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Mars I


Phobos [FOH-bohs] (fear) is a moon of Mars and was named after an attendant of the Roman war god Mars. Phobos is a dark body that appears to be composed of C-type surface materials. It is similar to the C-type (blackish carbonaceous chondrite) asteroids that exist in the outer asteroid belt. Some scientists speculate that Phobos and Mars' other moon, Deimos, are captured asteroids. However, other scientists point to evidence that contradicts this theory. Phobos shows striated patterns which are probably cracks caused by the impact event of the largest crater on the moon.

Phobos Statistics
 Discovered byAsaph Hall 
 Date of discovery1877 
 Mass (kg)1.08e+16 
 Mass (Earth = 1)1.8072e-09 
 Radius (km)13.5x10.8x9.4 
 Radius (Earth = 1)2.1167e-03 
 Mean density (gm/cm^3)2.0 
 Mean distance from Mars (km)9,380 
 Rotational period (days)0.31910 
 Orbital period (days)0.31910 
 Mean orbital velocity (km/sec)2.14 
 Orbital eccentricity0.01 
 Orbital inclination (degrees)1.0 
 Escape velocity (km/sec)0.0103 
 Visual geometric albedo0.06 
 Magnitude (Vo)11.3 

Animations of Phobos

Views of Phobos

High Resolution Mosaic of Phobos
This high-resolution mosaic of Phobos was created from three Viking orbiter images. The striking feature in this image is the giant Stickney crater. Grooved fractures caused from the impact that created Stickney extend away from the crater. (Copyright © 1997 by Calvin J. Hamilton)

Six Views of Phobos
This picture shows six different orientations of Phobos. The images were rendered by using Peter C. Thomas' model and the USGS airbrush map of Phobos. (Courtesy A. Tayfun Oner)

This image was taken by the Viking Orbiter spacecraft in 1977. Striated patterns can be seen in this image. These are probably cracks caused by the impact event of the Stickney crater shown below. (Courtesy NASA)

Stickney Crater
One of the most striking features of Phobos, aside from its irregular shape, is its giant crater Stickney. Because Phobos is only 28 by 20 kilometers (17 by 12 miles), the moon must have been nearly shattered from the force of the impact that caused the giant crater. Grooves that extend across the surface from Stickney appear to be surface fractures caused by the impact. Near the crater, the grooves measure about 700 meters (2300 feet) across and 90 meters (295 feet) deep. However, most of the grooves have widths and depths in the 100 to 200 meters (328 to 655 feet) and 10 to 20 meters (33 to 65 feet) ranges, respectively. (Courtesy NASA/JPL)

Stickney Crater Another View
This image shows a slightly different view of the Stickney crater. A crater within the Stickney crater is visible. (Courtesy Calvin J. Hamilton)

Topographic Map of Phobos
This is a topographic map of Phobos. It is based upon the shape model of Phil Stooke. As with all maps, it is the cartographer's interpretation; not all features are necessarily certain given the limited data available. This interpretation stretches the data as far as possible. (Courtesy A. Tayfun Oner)

Conformal Projection of Phobos
This shows two different views of Phobos in a Morphographic Conformal Projection. One view shows the leading side and the other the trailing side. (Courtesy Phil Stooke, NSSDC, and NASA)


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Copyright © 1997 by Calvin J. Hamilton. All rights reserved.