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