The Sun Sets on Mars (Still)
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.
Images through the different filters were obtained with one-minute gaps during the sunset. Because of some saturation in the pre-sunset images, it was not possible to generate color composites for those images. Therefore, to approximate what the scene would have looked like to the human eye, a color image was made from the first post-sunset sequence of filters. The color seen in the first post-sunset image was then "painted over" each pre-sunset image in the sequence. In this fashion, the position and brightness of the Sun is taken from each individual image, but the color of the Sun is taken from a single set of post-sunset sky images, and so is not particularly accurate (previous missions have shown that the Sun is actually closer to white during martian sunsets).
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.
Image Credit: NASA/JPL/Texas A&M/Cornell
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