I have his reply, if you want to read it. He still votes for a blue sky on Mars and I cannot argue with him becuase I do not know enough. One of his statements is that if blue is removed from the spectrum the resulting colour is yellow NOT red; which could be perfectly true; I just do not know!
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Hi Donn,
Thanks for your response. I found it interesting.
It seems you have put considerable effort into all this.
The links below affirm the fact that the colour of the Martian sky can be non-other than blue.
It appears that NASA have NOT, from the evidence at hand (essentially 'proven' by Keith Laney's processing - see below), been 'changing' the colours; but they most certainly HAVE been mis-representing the colours pictures to the public by incorrectly combining the colour channels from the
Pancam sources.
Of course, I agree with you that the Martian sky, like that our planet, can change depending on conditions; however, the Martian sky is generally a rather light blue (as could be expected, due to it's thin atmosphere) as is proven by the (truly) True Colour pictures composed by Keith Laney, the
NASA/Ames image processor, as taken from NASA's own original data (no, it has not been modified by Keith):
http://www.keithlaney.com/spirit_color_images.htm
Here Keith explains the process, very clearly as far as I'm concerned, of obtaining True Colour images from the raw NASA feed: http://www.keithlaney.com/spirit_color_images_calibration.htm
His processing of the colours in the correct way makes complete sense to me and I don't know why I didn't think of this in the first place (maybe because I'm not a space image processor *LOL*).
The reason the sky looks blue', and these are NASA's own words:
"The visible violet light has a wavelength of about 400 nm. Within the visible wavelength spectrum, violet and blue wavelengths are scattered more efficiently than other wavelengths. The sky looks blue, not violet, because
our eyes are more sensitive to blue light (the sun also emits more energy as blue light than as violet)."
(Hence this should apply to ANY planet with an atmosphere, such is the case with Mars.)
See:
http://eosweb.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html
I really don't wish to see this kind of thing turn into an argument and perhaps it would be a good idea for us to sit down and discuss it sometime.
To keep things objective, I have to point out one or two erroneous conclusions you have put forward in your response document:
'Point 2'
"The dust, some of the particles being large enough, reflects the other colours and because the blue (And others? Unsure.) has/have been subtracted this causes a general scattering of red - such that wherever you look there is a supply of red light. I emphasize this because the red light is not
"effectively removed" at all; no more so than blue light is removed from Earth's sky by Raleigh scattering."
Specifically, I am referring to the statements, "These have a special effect on the higher-frequency wavelengths, they absorb blue light.", and (this),"causes a general scattering of red". -This is simply NOT the case: An absorption of Blue would cause a reflection (and some scattering) of Yellow, NOT Red-
Also, I do not see how you can say that it is a fundamental mistake to say that, "The secondary effect of the dust particles would be one of scattering the incoming light, and thus causing objects on the ground to appear turquoise (or 'bluish') by effectively removing the red component from the
direct light." I would certainly say that my wording here is perhaps NOT 100% accurate, and for that I apologise, since I do NOT intend to imply that ALL of the 'red component' from the direct light is removed. I would say, however, that a large percentage would potentially be removed, depending on the degree of scattering, in the case of having a 'red-scattering' Martian atmosphere.
I certainly do NOT mean to imply that the Red light which would be scattered in this case will NOT reach the surface; it will, however, reach it in diffuse form (the degree of which would depend on a number of factors).
This would effectively mean that one could expect to see Red shadows (of
varying degrees, of course, depending upon the variables).
You asked me to show you a link to show you that the incoming light on Earth
has a Yellow-tinge, as I asserted earlier. I did a quick search and found
this http://www.atmos.umd.edu/~ezra/whyblue/whypaper.html which you might want to
look at. I quote from it: "Since a lot of the blue light is no longer in
the original direction, the remaining light becomes more yellow."
An extension of my statement that the direct sunlight reaching the Earth's
surface is tinged with Yellow, and something which I did NOT want to get
into earlier since it is diverging from the point of the discussion, is that
due to the direct Sunlight on Earth having (some of) the Blue spectrum
removed from it one can expect all terrestrial shadows, especially at Noon,
to be Bluish (which they ARE, but we do NOT notice this since the direct
Sunlight is way too bright and creates too much contrast). The Blue shadows
on Earth are, of course, due to the diffuse Blue light scattered in the
atmosphere. The fact that the direct Sunlight illuminating everything on
the surface of our planet is in fact Yellow light is something that most
people are NOT aware of since our eyes are naturally adjusted to this
'natural' light.
It is important for us to distinguish between Raleigh Scattering and dust
particle scattering, and do NOT think that either of us have been clear on
this. One thing that has become clearer to me is that Raleigh Scattering
occurs due to the fact that Blue (and violet) light scatters much easier
than do the other colours in the spectrum (this re-affirms the point that
virtually ALL planets with atmospheres will exhibit Blue light Raleigh
Scattering).
Dust particle reflection (of which scattering is a secondary effect, though
they can be argued as being one and the same in most cases, I believe), will
of course, affect the general 'ambient' (diffuse) colour of it's
surroundings and align it with the particular colour spectrum reflected by
the particles (i.e. 'Yellow dust', especially in the lower atmosphere, will
have an effect of causing objects on the surface to appear more 'Yellow', if
the particles are in close proximity to the surface and their density is
high enough).
This is straight-forward physics and it needn't be as complex as some people
believe it to be. We should keep the fundamental Light Spectrum physics in
mind when we look at these things, and it is indeed very simple: Absorption
and Reflection of varying wavelengths of light - that is all that is going
on here - Subtraction and Addition - there is NO 'in-between'.
The statement quoted by someone in your Response document, "The reddish
particles PREFERENTIALLY ABSORB BLUE LIGHT and effectively act as mirrors by
scattering the remaining wavelengths: the color of the atmosphere is
therefore pinkish, like the particles themselves", is incorrect since the
'remaining wavelengths' would form Yellow, NOT 'Pink' or even Red; unless
they absorb the Turquoise (Cyan) colours, which is probably the case. So,
the Limonite is absorbing the Cyan wavelengths? Ok, so then Red is being
reflected and scattered, which would mean that we could have a limited 'Red
colouring' of objects on the surface; however, this will remain very limited
and will be DIFFUSE in nature which would mean that the Direct Sunlight
coming through will create too much of a contrast of Pure Light in order to
distinguish it since the Direct Sunlight is by far much more intense than
the reflected (diffuse) light created by the Limonite. If this were NOT the
case (where we have Greater Diffusion than Direct Light), we would see
badly-defined shadows on the surface.
Anyway, I think we now have our answers as to the True Colours of Mars,
thanks to Keith Laney.
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So, what say you? I'm back to square 1 !