Why Mars has blue sunsets and Earth has red.

On Mars, the sky immediately around the setting sun appears pale blue — not red, as on Earth. The mechanism is Mie scattering. Suspended atmospheric dust grains on Mars are micron-scale iron-oxide particles (predominantly Fe₂O₃, hematite), comparable to or larger than the wavelengths of visible light. Unlike Rayleigh scattering — which dominates in Earth's molecular atmosphere and scatters short wavelengths (blue) approximately isotropically — Mie scattering by these dust particles preferentially forward-scatters short-wavelength light into a tight cone around the solar disk. At sunset, when sunlight traverses the longest atmospheric path through the dust column, the surrounding sky reads warm amber-orange while the near-sun region appears blue. The same dust that gives the daytime Martian sky its butterscotch colour inverts the sunset palette relative to Earth.
Mars's surface atmospheric pressure is approximately 0.6% of Earth's, too thin for molecular Rayleigh scattering to dominate the visible-light optics. Atmospheric dust at Gale Crater has been retrieved at effective radii near 1.5 µm and is lifted by convective vortices and regional storms, with grains remaining suspended on timescales of weeks to months. NASA's Curiosity rover captured a clearly resolved colour image of a Martian blue sunset on Sol 956 (April 15, 2015), consistent with predictions from Mie theory and with earlier observations from Viking, Pathfinder, Spirit, and Opportunity.

References

Mie, G. (1908). Beiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen. Annalen der Physik, 330(3), 377–445.
Vicente-Retortillo, Á., Martínez, G. M., Rennó, N. O., Lemmon, M. T., & de la Torre-Juárez, M. (2017). Determination of dust aerosol particle size at Gale Crater using REMS/UVS and Mastcam measurements. Geophysical Research Letters, 44, 3502–3508. doi:10.1002/2017GL072589
Lemmon, M. T., et al. (2024). The Mars Science Laboratory record of optical depth measurements via solar imaging. Icarus, 408, 115821.
Image: NASA/JPL-Caltech/MSSS/Texas A&M Univ. — PIA19400, Curiosity Mastcam, Sol 956.

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