Another Lunar Revolution

 

Sabine Baeß, Tassilo ThierbachFigure skating is, of course, the best known example of angular momentum at work. A spinning skater rotates more slowly as they extend their arms and more rapidly as they bring their arms closer in. It works the same way for the Earth-Moon system. Now here is the important question: has the angular momentum of the Earth and Moon remained the same since the Giant Impact?

Back in May of last year, I wrote an entry suggesting that the Moon-forming Giant Impact model was on life-support. Of course it was a Mark Twain moment, “reports of my death are greatly exaggerated” and all that, because the model re-emerged at the end of 2012 in rude health, re-energized (literally) by a duo of papers published in Science.

Earlier versions of the Giant Impact model took the present angular momentum of the Earth-Moon system to be a primary constraint. This meant that the collision between the incoming projectile body and the proto-Earth would have had a relatively low energy. As a result, the disk from which the Moon formed should have contained a high proportion of projectile material, possibly as much as 80%. In view of the relatively heterogeneous composition of most Solar System bodies, one implication of this model is that the Earth and Moon should be geochemically distinct. In fact this is far from being the case. In terms of their oxygen, silicon, tungsten and titanium isotopes, the Earth and the Moon are essentially identical.

In a break with previous models, Matija Ćuk and Sarah Stewart now propose that the angular momentum of the Earth-Moon system may initially have been significantly higher following the Giant Impact and was then reduced through an orbital resonance between the Sun and Moon, this is termed an “evection resonance”. Ćuk and Stewart suggest that without the angular momentum constraint a range of impact scenarios are compatible with the observed geochemical composition of the Earth and Moon. In a second paper published in the same issue of Science, Robin Canup builds on the work of Ćuk and Stewart to present a model for the formation of the Earth and Moon via an impact between two planets of roughly equal mass.

The scenarios presented in both papers are of significantly higher energy than the earlier versions of the Giant Impact model. As a result, not only is mixing between the projectile and proto-Earth more efficient, but also temperatures in the aftermath of the collision significantly higher. Canup points out that the disc of material produced in her model is significantly hotter than earlier models i.e. with 50 to 90% of the mass as vapour rather than 10 to 30%. As a result, the disc would be well mixed with respect to both volatile and refractory components. While this feature of the new models helps to explain the geochemical similarities between the Earth and the Moon, it does not solve the mystery of why the Moon is so depleted in volatiles. Ćuk and Stewart suggest that this feature might reflect separation of refractory and volatile material during formation of the Moon from the debris disk. However, they admit that this process needs further detailed study.

Of course, to make real scientific progress a new model needs to be testable. Alex Halliday, in his perspectives article, suggests a number of ways in which these new models can be evaluated using geochemical evidence. He points out that the silicon isotopic composition of the Earth carries the signature of high-pressure core formation. This signature was transferred to the Moon. If the Earth grew significantly in size during the Giant Impact, as suggested by the Canup model, then some further fractionation of silicon isotopes in the Earth ought to be detectable. The preservation of deep reservoirs displaying noble gas isotopic heterogeneities appears to be inconsistent with the high levels of mixing associated with the new impact models. Finally, Halliday suggests that tungsten isotopic signatures in the Moon and Earth may also help to discriminate between the various impact scenarios.

Clearly these new models will come under sustained scientific scrutiny in the coming years. However, there seems little doubt that they represent a very significant milestone in understanding the development of the Earth-Moon system. In addition, some of the processes invoked by these models will undoubtedly have implications for our understanding of the early evolution of other solar system bodies.

newPicture1

A new giant impact model for the formation of the Earth-Moon system by Robin Canup has been published in Science. Shown is an off-centre, low-velocity collision of two protoplanets containing 45 percent and 55 percent of the Earth’s mass. Color scales with particle temperature in Kelvin, with blue-to-red indicating temperatures from 2,000 K to in excess of 6,440 K. After the initial impact, the protoplanets re-collide, merge and form a rapidly spinning Earth-mass planet surrounded by an iron-poor proto-lunar disk containing about 3 lunar masses. The composition of the disk and the final planet’s mantle differ by less than 1 percent. (image and caption: Southwest Research Institute)

Blog image credit top: Wikipedia

 Richard Greenwood

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