In Mars the sunset is Blue

SCIENCE

Contemplate this image for a few seconds, realize all the engineering effort, all the technological and scientific progress, the vast amount of human resources that were invested in achieving this photo. We are watching the sunset on ANOTHER PLANET that at its closest point to us is more or less 70 million kilometers away from Earth (here you can see the current Mars-Earth distance). Think about it. Visualize Mars in your mind…that little red dot in the sky, think about what it would be like to approach it little by little until you reach something as immense as a planet — even if it is almost half the size of the Earth —, that makes me shiver. Because it’s incredible.

On this rust-colored planet, the Sun’s glow at sunset is pale light blue while the rest of the sky looks subtle yellow/reddish rust color, practically the opposite of what we see on our planet. And it is because the color of the sky and the sunset/sunrise depends on the composition of the atmosphere.

But first we must remember that visible light is composed of light of all colors that combined give white light (cover of The Dark Side of The Moon in mind).

And just like a prism, the atmosphere can also split this white light coming from our Sun. On Earth, it is mostly composed of 78% nitrogen (actually dinitrogen — N2 two nitrogen atoms bonded together) and 21% dioxygen O2 (what is commonly called “oxygen”). These gases are very good at scattering smaller wavelengths, which correspond to colors like blue and violet, that’s why they give those hues to our sky. And what do I mean by scattering? Well, in physics this phenomenon is known as Rayleigh scattering.

This image explains it beautifully:

What we see here is essentially the same thing that happens in the sky at sunset. Here we see a type of opalescent glass that scatters the blue light so much that it only lets the orange/red light through. That’s why the mineral looks blue. Now imagine that your eyes are the white surface and the sky is the glass. Shazam! Red sunset.

On Mars it is a different story, the atmosphere is 98% CO2, and it is very, very thin, so thin that the atmospheric pressure on Mars (0.006 atm) is less than 1% of what it would be here on Earth at sea level (1 atm) [1]. It would take a climb to 35 km altitude on Earth to experience an atmospheric pressure similar to that of Mars (Mount Everest peak is 8 km). But the most interesting thing here is that it has lots and lots of iron oxide dust, especially iron(III) oxide (Fe2O3), what we commonly call “rust”. This fine dust is very good at scattering larger wavelengths: yellows, oranges and reds, in essence, the opposite of what happens on Earth.

Size matters:

Let’s make a brief parenthesis to talk about sizes. A molecule of N2 — dinitrogen, the main component of our atmosphere, is about 370 pm (picometers), while a particle of Martian dust is about 3 µm). But these units don’t tell us much, so let’s put it into perspective. A picometer or pm equals 0.001 or 10–3 nanometers (nm), and a nanometer equals 0.001 10–3 micrometers (µm), which in turn equals 0.001 or 10–3 millimeters. So imagine you take a ruler, measure 1 millimeter, take that millimeter and divide it into 1000 parts, we have 1000 µm; then take 1 of those parts and divide it again into 1000 parts, we now have 1000 nm! Let’s do it one last time, and we get 1000 pm! Hmm… But that’s still very abstract and intangible, let’s look up some real references:

-mm: a flea is ~5 mm.

-μm: the thickness of human hair varies between 17–181 μm; a red blood cell, measures ~7 μm; Here’s our Martian dust particle 3 µm; bacteria typically measure between 1–4 μm.

-nm: Visible light wavelength ranges from 400–700 nm; HIV virus typically measures ~120 nm; a cell membrane width is 6–10 nm; DNA double helix diameter is ~2 nm;

-pm: here would be our N2 from the atmosphere with ~370 pm; size of a water molecule H2O ~280 pm; radius of a hydrogen atom — or also called Bohr radius — is 53 pm; smallest wavelength of X-rays is ~5 pm.

A Martian dust particle is then 10 000 bigger than the N2 molecule!!!! I just find this size comparison awesome.

And hence the effect becomes more intense at sunset/sunrise, but what is so strange about sunsets and sunrises that make the sky and the sun look so different? Well this has to do with the angle at which the sun is with respect to the observer, and the amount of atmosphere that light must pass through before it reaches our eyes.

When the Sun is close to the horizon, its light has to pass through more atmosphere than when it is just above us at the zenith. Here it helps to imagine white light as an army of millions of little light “soldiers” of all the colors of the rainbow, blue, red, yellow, etc… And these are now facing another army, the atmosphere, it has mainly N2 and O2 “soldiers”. They clash face to face, let the battle begin: blue light soldiers crash much more against the N2 and O2 soldiers and only a few make it to the other side, the others go through almost unnoticed and untouched. Now if we increase the amount of N2 and O2 soldiers (more atmosphere) then at a certain point the blue light soldiers simply stop passing through, and we are left with only the red and yellow ones.

In brief, this intensifies the effects of the atmosphere, whatever planet it is. So on Earth the blues are filtered leaving only yellow/red tones to pass and on Mars the opposite, the yellow/red tones are dispersed and filtered and only the bluest ones pass. And that’s why the sunset on Mars looks blue.

References:

  • Halliday, David, Robert Resnick, and Jearl Walker. Fundamentals of physics. John Wiley & Sons, 2018.
  • Franz, Heather B.; Trainer, Melissa G.; Malespin, Charles A.; Mahaffy, Paul R.; Atreya, Sushil K.; Becker, Richard H.; Benna, Mehdi; Conrad, Pamela G.; Eigenbrode, Jennifer L. (1 April 2017). “Initial SAM calibration gas experiments on Mars: Quadrupole mass spectrometer results and implications”. Planetary and Space Science
  • Clancy, R. T., et al. “A new model for Mars atmospheric dust based upon analysis of ultraviolet through infrared observations from Mariner 9, Viking, and Phobos.” Journal of Geophysical Research: Planets 100.E3 (1995): 5251–5263.
  • What Does a Sunrise-Sunset Look Like on Mars? — NASA Solar System Exploration

I am a science communicator, biologist, microbiologist, and musician. I like to learn, research and explain things.

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