Solving the mystery of Mars’s surreal, blue sunsets as we explore the red planet

Dec 6, 2018

Mars, the Red Planet, is Earth’s closest planetary neighbor, yet the differences between the two worlds are nothing short of surreal. As NASA’s robotic rovers have already observed, the Martian soil and its sky are both reddish in color; meanwhile, its sunsets are a stunning, spectacular blue.

So, what strange forces are behind these alien, aerial phenomena? Just like the age-old question “Why is the sky blue?” there is a legitimate, scientific answer to be had—but with a bit of a twist.

First, we have to examine our Martian neighbor a little bit closer. What do we see?

The frigid desert landscape of Mars is literally made of rust (hence its red hue), which differs vastly from Earth’s own landscape, which contains plenty of water and life. And this difference has something to do with the relatively high volume of dust particles in Mars’s atmosphere.

NASA
NASA

In turn, the Martian atmosphere is incredibly thin, with a pressure of around 1 percent of Earth’s, and is mostly composed of carbon dioxide—plus, as we have seen, the high quantity of dust in the air.

It’s what happens when sunlight shines through these atmospheres of various compositions, though, that’s key to explaining their color.

Like on Earth, when sunlight (which is composed of an array of different wavelengths) enters Mars’s atmosphere, it is scattered by molecules and dust particles. This causes wavelengths of particular colors to be scattered more than others, which then enter our eyes.

The high dust levels on Mars in particular tend to scatter more red light, thus giving the sky its reddish hue.

On Earth, our atmosphere is composed of mostly oxygen and nitrogen molecules, which scatter more blue light—as well as other wavelengths that our eyes are less well adapted to seeing, such as violet. (So, hopefully, this helps answer that age-old question!)

As for the surreal, blue sunsets on Mars, with the reversal of red sunsets on Earth, here too we have science to help explain them.

We terrestrials know that when the sun sets, sunlight must travel through a longer distance of atmosphere. That causes more light to be scattered than during other times of the day, which, combined with the ash and dust caused by volcanoes and fires (not to mention pollution) off in Earth’s horizon, tends to scatter more red light—hence the often-beautiful sunsets we Earthlings are familiar with.

Courtesy of Neal Simpson

While on Mars, the increased diffusion of light results in an unusual, yet no less spectacular, blue aureole surrounding the sun like a halo as it sets on the Martian landscape.

Humans have (literally) come a long way, from when our ancestors named planets after ancient gods, to now when we have extended our reach and our senses tens of millions of miles from our home to come in contact with other worlds.

NASA

Thus far, several Mars missions have captured video and images of phenomena that leave us in amazement as we try to reconcile the extraterrestrial with life as we know it.

Footage from robotic rovers Curiosity, Spirit, and Opportunity has whet our appetite to open this door. Going forward, we eagerly await what recently landed rover InSight will capture and send back to us, with a future, fully manned mission on the horizon.

NASA
NASA