Phobos and Deimos: At correct relative scales 7.11
Phobos (left) and Deimos (right) at the correct relative scales, as seen by the Viking orbiters. Deimos appears less cratered only because the resolution of the original image is poorer than for Phobos.
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Image Credit: NASA  

Satellite Facts Phobos:

Distance from planet's centre: 9377 km

Radius (km): 13.1 x 9.3

Orbital Period: 7.65 hours

Mass: 1.08 x 1016 kg

Density (g/cm3): 1.9

Satellite Facts Deimos:

Distance from planet's centre: 23463 km

Radius (km): 7.8 x 5.1

Orbital Period: 30.3 hours

Mass: 1.8 x 1015 kg

Density (g/cm3): 1.8
Phobos (left) and Deimos (right) at the correct relative scales, as seen by the Viking orbiters. Deimos appears less cratered only because the resolution of the original image is poorer than for Phobos. Note the shadows in hollows (mostly impact craters) near the day-night boundary on each. The largest crater on Phobos, named Stickney, is 10 km across. The image on the lower right is 1.4 km wide detail from a Mars Observer view of the surface just beyond Stickney's outer edge, obtained from a range of about 1080 km. In addition to craters down to the limit of resolution, there are several boulders scattered across the surface.  
Text credit: David A. Rothery Return to top of page