Since Viking 1 landed on Mars in 1976, rovers have captured and shared images from the red planet, including views of sunrises and sunsets. The Perseverance rover recently shared an image of a blue-hued sunset on Mars, unlike the ones that we usually see on the earth. We take a look at what causes this.
The atmosphere on Mars is largely composed of carbon dioxide, with a very small percentage of nitrogen and only trace amounts of oxygen. Dust particles are also an abundant component of the Martian atmosphere. These particles are why sunsets appear blue on Mars’s surface.
The sun emits electromagnetic radiation in a range, from high-frequency gamma rays to low-frequency radio waves, and includes the visible spectrum in between. But which colours are visible to human eyes depends on the medium through which light passes and how it interacts with the constituents of the medium – which can absorb, reflect or scatter different wavelengths.
To a person standing on the Martian surface and looking at a sunset, light from the sun has to travel through more of the planet’s atmosphere than if the sun is directly above the person. And there is a peculiar result of this longer journey through the Martian air. In the words of Mark Lemmon, writing as a member of the Mars Pathfinder imaging team:
“The blue colour near the sun is not caused by clouds of water ice, but by the Martian dust itself. The dust in the atmosphere absorbs blue light, giving the sky its red colour, but it also scatters some of the blue light into the area just around the sun because of its size. The blue colour only becomes apparent near sunrise and sunset, when the light has to pass through the largest amount of dust.”
This phenomenon – of some wavelength of light getting lost – is called interstellar extinction. Its extent depends on the absorption of light as well as other interactions with dust particles in its path.
“When the blue light scatters off the dust, it stays closer to the direction of the sun than light of other colours does,” Dr. Lemmon, now of Texas A&M University, College Station, and a member of the science team of the Curiosity rover mission, said in a different statement. “The rest of the sky is yellow to orange, as yellow and red light scatter all over the sky instead of being absorbed or staying close to the sun.”
