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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
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.