The freezing of a’slushy’ ocean of magma may be responsible for the makeup of the Moon’s crust, according to scientists.
The researchers from the University of Cambridge and the Ecole Normale Supérieure de Lyon presented a novel crystallisation model in which crystals remained suspended in liquid magma for hundreds of millions of years while the lunar’slush’ froze and solidified. The findings were published in the journal Geophysical Review Letters.
Apollo 11 astronauts obtained samples from the lunar Highlands more than fifty years ago. These huge, pale portions of the Moon that can be seen with the naked eye are composed of comparatively light rocks known as anorthosites. Anorthosites first appeared in the Moon’s history, between 4.3 and 4.5 billion years ago.
Similar anorthosites, generated by magma crystallisation, may be found on Earth in fossilised magma chambers. The vast amounts of anorthosite discovered on the Moon, on the other hand, would have necessitated the formation of a massive global magma ocean.
The Moon, according to scientists, arose when two protoplanets, or embryonic worlds, collided. The bigger of these two protoplanets evolved into the Earth, while the smaller evolved into the Moon. As a result of this impact, the Moon became very hot – so hot that its whole mantle was molten magma, or a magma ocean.
“Since the Apollo period, it has been assumed that the lunar crust was produced by light anorthite crystals floating at the top of the liquid magma ocean, with larger crystals consolidating near the ocean bottom,” co-author Chloé Michaut of Ecole Normale Supérieure de Lyon stated. “This ‘flotation’ hypothesis explains how the lunar Highlands developed.”
However, numerous lunar meteorites have been analyzed since the Apollo missions, and the Moon’s surface has been intensively investigated. The composition of lunar anorthosites seems to be more heterogeneous than that of the original Apollo samples, which contradicts a flotation scenario in which the liquid ocean is the common source of all anorthosites.
The anorthosite age range (over 200 million years) is difficult to reconcile with an ocean of practically liquid magma with a typical solidification duration of less than 100 million years.
“Given the variety of ages and compositions of the anorthosites on the Moon, and what we know about how crystals settle in solidifying magma,” said co-author Professor Jerome Neufeld of Cambridge’s Department of Applied Mathematics and Theoretical Physics.
To discover this process, Michaut and Neufeld created a mathematical model.
Crystal settling is difficult under the low lunar gravity, especially when the convecting magma ocean is rapidly stirring. If the crystals stay suspended as a crystal slurry, when the slurry’s crystal concentration surpasses a certain threshold, the slurry thickens and becomes sticky, making deformation sluggish.
This rise in crystal content is most pronounced at the surface, where the slushy magma ocean cools, resulting in a hot, well-mixed slushy core and a slow-moving, crystal-rich lunar ‘lid.’
“We assume the lunar crust originated in this stagnant ‘lid,’ when lightweight, anorthite-enriched melt percolated up from the convecting crystalline slurry below,” Neufeld said. “We propose that the early magma ocean’s cooling produced such intense convection that crystals stayed suspended as a slurry, much like the crystals in a slushy machine.”
Enriched lunar surface rocks are thought to have originated in magma chambers within the lid, which explains their variety. The findings imply that the timeline for the creation of the lunar crust is many hundreds of millions of years, which coincides to the ages of the lunar anorthosites.
Serial magmatism was offered as a plausible process for the production of lunar anorthosites at first, but the slushy model reconciles this notion with that of a worldwide lunar magma ocean.
The European Research Council provided funding for the study.