Viking 1 lander image of a martian sunset over Chryse Planitia. In this image the sun is 2 degrees below the local horizon. The banding in the sky is an artifact produced by the incremental brightness levels of the camera. This image was taken on the sol 30, at 19:13 local time (10:40:31 on 20 August 1976 (UTC)). The camera is pointing towards the southwest.
I used the original 12a240.blu, 12a240.grn and 12a240.red images from the NASA Viking image archive, converted them to .png, manually removed the noise, merged them into one image (almost matching true color; see here for the channel mixing process) and finally increased the brightness with 35.