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As a bonus, mission scientists were able to use radio communication signals from Pathfinder to measure the rotation of Mars. Daily Doppler tracking and less frequent two-way ranging during communication sessions determined the position of the lander with a precision of 100 meters. The last such positional measurement was done by Viking over 20 years ago. In the interim, the pole of rotation has precessed — that is, the tilt of the planet has changed, just as a spinning top slowly wobbles. The difference between the two positional measurements yields the precession rate. The rate is governed by the moment of inertia of the planet, a function of the distribution of mass within the planet. The moment of inertia had been called "the single most important number about Mars that we don’t know."
From Pathfinder’s determination of the moment of inertia we now
know that Mars must have a central metallic core of between 1,300 and 2,400
kilometers in radius. With assumptions about the mantle composition —
derived from the compositions of the martian meteorites and the rocks measured
by the rover — scientists can now start to put constraints on interior
temperatures. Prior to Pathfinder, the composition of the martian meteorites
argued for a core, but the size of this core was completely unknown. The
new information about the interior will help geophysicists to understand
how Mars has evolved over time. In addition to the long-term precession,
Pathfinder detected an annual variation in the planet’s rotation rate,
which is just what would be expected from the seasonal exchange of carbon
dioxide between the atmosphere and ice caps.
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| 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. |
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| 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. |