OFFICE OF PUBLIC INFORMATION
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA  91109.  TELEPHONE (818) 354-5011

PHOTO CAPTION
VIKING 1-91
P-18612 (Phobos)
FEBRUARY 22, 1977

Viking Orbiter I flew within 300 miles (480 kilometers) of Mars’ 
inner satellite, Phobos, to obtain the pictures in this mosaic of 
the asteroid-size moon.  Range varied during the three-picture 
sequence last Friday (February 18) from 480 to 660 kilometers 
(410 miles).  As seen here, Phobos is about 75% illuminated and 
is about 21 kilometers (13 miles) across and 19 kilometers (11.8 
miles) from top to bottom.  North is at top.  The south pole is 
within the large crater, Hall (5 kilometers in diameter), at 
bottom center where the pictures overlap.  Some features as small 
as 20 meters (65 feet) across can be seen.  Remarkable features 
include striations, crater chains, a linear ridge, and small 
positive features which appear to be resting on the surface.  A 
long linear ridge is seen starting near the south pole and 
extending to the upper right.  A very sharp wall at the 
intersection of two kilometer-size craters is seen along this 
ridge at right.  A series of craters runs horizontally in the 
picture which is parallel to the orbit plane of Phobos.  These 
crater chains are commonly associated with secondary cratering by 
ejecta from larger impacts.  A surprising discovery has been made 
of what apparently resembles hummocks or small positive features.  
These features, primarily seen near the terminator (right), are 
about 50 meters (165 feet) in size and may be surface debris from 
previous impacts.  The Orbiter took these pictures during its 
242nd revolution of the planet.  Viking I was launched on August 
20, 1975, and went into orbit around Mars on June 19, 1976.

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