does 03 (ozone) help scrub fine dusts out of Earth's atmosphere, the scattering from the 20% of 02 molecules is said to be responsible for most of the Rayleigh blue in Earth's atmosphere, so that the reducing atmosphere of the early Earth would have been a different colour (unfortunately I've mislaid the reference for this interesting point). >>If you see a blue car under a sodium lamp it might look like >>mud, and if (for the sake of argument) sunlight everywhere had >>that same narrow sodium emission peak then "mud" would be the >>"true colour" of all similar cars. >Just like the Earth, the source of light on Mars is the Sun >which is predominately yellow in colour, not red. Unlike Sodium >light, the solar spectrum is not limited to just two nearly >identical wavelengths of yellow light so your example is not a >valid one. Surely you can understand the difference between an illustration of principle and a literal model? And thank you, we are all aware that Mars is illuminated by the same sun as Earth, but the colour of the light incident on a rock or a flag is not just the emission spectrum of the sun, even on Earth, and the difference is likely to be greater on Mars. You point out that the sun's light is predominant yellow - indeed so, and the degree to which direct sunlight on Mars dominates over the blueing effect of Rayleigh scattering (very small compared to that on Earth) will tend to emphasise that warm colour. The region of the Earth's surface around you receives a contribution in the blue part of the spectrum from photons which, were it not for the large-angle Rayleigh scattering high in the atmosphere, would never reach it and would contribute nothing to the colour balance at your retina or camera. These are real blue photons, a real net blueing of the light reflected by objects around you, not some illusion. If the atmosphere were to leak away this indirect light would become progressively less bright, leaving the total incident light increasingly dominated by the yellow solar emission spectrum, i.e. the net colour balance would become warmer. How much warmer when you get down to 1/100th atmosphere? I don't know, but some. And what allowance should be made for scattered light from suspended dust?O for the light reflected up from the surrounding rusty terrain? The point is that corrections to your Martian colour settings have to be made against a Martian reference spectrum, and there is some legitimate debate about this. Crying "conspiracy" seems way too easy. <snip> >>From a strictly objective point of view the "true" colour of a >>flag on Mars will be the spectrum it reflects in Martian light, >>not the one you may be familiar with. Does your theory about >>NASA suppressing the "correct" Viking images allow for this? >Martin, when you examine pictures of the Martian sky at higher >elevations taken by the two Viking landers in 1976, as well as >pictures taken by all other NASA landers, you will see that the >colour of the sky is NOT red. No need to shout, Nick, since I never said it was. One would expect to see the maximum blueness in views at "higher elevations", with the caveat that what you are seeing is not going to be a realistic (i.e., naked eye) sky brightness. But the iconic Viking images which are at issue show horizon views, and in many similar views from Pathfinder, Spirit and Opportunity I see similar sky tones, with some variation presumably due to changing dust concentrations and sun positions. >This morning commuters from the suburbs driving to work in >Toronto will have noticed a brown haze low over the city. Like >the Mars landers pictures, if a commuter were to snap a picture >of the city skyline and send it to a friend who has never been >to Toronto, he/she may also come to the erroneous conclusion >that the colour of the sky over Toronto is brown! In fact, the >sky above the brown haze all the way to the zenith was a bright >clear blue colour. So you acknowledge that even on a planet with your bright and clear "blue Earth-like sky", photos could show a sepia-coloured skyline caused by dust haze. How much _more_ likely is this under a much darker Mars-like sky filled with suspended oxide dust? The Viking sky colour therefore doesn't make your case for a conspiracy of suppression, even if your guess about the bright blue of the Martian zenith is correct. So we revert to the flag and callibration swatches on the the lander, remembering that we are likely to be in a situation where, compared to Earth, direct illumination is reduced, Rayleigh blueing is dramatically reduced, dust haze reddening is dramatically increased, and reflected ground light will also be generally red (unless you are also wanting to 'correct' hundreds of years of remote ground- and space-based optical and instrument observation of the Red Planet) <snip> >If JPL staff did not make those colour adjustments to the first >images obtained from the surface of Mars back in 1976 (a few of >these "uncorrected" images did make it in magazines soon after >they were displayed live from Mars), I believe that man would >have already settled on Mars and become a multi-planet species. The 'original' images were as seen on monitors where the colour settings had just been left at default values "around the middle" of their ranges, according to Levin, and JPL staff subsequently altered or 'tweaked' these to turn the rather more blue-grey colour balance to a warmer one. NASA does not appear to argue with this. It happened, and the original balance that Levin saw was also seen by the press and the world - the first released picture _was_ less red than the subsequent versions. NASA's explanation is that this was a raw false-colour picture released because of a cock-up due to haste, before corrections had been made for unavoidable imperfections in the colour filters (in particular, a significant leakage into the blue channel from the IR). I've seen scepticism expressed - how likely is such a highly convenient cock-up? The conspiracy theory is that this was a raw _true-colour_ picture released because of a cock-up due to haste, which was rapidly covered up because the pictures were part of a plot to deceive the public. Inverting the sceptical response, how likely is such a highly _in_convenient cock-up in the context of what is alleged to be a massive conspiracy of deception? The idea behind the Levin story seems to be that just leaving your colour settings at a default median value gives you a raw 'true' colour, whilst active correction is a sort of cheat. But I don't think this means much where the colour response of filters is not flat, where it is understood ahead of time that the balance is going to have to be callibrated against some seat-of-the-pants reference standard (even if the difficulty and sensitivity of that task were not appreciated in the heat of the moment), and where that standard remains even today a matter of debate. The Levins themselves (father and son) wrote a paper in 2003 called Solving The Color Callibration Problem which (by definition), acknowledges that there is a problem and points out that there is "no consensus" on what are the properly callibrated colours of soil, rocks and sky on Mars, precisely because there is "great uncertainty in the illumination spectrum" which means that "whilst the reflectivity of the [callibration] charts is well known, the spectrum of their illumination on Mars is not." I think that's something like what I was saying. http://mars.spherix.com/spie2003/SPIE_2003_Color_Paper.htm They offer some ancillary arguments for adopting a callibration that favours a bluer result, some of which I agree are interesting (if reddened reflected light dominates over direct light then this might not be consistent with the existence of strong shadows, and Pathfinder's solar cells would not have been efficient in red light). But it's really a much more subtle and difficult issue than it seems when you first hear about "original" images being suspiciously "altered". Maybe Tom Mutch, leader of the original Viking imaging team, gave the best and most sensible _qualified_ answer to the question of how realistic the lander images are: http://www.hq.nasa.gov/office/pao/History/SP-4212/ch11-4.html "In summary," Mutch said, "the color of the Martian scene, perceived by the necessarily abnormal eyes of Viking, is elusive. In response to the inevitable question: 'Is that exactly how it would look if I were standing on Mars?' a qualified 'yes' is in order." Maybe soon someone will be able to go there and have a proper look. Then they can paint the scene for us. We'll just have to hope they have 'normal' colour vision. Martin Shough Listen to 'Strange Days... Indeed' - The PodCast See: http://www.virtuallystrange.net/ufo/sdi/program/
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