Why is the sky blue and Mars red?

[Donn]

Hi, first post here.

 

Have been trying to convince this conspiracy theorist (CT) that the red colour on Mars is perfectly natural and in the course of this have discovered that I don't know the basics of light and atmosphere colour.

I have looked it up on HowStuffWorks but I have some questions.

If anyone here can help me to answer that would help me to argue with the CT.

 

(If this is the wrong place to ask such a question, please point me in the right direction.)

 

If you look up at the blue sky you are seeing many things:

A. Blue light scatter-reflected billions of times such that when it hits your eye it comes from many many many sources.

 

B. The other wavelengths, some blue still mixed in, that are mixing together as they hit your eye to be seen as white.

B2. Could we say from this that unreflected light from the sun is always white?

 

C. However there is far more blue light coming in so you don't notice the white.

C2. Perhaps you notice it as a glare or a brightness?

 

If there is a massive dust-storm or huge fires or terrible smog-pollution we can see the effects of absorption of blue-light.

D. Does this then remove most of the blue light and scatter the reddish light through the dust, thus painting it red/brown/tan.

D2. Or is the reddish colour of such scenes due only to the absorption of blue light?

D3. And if so, should we see the reddish light at all since it's not reflecting off anything?

 

E. On Mars, if the dust absorbs the blue light can you explain the blue sunset effect?

 

Oh, one more:

 

On Earth, an explanation I have read for the yellow/red/orange sunsets is that because of the further distance the light has to travel through the atmosphere, by the time the light reaches your eye all the blue light has scattered away and so we only see the reds etc.

F. So, where did the blue scatter to?

G. Is the y/r/o colour due to scattering of these colours too?

H. Is this due to dust in the air or would the molecules that usually make blue sky work the same magic?

 

Thanks for any help.

[Cap'n Refsmmat]

A= right!

B=yes...

B2=YES!

C=No, it's like the atmosphere is visible. The atmosphere's molecules have a tiny bit of blue. You just can't see it over short distances.

C2= glare is reflected light

D= simply put, the smog is orange.

D2=yes

D3=it IS reflecting! off the dust!

E=blue sunset effect?

F=no! see earlier! it's to YOU!

G=it's dust! with more atmosphere in the way, it's more visible!

H=yes, dust!

Mars is red because A, not much oxygen, and B, it's really rusty.

[Donn]

Cap'n thanks,

 

Sunset answer (F) is not yet clear to me: Where did the blue go?

 

The Blue sunset effect (E), see <http://www.exo.net/~pauld/Mars/1viewsurface/viewfrommars.html>

 

I am asking about reflection and absorption because the CT thinks 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."

 

So I want to understand what happens to light as it travels through the layers of the atmosphere on Mars from space to soil.

 

Thanks.

 

PS: I have attached the CT's email to me, so people on this board can read it and perhaps comment. I have to go offline soon and will only be back tomorrow night!

ctheory.txt

[blike]

Check out this link over at NASA:

 

http://humbabe.arc.nasa.gov/mgcm/faq/sky.html

 

Some excerpts:

 

The Earth's blue sky is caused by Rayleigh scattering of light, a process first described by the British physicist Lord Rayleigh (1842-1919). Rayleigh scattering is the scattering of light by objects that are small in comparison to the wavelength of the light. Visible light from the sun contains a mixture of colors, each color corresponding to a different wavelength. Sunlight streaming into the Earth's atmosphere from one direction is scattered in all directions as it encounters air molecules. Shorter wavelengths of light (the blue end of the spectrum) are scattered more than longer wavelengths (the red end of the spectrum).

 

Measurements also showed that the Martian atmosphere always had some fine dust suspended in it. The dust particles vary in size from smaller than visible wavelengths (0.4 - 0.7 micrometers) to as large as several tens of micrometers. (A micrometer is one-millionth of a meter, or about 0.00004 inches). Sky color measurements from Viking Lander 1 have been used with computer simulations of light scattering to estimate that the dust particles contained about 1% by volume of an iron oxide mineral known as magnetite (a black, opaque material). This mineral absorbs sunlight more effectively at blue wavelengths than at red wavelengths. Scattering (including absorption) of sunlight by the dust particles in the Martian atmosphere therefore accounts for the sky color. The scattering is more complicated than the simple Rayleigh case because the dust particles both reflect and absorb the sunlight, and because the presence of 'large' particles leads to more uniform scattering among the different wavelengths.

[blike]

hrm, let me have a gander at his paper.

[blike]

I don't know too much about light and all its properties, but just in-case he starts pointing to the color-wheel mohagana, here's another good link.

 

http://www.atsnn.com/story/30048.html

 

"This article is a brief summarised explanation of how the PanCam on the Mars Spirit Rover operates, in relation to the strange appearance of the calibration sundial in some pictures. The question was first raised by ATS member AArchAngel, and has been discussed at length in this AboveTopSecret forum thread and ATSNN story:

thread "

[blike]

In colour-corrected images (like the one I sent you), the Martian sky appears to be Light Blue, similar to Earth's atmosphere (this is confirmed by Hubble Telescope images).

 

It's worth mentioning that the Hubble pictures of Mars are composites of various filters.

[Cap'n Refsmmat]

In those color images, the camera was not calibrated! They then calibrated it to fix it. (one wonders why they didn't do it earlier)

[Donn]

I see there is not much in the way of answers on this forum. A pity, I had hoped for some serious detail from those in the know.

 

Can I add a question?

 

Q: If you sent a beam of light on Mars through a prism, what would the result be? Would there be a gap in the blue spectrum?

[JaKiri]

Donn said in post # :

I see there is not much in the way of answers on this forum. A pity, I had hoped for some serious detail from those in the know.

 

Well, there's a quote from the nasa page that explains the phenomenon, although it's debatable if they should be counted as 'in the know'.

[blike]

Seems to me like you have all the information you need. Just pick out where his assumptions are wrong:

 

In the presence of a high density of reflecting and scattering particles

(such as red dust), the vertically-facing parts of the lander will be

coloured enough to make a difference in their colour. If the air was filled

with enough dust, for example; let us say that it is dense enough so as to

reduce the overall atmospheric visibility to 50% (50% Sunlight gets through

to the lander, 50% is reflection off dust particles) then we would of course

have a situation where the colouration of the vertical parts of the lander

will be affected by as much as 50% of the pigment of the dust particles.

 

That's completely wrong. 50% of the light is filtered over the span of the whole atmosphere (from space to the observer). That doesn't mean a difference in the color of a rover standing three feet tall.

 

"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."

 

Firstly, the red component is not removed. It's the blueish hues that are absorbed by the dust. The red is somewhat scattered, but the martian atmosphere is about 100 times thinner than our atmosphere, and thus, scatters less red light. Secondly, the same dust that is in the air is all over the ground. Whatever blue happened to make it through the atmosphere would be absorbed again by the dust in the ground. This dust would undoubtedly remove blue from the spectrum again, making the ground appear redder than the atmosphere.

 

With such a scattering scenario, where the light

scattered is in the red spectrum and the majority of direct Sunlight is able

to reach the surface, the effect will be that objects on the ground will

appear with a turquoise/cyan hue, NOT red/orange or even pink (this

turquoise effect, of course, takes place due to part of the red spectrum in

the direct Sunlight being removed by the scattering).

 

Reyleigh scattering on earth occurs by particles smaller than the wavelength of the light it's scattering. Thus, its pretty selective for which wavelengths it scatters. On mars, the atmosphere is much thinner, so Reyleigh scattering is much less significant by the atmosphere. However, dust particles in the atmosphere will indeed scatter light. They are much bigger than the wavelength of most (visible) light, and thus, they scatter ALL light pretty uniformly, not just red light.

 

With such a scattering scenario, where the light

scattered is in the red spectrum and the majority of direct Sunlight is able

to reach the surface, the effect will be that objects on the ground will

appear with a turquoise/cyan hue, NOT red/orange or even pink (this

turquoise effect, of course, takes place due to part of the red spectrum in

the direct Sunlight being removed by the scattering).

 

Wrong. See above. Iron Oxide does not selectively scatter red light, but it does abosorb blue hues.

 

Here on Earth we have a Blue sky (due to scattering by gases in the

atmosphere), which means that the direct Sunlight reaching the surface has a

Yellow tinge, colouring everything in a Yellow hue (which our eyes

automatically 'filter' out).

 

Wrong. Scattering gives the atmosphere a uniform, ambient blue, it does not remove blue light.

[Radical Edward]

Cap'n Refsmmat said in post # :

In those color images, the camera was not calibrated! They then calibrated it to fix it. (one wonders why they didn't do it earlier)

 

it was callibrated, but it was decallibrates because of large patches of algae which they want to keep hidden as part of the great martian illuminati conspiriacy.

[Donn]

Thanks blike,

I was starting to despair of getting help online for this kind of thing. I have been beavering away and have a summary of my argument which I will now reappraise in the light of your info.

 

Could you tell me if a prism would reveal that blue is absent on Mars? Is this a viable analogy to use - I want to give him something solid that he can wriggle out of.

[YT2095]

I`m sure they wouldn`t have overlooked the idea of putting down several samples of KNOWN colors with specific hues in the machine itself so that it can calibrate from there using Martian light and making the appropriate RGB adjustments. I know I would have done, so they MUST HAVE

[blike]

Donn said in post # :

Thanks blike,

Could you tell me if a prism would reveal that blue is absent on Mars? Is this a viable analogy to use - I want to give him something solid that he can wriggle out of.

 

Well, blue wouldn't be completely filtered out. Most certainly there is a good deal of blue wavelengths still hitting the surface of mars (otherwise the blue on the calibration color wheel wouldn't show up). You'd probably see some blue, but definately not as strong as you would see it on earth.

 

One thing to note about CT's and psuedoscience: They always will find a way to wiggle out of anything! Good luck whippin him into shape.

 

Can I read your response to him?

[Donn]

blike - I should have spotted that blue calibration chip!

Still, I would have liked to have some 'shocking' evidence - I was kind of hoping that there was a prism on the lander! Well, I suppose the CT would just claim that it was being faked too.

 

I am almost done my response, and I will certainly attach it to this forum - if it's too big, I will cut it down some.

 

Regards.

[Cap'n Refsmmat]

I doubt you could make that big of a response.

[Donn]

I meant big in KB size - I have some pictures and so forth in the document.

[Donn]

blike,

I have attached my response as a zip file - it contains some images too. I use Open Office Org and inline images do not work in saved .doc files, hence the zip.

 

Hope it makes sense...

 

I will also post his reply, when I get it.

mars_colour_issue.zip

[Donn]

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!

______________________________________

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.

--------------------------------------

 

So, what say you? I'm back to square 1 !

[Cap'n Refsmmat]

Couldn't that dust he talks about be red dust? Oh, and he seems to be assuming our atmosphere is the same as Mars', which it isn't. Mars is 90 some percent carbon dioxide, with little nitrogen and even less oxygen. So the light is scattered differently.

Also, he seems to assume that blue light is removed from Earth's atmosphere by Raleigh Scattering, but if it was, I couldn't see any blue! No blue light! And, of course, he seems to think that they know what Martian dust is made of, when they don't.

The easiest way to prove Mars has a red atmosphere is get a telescope. Earth looks blue because of our atmosphere (somewhat) and when you look from space, to just above the horizon, you see blue. I believe the same for Mars.

[Cap'n Refsmmat]

Donn said in post # :

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!

No, the remaining primary colors (red and yellow) equal ORANGE! Also, he says that if you also remove cyan blah blah, but removing blue also removes all colors with blue in them!

[Donn]

Cap'n - can you point me to a source of information about this colour removal question.

 

For e.g. To remove blue seems to mean *only* blue, thus leaving green (as next in the spectrum) - so what does light minus blue look like?

 

you said:

but removing blue also removes all colors with blue in them

But that seems strange to me, the other colours don't 'have blue in them' they are pure wavelengths unrelated to blue.

 

You see my problem? Accurate answers to simple little questions about light and colours?

[Cap'n Refsmmat]

Whoops, it doesn't remove ALL the blueish, it depends on the dust. It may remove some, all, or none of the mixed blue colors may be removed.

I don't have a source, but if you look at a color wheel, you see that without blue, the middle of the rest is orange. The eye may be sensitive to different things, but I'm not sure. You could search for it.