"Long before it's in the papers"
March 25, 2016

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The Moon’s spin axis shifted, scientists say

March 25, 2016
Courtesy of NASA
and World Science staff

New re­search sug­gests that the spin ax­is of Earth’s moon shifted by about five de­grees roughly three bil­lion years ago. The ev­i­dence of this is recorded in the dis­tri­bu­tion of an­cient lu­nar ice, ac­cord­ing to sci­en­tists.

“The same face of the Moon has not al­ways point­ed to­wards Earth,” said Mat­thew Siegler of the Plan­e­tary Sci­ence In­sti­tute in Tuc­son, Ar­i­zo­na, lead au­thor of a pa­per in the March 23 is­sue of the jour­nal Na­ture. “As the ax­is moved, so did the face of the ‘man in the moon.’ He sort of turned his nose up at the Earth.”

Thiscross-sec­tion of the Moon high­lights vol­a­tile, or eas­i­ly evap­o­rat­ed, ma­te­ri­als at the poles (in pur­ple), and how they trace an an­cient spin pole. The re­o­ri­en­tation from that an­cient spin pole (red ar­row) to the pre­s­ent-day spin pole (blue ar­row) was driv­en by the for­ma­tion and ev­o­lu­tion of the Pro­cel­larum—a re­gion on the near side of the Moon as­so­ci­at­ed with a high abun­dance of heat-producing el­e­ments (green), high heat flow, and an­cient vol­can­ic ac­tiv­i­ty. (Cred­its: James Tut­tle Keane, U. of Ar­i­zo­na)


Sci­en­tists at mul­ti­ple in­sti­tu­tions did the re­search as part of NASA’s So­lar Sys­tem Ex­plora­t­ion Re­search Vir­tu­al In­sti­tute based at NASA’s Ames Re­search Cen­ter in Sil­i­con Val­ley, Ca­lif.

Wa­ter ice can ex­ist on Earth’s moon in ar­eas of per­ma­nent shad­ow. If ice on the moon is ex­posed to di­rect sun­light it evap­o­rates in­to space. 

The scien­tists re­ported ev­i­dence that a shift of the lu­nar spin ax­is bil­lions of years ago en­abled sun­light to creep in­to ar­eas that were once shad­owed and likely pre­vi­ously con­tained ice. 

The re­search­ers found that the ice that sur­vived this shift ef­fec­tively “paints” a path along which the ax­is moved. They matched the path with mod­els pre­dict­ing where the ice could re­main sta­ble and in­ferred the moon’s ax­is had moved by about five de­grees. 

This is the first ev­i­dence that the moon un­der­went such a dra­mat­ic change in ori­enta­t­ion, the authors said. It also im­plies that much of the po­lar ice on the moon is bil­lions of years old.

The change in ax­is can only have been caused by a change in the mass, or “weight,” dis­tri­bu­tion of ma­te­ri­al in­side the Moon, the re­search­ers said.

As­tro­no­mers al­so re­ported ear­li­er this month that—a­round the same time the Moon’s changes are sup­posed to have oc­curred—Mars’ out­er lay­ers swiv­eled around the co­re of the plan­et by about 20 to 25 de­grees. But in that case, the plan­et’s spin ax­is did­n’t change.


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New research suggests that the spin axis of Earth’s moon shifted by about five degrees roughly three billion years ago. The evidence of this is recorded in the distribution of ancient lunar ice, according to scientists. “The same face of the moon has not always pointed towards Earth,” said Matthew Siegler of the Planetary Science Institute in Tucson, Arizona, lead author of a paper in the March 23 issue of the journal Nature. “As the axis moved, so did the face of the ‘man in the moon.’ He sort of turned his nose up at the Earth.” Scientists at multiple institutions did the research as part of NASA’s Solar System Exploration Research Virtual Institute based at NASA’s Ames Research Center in Silicon Valley, Calif. Water ice can exist on Earth’s moon in areas of permanent shadow. If ice on the moon is exposed to direct sunlight it evaporates into space. Authors of the Nature article show evidence that a shift of the lunar spin axis billions of years ago enabled sunlight to creep into areas that were once shadowed and likely previously contained ice. The researchers found that the ice that survived this shift effectively “paints” a path along which the axis moved. They matched the path with models predicting where the ice could remain stable and inferred the moon’s axis had moved by approximately five degrees. This is the first physical evidence that the moon underwent such a dramatic change in orientation and implies that much of the polar ice on the moon is billions of years old. The change in axis can only have been caused by a change in the mass, or “weight,” distribution of material inside the Moon, the researchers said. Astronomers also reported earlier this month that—around the same time the Moon’s changes are supposed to have occurred—Mars’ outer layers at one time swiveled around the core of the planet by about 20 to 25 degrees. But in that case, the planet’s spin axis didn’t change.