When astronauts brought rock samples from the lunar surface back to Earth for analysis, it was the beginning of the detailed study of the Moon. The pale areas on the Moon are easily seen from Earth. The highlands are made of a light rock called anorthosite, which was formed around 4.3 billion years ago.
There is a mystery surrounding the anorthosite formation on the Moon. The age of the anorthosite highlands doesn't match the time it took for the Moon's ocean to cool. The scientists behind the new study think they have solved the mystery.
There is uncertainty surrounding the formation of lunar anorthosites. Their formation involves fractional crystallization, where anorthite mineral crystals are removed from magma as they form, with lighter crystals rising to the surface. Some of the details of their formation are not clear.
The rock that forms the highlands is the anorthosite. Anorthite is a mineral. Anorthosites are rocks that are rich in anorthite.
The Moon was created by a collision between two planets. One of the planets became the Earth and the other the Moon. The entire mantle was molten after the crash. This ocean is called a magma ocean by geologists.
Evidence seemed to show that the anorthosites on the lunar highlands were formed by fractional crystallization when scientists studied the Apollo 11 samples. Light anorthite crystals rose to the top of the magma oceans. The Highlands is mostly anorthite.
There is an older model of anorthosite on the Moon. The new research suggests a different mechanism for formation. The older model can not solve an age discrepancy. The image is from the English-language Wikipedia, CC BY-SA 3.0.
There are problems with that explanation, and a pair of scientists think they have a better answer. They have presented their work in a paper. The paper was published in the A.G.U.'s Geophysical Letters.
The lunar crust is thought to have been formed by light anorthite crystals floating at the surface of the liquid magma ocean, with heavier crystals solidifying at the ocean floor. The model explains how the lunar Highlands may have formed.
The conclusion is based on samples from Apollo11, and present-day researchers have more tools and evidence at their disposal. The analysis of lunar meteorites and the analysis of the Moon's surface differ from the earlier conclusion. The lunar anorthosites seem to be more heterogeneous. The surface of the rock is rich in anorthites. The findings suggest that our understanding of the moon's ocean isn't complete.
The anorthosites in the lunar highlands were created in the ancient magma ocean.
The age range in the anorthosites and how long it took the ocean to cool are clues to what happened. The ocean solidified in 100 million years, but the anorthosites are over 200 million years old.
The dark areas of the moon are called maria, Latin for "seas." basaltic lava flows created the maria. The highlands were formed when the moon's ocean cooled and solidified. Gregory H. Revera is the author of CC BY-SA 3.0.
The co-author of the paper said that given the range of ages and compositions of the anorthosites on the Moon, and what we know about how crystals settle in solidifying magma, the lunar crust must have formed through some other mechanism.
A mathematical model was developed to identify this mechanism. It is difficult for heavier crystals to settle in the lower gravity of the Moon. The convective currents in the ocean discourage settling. The researchers found that the ocean could have formed a kind of slurry, where the crystals remained suspended rather than rising or falling. There is a critical threshold. When the crystal content is above the threshold, the slurry becomes more elastic and slows down.
The authors say that reaching the critical crystal fraction may result in a prolongated magma ocean stage.
The surface of the slurry cools quicker than the interior. In the lunar highlands, we see a rich, anorthite-rich crust and a slushy interior.
The figure shows the new model of formation on the Moon. The green area is the still-convecting crystal slush and the stagnant lid is the anorthite rich crust. Autectic means a homogeneity of materials. The eutectic crust is mostly anorthite. The image is of Neufeld and Michaut.
Neufeld said that they believe the lunar crust formed as lightweight, anorthite-enriched melt from the convecting crystalline slurry below. The cooling of the early magma ocean may have caused the crystals to remain suspended as a slush.
The press release states that enriched lunar surface rocks likely formed in magma chambers within the lid.
The research explains the discrepancy between the age of the anorthosites and the time it took for the ocean to solidify. The age of the anorthosites in the lunar highlands was 200 million years younger than the magma ocean's age of 100 million years.
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