Polar Hydrogen Deposits Paint Ancient Path of Moon Spin Axis Shift

Summary: A March 23 Nature letter finds that polar hydrogen deposits paint the ancient path of 5.5-degree moon spin axis shift about 3 billion years ago.

Polar hydrogen map of northern and southern hemispheres identifies locations of moon’s ancient and present poles; lighter areas mark higher hydrogen concentrations, darker areas indicate lower hydrogen concentrations; LCROSS marks Oct. 9, 2009, impact site of Lunar Crater Observation and Sensing Satellite: James Keane, University of Arizona; Richard Miller, University of Alabama at Huntsville, CC BY 2.0, via NASA

Polar hydrogen deposits, requiring permanent shadow for survival, paint the ancient path of a moon spin axis shift of about 5.5 degrees approximately 3 billion years ago, according to a letter by nine co-authors published online Wednesday, March 23, 2016, in Nature.

“This was such a surprising discovery. We tend to think that objects in the sky have always been the way we view them, but in this case the face that is so familiar to us -- the Man on the Moon  -- changed,” lead author Matthew A. Siegler explains March 23 for SMU Research blog. Siegler is a planetary astronomer affiliated with the Planetary Science Institute in Tucson, Arizona, and with Roy M. Huffington Department of Earth Sciences at Southern Methodist University (SMU) in Dallas, Texas.

Siegler and co-author Michael J. Poston, a postdoctoral fellow in geochemistry at the California Institute of Technology (Caltech) in Pasadena, California, pose the question of the moon spin axis shift, known as true polar wander, after examining puzzling polar hydrogen deposit locations detected as low energy neutrons by orbiting neutron spectrometers for NASA’s Lunar Prospector and Lunar Reconnaissance Orbiter (LRO) missions.

Hydrogen is the dominant molecule (H2O) in water and ice. The anomalous hydrogen deposits are presumed to be in the form of water ice. Preserved in craters, the deposits escape the exposure to direct sunlight that boils ice off into space.

Co-author Richard S. Miller, distinguished professor of astrophysics at the University of Alabama in Huntsville, refers to neutron data provided by the Lunar Prospector mission’s low polar orbits from January 1998 to July 1999 to map present locations of lunar water ice. Siegler and Miller discover the equidistant but exactly opposite directional offset of the hydrogen concentrations from each pole. The path painted by the polar hydrogen concentrations reveals a moon spin axis shift of 125 miles (200 kilometers) at each pole. The relocation approximates the shortest surface distance, known as great circle distance, on Earth between Philadelphia, Pennsylvania, and Washington, D.C.

The estimated start date for the moon spin axis shift of 125 miles (200 kilometers) at each pole is 3 billion years ago: Nature News & Views @NatureNV via Twitter March 24, 2016

“The maps show four key features,” Siegler and his colleagues explain for SMU Research blog. “First, the largest quantity of hydrogen is offset from the current rotation axis of the moon by roughly 5.5 degrees. Second, the hydrogen enhancements are of similar magnitude at both poles. Third, the asymmetric enhancements do not correlate with expectations from the current thermal or permanently shadowed environment. And lastly, and most significantly, the spatial distributions of polar hydrogen appear to be nearly antipodal.”

An internal event, volcanic activity, is credited with sufficiently altering the moon’s density to provoke a moon spin axis shift. Co-author James T. Keane, a graduate student in planetary dynamics at University of Arizona’s Lunar and Planetary Laboratory in Tucson, identifies the vast, basaltic Procellarum region on the western edge of the moon’s near side as the only feature matching the moon spin axis shift parameters of direction and of amount of density change.

“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,” notes the interdisciplinary team of specialists in astrophysics, geochemistry, planetary astronomy and planetary dynamics in the study’s abstract.

The estimated start date for the moon spin axis shift painted by the path of polar hydrogen concentrations is placed at about 3 billion years ago. The eventful relocation likely unfolded over the course of 1 billion years and created the gigantic, dark-patched Procellarum basin that is known popularly as the face of the Man in the Moon.

“Billions of years ago, heating within the moon’s interior caused the face we see to shift upward as the pole physically changed positions,” Siegler explains for SMU Research blog. “It would be as if Earth’s axis relocated from Antarctica to Australia. As the pole moved, the Man on the Moon turned his nose up at the Earth.”

Siegler, Miller and co-author David J. Lawrence, staff scientist at The Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Maryland, participate in the Volatiles, Regolith and Thermal Investigations Consortium for Exploration and Science team. The team is one of nine teams funded by NASA Ames Research Center’s Solar System Exploration Research Virtual Institute (SSERVI) in Silicon Valley, California.

The moon spin axis shift study’s other four co-authors are Matthieu Laneuville, assistant professor of geophysics at Tokyo Institute of Technology’s Earth Life Sciences Institute; David A. Paige, professor of planetary science at the University of California, Los Angeles; Isamu Matsuyama, Unversity of Arizona’s Lunar and Planetary Laboratory; and Arlin P.S. Crotts, astrophysicist and professor of astronomy at Columbia University. Dr. Crotts passed away Nov. 19, 2015, in the interval between Nature’s receipt of the study July 13, 2015, and acceptance Jan. 21, 2016.

Lunar cross-section highlights ancient and present polar locations and identifies Procellarum basin (Man in the Moon) formation as driving force for axial shift: James Tuttle Keane/University of Arizona, CC BY 2.0, via NASA

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

ancient and present moon poles: http://www.nasa.gov/ames/feature/lunar-polar-ice-reveals-tilting-axis-of-earth-s-moon

The estimated start date for the moon spin axis shift of 125 miles (200 kilometers) at each pole is 3 billion years ago: Nature News & Views @NatureNV via Twitter March 24, 2016, @ https://twitter.com/Topiy13/status/712941279760375810

lunar cross-section: James Tuttle Keane/University of Arizona, CC BY 2.0, via NASA @ http://www.nasa.gov/ames/feature/lunar-polar-ice-reveals-tilting-axis-of-earth-s-moon

For further information:

GeoBeats News. "Study: The Moon's Axis Likely Shifted 3 Billion Years Ago." YouTube. March 23, 2016.
Available @ https://www.youtube.com/watch?v=lF7di26Nqxk

“NASA data leads to rare discovery: Earth’s moon wandered off axis billions of years ago.” SMU Research. March 23, 2016.
Available @ http://blog.smu.edu/research/2016/03/23/nasa-data-leads-to-rare-discovery-earths-moon-wandered-off-axis-billions-of-years-ago/

Nature News & Views @NatureNV. "Evidence that the Moon may have shifted its orientation." Twitter. March 24, 2016.
Available @ https://twitter.com/Topiy13/status/712941279760375810

Siegler, M.A., et al. "Lunar true polar wander inferred from polar hydrogen." Nature, vol. 531, no. 7595 (2016): 480-484. DOI: 10.1038/nature17166
Available @ http://www.nature.com/nature/journal/v531/n7595/full/nature17166.html

Williams, Kimberly K. “Ancient Polar Ice Reveals Tilting of Earth’s Moon.” NASA. March 23, 2016.
Available @ http://www.nasa.gov/ames/feature/lunar-polar-ice-reveals-tilting-axis-of-earth-s-moon