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Scientists: The Moon is slow ly shrinking

Aug. 20, 2010
Courtesy of the Smithsonian Institution
and World Science staff

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.


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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. “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 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. Watters and colleagues reported their findings in in the Aug. 20 issue of the research journal Science. 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 orbiter, which circles the Moon (see here for a fine video showing its orbit and how it was launched 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.