Abstract: A spherical harmonic solution of the Mars gravity field field to degree and order 80 (Goddard Mars Model 2B) has been developed using X band tracking data of Mars Global Surveyor (MGS) from October 1997 to February 2000, including data from the Hiatus, Science Phasing Orbits and Mapping phases of the mission. During the Mapping mission, MGS was located in a near-polar (92.7\deg inclination), and near-circular orbit, at a mean altitude of 400 km. The tracking data from this orbit provide a detailed, global, and high resolution view of the gravity field of Mars. Mars gravity solutions are stable to 60x60 even without application of a Kaula power law constraint. The Valles Marineris is resolved distinctly with lows reaching -450 mGals. Olympus Mons and its aureole are both separately resolved and the volcano has a peak anomaly of 2950 mGals. Crossovers formed from the Mars Orbiter Laser Altimeter (MOLA) data have been incorporated into GMM-2B, and improve MGS orbit performance. The RMS of altimeter crossover residuals of MGS orbits in mapping determined solely from the radiometric tracking data using GMM-2B is 1.9 m. Using GMM-2B, adjacent six day arcs in mapping from May to December 1999 that overlap by an average of 1.25 days, are consistent by 1 to 2 m in the radial direction, and 8 to 10 m in total position.

Download the spherical harmonic coefficients. Download the format description. Download the coefficients in GEODYN format. View abstract of companion paper on the use of MOLA crossovers by Neumann et al. (2000). MGS Reveals the Internal Structure of Mars (Zuber et al., Science, 287, 1788-1793,2000) Other Links: Mars Global Surveyor. MGS Radio Science Team. Mars Orbiter Laser Altimeter (MOLA) Planetary Data System (Geophysics Node).

Tracking Data in GMM-2B

Mission Phase	Periapse Ht. (km)	No. of Arcs	No. of Obs
Hiatus	170	2	24,119
SPO-1	170	8	31,001
SPO-2	170	16	157,972
GCO	370	9	76,813
Mapping	370	47	665,210
total			955,115

Tracking data coverage in GMM-2B: (a) Doppler observations obtained below 500 km altitude during the Science Phasing Orbit (SPO). During SPO, MGS was located in an elliptical 11.6 hour orbit, whose periapsis was in the Northern Hemisphere of the planet. Therefore low altitude Doppler data in this phase were obtained only over this region of the planet. (b) Doppler observations from the Gravity Calibration Orbit and Fixed High Gain Antenna Mapping (February - March 1999). These Doppler data represent the first global observations of the gravity field of Mars obtained from the low-altiude Mapping orbit. (c) Doppler observations from later in Mapping (May 10 1999 to February 29 2000), obtained after deployment of the High Gain Antenna.

The degree variances for GMM-2B are compared to the degree variances from GMM-1 to illustrate the field quality and resolution obtained with the MGS X band tracking data compared to solutions derived solely from the historic S band Viking Orbiter and Mariner 9 data. Whereas for GMM-1 [ Smith et al., 1993], the coefficients had approximately 100 percent error by degree 20, with the new MGS-derived field, the coefficients do not reach 100 percent error until degree 60. Below degree 20, the MGS data have improved the knowledge of the Mars gravity field by two to three orders of magnitude. A solution without a Mars Kaula constraint (mgm1004e) is stable to approximately degree 60, before the coefficient power departs from the expected Kaula power law for Mars of 13 x 10-5/l2

The correlations of the GMM-2B gravity field coefficients were computed using a spherical harmonic expansion of the topography to degree and order 90 (derived from the MOLA data). The global correlations are compared for GMM-1 and Mars50c (two models determined solely from the Viking and Mariner 9 data) and GMM-2B (determined from the MGS data). The average global correlations to degree 50 are 0.42 for GMM-1, 0.64 for Mars50c, and 0.78 for GMM-2B. The correlation drops off sharply after degree 60, indicating a deficit in signal in the gravity field coefficients above this degree. This is caused by the resolution and sensitivity of the MGS tracking data, and does not necessarily reflect the character of the Martian gravity field.

Model highlights.

The anomaly maps shows the classic features identified in the gravity fields derived from the Viking Orbiter and Mariner 9 tracking, such as Olympus Mons , the Tharsis Montes , Elysium , and Isidis. However with GMM-2B many of these features now appear with greater power. Valles Marineris appears as a quasi-continuous mass deficit with lows reaching -450 mGals. Olympus Mons is now resolved with an anomaly of 2950 mGals, and the signature of the aureole to the northwest appears distinctly in the gravity anomaly map. Hellas appears as a general low of -50 to -150 mGal with a complicated structure. The muted expression of this feature as a gravity anomaly, compared to its size (1800 km across, and 12 km from rim to floor [ Smith et al., 2000]} indicates it is largely isostatically compensated [ Smith and Zuber, 1996]}. In the northern polar regions, several anomalies of -200 to +200 mGals appear between 70 deg N and 90 deg N, however none seem to correlate directly with visible topography or the location of the polar cap. In the southern polar regions, a small gravity high of about 100 mGal occurs near the South Pole, and may correlate with the presence of the polar layered terrains. The power of gravity anomalies in the southern hemisphere appears muted compared to the anomalies in the equatorial regions and the northern hemisphere [ Smith et al., 1999b; Zuber et al., 2000a]. The most prominent anomaly, situated just south of the Hellas Basin at 62 deg E, 58 deg S, is a gravity high of 250 mGal and is associated with Amphitrites Patera, an ancient volcanic shield. Apollinaris Patera, another volcanic shield, is visible in GMM-2B (175 deg E, 10 deg S) as a small but prominent gravity high of 225 mGals. This feature was not even discernible in the gravity models derived from the historical Viking and Mariner 9 data. A gravity high of 160 mGal is resolved in the Argyre basin. GMM-2B resolves anomalies of 100 to 200 mGals in the Utopia basin. Anomalies of that magnitude were already apparent in the GMM-1 and Mars50c fields, but did not correlate with topography prior to MOLA [ Zuber et al., 2000a].}

Gravity Anomaly Images of GMM-2B

More Gravity Anomaly Images of GMM-2B

Gravity anomaly images over shaded relief maps of the MOLA topography

Valles Marineris	Hellas	Alba Patera

Argyre	Elysium	Isidis

Tharsis Montes