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




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