Mars moon Phobos leaves a trail as it sets in the east over the shoulder of Mount Sharp, in the first of six time- exposure frames taken over a 10-minute period by Curiosity’s right-side Navcam. The full series of images is here. (The white speckles are noise in the imager.)
NASA description: This image was taken by Navcam: Right B (NAV_RIGHT_B) onboard NASA’s Mars rover Curiosity on Sol 613 (2014-04-28 04:48:22 UTC).
Sol 613 raw images (from all cameras), and Curiosity’s latest location map.