Daddy, why is the sky blue?” Well son, it’s because of bacteria What do you mean?

It may be the most classic curious  kid question of all time.

but the answer is a lot more  interesting than you probably think. Because the sky hasn’t always been blue.

And we have life itself to thank for  changing the color of the sky not once,  but twice. [Intro music] Whatever color the sky might  be outside your nearest window, it gets that color because of two things: The Sun, to provide a light source, and all the stuff in the air  between the Sun and your eyeballs. We’ll get into the nuances of “how” later. but these days, most of that stuff is one of two gas molecules: nitrogen and oxygen. Combined, they account for basically 99% of the gas  molecules in the Earth’s atmosphere, with the leftover 1% going to compounds like argon, carbon dioxide, and methane. but four and a half billion years ago, when our planet was just a baby, things were very different.

In fact, the Earth was born  with a very lightweight, mostly hydrogen atmosphere. but something… perhaps a collision with a large proto-planet, perhaps radiation from our also-a-baby Sun… quickly stripped that atmosphere away. So the Earth had to grow itself a new one, using volcanoes and other geologic events to spew out gasses that had  previously been trapped in and beneath the surface. This second atmosphere began growing before any life existed, around 4.3 billion years ago.

And if you could hop in a time machine, you’d see that just like today, it was mostly made of nitrogen. There was basically no oxygen, and it was chock full of carbon dioxide. You might also encounter an occasional molecule of ammonia or methane, but these were quickly broken apart by sunlight, or used up in chemical reactions  on the planet's surface.

This ancient daytime sky probably looked like a perpetually overcast day, with a gray-ish blue-ish green-ish tint. And if you stuck around until twilight, the sky wouldn’t be a deep blue. It would actually be a lot more yellow-ish.

And to explain why, we can finally take a step back and talk about where the many  colors of the sky can come from Basically, it’s all to do  with how light can interact with different particles in different ways. When it comes to the sky being some shade of blue, the most important interaction is something known as Rayleigh scattering, which was named after a dude who wrote about it in the late 1800s. Now, Rayleigh scattering only happens when the wavelength of the  light that’s hitting a particle is way longer than the size of that particle.

For example, the nitrogen  molecules in our atmosphere are a fraction of a nanometer long but visible light comes in wavelengths that are hundreds of nanometers. So all that nitrogen in both  the ancient and modern Earth air is doing a bunch of Rayleigh scattering. And ditto for similarly simple molecules like oxygen, carbon dioxide, and methane. but there’s a second component  to Rayleigh scattering: shorter wavelengths of light scatter a lot better.

So blue light has a much easier  time getting into our eyeballs when we look up during the day. but depending on what else is in the sky, another kind of scattering  may dictate the final color. That’s because our planet’s air is also filled with bigger particles. Stuff like water droplets  in clouds, and bits of ash after a wildfire or volcanic eruption, and dust.

Rayleigh scattering doesn’t  apply for these larger particles. Instead we’re dealing with Mie scattering, where all light gets scattered  pretty much the same amount. That’s why overcast days on Earth look so drearily gray.

And finally, a sky’s color can be influenced not by how light scatters, but how it’s prevented from scattering… because the particle absorbs it For example, Mars’ atmosphere  is 95% carbon dioxide, which as I mentioned is still much smaller than visible light wavelengths. but if you’ve seen photos taken by any Martian lander or rover you’ll know a Martian day is as  far from blue as you can get. There are two main reasons for that. One, the air is so thin that there’s actually not much Rayleigh scattering going on in the first place.

The only time you’ll see a blue hue in the sky is during sunrise or sunset, when the Sun’s light has to travel through more of Mars’s atmosphere. And two, there’s a ton of dust floating around. And those dust molecules are super good at absorbing blue light.

Now, when I hear “absorption”, I usually think of, like a sponge. but for the absorption of light, we need to picture an atom. It’s actually the electrons in the atom that are doing the absorbing, but they’ll only absorb a bit of light if it meets their incredibly picky requirements. That’s because electrons only exist in specific energy levels around an atom’s nucleus.

You can think of it kind of like a set of stairs. You can hop up one step, or two, or even twelve,

but you can’t land between steps. So an atom’s electrons can only absorb specific wavelengths of light.

Which wavelengths those are depends on a bunch of different factors, like what kind of element the atom is, whether that atom is bound  up in a larger molecule, and how many electrons are in the whole setup. but getting back to Mars, the picky electrons in that dust  prefer the flavor of blue light, and preferentially absorb it away. That means there’s only really  reddish light left for us to see! So now that we’ve got the basics down, we can return to ancient Earth.

The deep, rich shade of blue we now see at twilight actually comes from ozone absorbing yellow, orange, and red light, leaving blues and purples behind. It’s kind of like the opposite of what’s going on on Mars. Ozone is a molecule made of three  oxygen atoms stuck together, and it wasn’t present at this  point in the Earth’s timeline.

So without it soaking up a bunch of yellow-y wavelengths, twilight would have looked  more yellow by comparison. but around 3.7 billion years ago, the Earth’s sky started to change color. Life had arrived, and it wound up turning the sky a murky shade of…orange. The key players at that time  were methanogenic bacteria, which meant that they were generating methane by just going about their microscopic lives.

Much like plants produce oxygen, or we exhale CO2, they were pumping methane into the atmosphere. Technically there were some geologic processes that helped fill the air with methane, but it was these bacteria that  did most of the heavy lifting. And with a consistent supply of methane molecules getting pumped into the atmosphere, there was plenty for the Sun to break apart.

Those pieces and bits could then engage in some chemical reactions to form much bigger and beefier molecules we collectively call haze. but there are lots of different types of hazes, and not all of them interact with light in the same way. You can see a great example  of this happening right now in the atmosphere of Saturn’s moon Titan. If you look at it from the right angle, you can see two major haze layers that come in two different colors.

The lower layer is orange-ish,  and the upper layer is blue. As for the haze indirectly made by Earth’s early methanogenic life, it was great at absorbing  shorter wavelengths of light. So the more methane life was churning out, the more haze there was to absorb blue light.

And with less blue light  available to get scattered by all the nitrogen and other small molecules in Earth’s atmosphere, the sky would appear more orange! So life is responsible for  turning a typical Earth day from some shade of blue to some shade of orange.

but it wasn’t done, yet. Because it wasn’t done evolving.

Around 2.4 billion years ago, microbes we call cyanobacteria stumbled into developing a type of photosynthesis that could produce oxygen. They might not have been the first  photosynthesizers on the planet, but they were totally the most important. And when these new bacteria  started belching oxygen into the atmosphere, they followed in their methanogenic cousins footsteps and once again shifted the color of the sky.

Except for a few weird scenarios, planets can’t make oxygen gas without some form of living thing. but to change the atmosphere of an entire planet, you almost certainly need  life to keep pumping it out. That’s because oxygen is super reactive with basically everything. So the first batches of oxygen  that these cyanobacteria were making didn’t end up in the air at all.

They reacted with the ocean, with rocks, and even from gas leaking out of volcanoes.

but eventually, those reactions ran out of steam. There wasn’t as much stuff  for oxygen to react with, so it just floated up and started  accumulating in the atmosphere. This moment in Earth’s history was so impactful, scientists gave it a fancy name: The “Great Oxidation Event”,  although the word “Event” suggests it was a lot shorter than it really was.

Because it took those little  critters about 200 million years to oxygenate the atmosphere. By which I mean bring the atmospheric composition from barely any oxygen up to  maybe 10% of today’s levels. And yeah, that’s isn’t particularly  helpful for a human time traveler.

And nor was it directly responsible for changing the color of the sky. Because remember, Earth’s atmosphere has been mostly  nitrogen gas all this time. And nitrogen loves scattering blue light, as much as molecules can “love” doing anything. but until the rise of cyanobacteria, a bunch of haze was getting in the way.

So oxygen’s real job was to react with  those haze molecules and clear them out,   leaving the blue light free to scatter and shine Some of that oxygen was also able to react to form  the ozone that turned our twilights dark blue. Plus, there’s also the fact that an oxygen-filled atmosphere is super toxic to methane-making bacteria. So soon enough, most of them weren’t even around to replenish the ingredients you’d need to make more haze.

Not only was oxygen mopping up the haze, it was also cutting off the source. When cyanobacteria permanently changed the Earth to an oxygen-rich planet, the sky finally became the  familiar blue we know today. So the next time someone asks you why the sky is blue, now you’ll have a much more interesting story to tell them.

It was kinda blue, then kinda orange, and then blue again… all thanks to life! If we’ve just shattered your world, you should take a moment to thank the book that gave us the idea for this video. It’s called Becoming Earth,  and while it’s not a sponsor, it is full of great stories about our planet. [ OUTRO ]