also be used to study the compensation of surface topography, which can provide
information on the mechanical properties and state of stress of planetary lithospheres.
The gravity fields of planetary bodies other than the Earth have been determined almost
exclusively from Earth-based tracking of robotic spacecraft. Our knowledge of the
gravity fields of the outer planets is limited to estimates of their mass and a few of the
low degree geopotential coefficients determined from planetary flybys (Thomas, 1991).
However, for the Moon, Venus, and Mars, fairly detailed gravity models have been
developed using Doppler tracking of spacecraft inserted into elliptical orbits about these
planets. Substantial improvements in the gravity models for these bodies has been gained
by reprocessing the historical tracking data using improved solution techniques.
During the 1990s, planning for future lunar satellite missions led to the observation that
the available lunar gravity models predicted vastly different orbit behavior, resulting in
large uncertainties in the predicted fuel requirements. This in turn led to a complete
reanalysis of the Doppler tracking from Lunar Orbiters I, II, III, IV, and V and the Apollo
15 and 16 sub satellites by Konopliv et al. (1993a) which resulted in the development of
an improved 60 x 60 gravity model. The other major development in modeling the lunar
gravity field has been the success of the Clementine mission. Zuber et a1. (1994)
obtained significant improvements to the gravity model through the analysis of the
Clementine tracking data, although the gravity model on the lunar farside is still not well
determined. In addition, the Clementine altimeter has provided substantial improvements
in the topography model for the Moon (Zuber et al., 1994), which has allowed
unprecedented geophysical studies of the Moon's interior.
The detailed gravity field of Mars has been obtained principally from the tracking data
of Mariner-9, Viking-I, and Viking-2. In preparation for the Mars Observer mission,
Smith et al. (1993) performed a thorough reanalysis of these same data and developed a
gravity model of 50th degree (Goddard Mars Modell - GMM-l). The superior spatial
resolution of this model has resulted in an imprOVed understanding of the geophysics of
Mars as well as improved orbit determination accuracies for satellites orbiting Mars.
These results will be important elements in the measurement of Martian topography using
satellite altimeters. Konopliv and Sjogren (1995) also recently developed an improved
gravity field model for Mars which appears to be an improvement over GMM-l. This
model will likely be considered the state-of-the-art until tracking data are obtained from
NASA's planned Mars Global Surveyor (MGS) mission.
During the last four years substantial advances have been made in modeling the gravity
field of Venus through improved analyses of the Pioneer Venus Orbiter (PVO) data
(Nerem et al., 1993; Reasenberg and Goldberg, 1992) and the analysis of tracking data
from the Magellan spacecraft (McNamee et al., 1993; Konopliv et al., 1993b). Priori to
the Magellan mission, the gravity field of Venus had largely been determined through the
tracking of PVO. A significant advance occurred in 1994 when aerobraking was used to
circularize the Magellan orbit. Tracking data from this phase of the mission have already
allowed dramatic improvements in our knowledge of the gravity field of Venus,
especially in the polar regions (Konopliv et al., 1994). The development of improved
models of the gravity field of Venus, in concert with improved topography models
developed using PVO and Magellan radar altimeter data (Ford and Pettengill, 1992), have
resulted in an improved understanding of the geophysics of Venus (e.g. Banerdt et al.,
1994; Bindschadler et aI., 1992; 1994; Johnson and Sandwell, 1994).
FUTURE DEVELOPMENTS
The development of new applications of global gravity field models will clearly depend
on the improvements that can be gained in the models. Tremendous advances can be
expected in measuring the ocean circulation and monitoring mass redistribution in the
Earth system if the gravity field and its temporal variations can be ac(:urately determined.
Although not yet flown, there continues to be a number of studies of dedicated satellite
6
information on the mechanical properties and state of stress of planetary lithospheres.
The gravity fields of planetary bodies other than the Earth have been determined almost
exclusively from Earth-based tracking of robotic spacecraft. Our knowledge of the
gravity fields of the outer planets is limited to estimates of their mass and a few of the
low degree geopotential coefficients determined from planetary flybys (Thomas, 1991).
However, for the Moon, Venus, and Mars, fairly detailed gravity models have been
developed using Doppler tracking of spacecraft inserted into elliptical orbits about these
planets. Substantial improvements in the gravity models for these bodies has been gained
by reprocessing the historical tracking data using improved solution techniques.
During the 1990s, planning for future lunar satellite missions led to the observation that
the available lunar gravity models predicted vastly different orbit behavior, resulting in
large uncertainties in the predicted fuel requirements. This in turn led to a complete
reanalysis of the Doppler tracking from Lunar Orbiters I, II, III, IV, and V and the Apollo
15 and 16 sub satellites by Konopliv et al. (1993a) which resulted in the development of
an improved 60 x 60 gravity model. The other major development in modeling the lunar
gravity field has been the success of the Clementine mission. Zuber et a1. (1994)
obtained significant improvements to the gravity model through the analysis of the
Clementine tracking data, although the gravity model on the lunar farside is still not well
determined. In addition, the Clementine altimeter has provided substantial improvements
in the topography model for the Moon (Zuber et al., 1994), which has allowed
unprecedented geophysical studies of the Moon's interior.
The detailed gravity field of Mars has been obtained principally from the tracking data
of Mariner-9, Viking-I, and Viking-2. In preparation for the Mars Observer mission,
Smith et al. (1993) performed a thorough reanalysis of these same data and developed a
gravity model of 50th degree (Goddard Mars Modell - GMM-l). The superior spatial
resolution of this model has resulted in an imprOVed understanding of the geophysics of
Mars as well as improved orbit determination accuracies for satellites orbiting Mars.
These results will be important elements in the measurement of Martian topography using
satellite altimeters. Konopliv and Sjogren (1995) also recently developed an improved
gravity field model for Mars which appears to be an improvement over GMM-l. This
model will likely be considered the state-of-the-art until tracking data are obtained from
NASA's planned Mars Global Surveyor (MGS) mission.
During the last four years substantial advances have been made in modeling the gravity
field of Venus through improved analyses of the Pioneer Venus Orbiter (PVO) data
(Nerem et al., 1993; Reasenberg and Goldberg, 1992) and the analysis of tracking data
from the Magellan spacecraft (McNamee et al., 1993; Konopliv et al., 1993b). Priori to
the Magellan mission, the gravity field of Venus had largely been determined through the
tracking of PVO. A significant advance occurred in 1994 when aerobraking was used to
circularize the Magellan orbit. Tracking data from this phase of the mission have already
allowed dramatic improvements in our knowledge of the gravity field of Venus,
especially in the polar regions (Konopliv et al., 1994). The development of improved
models of the gravity field of Venus, in concert with improved topography models
developed using PVO and Magellan radar altimeter data (Ford and Pettengill, 1992), have
resulted in an improved understanding of the geophysics of Venus (e.g. Banerdt et al.,
1994; Bindschadler et aI., 1992; 1994; Johnson and Sandwell, 1994).
FUTURE DEVELOPMENTS
The development of new applications of global gravity field models will clearly depend
on the improvements that can be gained in the models. Tremendous advances can be
expected in measuring the ocean circulation and monitoring mass redistribution in the
Earth system if the gravity field and its temporal variations can be ac(:urately determined.
Although not yet flown, there continues to be a number of studies of dedicated satellite
6