The good folks over at NASA recently released a new Vine from the red planet. Back in November, 2010, scientists instructed the Opportunity rover to focus its cameras on the horizon and take photos of the sun as it set on the martian landscape. After beaming the images back to Earth, they were stitched together to create a stunning video of a true sunset on another world, but this was not like a sunset on Earth... this sunset was blue.
video of the martian sunset as seen by the Opportunity rover
To understand why a setting sun on Mars is blue, we need to first ask another question: why is the martian sky red?
Over the years, scientists have amassed hundreds of images of the martian surface taken by various landers and rovers. These pictures reveal a sky above with an orange-red (sometimes called butterscotch) color.
This is because the atmosphere is constantly filled with dust particles. These particles are particularly good at taking in white sunlight and scattering the orange and red wavelengths in all different directions while allowing the blue part of the spectrum to pass through unhindered.
When the sun is setting (or rising) on Mars, it is very low on the horizon. Because of this, the sunlight has to travel through much more atmosphere before reaching your eyes (or, in this case, the rover's lens). This means that by the time the light reaches you, most of the red end of the spectrum has been scattered (i.e. filtered out), leaving only the blue wavelengths left to hit your eyes. It is this phenomena that makes the sun appear blue.
Same Physics, Different Conditions
This is the same thing that happens on earth, with the difference being that the various atoms in our atmosphere (mainly oxygen and nitrogen) are good at scattering blue wavelengths (making the sky blue) while allowing the other wavelengths to pass through. So, during sunset and sunrise on our planet, most of the blues have already been filtered out, leaving us to witness a sun that appears reddish-orange.
For more information on light scattering, see Blue Skies and Orange Sunsets.