Paula Cleggett-Haleim
Headquarters, Washington, D.C.                                        
January 28, 1992
(Phone:  202/453-1547)

Jim Doyle
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone:  818/354-5011)

RELEASE:  92-14

MAGELLAN BEGINS THIRD VENUS MAPPING CYCLE

   The Magellan spacecraft began its third radar mapping cycle 
of the surface of Venus on Friday, Jan. 24, and controllers at 
NASA's Jet Propulsion Laboratory, Pasadena, Calif., said today 
mapping was successful despite earlier transmitter problems. 

   The second mapping cycle ended Jan. 15, and controllers 
then successfully performed a planned week of battery 
recharging. 

   The spacecraft experienced difficulty with one of its radio 
transmitters earlier this month.  Mission Director Doug 
Griffith reported that the project will now use the backup 
transmitter to continue mapping.  The backup transmitter was 
turned off last year because of a spurious signal that 
degraded data transmission back to Earth. 

   Mapping is now continuing, Griffith said, after a new 
operational mode was devised for the backup transmitter which 
reduced the data transmission rate from 268,000 to 115,000 
bits per second. 

   Mission controllers also had to contend with interference 
on the spacecraft's radio signal caused by the spurious 
signal.  That problem is being managed by choosing one of the 
two possible data carrier tones to avoid the spurious signal 
frequency. 

   For the first five orbits starting on Jan. 24, controllers 
tested various configurations of the data carrier tones to 
analyze the spurious signal.  Finally, the spurious signal 
settled at a frequency which allowed satisfactory radar data 
to be transmitted. 
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   The transmitter problem is still under investigation by a 
special radio anomaly team that is trying to determine the 
most probable failure mechanism in both transmitters and 
devise additional methods to optimize use of the backup 
transmitter. 

   Now that mapping is continuing, stereo mapping of selected 
targets as well as mapping of areas not previously imaged are 
underway.  The first stereo imaging target is Maxwell Montes, 
the highest mountain on Venus. 

   Additionally, Project Scientist Steve Saunders said a 
global survey based on the data acquired during the first two 
cycles has found that about 85 percent of the planet is 
covered by volcanic rocks, mostly lava flows that form the 
great plains. 

   Much of the remaining 15 percent, he said, is high 
standing, chaotic material that is faulted and fractured.  
"Little can be determined about the composition or nature of 
the rocks that form these highlands," he said. 

   One possible explanation is that the highland rock 
represents crustal material that formed soon after the final 
accretion of the planet 4.6 billion years ago, Saunders said. 

   The high, ridged material is seen in the continent-like 
terrains such as Aphrodite Terra and Maxwell Montes.  "These 
regions often appear as islands surrounded by the more recent 
lavas that form the volcanic plains," Saunders said. 

   All of the terrestrial planets appear to have formed 
relatively low density crusts, Saunders said.  "Little of the 
ancient crust remains on Earth because of the constant 
churning of plate tectonics."  While Venus does not have 
Earth-like tectonic plates, it has a long, complex history of 
deformation with many episodes of tectonics and faulting. 

   "The Magellan data will provide the key to understanding 
that complex history and perhaps to the understanding of many 
of Earth's geologic puzzles," Saunders said. 

   Magellan already has mapped over 95 percent of Venus.  The 
primary  objective for the Magellan mission called for one 
mapping cycle to obtain images of 70 percent of the planet.  
The Jet Propulsion Laboratory manages the Magellan Project for 
NASA's Office of Space Science and Applications. 
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