Most scientists agree that the Earth has pretty much always had its moon. Details of the moon\u2019s composition (in particular the \u201cisotopic mixture\u201d of heavier and lighter versions of various elements) are too similar to the Earth\u2019s for it to have been captured from somewhere remote. However, some compositional details differ enough to rule out the idea that the moon is simply a chunk of the Earth that broke loose.<\/p>\n
The details of how the moon formed, and when, have long been debated. Now two new studies shed fresh light on the process \u2013 even pinning down a date.<\/p>\n
For more than 30 years, the prevailing view of the formation of our moon has been the \u201cgiant impact hypothesis<\/a>\u201d. The precursors to the current four rock planets \u2013 Mercury, Venus, Earth and Mars \u2013 appear to have been dozens of smaller bodies known as \u201cplanetary embryos\u201d. According to the giant impact hypothesis, our moon formed as the result of the last of a series of \u201cgiant impact\u201d mergers between planetary embryos that eventually formed the Earth. In this last collision, one embryo was nearly Earth-sized and the other approximately Mars-sized. The merged body resulting from this became the Earth. Debris flung out from the impact, most of which came from the rocky part of the smaller body, gathered in orbit to become the moon.<\/p>\n