Is Mars really red? A physicist explains the planet's reddish hue
Is Mars really as red as people say it is? – Jasmine (14), Everson, Washington
People from cultures throughout the world have been looking at Mars since ancient times. Because it appears reddish, it has often been called the red planet.
The English name for the planet comes from the Romans, who named it after their god of war because its colour reminded them of blood. In reality, the reddish color of Mars comes from iron oxide in the rocks and dust covering its surface.
Your blood is also red because of a mixture of iron and oxygen in a molecule called haemoglobin. So, in a way, the ancient connection between the planet Mars and blood wasn't completely wrong.
Rust, which is a common form of iron oxide found here on Earth, also often has a reddish colour.
In my current research on exoplanets, I observe different types of signals from planets beyond Earth. Lots of interesting physics goes into how researchers perceive the colours of planets and stars through different types of telescopes.
Observing Mars with probes
If you look closely at pictures of Mars taken by rovers on its surface, you can see that most of the planet isn't purely red, but more of a rusty brown or tan colour.
Probes sent from Earth have taken pictures showing rocks with a rusty colour. A 1976 picture from the Viking lander, the very first spacecraft to land on Mars, shows the Martian ground covered with a layer of rusty orange dust.
Not all of Mars' surface has the same colour. At the poles, its ice caps appear white. These ice caps contain frozen water, like the ice we usually find on Earth, but they are also covered by a layer of frozen carbon dioxide – dry ice.
This layer of dry ice can evaporate quickly when sunlight shines on it and grows back again when it becomes dark. This process causes the white ice caps to grow and shrink in size depending on the Martian seasons.
Beyond visible light
Mars also gives off light in colours that you cannot see with your eyes, but scientists can measure them with special cameras on their telescopes.
Light itself can be thought of not only as a wave, but also as a stream of particles called photons. The amount of energy carried by each photon is related to its colour. For example, blue and violet photons have more energy than orange and red photons.
Ultraviolet photons have even more energy than the photons you can see with your eyes. These photons are found in sunlight, and because they have so much energy, they can damage the cells in your body. You can use sunscreen to protect yourself from them.
Infrared photons, on the other hand, have less energy than the photons you can see with your eyes, and you don't need any special protection from them.
This is how some types of night-vision goggles work: they can see light in the infrared spectrum as well as the visible colour spectrum. Scientists can take pictures of Mars in the infrared spectrum using special cameras that work almost like night-vision goggles for telescopes.
The colours on the infrared picture aren't really what the infrared light looks like, because you can't see those colours with your eyes. They are called 'false colours', and researchers add them to look at the picture more easily.
When you compare the visible colour picture and the infrared picture, you can see some of the same features – and the ice caps are visible in both sets of colours.
Nasa's Maven spacecraft, launched in 2013, has even taken pictures with ultraviolet light, giving scientists a different view of both the surface of Mars and its atmosphere.
Each new type of picture tells scientists more about the Martian landscape. They hope to use these details to answer questions about how Mars formed, how long it had active volcanoes, where its atmosphere came from and whether it had liquid water on its surface.
Astronomers are always looking for new ways to take telescope pictures outside the regular visible spectrum. They can even make images using radio waves, microwaves, X-rays and gamma rays. Each part of the spectrum they can use to look at an object in space represents new information they can learn from.
Though people have been looking at Mars since ancient times, we still have much to learn about our fascinating neighbour. DM
First published by The Conversation.
David Joffe is an associate professor of physics at Kennesaw State University in Georgia, US.
This story first appeared in our weekly Daily Maverick 168 newspaper, which is available countrywide for R35.
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