These two images taken by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) show Mars' two small moons, Phobos and Deimos, as seen from the Mars Reconnaissance Orbiter's low orbit around Mars. Both images were taken while the spacecraft was over Mars' night side, with the spacecraft turned off its normal nadir-viewing geometry to glimpse the moons. The image of Phobos, shown at the top, was taken at 0119 UTC on October 23 (9:19 p.m. EDT on Oct. 22), and shows features as small as 400 meters (1,320 feet) across. The image of Deimos, shown at the bottom, was taken at 2016 UTC (12:16 p.m. EDT) on June 7, 2007, and shows features as small as 1.3 kilometers (0.8 miles) across. Both CRISM images were taken in 544 colors covering 0.36-3.92 micrometers, and are displayed at twice the size in the original data for viewing purposes.
Phobos and Deimos are about 21 and 12 kilometers (13.0 and 7.5 miles) in diameter and orbit Mars with periods of 7 hours, 39.2 minutes and 1 day, 6 hours, 17.9 minutes respectively. Because Phobos orbits Mars in a shorter time than Mars' 24 hour, 37.4-minute rotational period, to an observer on Mars' surface it would appear to rise in the west and set in the east. From Mars' surface, Phobos appears about one-third the diameter of the Moon from Earth, whereas Deimos appears as a bright star. The moons were discovered in 1877 by the astronomer Asaph Hall, and as satellites of a planet named for the Roman god of war, they were named for Greek mythological figures that personify fear and terror.