Exploratour - Evolution of the Solar System

This is a drawing of the evolution of moons.
Image from: The New Solar System

The co-formation theory explains the origins of moons as objects which formed out of the initial cloud of gas and dust at the same time and roughly the same place as the parent planet. As shown in this picture, while they were forming in the solar nebula, the beginning of the moons-to-be (called proto-moons) drew material to themselves from the cloud of gas and dust around them. This theory seems to explain the many icy moons around the giant planets in the outer solar system.

The temperature difference which played a role in the formation of the planets seems also to have played a role in the formation of these moons. Just as the planets became more and more icy the further from the sun, moons seem to be more and more icy the further from the planet they orbit. For example, Jupiter's moon Io (close to Jupiter) is very rocky, but Europa (farther from Jupiter) has more ice, Ganymede and Callisto (further still from Jupiter) have lots more ice.
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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