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Episode 113 - The Blue-Haze Limb of Mars

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Recap: The True Color of Mars conspiracy was dispelled in Episode 74, but a niggling part remained: What about the haze of blue around the limb of Mars seen in many photos of it from Earth orbit, especially from the Hubble Space Telescope? This part of the conspiracy has a surprising result -- it's true!

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Claim: This all gets back to a previous episode I did, #74, “The True Color of Mars.” Way back then, last May, I addressed the claim that NASA (and all other space agencies that have been to Mars, but conspiracists tend to ignore them) has been hiding the True color of Mars from us sheeple here on the ground. That in reality, it looks just like Earth. The reason they’re hiding it is possibly many different ones, including they never went there, or that there’s alien life there, but they’re hiding it; in the episode, I went through and explained that NASA isn’t hiding the “True” color of Mars, and I explained how you can tell. But there’s a little-talked about other part of that conspiracy, one that conspiracists who are “In the Know” about it will leap to: Real, genuine images ¡from NASA itself! taken by the Hubble Space Telescope show a blueish haze near the limb of the planet — the edge of the disk. Surely this means that all the coverup is true, that the sky on Mars really is blue.

A Real Phenomenon?

Last episode, I got back down to basics and talked about the steps in any investigation: First, find out if the phenomenon is real; if it is, then second, find out all of the possible explanations and then determine which is most likely to be correct. Last episode, we never got past that first part, because the data show Mercury’s magnetic field was not actually changing.

But now, we repeat the questions. And, you can find real images from NASA, in as close to true-color as we can estimate, that show Mars as a reddish/orangish disk and then right along one of the limbs, or sometimes the whole disk, is a faint ring of blue.

It is real. So now we have to ask, “What could cause it?” And, “If the limb of the planet - the upper atmosphere - appears blue from Earth, would the entire atmosphere appear blue from the surface of the planet?”

Earth’s Atmospheric Color

On Earth, from the ground, the sky appears to be blue due to a process called Rayleigh scattering, named for 19th-20th century British physicist, Lord Rayleigh (well, John William Strutt, 3rd Baron of Rayleigh, but we call him “Lord Rayleigh” for short).

The process is where light is scattered – distributed in all directions – by particles much smaller than the wavelength of that light. This can be done by “particles” as in dust, and also by “particles” as in molecules. In Rayleigh scattering, there is a very strong inverse-dependence on the wavelength of light, meaning that shorter wavelengths of light are scattered more strongly than longer wavelengths. Blue light has a shorter wavelength than red by roughly 25%, meaning that it is scattered about 3–4 times more than red light. Since blue light is scattered more, then it appears to come from everywhere.

At this point for Earth, you would therefore expect a blue sky: Light from the sun is scattered throughout the atmosphere, but blue light much more than red, therefore the sky will appear bluer.

In addition to this, the various molecules in the atmosphere will not only scatter light, but they can absorb it. Molecular nitrogen (N2) is the dominant component of the atmosphere, but molecular oxygen (O2) also plays a role. N2 is very transparent to visible light, but O2 and high-altitude O3 (ozone) tend to absorb light in the ultra-violet end of the spectrum, barely leaking into visible violet. Meanwhile, carbon dioxide (CO2) will also absorb light, but this is in the infrared wavelengths (hence why it is a greenhouse gas).

So at this point, I’ve established that Earth’s atmosphere should appear bluish, primarily because of Rayleigh scattering and molecules that do not really affect visible-light absorption. Even though there are other molecules that are in Earth’s atmosphere, they aren’t really numerous enough to do much for the overall color.

But there’s also dust. Dust on Earth is typically the size of about 10 µm, and particles that are 10 times smaller or 10 times larger are <10% as prevalent. These dust particles are much larger than individual molecules, and so will not contribute to the Rayleigh scattering of visible light.

What they CAN do is scatter light in what is known as Mie scattering, which is the scattering of light of a SIMILAR wavelength to the particles. But again, the dust particles are mostly large, 10 µm, which is in the infrared area of the spectrum. So, they will have a minimal effect on the visible color of the atmosphere from Earth’s surface (where there are the most dust particles) or when seen from space.

As far as visible light is concerned, therefore, on Earth you’re almost exclusively talking about Rayleigh scattering, which is why it’ll all look blue.

Mars’ Atmospheric Color

Contrast this with Mars. Mars does have an atmosphere, though it’s much thinner than Earth’s with only about 1% the pressure at the surface. It’s also predominantly CO2. Rayleigh scattering will again take effect and cause the atmosphere to appear bluish, IF nothing else were to take effect. From space, this is why it appears blue, for the exact same reason Earth’s atmosphere appears blue from space.

From the surface is a different case. Dust on Mars tends to be most prevalent at 1.5 µm, which is 10x smaller than it is on Earth. Even though this is still in the infrared, it is close enough to red (visible red light is half that wavelength) for Mie scattering to begin to impose a red-like color. So you have a different situation on Mars: The dust is smaller, meaning that the scattering that takes place on Earth in the infrared part of the spectrum from Mie scattering, can start to be visible in red light on Mars.

But besides scattering, the dust itself is derived from the surface material, and measurements by the Viking 1 Lander showed that the dust is approximately 1% iron oxide, also known as rust. Rust is red, meaning that it absorbs blue light and reflects and scatters red light. Meaning that you have a bunch of particles suspended in the lower atmosphere of Mars, near the surface, that are absorbing blue light and scattering red light all around.

But again, Mars’ atmosphere is very thin, and although the surface gravity is less than Earth’s, it is not enough to loft the dust many kilometers off the Martian surface into the upper atmosphere.

So, let’s put this together: Mars’ basic atmospheric gases will give it a blue tint, which we see from spacecraft observations like HST. Meanwhile, atmospheric dust is red itself and will preferentially scatter red light, yet it is most prevalent close to the ground. Therefore, we would expect to see a bluish atmospheric limb of Mars from space, but a reddish atmospheric haze when on the surface.

So we have a case where if we follow the basic, known physics, and we use independent observations of atmosphere composition, dust composition, and surface composition, from these basic observations we would actually expect what we see: The bluish atmospheric limb of Mars from space, but a reddish atmospheric haze when on the surface.

“But, why are Martian sunsets blue?” —might scream a conspiracist. The reason is that the red light has been scattered out by the dust. The setting sun goes through so much more atmosphere than when the sun is high overhead, and so more and more red light is scattered around by those dust particles. This leaves primarily the blue light to come through since Rayleigh scattering plays a smaller, relative role to the dust.

You can think of this as a situation where Rayleigh scattering is going on, so the sky would appear blueish, but the dust scattering that makes things reddish is much stronger. At sunset, the dust scattering red light is so strong that it’s extracted the red light, and so the blue light scattered by Rayleigh scattering is what’s seen close to the sun at sunset.

Science Is Hard but Conspiracy Is Easy

Science is hard, but conspiracy is easy.

And, this is a very good example of that: It would be much easier to simply look at the blue-haze-on-the-limb images, the red-sky-from-the-surface images, and literally just stop the thought process there and say it’s a conspiracy.

Going through the process of understanding different types of light scattering, researching or even measuring forms of dust and their sizes and colors, and putting that all together is much harder, involves much more thinking, and involves even designing experiments to measure and test ideas.

To put it bluntly, despite all the cognitive dissonance that conspiracy thinking requires, going the conspiracy route is simply lazy.

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Comments to date: 1. Page 1 of 1. Average Rating:

Robert Szasz   SF bay area

3:12am on Sunday, December 11th, 2016 

Perhaps a shorter version for non edge cases is that on Earth, blue is scattered most strongly, the sky is blue, sun is yellowish , sunset is orange/red. On Mars red/orange is scattered most strongly, the sky is orange, the sun is tinged blue, and the sunset is too.

If the absorption/scattering wavelengths were much closer things could get complicated, but earth and Mars are relatively simple.

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