Scientists: The Moon is slowly shrinking

NASA’s Lu­nar Re­con­nais­sance Or­biter sat­el­lite is re­veal­ing land­forms that in­di­cate the moon is shrink­ing, re­search­ers say.

Re­search­ers are ex­am­in­ing so-called lo­bate scarps, a type of “thrust fault.” A thrust fault is a break in a plan­e­tary or lu­nar crust in which orig­i­nally low-lying lay­ers of rock are pushed up over high­er lay­ers.

These fea­tures can be pro­duced as a re­sult of a shrink­ing plan­et or moon, ac­cord­ing to Thom­as Wat­ters of the Cen­ter for Earth and Plan­e­tary Stud­ies at the Smith­so­nian In­sti­tu­tion’s Na­tional Air and Space Mu­se­um in Wash­ing­ton, D.C.

As the lu­nar in­te­ri­or cooled and con­tract­ed the en­tire Moon shrank, re­search­ers say. As a re­sult its brit­tle crust rup­tured and formed dis­tinc­tive land­forms known as lo­bate scarps. In a particularly dra­mat­ic ex­am­ple, a thrust fault pushed crus­tal ma­te­ri­als (ar­rows) up the side of the far­side im­pact crat­er named Greg­o­ry (2.1°N, 128.1°E). By map­ping the dis­tri­bu­tion and de­ter­min­ing the size of all lo­bate scarps, the tec­ton­ic and ther­mal his­to­ry of the Moon can be re­con­struct­ed. (Cred­it: NA­SA/GS­FC/Ari­zona State U./Smith­son­ian)

“One of the re­mark­a­ble as­pects of the lu­nar scarps is their ap­par­ent young age,” Wat­ters said. “Rel­a­tively young, glob­ally dis­trib­ut­ed thrust faults show re­cent con­trac­tion of the whole moon, likely due to cool­ing of the lu­nar in­te­ri­or. The amount of con­trac­tion is es­ti­mat­ed to be about 100 me­ters [yards] in the re­cent past,” about a hun­dred mil­lion to a bil­lion years ago, he added.

The lower estimate would put the shrink­age speed at one mi­cron (thou­sandth of a milli­me­ter) per year. Wat­ters and col­leagues re­ported their find­ings in in the Aug. 20 is­sue of the re­search jour­nal Sci­ence.

The moon formed in a cha­ot­ic en­vi­ron­ment of in­tense bom­bard­ment by as­ter­oids and me­te­ors, sci­en­tists say. These col­li­sions, along with the de­cay of ra­di­o­ac­t­ive el­e­ments, made the moon hot, but it cooled off as it aged.

The team be­lieves the scarps are among the fresh­est fea­tures on the moon, in part be­cause they cut across small crat­ers. Since the moon is con­stantly bom­barded by me­te­ors, fea­tures like small crat­ers are quickly de­stroyed by oth­er im­pacts and don’t last long. If a small crat­er has been dis­rupted by a scarp, the scarp formed af­ter the crat­er and is even young­er.

Lo­bate scarps were first rec­og­nized in pho­tos tak­en near the moon’s equa­tor by the Apol­lo 15, 16 and 17 mis­sions. The new Or­biter, which cir­cles the Moon (click here for a fi­ne vi­deo show­ing its or­bit and how it was moved the­re), has re­vealed 14 ad­di­tion­al scarps. These faults are glob­ally dis­trib­ut­ed and not clus­tered near the moon’s equa­tor, as was pre­vi­ously thought, Wat­ters said.

This con­firms that the scarps are a glob­al phe­nom­e­non, mak­ing a shrink­ing moon the most likely ex­plana­t­ion for their wide dis­tri­bu­tion, ac­cord­ing to the team.

“The ul­tra­high res­o­lu­tion im­ages from the [Or­biter’s] Nar­row An­gle Cam­er­as are chang­ing our view of the moon,” said study co-author Mark Rob­in­son of Ar­i­zo­na State Uni­vers­ity, prin­ci­pal in­ves­ti­ga­tor of the Lu­nar Re­con­nais­sance Or­biter Cam­era. “We’ve not only de­tected many pre­vi­ously un­known lu­nar scarps, we’re see­ing much great­er de­tail on the scarps iden­ti­fied in the Apol­lo pho­tographs.”

Lo­bate scarps are found on oth­er worlds in our so­lar sys­tem, in­clud­ing Mer­cu­ry, where they are much larg­er.

“Lo­bate scarps on Mer­cu­ry can be over a mile high and run for hun­dreds of miles,” said Wat­ters. Mas­sive scarps like these lead sci­en­tists to be­lieve that Mer­cu­ry was com­pletely mol­ten as it formed. If so, Mer­cu­ry would be ex­pected to shrink more as it cooled, and thus form larg­er scarps, than a world that may have been only par­tially mol­ten with a rel­a­tively small co­re. Our moon has more than a third of the vol­ume of Mer­cu­ry, but since the moon’s scarps are typ­ic­ally much smaller, the team be­lieves the moon shrank less.