Rotational and Orbital Dynamics
Mars' interior is
simply modeled as a core and mantle with a thin crust, similar to Earth. Mars' size and
total mass have been determined by previous missions. Given four parameters, the core size
and mass and mantle size and mass can be determined. The combination of Pathfinder Doppler
data with earlier data from the Viking landers has determined a third parameter, the
moment of inertia, through measurement of Mars' precession rate. A fourth measurement is
needed to complete the interior model. This may be achieved through future Doppler
tracking of Pathfinder, since the presence of a fluid core may be detectable through its
effect on Mars' nutation. The determination of the moment of inertia is a significant
constraint on possible models for Mars' interior. If the core is as dense as possible
(i.e. completely iron) and the mantle is similar to Earths' (or similar to the SNC
meteorites thought to originate on Mars) then the minimum core radius is about 1300 km. If
the core is made of less-dense material (i.e. a mixture of iron and sulfur) than the core
radius is probably no more than 2000 km.
The Rotation and Orbital Dynamics
experiment is based on measuring the Doppler range to Pathfinder using the radio link.
Mars' rotation about its' pole causes a signature
in the data with a daily minimum when the lander is closest to the Earth. Changes in the
daily signature reveal information about the planetary interior, through its effect on
Mars' precession and nutation. The signature also is sensitive to variations in Mars'
rotation rate as the mass of the atmosphere increases and decreases as the polar caps are
formed in winter and evaporate in spring. Long-term signatures in the range to the lander
are caused by asteroids perturbing Mars' orbit. Analysis of these perturbations allows the
determination of the masses of asteroids.