PDS_VERSION_ID        = PDS3                                                  
RECORD_TYPE           = STREAM                                                
OBJECT                = TEXT                                                  
  PUBLICATION_DATE      =  " "                                                
  NOTE                        = "Experiment description for the               
                                 Satellite Field and Mass Determination       
                                 Experiment, which includes experiments       
                                 from Ganymeda, Callisto, and Europa.         
                                 Please note this description is only on      
                                 Gravity Field Measurements at Ganymede.      
                                 Conducted starting in 1996 through 1997      
                                 (DOY 179 through DOY 311). Formatted for     
                                 display or printing with up to 78 constant-  
                                 width characters per line."                  
END_OBJECT            = TEXT                                                  
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     GANYMEDE GRAVITY FIELD MEASUREMENT                                       
                                                                              
     Because the Galilean satellites respond to comparable perturbations from 
rotation and tides, an independent test of hydrostatic equilibrium is         
possible.  It was concluded as early as 1977 that nonhydrostatic components   
in Io and Ganymede should be negligible, and hence at least for these two     
satellities, a Jupiter orbiter should yield an excellent discrimination       
from an ensemble of interior models.  The Galileo mission team therefore      
scheduled two close flybys of Ganymede on the first two orbits (G1 and G2);   
the first flyby on June 1996 was a near-equatorial pass at an altltude of     
825 km, while the second on September 1996 was a near-polar pass at an        
altitude of 261 km.  The polar pass was most sensitive to the rotational      
components in Ganymede's gravitational field, while the equatorial pass       
was most sensitive to tidal components.                                       
                                                                              
                                                                              
      Coherent radio Doppler data are available for both G1 and G2.  These    
data show clearly that Ganymede has differentiated into a core and mantle.    
Combined with the discovery of an intrinsic magnetic field by the             
Magnetometer team, the gravity data suggest that Ganymede has a metallic      
core of radius 400-1,300 km surrounded by a silicate mantle, which is in      
turn enclosed by an ice shell ~ 800 km thick.  Depending on whether the core  
is pure iron or an alloy of iron and iron sulfide, it could account for as    
little as 1.4% or as much as one-third of the total mass.