Planetary Science Institute
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The earliest dynamical and thermal history of the Moon is not well understood. Surprisingly, this history may be recorded by the hydrogen deposits near the Lunar poles. These deposits (likely water ice) can only survive if they remain in permanent shadow. If the Moon's orientation has changed, so have the location of the shadowed regions. The polar hydrogen deposits have been mapped by orbiting neutron spectrometers, and their observed spatial distribution does not match the expected distribution of water ice inferred from present-day lunar temperatures. This is in stark contrast to the volatile distribution observed within a similar thermal environment at Mercury's poles. Here we show that polar hydrogen preserves evidence that the Moon's spin axis has shifted - the hydrogen deposits are antipodal, displaced equally from each pole along opposite longitudes. From the direction and magnitude of the inferred reorientation, and analysis of the lunar figure, we find that this change in the spin pole, known as true polar wander, was caused by a low-density thermal anomaly beneath the Procellarum region. Radiogenic heating within this province resulted in major mare volcanism and altered the density structure of the Moon, changing its moments of inertia. This resulted in true polar wander consistent with the observed remnant polar hydrogen. This anomaly still exists and partly controls the current orientation of the Moon. Procellarum was most geologically active early in lunar history, implying that polar wander initiated long ago and that a large portion of the measured polar hydrogen is ancient, recording early delivery of water to the inner solar system. This hypothesis not only provides a uniquely viable explanation for the antipodal distribution of lunar polar hydrogen, but also connects polar volatiles to the geologic and geophysical evolution of the Moon and the bombardment history of the early solar system.