A simulation of sunsets on other worlds: from Venus to Titan

When we think about exploring other planets and celestial bodies, we tend to focus on the big questions. How would astronauts live there when they are not working? What kinds of strategies and technology would be necessary for people to be there in the long term? How could gravity, the environment, and radiation affect humans who choose to make places like the Moon, Mars, and other bodies to place their home? We tend to overlook simple things …

For example, what will it be like to look at the sky? How will Earth, stars, and any orbiting moon appear? And what will it look like at sunset? These are things that we take for granted here on Earth that we don’t really meditate on much. But thanks to NASA, we now have a tool that simulates what the sunsets of other bodies would look like in the Solar System, from the infernal surface of Venus to the dense atmosphere of Uranus.

This simulation was created by Gerónimo Villanueva, a planetary scientist at NASA’s Goddard Space Flight Center who developed it while working on a computer modeling tool for possible missions to Uranus, which NASA is considering sending a probe into a near future. If such a mission is carried out, the probe would descend into Uranus’ atmosphere and use this tool to obtain spectra and determine its composition.

As you can see, Villaneuva’s simulation compares what a sunset would look like on various worlds using Earth as the “control group”, both on a clear night and when the sky is overcast. A Uranian sunset, in contrast, appears as a rich blue glow in the sky that gradually turns deep blue as the sun sinks to the horizon. This color is caused by the interaction of sunlight with Uranus’ atmosphere, which is rich in hydrogen, helium, and methane.

These gases absorb the longest wavelengths in the spectrum (red, orange, and yellow) and the shortest wavelength photons (blue and green) to scatter and collide with other molecules in the atmosphere. This is similar to what happens in Earth’s atmosphere on a clear day, where light is scattered by interacting with our atmosphere. When this happens, the shorter wavelength blue photons scatter further, making the sky appear blue.

Meanwhile, a Venus sunset (a Cytherean sunset!) Appears as a misty yellow, gradually turning dark brown as the Sun sinks to the horizon. This is the result of sunlight having a difficult time piercing the extremely dense and toxic atmosphere of Venus, which is predominantly composed of carbon dioxide with traces of nitrogen and other gases.

Mars experiences something similar, with gray-brown hazy skies and intense brightness as it reaches the horizon. If you look closely, you will also see how the Sun appears blue just before it sinks below the horizon. This is a familiar sight for followers of the Opportunity and subsequent rover missions, which have witnessed a blue sunset on Mars on more than one occasion.

Sunset photographed from Gale Crater by the Mars Curiosity rover on April 15, 2015. The four images shown in sequence here were taken in 6 minutes, 51 seconds, using the rover’s Mastcam left eye. Credit: NASA / JPL-Caltech

Lastly, there’s Titan, where the atmosphere appears a deep, misty orange color (exactly as it appears from space) that turns dark brown when the sun sinks toward the horizon. This hazy color and appearance is due to Titan’s unusual atmosphere, which is composed primarily of nitrogen (95%) and contains high concentrations of methane and other carbon-rich organic molecules.

To validate the accuracy of this tool, Villanueva simulated known sky colors from Uranus and other worlds. The animations show views of the entire sky that mimic what it would be like to watch from the surface of another body using a super wide lens. The halos of light that appear in some cases towards the end are produced by the way in which light is scattered by particles, including dust or mist, that are suspended in certain atmospheres.

These simulations are now part of the Planetary Spectrum Generator, a popular online tool developed by Villaneuva and his colleagues at NASA Goddard (which was used to produce the animation below). This generator allows scientists to replicate how light is scattered through the atmospheres of planets, moons, comets, and even exoplanets, allowing them to measure their compositions.

But for the rest of us, it serves to illustrate what it would be like to look from a strange landscape. The world may be strange and its skies have a different color, but what there is is still familiar to us.

Further reading: NASA