Phobos Overflying the Mouth of Ares Vallis. Three mosaics of pictures from Viking Orbiter 1 show Phobos passing beneath the spacecraft with the surface of Mars in the background. These mosaics, taken about a minute apart, show an apparent motion of Phobos across the surface of Mars of about 50 km. Orbiter 1 was 13 700 km above the Margaritifer Sinus region of Mars, and Phobos was 6700 km from the spacecraft. Phobos, four times darker than Mars, appears black against Mars in these unenhanced pictures. This region of Mars contains chaotic terrain along the equator; it is near the head of Ares Vallis, a major channel leading to Chryse basin where Viking Lander 1 is located. North is to the upper left. [451A03-10; 1° N, 19° W]