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4 years ago in Astrophysics , Planetary Science By Neethi

Why do Martian sunsets appear blue while Earth sunsets appear red?

 I teach introductory planetary science, and the textbook answer is unsatisfying for curious students who ask how the dust does this. I need a clear, physical explanation grounded in Mie scattering theory: how does the size distribution and composition of the suspended dust particles lead to a dominance of forward-scattered blue light at low solar angles?

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By Veena Answered 2 years ago

You're right to move beyond the hand-waving. The key is the particle size. Earth's Rayleigh scatterers are tiny gas molecules, which scatter short (blue) wavelengths much more efficiently, removing them from the direct beam. Martian dust particles are larger, roughly 1-2 microns in size, which is comparable to visible light wavelengths. This engages Mie scattering, which has a much weaker wavelength dependence. Crucially, these larger dust particles are efficient at forward-scattering blue light. At sunset, sunlight passes through a long atmospheric path, and this forward-scattered blue light is directed toward your eye, while the redder light is scattered away more isotropically.

 

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