Dr. Richard S. Miller’s research says the poles of the moon might have shifted over time, causing it to flop over.
Michael Mercier | UAH
Could the moon once have flopped over on its side like a child's top?
Yes, says a University of Alabama in Huntsville (UAH) scientist in Nature magazine, after he and collaborators discovered antipodal water deposits
on the moon that may indicate its poles have shifted over time. An antipodal water distribution pattern means that water deposits at the north and south
lunar poles point in opposite directions.
"This is unexpected, since if the water is 'recent' it should be distributed approximately uniformly around the poles," says Dr. Richard S. Miller, a UAH
physicist who joined with two other primary scientists, Dr. Matt Siegler of the Planetary Science Institute and James Keane of the University of Arizona,
in the research.
"This led us to postulate that the off-axis and antipodal nature of the water deposits might hint at an ancient pole," Dr. Miller says. "If the water
distribution is indicative of ancient lunar poles - called paleo-poles - it suggests the moon tipped over, or re-oriented, sometime in the past."
This sounds crazy, but as we looked at other models and data it all appears to be supported.
That would mean that billions of years ago the moon was oriented differently, with a different set of poles, and then tipped over to its current
orientation.
"This sounds crazy, but as we looked at other models and data it all appears to be supported," Dr. Miller says.
The University of Arizona's Keane - an expert in planetary structure, evolution and orientation - modeled the moon's internal evolution and was able to
show that lunar reorientation is possible if a bubble of low-density material formed below the surface and caused the moon to tip over like a top.
"Coincidentally, the location of this sub-surface feature appears to be consistent with a curious geological feature called the Procellarum KREEP Terranae
(PKT), a large surface feature we can see at night," says Dr. Miller. "It also is supported by recent observations from NASA's Gravity Recovery and
Interior Laboratory (GRAIL) mission, which measured the moon's gravity and structure."
Another piece of the puzzle comes from thermal studies by Dr. Siegler at the Planetary Science Institute that modeled the thermal environment at the lunar
pole regions. Using those models, Dr. Miller was able to show that the current temperature environment is not a good measure of where water ice should
exist.
"However, when we include where ice could exist when the moon was oriented about its paleo-poles, then we get a very good match to the data," he says.
Each piece of the analysis independently supports a hypothesis that the moon was originally oriented differently and, because of the sub-surface evolution
in the PKT, it re-oriented to its current position, a phenomenon known as True Lunar Polar Wander.
The neutron data comes from NASA's Lunar Prospector and Lunar Reconnaissance Orbiter.
"If true, this is the first time neutrons have been used as a tracer of the interior evolution of a planetary body," says Dr. Miller. "It brings together
expertise in orbital geochemistry, nuclear physics, thermal physics, dynamics, etc."
Polar hydrogen deposits at the North (left) and South (right) lunar poles as seen viewed from the North pole through the Moon. The figure illustrates the antipodal nature of the lunar hydrogen (water), and shows the current and ancient lunar poles derived using these lunar deposits.
J.T. Keane/University of Arizona
Dr. Miller and his collaborators presented their work at the 46th Lunar and Planetary
Science Conference, held annually in Houston. The research is based fundamentally on Dr. Miller's analysis of the lunar pole regions and the search for
water.
"Over the past few years I've developed new state of the art statistical analysis techniques to better understand, detect and map the water at the lunar
poles," he says. "We do this by detecting neutrons, and wherever we see a deficit of those particles it implies the presence of hydrogen, likely in the
form of water."
Because of these techniques, Dr. Miller has been able to characterize key parameters such as the abundance, distribution and depth of that lunar water.
Dr. Miller is associated with work being done on lunar mapping by the Johns Hopkins University Applied Physics Lab, which leads the
Volatiles, Regolith and Thermal Investigations Consortium for Exploration and Science (VORTICES) as part of NASA's Solar System Exploration Research
Virtual Institute (SSERVI), and this recent work was supported in part by VORTICES.
"It was a serendipitous discovery and wasn't part of the original set of goals, but it uses results from my VORTICES-related work, within the scope of
VORTICES science, and ultimately supports key parts of that investigation," Dr. Miller says.
A NASA "virtual research institute" composed of nine lead research teams from seven states based and managed at NASA's Ames Research Center at Moffett
Field, Calif., SSERVI "takes what was done in NASA's Lunar Sciences Institute and expands it to the solar system as a whole," says Dr. Miller.
VORTICES brings together scientific expertise across a broad range of disciplines. In addition to UAH, other collaborating institutions include NASA
Goddard Space Flight Center, the Jet Propulsion Laboratory, the Georgia Institute of Technology, the Lunar and Planetary Institute, Mt. Holyoke College,
Johns Hopkins University, the University of Alaska, the University of Hawaii and the University of Maine.
The five-year effort is focused on a deeper understanding of regolith, or soil, and volatiles - including water and hydroxide - on airless planetary bodies
like the moon, asteroids and other bodies in the solar system.