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Lunar true polar wander inferred from polar hydrogen

Author: 
Matthew Siegler
Topic: 
Geophysics
Delivered As: 
Oral
Abstract Text: 

The earliest dynamic and thermal history of the Moon is not well
understood. The hydrogen content of deposits near the lunar
poles may yield insight into this history, because these deposits
(which are probably composed of water ice) survive only if they
remain in permanent shadow. If the orientation of the Moon has
changed, then the locations of the shadowed regions will also
have changed. 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 finding
is in contrast to the distribution of volatiles observed in similar
thermal environments at Mercury’s poles. Here we show that
polar hydrogen preserves evidence that the spin axis of the Moon
has shifted: the hydrogen deposits are antipodal and displaced
equally from each pole along opposite longitudes. From the
direction and magnitude of the inferred reorientation, and from
analysis of the moments of inertia of the Moon, we hypothesize
that this change in the spin axis, known as true polar wander, was
caused by a low-density thermal anomaly beneath the Procellarum
region. Radiogenic heating within this region resulted in the bulk
of lunar 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 thermal anomaly still exists and, in part, controls
the current orientation of the Moon. The Procellarum region was
most geologically active early in lunar history, which implies
that polar wander initiated billions of years ago and that a large
portion of the measured polar hydrogen is ancient, recording
early delivery of water to the inner Solar System. Our hypothesis
provides an explanation for the antipodal distribution of lunar
polar hydrogen, and connects polar volatiles to the geologic and
geophysical evolution of the Moon and the bombardment history
of the early Solar System.

Co-Authors: 
Simone Dell' Agnello"...
SSERVI Identifier: 
NESF2016-123

About SSERVI
Recognizing that science and human exploration are mutually enabling, NASA created the Solar System Exploration Research Virtual Institute (SSERVI) to address basic and applied scientific questions fundamental to understanding the Moon, Near Earth Asteroids, the Martian moons Phobos and Deimos, and the near space environments of these target bodies. As a virtual institute, SSERVI funds investigators at a broad range of domestic institutions, bringing them together along with international partners via virtual technology to enable new scientific efforts."