Formation and Crystallization of the Lunar Magma Ocean

According to the giant impactor theory, the Moon and the Earth formed from a mixture of the early Earth and the giant impactor.What would be expected for the compositions of two planets that shared material when they were forming? Part One of this volume discussed how the Earth's iron core formed, leaving lighter, nonmetallic elements above the core in the mantle and crust.The Moon, on the other hand, has either no iron core or a very small one. If the material from the Earth and the giant impactor were completely mixed and then separated into the Earth and the Moon, then they would have the same amounts of iron in them relative to their sizes.With its giant iron core, the Earth has much more iron in it than the Moon has. Because of this, people think that the giant impact that made the Moon happened after the Earth's core had formed, and that the Earth's core was left inside the Earth during the event. The Moon, then, ended up with mostly the lighter, silicon-rich mantle material and material from the giant impactor, and therefore has less iron than the Earth.

The Earth's core is thought to have formed within 5 to 20 million years after the Earth accreted (accumulated out of planetesimals), and the Moon-forming giant impact is thought to have occurred right afterward, at between 10 to 30 million years after formation of the planets. Ten million years is an eye-blink of geologic time: Ten million years is just 0.02 percent of the age of the solar system, equivalent to three minutes out of a 24-hour day. As the planetesimals formed by the giant Earth impact collided and stuck together to form the proto-Moon (meaning early, or pre-Moon), the proto-Moon heated up from the energy of the impacts and from heat given off by radioactive decay of some elements. The Moon is thought to have been molten to some depth when it was first formed. Its surface would literally have been a sea of liquid rock. This stage of lunar formation is called the magma ocean (the Earth may have had one as well; so may the other terrestrial planets).

After the majority of the planetesimals had been accreted to form the Moon, the Moon began to cool. The magma ocean, which was at first literally a red-hot ocean of liquid rock covering the entire Moon, began to crystallize into minerals. Because pressure encourages liquid to freeze into crystals by pressing the atoms together, crystallization begins at the bottom of the magma ocean. The liquid of the magma ocean first crystallized a mineral called olivine (this is the same mineral as the gemstone peridot; it is made of magnesium, silicon, iron, and oxygen). Olivine probably made a dense layer at the bottom of the magma ocean. As the magma ocean cooled further, a mineral called orthopyroxene also crystallized, and then finally two more minerals, called clinopyroxene and plagioclase, accompanied at the end by high-density titanium-rich oxides such as ilmenite.The minerals that formed from the magma ocean are called cumulates, because they are the solid residue of cooling liquid.

All of the cumulate minerals listed here are denser than magma and so would sink to the bottom of the magma ocean except the pla-gioclase (NaAlSi3O8 to CaAl2Si2O8), which is more buoyant than the liquid it crystallized from, and so it may have floated up to the surface of the Moon. In fact, the white areas of the Moon surrounding the dark round craters are called the lunar highlands (shown in the lower color insert on page C-8) and they are made mainly of this mineral. Since plagioclase is lighter than magma and since the lunar highlands are made mainly of plagioclase, these highlands are thought to be a prime piece of evidence that the magma ocean did once exist on the Moon. Without flotation in a magma ocean, accounting for plagioclase-rich highlands is difficult.

There are different estimates of how deep the magma ocean probably was on the moon:These estimates vary from 190 miles (300 km) deep to the whole depth of the Moon (imagine a completely molten blob of liquid rock in the sky where the Moon is now!). At the final stage of magma ocean cooling, a strangely composed final liquid is thought to have crystallized fairly close to the bottom of the lunar plagioclase crust.This final liquid was rich with potassium, a family of elements called the rare earth elements, and phosphorus. Lunar researchers in the 1970s named this material KREEP, after the scientific symbol of potassium (K), REE for the rare earth elements, and P for phosphorus. After KREEP crystallized, the Moon was mostly solid, with only a deep, partly melted area forming later as the interior of the moon began to heat from radioactivity.

The idea of a magma ocean resulting from the heat of accretion and core formation for many of the terrestrial planets is controversial, as it is for the Earth, but there is very good evidence for a magma ocean having occurred on the Moon. The existence of KREEP indicates that some large amount of magma was crystallized until only the dregs were left; there seems to be no other reasonable way to make KREEP.

An element called europium provides further evidence. In the very low oxygen environment of the Moon, europium loses an electron and thus becomes electrically compatible with the crystal structure of the mineral plagioclase. Europium does not fit into olivine, orthopyroxene, or clinopyroxene crystals, even when it has lost an electron. When olivine and the pyroxene minerals crystallized out of the magma ocean, they left the europium behind, making the magma ocean more concentrated in this element. The samples returned from the Apollo missions show that the plagioclase in the highlands is very much enriched in europium compared to elements on either side of it in the periodic table; this is called the positive europium anomaly.The dark lavas that fill the craters on the face of the Moon (these are called mare—pronounced "mah-rey"—basalts, after the Latin word mare, meaning sea) have a conspicuous lack of europium: The corresponding negative europium anomaly.The mare basalts melted from material deep in the Moon, europium-poor olivine and pyroxene that crystallized from the magma ocean before plagioclase did. Apparently these two materials, the highlands plagioclase and the deep mantle that melted to make the basalts, are two halves of the same original reservoir of material. A magma ocean, crystallizing and differentiating, is the most effective way to create these separate reservoirs.

Thus, many scientists think that the Moon began as a homogeneous mass: Any part of it contained the same mixture of elements as any other part of it. Through the gradual crystallization of the magma ocean and the settling or rising of cumulate minerals, the Moon began the process of differentiation. In this case, crystallization, settling, and floating are the processes that differentiated the early Moon. Later, when the interior of the Moon remelted to form the basalts that erupted into the giant craters on the near side of the Moon, another step in the differentiation of the Moon occurred.

Was this article helpful?

0 0

Post a comment