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D. Rubincam (NASA Goddard Space Flight Center)
Polar wander may occur on Triton because of volatile migration. Triton, with its low obliquity, can theoretically sublimate volatiles (mostly nitrogen) at the rate of ~1014 kg/yr from the equatorial regions and deposit them at the poles. Assuming Triton to be rigid on the sublimation timescale, after ~105 years the polar caps would become large enough to cancel the rotational flattening, with a total mass equivalent to a global layer ~120-250 m in depth. At this point the pole wanders about the tidal bulge axis, which is the line joining Triton and Neptune. Rotation about the bulge axis might be expected to disturb the leading side/trailing side cratering statistics. Because no such disturbance is observed, it may be that Triton’s surface volatile inventory is too low to permit wander.
On the other hand, its mantle viscosity might be low, so that any uncompensated cap load might be expected to wander toward the tidal bulge axis. In this case, the axis of wander passes through the equator from the leading side to the trailing side; rotation about this wander axis would not disturb the cratering statistics. Low-viscosity polar wander may explain the bright southern hemisphere: this is the pole which is wandering toward the sub-Neptune point. In any case the ``permanent" polar caps may be geologically very young.
A spacecraft mission to Triton which measures the dynamical flattening could give information about the accumulation of volatiles at the poles. Such information is best obtained by measuring gravity and topography from orbiters, as was done for Mars with the highly successful Mars Global Surveyor.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.