The Sun Sets on Mars
On Sol 20 of its journey, Mars Exploration Rover Opportunity woke up around 5:30 p.m. in the martian afternoon to watch the sunset. This is a still image from the rover's panoramic camera showing Sun just over the horizon. The rover is looking to the southwest. This image is only approximate true color, using infrared, green and violet filters, rather than the human red-green-blue, so that the maximum panoramic camera wavelength range could be covered by the observations, enhancing the scientific value of the measurements.
The rapid dimming of the Sun near the horizon is due to the dust in the sky. There is nearly twice as much dust as there was when the Mars Pathfinder spacecraft, which landed on Mars in 1997, imaged the sunset. This causes the Sun to be many times fainter. The sky above the Sun has the same blue tint observed by Pathfinder and also by Viking, which landed on Mars in 1976. This is because dust in the martian atmosphere scatters blue light forward toward the observer much more efficiently than it scatters red light forward. Therefore, a "halo" of blueish sky color is always observed close to the Sun. Only half of this halo can be seen in this image because the other half is below the horizon.
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