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9. Gravity and Figure


Achievement: High resolution mapping of martian global topography and gravity field.

Significance: Allows detailed description of Mars’s shape, remote determination of its interior structure and insights into its evolution.


Mars is not quite a sphere and its crustal density varies from region to region

13-second animation of MGS gathering gravity data. Animation: NASA/Goddard Space Flight Center Scientific Visualization Studio: http://tinyurl.com/ndeqmb

Topographical map of Mars from MOLA data

Topographical map of Mars from MOLA data. North is up. Red is highlands, blue lowlands. The north-south hemisphere dichotomy is visible (vide infra).

Mars is not quite a sphere and its crustal density varies from region to region. An orbiting spacecraft above the changing masses encounters slightly variable gravity that affects its motion. Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter tracked their own deviations from expected orbit with Doppler radar, mapping the gravity field that they experienced. MGS’s MOLA laser altimeter mapped the topography of the martian surface. The data sets combine to give information about Mars’s shape and its crustal composition and thickness.

Zuber, M. T. et al. (2007) The Mars Reconnaissance Orbiter radio science gravity investigation, , J. Geophys. Res., 112, E05S07.
Neumann, G. A. et al. (2004), Crustal structure of Mars from gravity and topography, J. Geophys. Res., 109, E08002.
Smith, D.E. et al. (2001), Mars Orbital Laser Altimeter: experiment summary after the first year of global mapping of Mars, , J. Geophys. Res., 106 (E10), 23,689.

Images: Smith, et al.


MOLA topographic global maps
MOLA topographic global maps. Image: PIA02028, NASA/JPL

Mars is unique among the planets in that its northern and southern hemispheres are very different. Most of the southern hemisphere is heavily cratered highlands with a thick crust (~80 km) and often intense crustal magnetism; most of the northern hemisphere is lowlands with a thinner crust (~40 km), little or no crustal magnetism, and its craters buried beneath younger material. The cause of this north-south dichotomy is intensely controversial, one of the deep questions of Mars. No current theory explains all the observed factors. Core convection has been suggested, as have an ancient northern ocean and a colossal meteorite impact.

Clifford, S.M. and Parker, T.J.(2001). The evolution of the martian hydrosphere: implications for the fate of a primordial ocean and the current state of the northern plains. Icarus 154, 40-79 and references cited therein.
Roberts, J.H. and Zhong, S. (2006). J Geophys. Res. 111, E06013.
Andrews-Hann, J.C. et al. (2008). Nature 453, 1212-1215.
Marinova, M.M. et al. (2008). Nature 453, 1216-1219.
Nimmo, F. et al. (2008). Nature 453, 1220-1223.
Wilhelms., D.E. and Squyres, S.W. (1984). Nature 309, 138-140.


Pressure from a magma plume below the southern highlands rotates the lithosphere to locate the plume below thinner crust

Reprinted by permission from Macmillan Publishers Ltd: Nature Geoscience 2 (1), 19 (2009), © 2009.

Pressure from a magma plume below the southern highlands rotates the lithosphere to locate the plume below thinner crust

Pressure from a magma plume below the southern highlands rotates the lithosphere to locate the plume below thinner crust. Reprinted by permission from Macmillan Publishers Ltd: Nature Geoscience 2 (1), 7 (2009), © 2009.

The main volcanic center appears to have formed beneath the southern highlands, about 40º south of the equator, and migrated north; it now straddles the boundary between southern highlands and northern lowlands

From the Tharsis Rise: Olympus Mons, the largest volcano in the Solar System. Image: National Space Science Data Center.


Like the Earth, Mars has an equatorial bulge from rotation. Its entire western hemisphere also bulges outward because of the Tharsis Rise, the largest volcanic region in the Solar System, which covers most of the hemisphere and contains five enormous shield volcanoes up to 30 kilometers high. The main volcanic center appears to have formed beneath the southern highlands, about 40º south of the equator, and migrated north; it now straddles the boundary between southern highlands and northern lowlands. A recent suggestion is that the center did not in fact shift, that a magma plume formed beneath the highlands and in fact remained stationary while the martian lithosphere rotated south (relative to the plume) until the hotspot emerged from beneath the highlands’ thicker crust. In that event the lithosphere would have had to rotate as a unit, since the martian surface is a single tectonic plate.

Zhong, S., Nature Geoscience 2 (1), 19 (2009); Nimmo, F., Nature Geoscience 2 (1), 7 (2009).


MGS’s data suggests that the martian core may still be at least partly liquid, though this is debated

Image: NASA/JPL-Caltech

MGS’s data suggests that the martian core may still be at least partly liquid, though this is debated.

Yoder, C. F. et al. Fluid core size of Mars from detection of the solar tide, Science 300, 299 (2003).

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