The Martian daytime sky is generally a butterscotch (yellow/brown) color. We know this from properly color-balanced pictures taken by the Viking landers in 1976 and Mars Pathfinder in 1997. True color pictures from both missions are shown on this page. (NOTE: Your computer monitor will only show the true color in the image (butterscotch) if it is capable of true color display; laptop displays, in particular, tend to show an inaccurate color).

The first Viking Lander images to be broadcast over TV in the 1970s showed a blue sky, later "corrected" to a pink sky. This was because of uncertainties in the initial image processing. Also, the lander had several color patches to calibrate the cameras which were partially covered with Martian dust thrown up during landing. However, further careful analysis of Viking Lander data revealed a Martian sky which is generally "butterscotch" (yellow/brownish) in color, except for the pink/red of sunset and sunrise. This was confirmed by Mars Pathfinder in 1997.

A true color image from Viking Lander 2, showing the spacecraft and part of Utopia Planitia, looking due south. The American flag, color grid, and bicentennial symbols on the spacecraft were used for correct color balance. The RTG (radioisotope thermal generator) cover is in the foreground with the flag. The S-band high-gain antenna is at the top center, and the other RTG cover is at the left. The image spans an azimuth of about 140 degrees. The image was taken at 12:20 lander local time. (Viking 2 Lander, 21C056; Date/Time (UT): 1976-11-02 T 15:24:49)

The Earth's blue sky is caused by Rayleigh scattering of light, a process first described by the British physicist Lord Rayleigh (1842-1919). Rayleigh scattering is the scattering of light by objects that are small in comparison to the wavelength of the light. Visible light from the sun contains a mixture of colors, each color corresponding to a different wavelength. Sunlight streaming into the Earth's atmosphere from one direction is scattered in all directions as it encounters air molecules. Shorter wavelengths of light (the blue end of the spectrum) are scattered more than longer wavelengths (the red end of the spectrum). An observer perceives blue light coming in from all directions so the sky appears blue. The most intense blues are usually seen between 10:00 a.m. and 3:00 p.m. on cloudless days, if you look at the sky about 45 degrees above the horizon with your back to the Sun. In the evening, when the sun is low in the sky, rays from the sun pass through a greater thickness of air. Blue light can get scattered sideways out of the beam leaving a red-colored sunset. If there were no atmosphere, the sky would appear black, like the lunar sky in Apollo pictures taken from the moon.

On Mars, pictures taken from the surface by the two Viking lander spacecraft showed a sky which was a yellow color. Measurements also showed that the Martian atmosphere always had some fine dust suspended in it. The dust particles vary in size from smaller than visible wavelengths (0.4 - 0.7 micrometers) to as large as several tens of micrometers. (A micrometer is one-millionth of a meter, or about 0.00004 inches). Sky color measurements from Viking Lander 1 have been used with computer simulations of light scattering to estimate that the dust particles contained about 1% by volume of an iron oxide mineral known as magnetite (a black, opaque material). This mineral absorbs sunlight more effectively at blue wavelengths than at red wavelengths. Scattering (including absorption) of sunlight by the dust particles in the Martian atmosphere therefore accounts for the sky color. The scattering is more complicated than the simple Rayleigh case because the dust particles both reflect and absorb the sunlight, and because the presence of 'large' particles leads to more uniform scattering among the different wavelengths. If the dust did not absorb any sunlight, the Martian sky would appear whitish, since all wavelengths would be scattered to similar degree, much like sunlight scattered by clouds. The atmospheric dust which provides the pink-yellow tint to the Martian sky is also responsible, due to its ubiquitous presence on the martian surface, for producing the characteristic red color of Mars seen by the naked eye. In general, Rayleigh scattering is a very small effect in the Martian atmosphere. However, at certain times and in certain places, clouds of extremely small dust particles give a blue cast to images taken from overhead. These are the so-called "blue hazes" observed in some cratered regions and parts of the Valles Marineris.

If the Martian atmosphere were to be completely cleansed of dust, the daytime sky would appear blue, just as our own sky because of Rayleigh scattering by the molecules (primarily carbon dioxide molecules) which make up the atmosphere. Pictures from the Hubble Space Telescope in the early 1990s suggested that the Martian atmosphere had much less dust loading than in the Viking years. So perhaps the Martian sky was a different color (i.e. closer to blue) compared to the Viking years. However, this idea is likely incorrect because Mars Pathfinder pictures showed essentially the same sky color and similar dust loading as the Viking landers.

A true color image from Mars Pathfinder showing the butterscotch-color sky (Image:PIA01546, sol 10).

Note that the image of the true-color sunset shown at the top of the FAQ page (click below on FAQ) shows a red sunset sky. The sun, however, is surrounded by a blue halo. Why? The dust in the atmosphere absorbs blue light, giving the sky its red color, but it also scatters some of the blue light into the area just around the Sun because of its size. The blue color only becomes apparent near sunrise and sunset, when the light has to pass through the largest amount of dust.