What is the colour of Martian sky?

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.

Pic. 1 Earth’s sky is darker near the zenith and fainter close to the horizon. It depends on the thickness of the atmosphere, Beacon Hill, Princes Risborough.

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).

Pic. 2 Earth’s sky around sunset. Longer (orange and red) wavelengths are scattered, which turns the sky into a yellowish color, Długie, Poland.

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).

Pic. 3 All remote objects seen on Earth have a blue appearance (bigger or smaller depending on the weather conditions), Chiltern Hills, Princes Risborough.

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.

Pic. 4 A first color photo from Mars made by Viking displays sky colors from yellowish (butterscotch) to brown, Utopian Plain (mars.nasa.gov).

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.

Pic. 5 Color composite images from Opportunity: a) true color rendering with all 6 visible spectrum filters, b) color rendering using 3 filters (L4, L5, L6), c) color rendering using 3 filters (L4, L5, L6) with average normalized sky spectrum, d) average splined spectrum using all 6 filters. (Bell et al. 2006).

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.

Pic. 6 Martian sky covered by Opportunity rover in different color rendering: 1 – The Lion King Panorama, true colors (pic. 5a); 2 – The Endeavor Crater rim from ‘Murray Ridge, color rendering with 3 filters (pic. 5b); 3 – The Endeavor Crater west rim, color rendering with 3 filters + normalized sky spectrum (pic. 5c) (mars.nasa.gov).

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.

Pic. 7 Martian sunset is seen from the Gusev crater and captured by Spirit rover on 19.05.2005. The photo shows the blue light scattering up to 28 arc degrees from the Sun. The biggest effect of bluish light is to be seen 5 arc degrees directly above the Sun, which was related to a big density of dust in the Martian lower atmosphere for that time being (Ehlers K, et al. 2014).

Pic. 8 The Martian sunset above the Gale crater, captured by the Curiosity rover. The higher-detailed picture shows exactly the same effect, however, the dust density seems to be bigger than in the previous picture 7. Moreover, the mountainous area doesn’t show the true horizontal line, hence the bluish tinge is the best noticeable just above the Sun (looks like a plume). The Sun’s direction has been zoomed in (mars.nasa.gov).

Pic. 9 For the first time the Martian sunset was captured by Mars Pathfinder in 1997. This picture clearly shows the same bluish light scattering effect with other sky color variations above Twin Peaks. In all 3 cases (pic. 7,8,9) Sun appears like a white circle with a bluish outline (mars.nasa.gov).

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).

Pic. 10 Martian twilight sky with dioxide carbon clouds, making the sky more violet. All images were done by Mars Pathfinder in 1997: 1 – Pre-dawn clouds over Mars, 2 – Pre-dawn Martian sky, 3 – A violet Martian sky, 4 – Whispy blue clouds, 5 – Dry ice clouds 30 deg above the horizon, 6 – Pink “stratus” clouds (mars.nasa.gov).

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).

Pic. 11 The thunderstorm coming on the Spirit rover area in July 2007. In the span of tens of minutes, the sky became dark and brown. The Greek letter tau shows the atmosphere opacity factor when thunderstorms come. Light level is reduced dramatically and the ground becomes invisible (mars.nasa.gov).

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.

Pic. 12 The Spirit rover sky photo sequence showing the relative color changes of the Martian sky in time. The survey had been carried out for 35 Martian days (sols) for picture A and 60 Martian days (sols) for picture B. During the days with better weather conditions, you can spot, that the sky closer to the zenith is darker. In other cases, the hue seems to be constant or the sky looks slightly brighter higher above the horizon (Bell et al. 2006).

The color of the Martian sky changes due to the arc distance from the Sun, likewise on Earth and different planets (pic. 13).

Pic. 13 The color variations of the Martian sky due to spherical distance from the Sun. Usually, the sky looks brighter when closer to the Sun: 1 – Spirit rover near ‘Husband Hill’, 2 – The panorama of Mars by Mars Pathfinder, 3 – The panorama of Rocheport by Opportunity rover (mars.nasa.gov).

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).

Pic. 14. The Curiosity rover low-angle self-portrait with 360-degree panorama. See the sky color variations from butterscotch on the right through the dark in the middle to reddish on the right. It means, that when the rover was making the panorama the weather conditions changed (mars.nasa.gov).

Pic. 15 Another example with various colors and hues of the Martian sky seen on the Opportunity white balanced) – a Billion Pixel View on Mars (mars.nasa.gov).

Pic. 16 The Curiosity self-portrait at ‘Murray Buttes’ was done probably during more stable weather conditions (mars.nasa.gov).

Pic. 17 The sky above Martian ‘Cape Tribulation’ near Endeavor Crater made by Opportunity rover (mars.nasa.gov).

Pic. 18, 19 The Curiosity rover 360 deg panorama on ‘Murray Buttes’: 2 sections showing the color of the sky: 18 – close to the Sun (white-butterscotch), 19 – opposite side of the Sun (brownish) (mars.nasa.gov).

Pic. 20 The ‘Twin Cairns’ Curiosity panorama shows the same effect, although it looks faint, probably because of the different color rendering.

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)

Pic. 21 The Mount Sharp Panorama in white-balanced colors made by the Curiosity rover shows the blue sky, however, there is no difference in hue between the above-the-horizon sky and the zenith sky (mars.nasa.gov).

Pic. 22. A various Curiosity picture color rendering, Mount Sharp (mars.nasa.gov).

Pic. 23 The white-balanced picture of the Curiosity rover land site (pic. 22c) (mars.nasa.gov).

Pic. 24 The panorama of ‘Marathon Valley‘ made by Opportunity rover, unprocessed colors (mars.nasa.gov).

Pic. 25, 26 Northern portions of Gale crater rim viewed from ‘Naukluft Plateau’ – picture made by Curiosity rover shows the emphasized bluish effect, which is bigger when looking closer to the Sun (upper picture), on the opposite side of the sky, has mixed tinge with bluish, dark and brown (bottom picture).

Pic. 27 The ‘Knudsen Ridge’ captured by the Curiosity rover with enhanced colors shows a bluish sky with a constant hue (mars.nasa.gov).

Pic. 28 The Mount Sharp captured by the Curiosity rover with a white balance effect, that compensates for the blue color absence and makes the sky appear bluer (mars.nasa.gov).

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?

Pic. 29 The Mars Pathfinder picture, captured during lower dust conditions, where the horizontal sky appears to be slightly brighter (mars.nasa.gov).

Pic. 30 The true colors Twin Peaks image made by Mars Pathfinder in 1999, showing the sky during low dust conditions. The upper part of the sky looks a little bit greenish, and the horizontal part has a butterscotch color (mars.nasa.gov).

Pic. 31 The Big Joe in the Chryse Planitia captured by Mars Pathfinder. The image clearly shows the sky with thin dry ice clouds. During those weather conditions, the sky was looking from butterscotch near the horizon to dark near to zenith (mars.nasa.gov).

Pic. 32 The Curiosity rover made a picture, which shows a sky during a fair-weather day. The hue of the sky is dark greenish (mars.nasa.gov).

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.

Pic. 33 The Curiosity rover self-portrait shows a quite dark image. During Martian days with a high level of dust, the Purkinje effect may play a role (mars.nasa.gov).

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.

Pic. 34 The Curiosity color view picture showing the Martian sky during probably the best weather conditions. We can notice, that visibility is quite good, making in effect a pristine clear sky with Rayleigh scattering influence. The sky appears to be yellowish-greenish just above the horizon and dark bluish upper with some greenish elements. Those greenish elements may be a result of the mixing between lightly scattered in the Martian atmosphere (Rayleigh scattering) in short wavelengths (blue) and light reflected by the Martian surface (reddish). This photo gives probably the answer about a clear Martian sky without dust (mars.nasa.gov).

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.

Pic. 35 Planet Earth seen from Mars. This is one of the most prominent objects on the Martian evening & morning sky (mars.nasa.gov).

Mariusz Krukar

References:

1. 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.
2. Ehlers K. et. al., 2014, Blue Moons and Martian sunsets, (in:) Applied Optics, vol. 53, Issue 9, 1808-1819.
3. Frisby JP (1980), Seeing: Illusion, Brain and Mind, Oxford University Press, Oxford
4. 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.

Links:

Mars Pathfinder 20 years anniversary
Why the sky on Earth is blue?
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