Jupiter and Saturn are better to look at through a telescope, but if you could view them up close, they’d appear almost drab compared to the ice giants, which are hues of blue and green. But why blue rather than red? Why don’t we have neon-green ice giants?
Earth: The Pale Blue Dot
The Earth itself is so famously blue that its blue hue, viewed from the vantage point of Saturn, inspired Carl Sagan’s bookPale Blue Dot.
The blue appearance of Earth comes down to the vast amounts of water, which covers some 70% of the surface, and to a lesser degree,the makeup of the atmosphere.
However, most things our solar system are decidedlynotblue. Jupiter and Saturn are both yellow-brown, Mars is rusty red, and Venus is an off-yellow color. Some of the moons have interesting hues of red, brown, or even off-green.
But blue? Not a drop beside us and the ice giants, Uranus and Neptune.
Why is Uranus Blue?
Unlike Earth, Uranus isn’t blue because of water, but rather methane. Yes, the same chemical you hear about in the context of cow flatulence, sewage, and natural gas.
While the chemicals are different, the color of Uranus is actually related to why Earth’s oceans are blue.
In the case of water and our oceans, red light is absorbed very quickly by water molecules and turned into heat. As a result, if you carry something red deep underwater, you’ll quickly find that it looks black, since there is no red light to reflect back at you! The only colors left are blue and some green.
This is not the same thing that causes blue skies. Blue skies occur because blue light scatters more than red light (Rayleigh scattering) which means blue light reaches you from multiple angles simultaneously, while redder light reaches you more directly. That gives the sky its diffuse luminous blue appearance. As the atmosphere gets thicker, longer wavelengths (like yellow, orange, and red) are increasingly scattered too, which is why the sky turns gold or even reddish during sunsets. The thicker atmosphere scatters even more of the light!
Something very similar happens with methane. Much like water, methane absorbs more red light than green or blue light. It also absorbs a ton of infrared light, which is what makes it such a potent greenhouse gas.
With the red light absorbed by the methane (and mostly turned into heat), the only light that can be reflected back is the green and blue light, which is what gives Uranus its signature blue-green color.
Where Does Uranus’s Methane Come From?
On Earth, we associate methane with living things. The digestion of cows, microorganisms breaking down old plant matter, termites, and wetlands.
But there certainly aren’t any cows on Uranus, and so far as anyone knows, there aren’t any microorganisms either. So where does the methane come from?
As it turns out, there are actually a few ways you can create methane without living organisms at all. In the case of Uranus, it probably came from the combination of simpler molecules, like carbon monoxide and hydrogen gas, in the early years of the solar system, a process imaginatively named “methanation.” Methane can also form as carbon gradually picks up hydrogen atoms until you reach methane, which is four hydrogen atoms bonded to one carbon atom (CH4). Either way, this chilled methane would have been collected by planestimals (tiny protoplanets) or hung around as methane-rich icy dust that eventually collided with Uranus and deposited methane.
On planets like Mars or moons like Titan, it is possible to combine water with different minerals to produce methane gas instead, a process called serpentinization. However, those bodies also hold the potential of a more intriguing answer: life. While Mars likely doesn’t support life today, it very probably could have in the distant past. And Titan, a moon of Saturn that is covered in methane and other organic molecules, may harbor life even today.
There are also several other ways to produce methane too, and likely some ways we’ve not even discovered yet.
Why Is Neptune a Different Color?
Neptune, much like Uranus, is blue because of the presence of methane in its atmosphere, and they’re generally similar in many ways. So why the different color?
Despite similar chemical compositions, the atmosphere of Uranus is literally just more hazy. No one is entirely sure what the haze is made of, but several studies have concluded that they’re likely the result of organic molecules reacting with sunlight and other chemicals in the ice giant’s atmosphere.A paper published in 2022suggests that the condensation of methane—effectively methane snow or hail—removes the haze from the upper layers of Neptune’s atmosphere, much like how rain can pull smog out of the air on Earth, resulting in a deeper blue color.
Interestingly,recent findings using the Hubble Space Telescopesuggest that the arrangement of these aerosols in the atmosphere of Uranus changes seasonally, giving rise to long-term color changes.
Despite decades of study, Uranus and Neptune are poorly understood, even compared to Jupiter or Saturn. However, NASA is on the job. If all goes well, a mission will launch for Uranus sometime by the mid-2023s and shed some light on the enigmatic world.
