Is there any explanation for the blue sky of Mars shown in this video of photos transmitted by the Chinese rover?
Zhurong Rover is driving on Mars sending back new images (Chinese Tianwen-1 mission) - May 22, 2021
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$\begingroup$ I wonder if they shifted the color spectrum toward blue to match what our eyes would expect to see on Earth. Hasn't NASA done this before? $\endgroup$– user39728Commented May 22, 2021 at 19:52
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2$\begingroup$ But yeah, that looks nice, and almost inviting. That ground, though, so sterile. My eyes just want to nuke those rocks and see green sprout somewhere. Not suggesting we do this, just saying. $\endgroup$– user39728Commented May 22, 2021 at 19:54
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2$\begingroup$ different but related: Why would Mars' sky appear blue at dawn and dusk, but red during the middle of the day (reverse of Earth)? and Elon Musk's comment youtu.be/S5V7R_se1Xc?t=40m20s and Did any lander or rover photograph the sun rising or setting during a Martian dust storm? and all the various photos in answersto this question. $\endgroup$– uhohCommented May 22, 2021 at 22:00
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$\begingroup$ What exactly do you mean with "Is there any explanation"? $\endgroup$– user2705196Commented May 24, 2021 at 14:03
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3 Answers
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Despite my comments about the sky not always being red, I think there is a simpler explanation.
This video was made by a "content" company on a monetized YouTube channel. The screenshot is simply a colorized version of a black and white image
Note that the notes below the video do not provide the source of the images. You can't check the source of their data, they offer it only in the form they want you to see.
from the "Front Obstacle Avoidance Cam B" shown later between about 00:39 and 00:43 in the video.
Edit: The original color analysis I had here is not conclusive as @Ruslan points out so I'll replace it. It's still viewable in the edit history. Instead, while not conclusive I'll compare a cropped bit of Ruslan's suggested PIA19070 with a cropped bit of the (likely) colorized image screenshot from the video.
Don't trust random monetized channels!
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14$\begingroup$ What scatterplots would you expect from a faithfully-colored image? The one from NASA with blue sky also has two-part scatterplots: i.sstatic.net/XVwb7.png. Though, it seems it's also been manipulated to not-quite-real colors: "The color has been approximately white-balanced to resemble how the scene would appear under daytime lighting conditions on Earth". $\endgroup$– RuslanCommented May 23, 2021 at 9:12
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4$\begingroup$ @Ruslan the suspicious thing is not so much that the scatterplot is “two-part”, but that it's two pretty straight lines both pointing to (0,0). (Plus some noise and clipping.) In the scatterplots you show, both of the regions are more irregular / cloud-shaped, which is more characteristic of actual physical light scattering around. $\endgroup$ Commented May 24, 2021 at 16:08
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$\begingroup$ @Ruslan that's a really good point and after seeing your results, mine don't really look very convincing. So I'll delete them (still viewable in edit history) and replace with a simple comparison of a bit of PIA19070 with the screenshot. It's not meant to be convincing by it's own. The key part of the answer is that the black and white image from Obstacle Avoidance Camera B at
00:41is the same as the colorized image shown at the beginning of the video. Thanks for your comment and analysis! $\endgroup$– uhohCommented May 24, 2021 at 19:38 -
4$\begingroup$ A spec sheet stating that that camera is b/w would be conclusive (maaaybe the youtuber had secret access to the original color image). But were it a color camera, it's hardly likely that image-conscious China would degrade its images to b/w before publishing them. $\endgroup$ Commented May 24, 2021 at 22:17
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2$\begingroup$ @CamilleGoudeseune There are some good avenues to pursue in answer(s) to Where can I find access to information provided by CNSA and Chinese scientists and reputable science authors? Knock yourself out! :-) $\endgroup$– uhohCommented May 24, 2021 at 22:25
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Why wouldn't the sky appear that way? The sky color has little to do with the color of the land. It is to do with scattering of light by particles in the air as we are looking away from the planet. Think about it, why isn't the Earth's sky green here in the Northwest where there is endless forest? Read the NASA article below for why the sky is "blue". When we see the planet Mars from a distance, then it is reflected light from the surface of the planet which is predominately red, and we are viewing towards the planet. Similarly, the Earth likes blue.
Reference Del Genio, Anthony D. (2003) Why is the sky blue? NASA. https://www.scientificamerican.com/article/why-is-the-sky-blue/
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2$\begingroup$ What color is the sky on Mars?. It's all relative. Is it about the wavelengths of light detected or the limitations of the physiology of the human eye? $\endgroup$– FredCommented May 25, 2021 at 9:34
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3$\begingroup$ The sky is blue because of Rayleigh Scattering, which happens to a degree relative to the thickness of the atmosphere. Given that Mars's atmosphere is much thinner than Earth's we would expect that effect to be much less pronounced $\endgroup$– KevinCommented May 25, 2021 at 19:38
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The sky of Mars is blue like the one of Earth. The ESA picture you can admire below the text shows unequivocally that the sky of the Red Planet is much like the one above the Sahara desert.
Mars' thin atmosphere. (ESA/DLR/FU Berlin/J. Cowart/CC BY-SA 3.0 IGO)