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  1. Making the Moon

    Based on a Southwest Research Institute press release

    The “giant impact” theory, first proposed in the mid-1970s to explain how the Moon formed, has now received a major boost. New computer simulations demonstrate how a single impact could yield the current Earth-Moon system. According to these new results, which appeared in the August 16 issue of Nature, the Moon is a chip off of the terrestrial block.

    The Earth-Moon system is unusual in several respects. The Moon has an abnormally low density compared to the terrestrial planets (Mercury, Venus, Earth, and Mars), indicating that it lacks high-density iron. While the Earth’s iron core is about 30 percent of the planet’s total mass, the Moon’s core constitutes only a few percent of its total mass. In addition, the angular momentum of the Earth-Moon system is quite large. It implies that the terrestrial day was only about five hours long when the Moon first formed close to the Earth. These characteristics provide strong constraints for giant impact models that try to explain the Moon’s formation.

    Previous models had shown two classes of impacts capable of producing an iron-poor Moon, but both were problematic. One model involved an impact with twice the angular momentum of the Earth-Moon system. This model required that a later event (such as a second large impact) altered the Earth’s spin after the Moon’s formation.

    The second model proposed that the Moon-forming impact occurred when Earth was about half of its present mass. This model required that the Earth accumulated the second half of its mass after the Moon formed. However, if the Moon also accumulated its proportionate share of material during this period, it would have gained too much iron-rich material — more than can be reconciled with the Moon today.

    The new simulations performed by Southwest Research Institute (SwRI) and University of California at Santa Cruz (UCSC) researchers show that a single impact by a Mars-sized object in the late stages of Earth’s formation could account for an iron-depleted Moon and the masses and angular momentum of the Earth-Moon system. This is the first model that can simultaneously explain these characteristics without requiring that the Earth-Moon system be substantially modified after the lunar forming impact.

    The models developed by SwRI and UCSC use the modeling technique known as smooth particle hydrodynamics, or SPH, which has been used in previous studies about planet formation and impact events.

    SPH is often utilized to model interactions between forming gas giant planets and their precursor disks,” says the paper’s lead author, Robin Canup, assistant director of the SwRI Space Studies Department in Boulder, Colorado. “The technique has also been used to model asteroid collisions and the breakup of comet Shoemaker/Levy-9.”

    In SPH simulations, the colliding planetary objects are modeled by a vast multitude of discrete spherical volumes, in which thermodynamic and gravitational interactions are tracked as a function of time.

    The new high-resolution simulations show that an oblique impact by an object with 10 percent the mass of the Earth can eject sufficient iron-free material into Earth-orbit to yield the Moon, while also leaving the Earth with its final mass and correct initial rotation rate. This simulation also implies that the Moon formed near the very end of Earth’s formation.

    “The model we propose is the least restrictive impact scenario, since it involves only a single impact and requires little or no modification of the Earth-Moon system after the Moon-forming event,” says Canup.

    UCSC Professor Erik Asphaug adds, “Our model requires a smaller impactor than previous models, making it more statistically probable that the Earth should have a Moon as large as ours.”

    The Moon is believed to play an important role in Earth’s habitability. Because the Moon helps stabilize the tilt of the Earth’s rotation, it prevents the Earth from wobbling between climatic extremes. Without the Moon, seasonal shifts would likely outpace even the most adaptable forms of life.

    “A Moon-less Earth with the same mass, rotation rate, and orbit as today would have the direction of its spin axis vary chaotically between 0 and 90 degrees on time scales as short as 10 million years,” says Darren Williams, Assistant Professor of Physics and Astronomy at Penn State University and NAI member. “At high obliquity, temperatures over mid-to-high latitude continents would reach near boiling 80 to 100 Celsius around the summer solstice under a 1-bar nitrogen- dominated atmosphere. Such temperatures would be damaging to all forms of water-dependent life on Earth today.”

    What’s Next?

    Modeling lunar formation is important to the overall understanding of the origin of the terrestrial planets. Because the Moon appears to have formed by an impact event, studying such gigantic impacts may tell us something about the formation of Earth-like planets throughout the Galaxy.

    “It is now known that giant collisions are a common aspect of planet formation, and the different types of outcomes from these last big impacts might go a long way toward explaining the puzzling diversity observed among planets,” says Asphaug.

    “Understanding the likelihood of Moon-forming impacts is an important component in how common or rare Earth-like planets may be in extrasolar systems,” adds Canup.