Sunsets on Mars glow blue rather than red, because the fine dust suspended in its thin air scatters sunlight so that the blue gathers into a halo around the setting sun while the rest of the sky stays yellow and orange, leaving the planet that looks rusty all day to end it with a cool blue glow.

NASA’s rovers have been photographing sunsets on Mars for two decades, and the colour still catches people off guard. The Spirit rover recorded a blue glow around the setting sun over Gusev Crater on 19 May 2005, its mission’s 489th sol. A decade later, on 15 April 2015, its mission’s 956th Martian day, the Curiosity rover captured its first colour sunset from Gale Crater with its Mastcam, and the sun went down inside a pool of blue while the rest of the sky held its rusty yellow and orange.

The planet that looks red all day ends the day with a cool blue tint near the sun. The reason has nothing to do with Mars being a mirror image of Earth, even though that is how the effect is usually described.

What the dust actually does

Mars has a thin atmosphere, roughly one percent of the pressure at Earth’s surface, and it is full of fine iron-rich dust. The size of those dust grains is the key. According to NASA, the particles are about the right size to let blue light through the atmosphere more efficiently than longer wavelengths, and to scatter that blue light forward, so it stays close to the direction of the sun.

Mark Lemmon, the Curiosity science-team member who planned the 2015 sunset observations, put the mechanism plainly in the JPL announcement: the fine dust is sized so that “blue light penetrates the atmosphere slightly more efficiently”. The longer-wavelength light, the yellows and reds, gets scattered across the whole sky instead. So the blue concentrates into a halo around the sun, and everything away from the sun keeps the familiar butterscotch colour of suspended dust.

Why “the opposite of Earth” is too neat

The common shorthand is that Mars simply reverses Earth: blue sunsets instead of red ones, a reddish daytime sky instead of a blue one. It is a tidy line, and it is not quite right.

Earth’s blue daytime sky and red sunsets come mostly from Rayleigh scattering, where the gas molecules in the air scatter short blue wavelengths far more than long red ones. The particles doing the work are tiny, much smaller than the wavelength of light. On Mars, the colour at dusk is governed by dust grains that are comparable in size to the wavelengths themselves, and those grains scatter light strongly in the forward direction. That forward-scattered light is what carries the blue toward an observer looking at the setting sun. It is a different optical process producing a superficially similar inversion, not the same physics run backwards.

The distinction matters for anyone trying to picture it accurately. The blue is not the sky of Mars turning the way Earth’s does at noon. It is sunlight being scattered by dust in a particular geometry, visible most strongly when you look more or less toward the setting sun.

The blue is not always there

Because the effect depends on dust, it varies with how much dust is in the air. When the atmosphere is relatively clear, the blue is muted rather than vivid.

The Perseverance rover photographed a sunset on 9 November 2021, its 257th sol, on a day with comparatively little dust aloft. NASA noted in its overview of Martian sunrises and sunsets that the result looked subdued compared with the average, precisely because there was less dust to do the scattering. The striking blue aureole that turns up in the best-known images is a high-dust phenomenon, not a guaranteed nightly show.

What the sunset images are for

These are not only pretty pictures, though they are that. Sunset and twilight observations are a working tool for studying the atmosphere. Looking sideways through the air at dusk, a rover sees light that has travelled a long, slanting path, which makes the images sensitive to how dust and ice are distributed with height.

The Curiosity team timed its 2015 observations between dust storms, with some dust still suspended high up, specifically to help assess the vertical distribution of dust in the atmosphere. Twilight imaging can also reveal high ice clouds. The colour is a byproduct of the same dust that scientists are trying to measure, which is part of why the images get taken in the first place.

The next useful data points will come the same way they have for twenty years: a rover pointing its camera at the horizon at the right moment, on a day with the right amount of dust, and sending home another frame that tells researchers something about the air it was shot through.