The Moon

The lunar interior

Structure and composition

Most of the knowledge about the lunar interior has come from the Apollo missions and from robotic spacecraft, including Galileo, Clementine, and Lunar Prospector, which observed the Moon in the 1990s. Combining all available data, scientists have created a picture of the Moon (see ) as a layered body comprising a low-density crust, which ranges from 60 to 100 km (40 to 60 miles) in thickness, overlying a denser mantle, which constitutes the great majority of the Moon's volume. At the centre there probably is a small iron-rich metallic core with a radius of about 350 km (250 miles) at most. At one time, shortly after the Moon's formation, the core had an electromagnetic dynamo like that of Earth (see geomagnetic field), which accounts for the remanent magnetism observed in some lunar rocks, but it appears that such internal activity has long ceased on the Moon.

Despite these gains in knowledge, important uncertainties remain. For example, there seems to be no generally accepted explanation for the evidence that the crust is asymmetrical: thicker on the Moon's far side, with the maria predominantly on the near side. Examination of naturally excavated samples from large impact basins may help to resolve this and other questions in lunar history.

 Internal activity of the past and present

The idea that the lunar crust is the product of differentiation in an ancient magma ocean is supported to some extent by compositional data, which show that lightweight rocks, containing such minerals as plagioclase, rose while denser materials, such as pyroxene and olivine, sank to become the source regions for the later radioactive heating episode that resulted in the outflows of mare basalts. Whether or not there ever was a uniform global ocean of molten rock, it is clear that the Moon's history is one of much heating and melting in a complex series of events that would have driven off volatiles (if any were present) and erased the record of earlier mineral compositions.

At present all evidence points to the Moon as a body in which, given its small size, all heat-driven internal processes have run down. Its heat flow near the surface, as measured at two sites by Apollo instruments, appears to be less than half that of Earth. Seismic activity is probably far less than that of Earth, though this conclusion needs to be verified by longer-running observations than Apollo provided. Many of the moonquakes detected seem to be only small “creaks” during the Moon's continual adjustment to gravity gradients in its eccentric orbit, while others are due to meteorite impacts or thermal effects. Quakes of truly tectonic origin seem to be uncommon. The small quakes that do occur demonstrate distinct differences from Earth in the way seismic waves are transmitted, both in the regolith and in deeper layers. The seismic data suggest that impacts have fragmented and mixed the upper part of the lunar crust in a manner that left a high proportion of void space. At depths beyond tens of kilometres, the crust behaves as consolidated dry rock.

Origin and evolution

With the rise of scientific inquiry in the Renaissance, investigators attempted to fit theories on the origin of the Moon to the available information, and the question of the Moon's formation became a part of the attempt to explain the observed properties of the solar system (see Solar system: Origin of the solar system). At first the approach was largely founded on a mathematical examination of the dynamics of the Earth-Moon system. Rigorous analysis of careful observations over a period of more than 200 years gradually revealed that, because of tidal effects (see tide), the rotations of both the Moon and Earth are slowing and the Moon is receding from Earth. Studies then turned back to consider the state of the system when the Moon was closer to Earth. Throughout the 17th, 18th, and 19th centuries, investigators examined different theories on lunar origin in an attempt to find one that would agree with the observations.

Lunar origin theories can be divided into three main categories: coaccretion, fission, and capture. Coaccretion suggests that the Moon and Earth were formed together from a primordial cloud of gas and dust. This scenario, however, cannot explain the large angular momentum of the present system. In fission theories a fluid proto-Earth began rotating so rapidly that it flung off a mass of material that formed the Moon. Although persuasive, the theory eventually failed when examined in detail; scientists could not find a combination of properties for a spinning proto-Earth that would eject the right kind of proto-Moon. According to capture theories, the Moon formed elsewhere in the solar system and was later trapped by the strong gravitational field of Earth. This scenario remained popular for a long time, even though the circumstances needed in celestial mechanics to brake a passing Moon into just the right orbit always seemed unlikely.

By the mid-20th century, scientists had imposed additional requirements for a viable lunar-origin theory. Of great importance is the observation that the Moon is much less dense than Earth, and the only likely reason is that the Moon contains significantly less iron. Such a large chemical difference argued against a common origin for the two bodies. Independent-origin theories, however, had their own problems. The question remained unresolved even after the scientifically productive Apollo missions, and it was only in the early 1980s that a model emerged—the giant-impact hypothesis—that eventually gained the support of most lunar scientists.

In this scenario the proto-Earth, shortly after its formation from the solar nebula about 4.6 billion years ago, was struck a glancing blow by a body the size of Mars. Prior to the impact, both bodies already had undergone differentiation into core and mantle. The titanic collision ejected a cloud of fragments, which aggregated into a full or partial ring around Earth and then coalesced into a proto-Moon. The ejected matter consisted mainly of mantle material from the colliding body and the proto-Earth, and it experienced enormous heating from the collision. As a result, the proto-Moon that formed was highly depleted in volatiles and relatively depleted in iron (and thus also in siderophiles). Computer modeling of the collision shows that, given the right initial conditions, an orbiting cloud of debris as massive as the Moon could indeed have formed.

Once a proto-Moon was present in the debris cloud, it would have quickly swept up the remaining fragments in a tremendous bombardment. Then, over a period of 100 million years or so, the rate of impacting bodies diminished, although there still occurred occasional collisions with large objects. Perhaps this was the time of the putative magma ocean and the differentiation of the ancient plagioclase-rich crust. After the Moon had cooled and solidified enough to preserve impact scars, it began to retain the huge signatures of basin-forming collisions with asteroid-sized bodies left over from the formation of the solar system. About 3.9 billion years ago, one of these formed the great Imbrium Basin, or Mare Imbrium, and its mountain ramparts. During some period over the next several hundred million years there occurred the long sequence of volcanic events that filled the near-side basins with mare lavas.

In an effort to unravel the history of this period, scientists have applied modern analytic techniques to lunar rock samples. The mare basalts show a wide range of chemical and mineral compositions reflecting different conditions in the deep regions of the mantle where, presumably because of heating from radioactive elements in the rock, primordial lunar materials were partly remelted and fractionated so that the lavas carried unique trace-element signatures up to the surface. By studying the past events and processes reflected in the mineral, chemical, and isotopic properties of these rocks, lunar scientists have slowly built a picture of a variegated Moon. Their findings have provided valuable background information for Earth- and spacecraft-based efforts to map how the content of important materials varies over the lunar surface.

Once the huge mare lava outflows had diminished, apparently the Moon's heat source had run down. The last few billion years of its history have been calm and essentially geologically inactive except for the continuing rain of impacts, which is also declining over time, and the microscopic weathering due to bombardment by solar and cosmic radiation and particles.