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Debate on the color of the sky of Mars:

I had not expected that my discussion that the sky of Mars is not red, or that the photographs of Mars are exaggeratedly colored in red by a majority of media would receive as many answers, some being praises, others being much more critical. Here an example and my answer.

Of course you must have initially read my page about this question.

Subject: Mars sky and percentages
Date : Nov. 3, 2002
From :

I read your article about the sky of March which would not be red but which would be blue: this is silly!...

Of course not, don't get irritated, I am teasing you.

I just write to have some precise details on your final improvements of images of Mars and its sky.

You claim that "... the red component having been exaggerated of 30%" then that "too much red there was added, 30% and too much green, 15%."

But 30% compared to what? Compared to the colors obtained or the initial colors?

If it is 30% compared to the obtained colors, your operation (- 30% of red and -15% of green) is correct.

But if it is 30% compared to the initial colors, the operation is not completely correct: if 30% of red and 15% of green are added, it is then necessary to remove 23% of red and 15% of green to find the original color.


Let's consider a gray pixel (R,G,B) (100,100,100)
to which one adds 30% of red and 15% of green (130,115,100)
if 30% of red and 15% of green are then removed, which gives give (91,97.75,100) and the initial gray is lost
but by removing 23% of red and 13% of green, one gets (100.1,100.05,100) and ouch! the gray was found.

That could correct errors of too weak luminosity, or colors a little greenish.

Anyway, your demonstration of the color of the sky remains correct.

X.X [Full name indicated, I do not publish it for privacy respect reasons]
DESS Ingénierie de l'Image Numérique.

Answer, November 3, 2002:

Dear X,

Thank you to have considered the subject, and of course I can only subscribe to your conclusion ;-)

With regard to the precision you ask, here is how that developed:

Initially, I did not personally come to the assertion of this exaggeration of the red. It would never have occurred to me that there may be any doubt there: like everyone did, I saw photographs of the surface of Mars, with a red or pink or beige sky, and I did not imagine that it may not be accurate.

It is only when, as I indicate it in the top of my page on this subject, I read by mere accident that a NASA scientist named Vince DiPietro caused some fuss by claiming that the colors of these photographs is not in conformity with the reality, that the rest unfold (Ron Levin who said that he personally witnessed the adjustment of the red hue for Viking, web pages by NASA trying to justify the adjustments which did not really convinced me, the argument between Hubble people and JPL on this subject and so on, as explained on my page.

Besides I had not immediately reacted; after all, red or pink or blue or gray, this is probably not so important. But, after contemplating many wonderful photographs of Mars by NASA, I ended up seeing red (sorry about that), too much red. The climax was probably this photograph among others presented on an educational page of the CNES (NASA's French counterpart) in connection with Mars:

(Source, see here also)

Honest, I think that for this photograph above, few readers would argue that its colors are accurate at all.

And thus, I thought perhaps that Mr. DiPietro had something of interest to tell us. Admittedly, I do not see really founded reasons to utter the "conspiracy" word; but I was dismayed enough to see that indeed, a kind of "tradition" perhaps, had everyone enhance the red hue of almost every Martian color picture unless it had the redness of a quite ripe tomato. I noted later that, with news results in the new field of exobiology, the trend is now to go back to pictures of a not-so-red Mars.

Now back at the sky, here is what I can say on the percentages issue:

The exact percentages do not seem to me firmly decided. The claims by Vince DiPietro appeared approximately two years ago on an Internet site dealing primarily with ufology and now disappeared. Vince DiPietro said there in an interview, two things in connection with these figures:

On the one hand, he said that the red component of the photographs had been inopportunely increased " by a third;" admittedly, it seems a little bit silly to interpret this as a strictly accurate figure and thus to consider 33.33% for the red. But it had quickly appeared to me that the exact percentage, whether is is 33, 30, 25 or 40%, plays only a secondary part in the case. I will give the reasons for that later.

On the other hand, he did not give a precise indication about a reduction in the green hue, but introduced into the discussion Ron Levin, son of Gil Levin which had developed the Viking LR experiment, one of the three experiments assembled to detect a possible life. Young Levin said he was on location when the engineers received the first color images, and he says that he saw greenish patches on the rocks which for him evoked fungi or lichen, and that the engineers carried out RGB balance adjustment until the green patches did not appear anymore with their green color. It is thus difficult to talk about exact percentage, I will explain further why this precision is secondary.

Lastly, when I changed the balance of the colors on some definitely reddish photographs, my only idea was the following: if I change these proportions until obtaining a blue sky, normally, the ground also, all the chromatic aspect of the image should be turned blue to some ridiculous amount. Worse, if there is "no blue" remaining in the image in the first place, and that I claim that by removing red, blue will appear, I should see my error in the sense that if there is no blue, it is not by decreasing the red by 30% that I would reveal any blue, it would just lead to obscure the photograph, to deteriorate it. This is not what occurred.

Agreed, this image handling is completely naive and uneducated. This is why that was still not enough to convince me.

The next step, for me, was to think that this handling of colors had to be to be ridiculous: NASA people are specialists, they inevitably took very great care of the calibration of the colors, and could not have left any amateurism in that, I thought.

And it is at this point that I realized that the photographs, not only the reprints in the paper media or on web sites, but also those proposed by NASA, have all the possible and conceivable colors, including the blue sky. Moreover, total uncertainty is apparent in the comments relating to their color balancing in many pages of NASA web sites.

At that moment, it occurred to me that I should put the horse before the cart: what makes the color of a sky? Why is the sky of our planet blue? Can the sky of Mars be red, or blue? Thus I realized that according to some scientists' opinion, the Martian sky was to be quite blue, according to the laws of the atmospheric optical system and the composition of the Martian sky.

Finally, that my handling is inexpert, that the percentages are vague for the red even arbitrary for the green becomes rather secondary.

Agreed, when you write: "But if it is 30% compared to the initial colors, the operation is not completely correct: if 30% of red and 15% of green are added, it is then necessary to remove 23% of red and 15% of green to find the original color, " you are absolutely right and I missed that. And in fact, if I remember well, for the first photographs on which I tried this reduction, I had the impression that I should not reduce the red by 30%, but by some 20-25%: the yks was already blue at 20%. I had in fact carried out several tests with several different values, what came out from was not basically different: by decreasing the red, there is no impression to impoverish the image, but rather of a restoration of a more natural color balance (once again, I mention above that this inexpert handling was not enough for me to conclude).

Actually, there is a a little unhappy "shortcut" in my introduction page, when I write "knowing that NASA increases the red by 30%," I should have had said "knowing that NASA's Vince DiPietro mentions that there is a third of red in excess."

In conclusion, I would be very interested if a specialist "corrected my corrections." As for the question "30% compared to what?," I fear that unfortunately I have no precise answer.

Best regards and many thanks.


My readers will find references, quotations, and photographs corresponding to this issue in the pages dealing with this question, and also, the opinions of other readers.

BEFORE writing to me about this, please read all the pages, please also read what others already wrote, please read my answers.

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This page was last updated on November 11, 2002