Ask Ethan: Why is the sky bright and red on Mars?

Published on May 30, 2018

Ask Ethan: Why is the sky bright and red on Mars?

Original author: Ethan Siegel
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The image from the Opportuny rover demonstrates part of the “Marathon Valley”, as it looks from the north. The image was made by the camera of the Pancam rover March 13, 2015.

You can learn a lot about the world by studying its atmosphere. For example, on Earth, the optical properties of light passing through our atmosphere tell us about its composition, reflection coefficient, density, and so on. If our atmosphere were much thinner and less dense, the sky would not be so blue, the sunsets would not be so red, and in general the brightness of the sky would be less. On Mars, the density of the atmosphere is 0.7% of Earth's, so that even with less gravity, its atmosphere is very thin and rarefied compared to ours. So why on the images obtained from the rovers, the Martian sky seems so bright? This is the question our reader asks:
We physicists know that the sky acquires the brightness due to the sunlight scattering on the matter of the atmosphere. The brightness of the sky is directly related to the mass of matter in the atmosphere. But what do we see in the images obtained with Curiosity, Spirit and Opportunity? Incredibly bright sky and incredibly vague mountains. How can you explain this?

Curiosity self portrait from 2015. This rover is the heaviest cargo that landed on Mars, and even weighs less than a ton. The quality of his camera, however, is enough to see the sky of Mars in the same colors in which the human eye would see him.

There is no doubt, the sky of Mars is red and bright, not such as one would expect, simply by extrapolating on the basis of the Earth’s atmosphere, turning it into a more subtle and rarefied one. This question is constantly becoming a topic of conspiracy theories, because what we see on the surface of Mars is very different from what we expected to see there. What could be the cause?

The distinctive features of Mars have evolved because of its geological history, but the color of the atmosphere strongly depends on how, when and where to look at it. It can vary from yellow-brown to red and blue, depending on a set of factors.

Before your imagination begins to make hasty conclusions, which some people will probably immediately begin to consider - for example, that all photos released by NASA are actually edited photos from the Earth - let me show you an image of the Earth that has not been modified .

US Air Force Base Al-Takadum in Iraq, also known as Air Base TQ , during a sandstorm on Earth

Does this photo remind you of photos of Mars? Probably, depending on what images of Mars come to your mind - and you might also think that the sky on Mars should not be yellow, but red.

In other words, you would prefer a similar photo: A

sandstorm in an unknown desert, May 2006

These photos were not taken at sunset, but they were taken under very specific conditions on Earth. Namely, they were made with a large number of particles in the atmosphere. If you have been to places with a lot of smog, air pollution or forest fires burning nearby , you have probably seen something comparable to what a sandstorm looks like in a desert or Martian atmosphere.

If you look at the photos of Mars obtained by various rovers in different places, you will see that the colors vary quite a lot.

The surface of Mars is terribly inhospitable for forms of life known to us. Mountains in the distance and the sky will look different depending on the density of particles in the atmosphere.

What is causing this discrepancy?

There are many factors that play a role. Rover cameras - a little different from each other - do play a role. But this is not the main factor in changing colors; There are physical explanations for the difference between landscapes and the sky. Among them:

  • The amount and density of dust in the Martian atmosphere during the shooting;
  • The physical size of dust particles, varying in time and space;
  • The location of the sun in the sky relative to the camera.

Variations of the picture that we see, looking at Mars, can demonstrate the elusive, but extremely important properties of this world and its atmosphere.

Images of Mars from Hubble, especially the areas with clouds and ice, demonstrate the blue color of this part of the world, which indicates the size of the ice particles in the atmosphere of Mars.

Why, for example, does the sky of Mars sometimes appear blue? The image above, taken from afar by the Hubble telescope, corresponds to what I first saw on the surface of Mars Pathhinder.- the sky, which can cover the clouds of water ice, consisting of very small particles. They are perhaps ten times smaller in size than ordinary Martian dust, and a thousand times smaller than the thickness of a human hair. These particles, being smaller than the wavelength of visible light, appear bright in the blue part of the world, but almost invisible in red light. From this we learn an important scientific lesson: ice particles in the clouds of Mars are extremely small.

On Earth, at sunset or sunrise, the sky on the opposite side of the Sun, free from particles, looks blue, despite the fact that the Sun and its surroundings look red. And what happens on Mars?

A wide angle photo of a sunset on Mars taken by the Spirit rover in 2005. Mark the bluish color next to the Sun and compare it with the reddish color of other parts of the sky.

The sky of Mars near the Sun appears blue, and the sky far from it - red. The disk of the Sun appears almost white, with a light blue tint. It is not associated with clouds or ice, but only with Martian dust permeating the atmosphere of the planet. The dust in the atmosphere, like the dust of sandstorms on Earth, absorbs blue light, making the sky mostly red. However, the dust disperses some blue light in the area near the Sun, due to the size of the dust particles. If you look at the sunset on Mars, as did Curiosity in 2015, you can see the incredible visual effect of the blue setting sun.

Sunset, view from Gale's crater; The photo was taken from Kyuriositi on April 15, 2015.

Mars does not look at all like we expected before we got there, because it is full of dust - not only on the surface of the red planet, but also in the entire atmosphere. Of course, sometimes there are sandstorms on Mars, which further enhance this effect, and sometimes the density and amount of dust increases or decreases, and this affects the picture. But this is not just a transparent atmosphere familiar to the Earth; dust is suspended in it, because of which remote objects are poorly visible. The dust particles on Mars are large, they mostly absorb blue light, which means that the light reflected from the particles will be predominantly red. Also, due to the large number of particles, the sky of Mars will be brighter than one would expect - brighter than if this dust were not.

Mars and its subtle atmosphere, as seen from the Viking spacecraft in the 1970s. The atmosphere has a bright red color because of the presence of dust in it.

We didn’t even need to land on Mars to see it: the Viking missions , approaching Mars in the 1970s, carefully considered the atmosphere of Mars from space. The red glow was clearly distinguishable from a distance of tens of thousands of kilometers, and would block more light than it would have if there were no dust. Scientists have even been able to understand the mineralogy of the Martian dust only on the basis of the effects of light absorption. Total:

  • Mars clouds are made up of tiny ice particles;
  • There is much more dust on Mars than on Earth;
  • Red dust;
  • It mainly absorbs blue light;
  • The red light is mainly reflected from it, on the line of sight of the Sun;
  • Because of this, the sky appears red, the sky near the Sun is blue, the distant mountains are blurred, and the clouds are blue.

Therefore, the atmosphere on Mars looks exactly like this, and this is confirmed by evidence gathered in decades. You can not apply the physics of the atmosphere of the Earth to Mars and hope for a meaningful result. Mars is, literally, a whole new world.