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This is an illustration of the Chicxulub impact.
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The Collisional Ejection Theory

Currently, the theory that best explains all the evidence is the collisional ejection theory. This theory holds that the Moon formed from debris ejected from the Earth when a large object (possibly as large as Mars) crashed into the Earth. Simulations of this scenario show that energy from such a collision produces a stream of vaporized rock from the impact. The Moon forms from this cooled material.

This theory explains many of the known properties of the Moon's orbit and composition. The ejected material would have coalesced in or near the ecliptic plane, putting the Moon into an orbit much like the one it has. The Moon, which is believed to have a small iron core, could have retained this core through the collision. Volatile elements would have been vaporized during the impact. Also, this collision could have tipped the Earth's axis, causing the seasons we now know.

The problem with this theory is that it does not seem very likely, although objections to the theory based on considerations of the angular momentum between the two objects have been resolved in recent computer models.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA