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New model reconciles the Moon's Earth-like composition with the giant impact theory of formation

Date:
October 17, 2012
Source:
Southwest Research Institute
Summary:
The giant impact believed to have formed the Earth-Moon system has long been accepted as canon. However, a major challenge to the theory has been that the Earth and Moon have identical oxygen isotope compositions, even though earlier impact models indicated they should differ substantially. A new model accounts for this similarity in composition while also yielding an appropriate mass for Earth and the Moon.

Shown is an off-center, 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 protolunar disk containing about 3 lunar masses. The composition of the disk and the final planet's mantle differ by less than 1 percent.
Credit: Image courtesy of Southwest Research Institute

The giant impact believed to have formed the Earth-Moon system has long been accepted as canon. However, a major challenge to the theory has been that Earth and Moon have identical oxygen isotope compositions, even though earlier impact models indicated they should differ substantially. In a paper published October 17 in the journal Science online, a new model by Southwest Research Institute (SwRI), motivated by accompanying work by others on the early dynamical history of the Moon, accounts for this similarity in composition while also yielding an appropriate mass for Earth and the Moon.

In the giant impact scenario, the Moon forms from debris ejected into an Earth-orbiting disk by the collision of a smaller proto-planet with the early Earth. Earlier models found that most or much of the disk material would have originated from the Mars-sized impacting body, whose composition likely would have differed substantially from that of Earth.

The new models developed by Dr. Robin M. Canup, an associate vice president in the SwRI Space Science and Engineering Division, and funded by the NASA Lunar Science Institute, involve much larger impactors than were previously considered. In the new simulations, both the impactor and the target are of comparable mass, with each containing about 4 to 5 times the mass of Mars. The near symmetry of the collision causes the disk's composition to be extremely similar to that of the final planet's mantle over a relatively broad range of impact angles and speeds, consistent with the Earth-Moon compositional similarities.

The new impacts produce an Earth that is rotating 2 to 2.5 times faster than implied by the current angular momentum of the Earth-Moon system, which is contained in both Earth's rotation and the Moon's orbit. However, in an accompanying paper in Science, Dr. Matija Ćuk, SETI Institute, and Dr. Sarah T. Stewart, Harvard University, show that a resonant interaction between the early Moon and the Sun -- known as the evection resonance -- could have decreased the angular momentum of the Earth-Moon system by this amount soon after the Moon-forming impact.

"By allowing for a much higher initial angular momentum for the Earth-Moon system, the Ćuk and Stewart work allows for impacts that for the first time can directly produce an appropriately massive disk with a composition equal to that of the planet's mantle," says Canup.

In addition to the impacts identified in Canup's paper, Ćuk and Stewart show that impacts involving a much smaller, high-velocity impactor colliding into a target that is rotating very rapidly due to a prior impact can also produce a disk-planet system with similar compositions.

"The ultimate likelihood of each impact scenario will need to be assessed by improved models of terrestrial planet formation, as well as by a better understanding of the conditions required for the evection resonance mechanism," adds Canup.

Canup used smoothed-particle hydrodynamics (SPH) to simulate the colliding planetary objects using 300,000 discrete particles whose individual thermodynamic and gravitational interactions were tracked with time.


Story Source:

The above story is based on materials provided by Southwest Research Institute. Note: Materials may be edited for content and length.


Journal Reference:

  1. Robin M. Canup. Forming a Moon with an Earth-Like Composition via a Giant Impact. Science, 17 October 2012 DOI: 10.1126/science.1226073

Cite This Page:

Southwest Research Institute. "New model reconciles the Moon's Earth-like composition with the giant impact theory of formation." ScienceDaily. ScienceDaily, 17 October 2012. <www.sciencedaily.com/releases/2012/10/121017141759.htm>.
Southwest Research Institute. (2012, October 17). New model reconciles the Moon's Earth-like composition with the giant impact theory of formation. ScienceDaily. Retrieved July 28, 2014 from www.sciencedaily.com/releases/2012/10/121017141759.htm
Southwest Research Institute. "New model reconciles the Moon's Earth-like composition with the giant impact theory of formation." ScienceDaily. www.sciencedaily.com/releases/2012/10/121017141759.htm (accessed July 28, 2014).

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