Imagine you’re standing on the rocky, rust-pink surface of Mars. You’ve just finished a hard day’s work helping to build the first human base on another planet, so you decide to take a break and watch the sunset. As you gaze west across the Martian desert, a small, wan sun sinks through the hazy, orange-brown sky. The light wanes, and the temperature drops from a balmy daytime high of -15° F to an evening chill of -120° F (good thing you’re wearing your spacesuit). The weak wind that has been kicking up dust devils all day drops away, leaving you in a silence deeper than any quiet on Earth.
Finally, you see what you’ve been waiting for. As the silvery disk of the sun approaches the horizon, the sky around you turns an ominous ash-gray. Only a broad, fan-shaped area directly above the sun remains bright—and it glows a cold, eerie, smoky blue.
That’s right—as a Martian colonist, you’d only see a blue sky at sunrise and sunset. To the human eye, the daytime sky on Mars can range from yellow-brown to pinkish-orange. But when the sun is near the horizon, it produces an icy blue glow. This is just the opposite of Earth’s serene blue skies and vivid orange sunsets.
What is it about the Martian atmosphere that makes the view so different?
To understand why the Red Planet’s sunsets are blue, it’s necessary to understand the differences between the atmospheres of Earth and Mars. Although Mars is the most Earth-like planet in our solar system, we wouldn’t find it an enjoyable place to live. Here on our comfortable, ocean-covered home, we’re used to a warm, 300-mile-thick layer of air that insulates us from the extreme temperatures of outer space. In addition, our atmosphere contains oxygen, an element that is necessary to all of our body’s operations. Earth’s air contains about 78% nitrogen, 21% oxygen, 0.93% argon, 0.04% carbon dioxide, and trace amounts of other gases, including water vapor.
The atmosphere of Mars, on the other hand, would be extremely toxic to us. Martian air is 95% carbon dioxide, 2.7% nitrogen, 1.6% argon, and only 0.13% oxygen (along with other trace gases).
A lungful of Martian air would quickly suffocate a human. But that’s not the only problem. The atmosphere of Mars is much thinner than Earth’s. On Earth, the atmospheric pressure at sea level is 14.7 pounds per square inch, or 1,013 millibars. Meanwhile, Mars’ average surface pressure is a mere 7 millibars.
So the air at the surface of Mars is about as dense as the Earth’s upper stratosphere. (Passenger jets fly in the lower stratosphere at about 30,000 feet; the upper stratosphere is twice that altitude at about 60,000 feet–12 miles up in the air!) So even if Mars’ atmosphere happened to have more oxygen, it wouldn’t be dense enough for us to breathe. In other words, keep that pressurized spacesuit on!
Incredibly, Mars’ scanty air still supports regular weather patterns. Because the tenuous atmosphere does not insulate the planet well, temperatures swing dramatically between day and night. When sunlight warms the frozen ground, air currents form, creating gusts of wind that blow up to 60 miles per hour. On Earth, wind speeds this high can uproot trees and severely damage buildings. But the Red Planet’s extremely low air pressure would make this gale feel like a gentle breeze.
So future Martian colonists won’t have to worry about windstorms tearing their buildings apart (or blowing over their spacecraft–that dramatic evacuation scene from the movie The Martian would never happen in real life!). But they will have to worry about the dust. Mars is famous for its dust storms, which can range in scale from local dust devils that are taller than Earth tornadoes, to global storms that engulf the entire planet and blot out the sun’s light for months.
How can Mars’ thin atmosphere support so much dust? For one thing, Mars’ gravity is only about 38% as strong as Earth’s, so soil particles can be picked up more easily by the wind and suspended in the air for longer periods of time. Also, Martian dust is extremely fine, with grains as small as 3 microns across (about the consistency of talcum powder). These grains are light enough to be carried high into the atmosphere, giving the sky its hazy appearance.
The dust is also responsible for those red skies and blue sunsets. To understand this, let’s pause and recall for a moment why our own sky is blue during the day. When white light from the sun enters Earth’s atmosphere, the light particles are deflected or scattered by the molecules in the air. Due to a phenomenon known as Rayleigh scattering, the tiny air molecules scatter blue light more effectively than any other wavelength. Thus, the light that reaches our eyes is dominated by the color blue, creating that gorgeous azure dome above our heads.
On Mars, however, it’s a different story. The Martian atmosphere is very thin, so there aren’t as many air molecules for the photons to bump into. Thus, Rayleigh scattering does not have much effect on the color of the daytime sky. But Mars’ air is chock-full of dust. The dust particles block much of the sunlight and scatter it differently than air molecules would. Also, the reddish iron oxide in the dust particles actually absorbs blue light, so that longer wavelengths (red and orange) dominate over the blue wavelengths. This creates the distinctive, dim orange or yellow hue of the Martian sky. (The daytime sky on Earth can actually turn orange under the right conditions, such as during a major dust storm.)
So what happens at sunset to make the Red Planet’s sky turn blue? It’s a complex process that is not fully understood, but most experts believe it is due to a type of light deflection called Mie scattering. Mie scattering occurs when there are relatively large particles in the air, such as dust grains or water droplets. While Rayleigh scattering throws photons in all directions, Mie scattering tends to angle the light forward. (This is what causes the blinding glare of car headlights when you’re driving in fog.)
Also, Mie scattering throws blue light a little bit farther forward than it throws red light. This effect is more pronounced at sunrise and sunset. Thus in the evening, the normal reddish color of the dust haze is dominated by blue light. The forward angling of the light also accounts for the unique fan-shaped halo above the sun.
In the end, the atmospheres of Earth and Mars are similar in at least one way: they are both subject to the same laws of physics. But the differences in air pressure and dust density create dramatic contrasts to the human eye, giving us the impression of two totally distinct—and beautiful—worlds. The next time you step outside to enjoy a brilliant red sunset on Earth, take a moment to appreciate our planet’s clear atmosphere, which makes this vivid display possible. Or you can imagine yourself as a future Martian, watching the silver-blue sun drop below the lip of a dusty crater. Both views are pretty spectacular.
Mary Woods is a graduate of Wyoming Catholic College, a freelance writer and editor, and an astronomy enthusiast.