For a time, then, after the impact, Earth would have had Saturn-like rings. Later on, the majority of the impactor’s mass accreted onto Earth’s surface. The collision sped up Earth’s rotation and melted the mantles of both planets and reformed them, creating the plagioclase feldspar. Planetary scientists believe the smaller protoplanet struck Earth at a grazing angle. Proto-Earth had 50 to 90 percent of its current size and mass, while the second planet - a body about the size of Mars - no longer exists. These two lines of evidence have led to the current thinking about where the Moon came from.Ībout 4.6 billion years ago, two planets floated in the space now occupied by the Earth-Moon system, according to most planetary scientists. The basin’s size - more than 1,500 miles (2,400 kilometers) across - shows that huge catastrophic collisions occurred. Another key piece of evidence centered on the solar system’s largest impact basin, the South Pole-Aitken Basin on the Moon’s farside. The solution had to be something like this: Soon after its formation, the Moon may have been covered by an ocean of liquid rock that crystallized as plagioclase feldspar over its surface. That’s not what scientists would have expected. But Apollo showed that much of the lunar crust is composed of anorthosite and similar rocks, with high concentrations of plagioclase feldspars.
The strangeness of this finding comes from the relative purity of the feldspars most minerals in nature are all mixed up into rocks. The samples also contain small amounts of pyroxenes and olivine. These minerals, composed of sodium and calcium aluminum silicates, are commonly found in Earth’s crust. The rock contains large amounts of a mineral class called plagioclase feldspars. Apollo 11 astronauts returned samples containing strange white pebbles that suggested the lunar highlands are composed of an igneous rock called anorthosite. The clues to a solution came from an unexpected direction. None of these ideas fully convinced astronomers or matched up with what planetary scientists knew about the Earth-Moon system. Their ideas included “co-accretion,” in which Earth and the Moon formed independently and then came together gravitationally “capture,” in which Earth gravitationally dragged the Moon into orbit after its formation and a near-miss encounter and “fission,” in which Earth’s interior belched out the Moon like the splitting of a cell. Learn More >.įor many years, planetary scientists struggled with ideas about the Moon’s origin. We’re excited to announce Astronomy magazine’s new Space and Beyond subscription box - a quarterly adventure, curated with an astronomy-themed collection in every box. Pluto also has a moon, Charon, that's large compared with its host planet.īringing the universe to your door. Earth is special in the solar system because it has such a large moon. Look at Venus with no moons, Mars with its two tiny potato-shaped moons, and Mercury with no moon. But there’s a much weirder aspect to Earth - the Moon is huge relative to its nearest neighbor. It’s the only one with lots of liquid water. Our planet is a strange one, judging by the standards of our solar system.
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