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. Phobos, four times darker than Mars, appears black against Mars in these unenhanced pictures.
North is to the upper left.