Why Are Mars’ Sunsets Blue?

Earth and Mars are a bit like mirror worlds. Mars is the Red Planet. Earth is the pale blue dot. Mars is a frigid desert. Earth is full of water and life. But there’s another curious difference. The sky on Mars is red, while its sunsets are blue.

This is similar to the reason why our sky is blue and our sunsets are red. The Sun’s light scatters depending on what’s in the atmosphere. Sunlight has a wide range of wavelengths, and molecules and dust particles only interact with certain waves. The color we see is due to the scattering of light by these particles.

NASA’s Curiosity Mars rover recorded this image of the sun setting at the close of the mission’s 956th Martian day, or sol, April 15, 2015, from the rover’s location in Gale Crater.  
NASA/JPL-CALTECH/MSSS

The atmosphere of Mars is extremely thin, with a pressure of about 1% that of Earth. It is made of carbon dioxide and contains a significant amount of dust. This fine dust scatters red light, making the sky appear reddish and allowing blue light to pass through. On Earth, the situation is reversed. Our sky gets its color from blue light bouncing off air molecules.

Because light has to travel a longer distance through the atmosphere at sunset, it scatters more. The color we see is all that’s left. We have a wider range of reds on Earth, which is amplified by ash from volcanoes and dust from fires. On Mars, we get a cool blue hue.

Perseverance, Curiosity, Spirit, and Opportunity – the indefatigable robotic rovers we’ve sent to the Red Planet, have witnessed and recorded the curious phenomenon. Interestingly, Earth and Mars are the only two places in the Solar System that have sunsets that we can observe.

Because Mercury lacks an atmosphere, we would see the Sun vanish as the temperature dropped from 427°C (801°F) to -173°C (-279°F) as we moved from day to night. It also has a very long day, rotating every 58 and a bit days. Going to Venus, on the other hand, would be even worse. The dense atmosphere and thick cloud cover would prevent the Sun’s rays from reaching us. And the extreme heat and acid rain would quickly melt our suits – and eventually our faces.

Maybe Titan could offer a rare sunset, occasionally, within its dense atmosphere. But for the time being, our Earthly sunset and videos of Martian ones are the best we can hope for.

READ MORE: Curiosity captures rare, colorful clouds on Mars


 

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