"Long before it's in the papers"
August 03, 2011

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“Big splat” may explain mountains on Moon’s far side

Aug. 3, 2011
Courtesy of UCSC
and World Science staff

A vast moun­tain­ous re­gion on the far side of the Moon may have formed that way be­cause of a long-ago col­li­sion with a smaller com­pan­ion moon, sci­en­tists say.

The strik­ing dif­fer­ences be­tween Moon’s near and far sides have been a long­stand­ing puz­zle. The near side is low and flat, while far side is high and moun­tain­ous, with a much thicker crust. 

The new stu­dy, pub­lished in the Au­gust 4 is­sue of the re­search jour­nal Na­ture, builds on the “gi­ant im­pact” mod­el for the or­i­gin of the Moon. In this the­o­ry, a Mars-sized ob­ject col­lid­ed with Earth early in the his­to­ry of the so­lar sys­tem and threw off de­bris that came to­geth­er to form the Moon. 

This im­age of the lu­nar far­side from NA­SA's Lu­nar Re­con­nais­sance Or­biter shows the moon's to­pog­ra­phy with the high­est el­e­va­tions (a­bove 20,000 feet) in red and the low­est ar­eas in blue. (Cred­it: NA­SA/­God­dard)

The new study sug­gests that this gi­ant im­pact al­so cre­at­ed an­oth­er, smaller body, in­i­tially shar­ing an or­bit with the Moon. This even­tu­ally fell back on­to the Moon and coat­ed one side with an ex­tra lay­er of crust tens of kilo­me­ters or miles thick.

“Our mod­el works well with mod­els of the Moon-form­ing gi­ant im­pact, which pre­dict there should be mas­sive de­bris left in or­bit about the Earth, be­sides the Moon it­self. It agrees with what is known about the dy­nam­i­cal sta­bil­ity of such a sys­tem, the tim­ing of the cool­ing of the Moon, and the ages of lu­nar rocks,” said Er­ik As­phaug, pro­fes­sor of Earth and plan­e­tary sci­ences at the Uni­vers­ity of Cal­i­for­nia San­ta Cruz.

As­phaug, who coau­thored the pa­per with post­doc­tor­al re­searcher Mar­tin Jutzi at the uni­vers­ity, has pre­vi­ously done com­put­er sim­ula­t­ions of the Moon-form­ing gi­ant im­pact. He said com­pan­ion moons are a com­mon out­come of such sim­ula­t­ions.

In the new stu­dy, he and Jutzi used com­put­er sim­ula­t­ions of an im­pact be­tween the Moon and a smaller com­pan­ion (a­bout one-thirtieth the weight of the Moon) to study the dy­nam­ics of the col­li­sion and track the ev­o­lu­tion and dis­tri­bu­tion of lu­nar ma­te­ri­al in its af­ter­math. In such a low-velocity col­li­sion, the im­pact does­n’t form a crat­er or cause much melt­ing. In­stead, most of the col­lid­ing ma­te­ri­al is piled on as a thick new lay­er of sol­id crust, form­ing a moun­tain­ous re­gion com­pa­ra­ble in ex­tent to the lu­nar far­side high­lands.

“Of course, im­pact mod­elers try to ex­plain eve­ry­thing with col­li­sions. In this case, it re­quires an odd col­li­sion: be­ing slow, it does not form a crat­er, but splats ma­te­ri­al on­to one side,” As­phaug said. “It is some­thing new to think about.” 

He and Jutzi hy­poth­e­size that the com­pan­ion moon was in­i­tially trapped at one of the gravita­t­ionally sta­ble “Tro­jan points” shar­ing the Moon’s or­bit, and be­came desta­bilized af­ter the Moon’s or­bit had ex­pand­ed far from Earth. “The col­li­sion could have hap­pened any­where on the Moon,” Jutzi said. “The fi­nal body is lop­sid­ed and would re­or­i­ent so that one side faces Earth.” 

Oth­er mod­els have been pro­posed to ex­plain the forma­t­ion of the high­lands. These include one pub­lished last year in the journal Sci­ence by Jutzi and other re­search­ers at UC San­ta Cruz, Ian Garrick-Bethell and Fran­cis Nimmo. Their anal­y­sis sug­gested that tid­al forc­es, rath­er than an im­pact, were re­spon­si­ble for shap­ing the thick­ness of the Moon’s crust.

“The fact that the near side of the Moon looks so dif­fer­ent to the far side has been a puz­zle since the dawn of the space age, per­haps sec­ond only to the or­i­gin of the Moon it­self,” said Nimmo. For now, he said, there is not enough da­ta to say which of the al­ter­na­tive mod­els of­fers the best ex­plana­t­ion. “As fur­ther space­craft da­ta and, hope­ful­ly, lu­nar sam­ples are ob­tained, which of these two hy­pothe­ses is more nearly cor­rect will be­come clear,” Nimmo said.

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A vast mountainous region on the far side of the Moon may have formed that way because a long-ago collision with a smaller companion moon, scientists say. The striking differences between Moon’s near and far sides have been a longstanding puzzle. The near side is low and flat, while far side is high and mountainous, with a much thicker crust. The new study, published in the August 4 issue of the research journal Nature, builds on the “giant impact” model for the origin of the Moon. In this theory, a Mars-sized object collided with Earth early in the history of the solar system and threw off debris that came together to form the Moon. The study suggests that this giant impact also created another, smaller body, initially sharing an orbit with the Moon, that eventually fell back onto the Moon and coated one side with an extra layer of solid crust tens of kilometers thick. “Our model works well with models of the Moon-forming giant impact, which predict there should be massive debris left in orbit about the Earth, besides the Moon itself. It agrees with what is known about the dynamical stability of such a system, the timing of the cooling of the Moon, and the ages of lunar rocks,” said Erik Asphaug, professor of Earth and planetary sciences at the University of California Santa Cruz. Asphaug, who coauthored the paper with postdoctoral researcher Martin Jutzi at the university, has previously done computer simulations of the Moon-forming giant impact. He said companion moons are a common outcome of such simulations. In the new study, he and Jutzi used computer simulations of an impact between the Moon and a smaller companion (about one-thirtieth the weight of the Moon) to study the dynamics of the collision and track the evolution and distribution of lunar material in its aftermath. In such a low-velocity collision, the impact doesn’t form a crater or cause much melting. Instead, most of the colliding material is piled on as a thick new layer of solid crust, forming a mountainous region comparable in extent to the lunar farside highlands. “Of course, impact modelers try to explain everything with collisions. In this case, it requires an odd collision: being slow, it does not form a crater, but splats material onto one side,” Asphaug said. “It is something new to think about.” He and Jutzi hypothesize that the companion moon was initially trapped at one of the gravitationally stable “Trojan points” sharing the Moon’s orbit, and became destabilized after the Moon’s orbit had expanded far from Earth. “The collision could have happened anywhere on the Moon,” Jutzi said. “The final body is lopsided and would reorient so that one side faces Earth.” Other models have been proposed to explain the formation of the highlands, including one published last year in Science by Jutzi and Asphaug’s colleagues at UC Santa Cruz, Ian Garrick-Bethell and Francis Nimmo. Their analysis suggested that tidal forces, rather than an impact, were responsible for shaping the thickness of the Moon’s crust. “The fact that the near side of the Moon looks so different to the far side has been a puzzle since the dawn of the space age, perhaps second only to the origin of the Moon itself,” said Nimmo, a professor of Earth and planetary sciences. “One of the elegant aspects of Erik’s article is that it links these two puzzles together: perhaps the giant collision that formed the Moon also spalled off some smaller bodies, one of which later fell back to the Moon to cause the dichotomy that we see today.” For now, he said, there is not enough data to say which of the alternative models offers the best explanation for the lunar dichotomy. “As further spacecraft data and, hopefully, lunar samples are obtained, which of these two hypotheses is more nearly correct will become clear,” Nimmo said.