The Mars Exploration Program has been launching for around 20 years since the first probe Mars Pathfinder done by NASA landed on the Red’s Planet surface. For this time another four rovers touched down on the Martian ground: Spirit & Opportunity (2004), Phoenix (2008), and Curiosity (2012). All of those missions beamed many photos of the Red Planet environment. Among tens of photos available on the main Mars, I selected those, which present the biggest fragments of the Martian sky.
Everyone saw the sky on Earth. Our sky appears blue because air scatters more blue wavelengths than red. This is a result of the scattering of sunlight. During the day a blue color is perceived by the human eye. On a typical sunny day, Earth’s sky blue gradient is the darkest in the zenith and light near the horizon (Pic. 1). This is because the light that comes overhead encounters 1/38 of the air mass that light coming along a horizon path encounters.
Another thing to consider is the Rayleigh scattering of sunlight in the atmosphere, which causes diffuse sky radiation. Diffusing sky radiation refers to the sunlight reaching the Earth’s surface, which previously has been scattered in the atmosphere. The sunlit sky is blue because air scatters short-wavelength light more than longer wavelengths. The situation is slightly different during the sunrise and sunset when longer wavelengths are scattered (pic. 2).
On top of that Earth’s surface also influences the sky’s color. The majority of Earth’s surface consists of water (blue color), from which eventually both the direct sunlight and light scattered in the atmosphere are reflected in blue color making the sky bluer. Standing on the Earth we can also see the blueness of the horizon and the object, that is situated further from us appears bluish. This is caused by a smallish part of the air–tiny hydrocarbon particles released by vegetation chemically react with ozone molecules, that selectively scatter blue light, giving the mountains, and remote objects a blue appearance (pic. 3).
The Martian sky appears to be different than Earth’s. People have seen it since the Viking Mars landers sent back the first color photos in 1977. Since that time many photos have been sent back to Earth showing a different appearance of the Red’s Planet sky. Generally speaking, is really hard to state the exact color of the Martian sky. Some images show it to have a red sky, others a butterscotch sky, and even, similarly like on Earth blue sky. Taking full 360-panorama photos by rovers before Curiosity seemed to be difficult because the rovers couldn’t see the full vista at once. They couldn’t see a whole sky simultaneously. It took a few days to complete the panorama. Rover had to make smaller pictures and then stick them all together. During this time weather could have changed and the sky would have a different color also.
The rovers are able to take pictures using three different primary color filters (RGB) using 14 filters aimed at geological stuff and providing maximum contrast to analyze. Each Rover is equipped with a calibration target. Everything is to be made all photos as realistic as possible. Moreover, each Rover has a photo calibration before leaving the Earth.
The aforementioned graph shows the photo rendering possibilities for the Opportunity rover, which is equipped with 6 photo filters. Below you can see the example of pictures done by using different color rendering.
Unlike Earth Mars has an atmosphere dominated by dust, which produces a bright orange and red sky color. The redness increases toward the horizon during midday time hours. Conversely to Earth’s conditions, the Martian sky seems to be the most brighter in its zenith. It may be caused by suspended atmospheric dust. Those micron-size dust aerosols cause the bluish glow of the sunset also. The Martian dust absorbs blue light more strongly than red. We can see it, especially when the Martian sun is low above the horizon. A bluish glow surrounding the sun around sunset is created by light scattered at small angles by dust particles. The glow with blue ting around the sun appears also during dust storms. In general, the blue glow should follow the sun as it traverses the Martian sky from sunrise to sunset. However, it will be most intense during sunrise and sunset due to the increased optical path length through the atmosphere. Blue light’s intensity dominates dust for scattering angles up to about 28 deg with the greatest dominance of angles up to 10 deg (pic. 7). The dust particles, that dominate the Martian atmosphere are much bigger than aerosols that exist in Earth’s atmosphere. The radius of dust particles is close to the wavelength of visible light. For these big particles, there is no simple law analog to the Raleigh light scattering law. Scattering for these particles is sensitive to size, shape, and composition. Blue sun and moon may appear on Earth also, but they can be seen very rarely, mainly during volcanic eruptions, when bigger particles of dust are emitted into the atmosphere.
Twilights on Mars last for a long time, because of the dust density in the atmosphere, which perfectly scatters the sunlight. During the low dust level, the observer would be able to see Martian clouds in the sky. There are very faint clouds, which look likewise noctilucent clouds seen from the Earth’s surface. Those clouds are higher than every cloud on Earth – up to 100 km. The clouds are most likely made of carbon dioxide and can be seen reflecting the sunlight against the darkness of the twilight sky, making it more violet (pic. 10).
There is a lot of variation in the Martian sky due to the level of dust influence. There are huge dust thunderstorms, that may cover continent-sized areas, which may last even a month. During that weather, the light level drops quickly and the sky color turns from butterscotch to reddish, even brown (pic. 11).
Those huge dust storms are caused by winds. Aside from winds, generated by the difference of the heating up the surface and especially by trade winds, that exist at low altitudes. At higher latitudes, a series of high and low-pressure areas dominate the weather. Because Mars is much dryer than Earth, and in consequence, dust raised by these winds tends to remain in the atmosphere longer than on Earth as there is no precipitation to wash it out.
The color of the Martian sky changes due to the arc distance from the Sun, likewise on Earth and different planets (pic. 13).
When rovers do the 360-degree panorama it takes time, even a few days. During this time the weather conditions may change significantly. See this situation below on the Curiosity 36 deg self panorama (pic. 14).
What how the sky be like when the dust would be completely gone? The newest photos provided by the Curiosity rover since 2012 display the most realistic and high-detailed color of the sky, which observation could see being on the Red Planet. Some of them show the bluish or greenish color of the sky. It may be explained by the color-corrected mosaics, enhanced colors, and color balance settings (pic. 21, 22)
However the most important for us are images made in true color rendering. When there have been no recent storms the sky should be blue. According to photos provided during the best weather conditions, the sky is to be seen like bright butterscotch near the horizon through yellow-green to dark greenish-bluish near the zenith. Should we believe that this sky color is right? A greenish hinge is to be noticed in some photos, but I think that we need to treat it as something between dark blue and fair butterscotch. Because Mars is extremely dry there, dust may be suspended in the atmosphere for a very long time. Even then, when the dust level is possibly the lowest can still affect the color of the sky, making it mixed with blue and butterscotch, which may result in some greenish tinge. See the picture below (pic. 34), which shows true Martian sky colors during the low dust conditions. The sky looks similar to this, seen on Earth’s surface. This picture brings us the answer to the question: How would the sky color change as the dust opacity decreases, hypothetically all the way to zero dust?
During the low dust conditions, the Martian sky is to be blue. The Rayleigh scattering will play the main role in this case. This light scattering effect will be much weaker than in Earth’s conditions because the Martian atmosphere is much thinner. Observers on Mars’s surface would see the faint blue-butterscotch or greenish-butterscotch sky near the horizon and dark blue sky near the zenith. Besides Mars’s surface is reddish, so it must reflect a reddish light only. This light should also influence the color of the Martian sky. Taking into account the thickness of the Martian atmosphere the sky near to zenith would look much darker than seen from Earth, and possibly the stars would be visible during the day. I think that we can compare the zenith of the Martian sky to the zenith of Earth’s sky when we are traveling by plane at around 10 km altitude. We need to know that aside from the rare atmosphere the big role plays a distance between Sun and Mars. Mars’s average distance from the Sun is around 230 million kilometers (1.52 AU), around 80 million kilometers further than Earth. It means, that our host star appears to be apparently smaller giving a smaller amount of light at once. Basically, the solar disk seen from the Martian surface consists of around 5/8 size as seen from Earth (around 21 arc minutes diameter). According to the inverse square law, Mars receives only 40% of light distributed to Earth. To imagine this light level is good to compare it with a slightly cloudy late afternoon on Earth or a partial solar eclipse day with around 60% obscuration. Obviously, it applies to the midday time hours in low-longitude areas where the Sun rises up near the zenith. For the morning, evening, and polar latitudes this light appears to be weaker, especially during huge dust storms. Considering fair weather conditions with the pristine sky, the atmosphere absorption effect would not be noticed on the Earth-scale, because the Martian atmosphere is nearly 100 times thinner than Earth’s, therefore, it should not impact the luminance of this planet. This is only theory. As I wrote above Martian atmosphere must be completely free of dust so the light absorption appears to be bigger. In the low light level conditions, which may be common throughout the Martian day with thick dust the Purkinje effect may play a big role.
The Purkinje effect tells us what is the human’s eye response to color in different levels of ambient light: for instance, red objects appear to darken faster than blue objects as the level of illumination goes down.
There are many variations in the color of the sky as reproduced in published images since many of those images have used filters to maximize their scientific value. For many years the sky on Mars was thought to be reddish than it is now believed to be. The newest images and measurements show that sky color properties may significantly differ due to weather conditions. All studies about the Martian sky may have a significant impact on future human missions to the Red Planet. The unique sky color differing so greatly from reddish-butterscotch to bluish-black may provide some psychological or physiological effects on astronauts. Possibly during fair weather conditions, the stars would be visible during the day, especially around sunrise and sunset. To know many more details we have to wait for forthcoming space missions, which will provide us with better photo documentation.
- Bell J. F. et. al., 2006, Chromacity of the Martian sky as observed by the Mars Exploration Rover Pancam instruments, (in:) Journal of Geophysical research – Planets, vol. 111, Issue E12.
- Ehlers K. et. al., 2014, Blue Moons and Martian sunsets, (in:) Applied Optics, vol. 53, Issue 9, 1808-1819.
- Frisby JP (1980), Seeing: Illusion, Brain and Mind, Oxford University Press, Oxford
- Thomas N. et. al., 1999, The color of the Martian sky and its influence on the ilumination of the Martian surface, (in:) Journal of Geophysical research – Planets, vol. 104, Issue E4, 8795-8808.
Mars Pathfinder 20 years anniversary
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Blue sky and blue haze
Mars sky colors and image processing
Information about Spirit & Opportunity rovers and photography of the Red Planet
Mars – atmosphere, climate, weather
Clouds on Mars – Mars Pathfinder gallery
Mars clouds information