Chapter 1: The Basics of Sky Color

To grasp why the sky has its distinct colors, it's essential to understand how we perceive objects.

Imagine directing a flashlight towards a wall. You might notice that, while the light illuminates the wall, the beam itself is often invisible until it interacts with particles like dust or fog. Why is that?

For us to observe anything, light must bounce off it and enter our eyes. This principle applies to all non-luminous objects. In total darkness, we cannot see anything because there’s no light reflecting off these objects.

The visibility of a beam of light traveling through air happens only when it interacts with tiny particles. Similarly, we don't see sunlight beams until they reach surfaces.

The fundamental takeaway is:

To perceive anything, light must reflect off it.

Section 1.1: The Earth's Atmosphere

The atmosphere comprises various gases, primarily nitrogen and oxygen. When sunlight encounters these molecules, a phenomenon known as Rayleigh scattering occurs. This occurs when light interacts with particles significantly smaller than its wavelength.

Visible light ranges from blue/violet light at approximately 400 nanometers to red light at around 700 nanometers. Though these measurements seem small, nitrogen and oxygen molecules are considerably smaller, being thousands of times less.

Subsection 1.1.1: Understanding Rayleigh Scattering

In Rayleigh scattering, light photons bounce off particles in random directions, which means some scatter backwards. Notably, shorter wavelengths (like blue) scatter more than longer wavelengths (like red).

When sunlight penetrates the atmosphere, more blue light disperses than red, allowing the latter to reach the ground predominantly. This scattering effect gives the sky its blue appearance.

Section 1.2: The Colors of Sunsets and Sunrises

Why do sunsets and sunrises appear red?

When the sun is near the horizon, its light must traverse a thicker layer of the atmosphere. This increased distance leads to more interactions with atmospheric particles. Consequently, much of the blue light scatters away, leaving the longer wavelengths—orange and red—to be seen.

So, would a thicker atmosphere always render the sky orange or red?

Indeed, this phenomenon can be observed on Venus, where the atmosphere is so dense that most shorter wavelengths scatter, resulting in a predominantly orange sky. However, the situation is more intricate.

Chapter 2: The Martian Atmosphere

Mars presents a different scenario. Despite having a thinner atmosphere than Earth, it often appears red in images captured by NASA missions.

This isn’t due to Rayleigh scattering; instead, the Martian atmosphere is laden with red dust particles that absorb blue light, giving the sky its reddish hue.

This effect can also be witnessed on Earth during dust storms, particularly in regions like Australia, where clouds of red dust can transform the sky into a striking red spectacle.

This video, "Why Does Mars Have Blue Sunsets?" delves into the unique atmospheric conditions on Mars that lead to its captivating sky colors.

In the second video, "Why sunsets on Mars are tinted blue | BBC Global," the discussion continues about the fascinating interplay of dust and light on the Red Planet.