The atmospheric pressure on the surface of Mars is 0.00609 atm on average and 0.012 atm at most in the Hellas Basin. On Earth, at these pressures, which are found at altitudes about 30-35 km (19-22 mi), the sky is very black and full of stars. On the Martian surface however the sky is still bright (namely orange) and no stars/planets are visible in daytime from what is considered the sea level (except the Sun and maybe the Earth). Is this due to the many dust in the Martian atmosphere only or are there other reasons too for why the sky is not black but still orange at these pressures?
It is very likely due to dust particles. The color of Martian sky is described to be yellow-brown "butterscotch" color during daytime (sometimes pinkish red) and blue during rising and setting of sun (very different from what happens on Earth). The typical color is because as the dust particles in the air are large relative to the wavelength of visible light, it absorb blue light and effectively act as mirrors by scattering the remaining wavelengths. Also, the presence of 1% by volume of magnetite in the dust particles intensify the color. Twilight lasts a long time after the Sun has set and before it rises due to that.
At times, the Martian sky takes on a violet color due to scattering of light by very small water ice particles in clouds.
For more information, read this excellent paper: The color of the Martian sky and its influence on the illumination of the Martian surface
For more on Mars Twilight see:
- At Mars' equator how long does twilight last?
- How bright is the Martian landscape during the long (up to two hour) twilight?
- How much more quickly does night come on Mars compared to Earth? Will it be easier to see Mercury there?
- How bright is twilight on Mars compared to Earth?
- Mars twilight photos