Lunar interior

Using fundamental knowledge of Earth’s interior along with data returned by missions to the Moon, scientists have been able to extrapolate theories about the composition of the lunar interior.

Overview

The contrasting light-colored highlands and darker maria of the Moon were very apparent to even the earliest observers. Early telescopic observations revealed the highlands to be rough, cratered, and mountainous, as compared to the smoother and less cratered maria. Galileo’s first impression of the lunar surface clearly drew a comparison to the land and sea regions of Earth. Later scientists, with only telescopic observations to guide them, assumed that Earth and its Moon probably had similar origins and should be quite alike in most respects. This assumption did not last long.

When scientists made their first calculations of the densities of the Earth and the Moon, they discovered an interesting fact. Earth has a density of 5.5 grams per cubic centimeter (g/cm³), compared to the Moon's density of 3.34 g/cm³. This seemed strange for two bodies found so close to each other in space. Later theories attributed the difference in densities to Earth’s having a much higher percentage of metal in its overall chemical composition. With the higher metal content, Earth would naturally have a greater mass and a higher density. Two distinctly different sets of characteristics would soon define each object, for it is mass that determines a planet’s internal pressure and temperature conditions. These two factors, in turn, determine whether a metal or silicate mineral will remain a solid or become a molten liquid. Once a molten liquid is produced, the process of density separation can take place and produce a distinct core.

Based on a density comparison between Earth and the Moon, it is believed that the Moon does not have a well-defined core like that of Earth. Experiments conducted on the Apollo 15 and Apollo 17 missions did indicate that the Moon’s heat-flow rate is about half that of Earth. Although the lunar interior is relatively hot, it is not sufficiently hot to produce a density separation of materials comparable to that which occurred on Earth. The higher iron content of the lunar crust also tends to support a planetary body that is not as well-differentiated as Earth.

The current model for the nature and chemical composition of the Moon’s interior has been primarily derived from lunar rock samples and the seismic experiments left on the Moon by the Apollo astronauts. This information, combined with data from both manned and unmanned orbital missions, has given scientists a much clearer idea of what constitutes the lunar interior. Originally, based on the relatively well-established models developed for Earth, it was assumed that the Moon’s interior should have experienced a similar history. As Apollo data started to pour in, it quickly became apparent that this was not going to be the case.

The interior of Earth is divided into a crust, mantle, and core. This division is based on the determination of the densities of various rocks and metals at specific depths. It does not represent the primordial Earth. One theory of the origin of Earth suggests that the planet formed as a result of the accretion of innumerable cold, solid bits of rock and metal into a relatively cold, chemically homogeneous body. Shortly after Earth’s formation, its internal temperature began to rise due to the decay of radioactive isotopes, the heat from accretion, and the mass of Earth itself. At this point, its interior temperature was well below the melting point of most metals and silicate minerals. Gradually, temperatures reached a point where melting occurred, producing a molten liquid. Within this molten magma, denser metals sank to the center of mass, forming the solid inner and liquid outer metallic cores. As a result, lower-density silicate minerals were displaced and moved upward to form the crust.

Even within Earth’s crust itself, rock materials of two distinctly different densities are believed to have separated into the lower-density continental crust and the higher-density oceanic crust. Sandwiched between the crust and core is the mantle, a large region of silicate minerals with variable densities appropriate to specific depths. This entire process is referred to as density separation, and it is believed to have taken place very early during the initial stages of Earth’s formation. Through this separation of molten metals and nonmetals, the generation of an electric current and magnetic field became possible.

If this process occurred on Earth, then could a similar process have taken place on the Moon? Perhaps the best indicator of the composition and structure of the lunar interior came from the Apollo Lunar Science Experiment Packages (ALSEPs), which included a seismometer to record “moonquakes.” On Earth, scientists use seismic waves to calculate the density and predict the composition of materials at various depths. The way seismic waves pass through various materials gives clues to their chemical and physical properties. The same should be true for the Moon and thus provide a detailed picture of the lunar interior. Apollo results indicate that the internal structure of the Moon is very different from that of Earth. Both the lunar crust and the mantle are much thicker relative to Earth’s and show no evidence of plate tectonics. Seismic evidence does indicate that the upper portion of the lunar crust has been shattered by countless meteoroid impacts; the crust is thought to be approximately thirty-four to forty-three kilometers thick, gradually progressing into a solid rock layer termed the lithosphere. Beneath approximately 1,000 kilometers (620 miles) of lithosphere lies the asthenosphere, a region where seismic waves have indicated the presence of a liquid or partially liquid environment, which may include the core. The core, if it truly exists, may consist of an iron sulfide mixture rather than a “pure” nickel-iron alloy. The presence of a significant amount of iron sulfide minerals could lower the melting temperature required to produce a liquid phase, thus enabling core formation at lower temperatures. The specific chemical composition of the lunar interior, along with the existing temperature and pressure conditions, are the defining factors as to whether or not the Moon has a core similar to Earth’s. Existing data suggest that the Moon does not have sufficient internal heat to produce a distinct metallic core. The fact that iron-bearing lunar materials are not magnetic also points to the weakness of the Moon's magnetic field, which is usually attributed to the presence of a liquid-solid metallic core.

Knowledge Gained

Beginning in the early 1960s, when US President John F. Kennedy challenged the American people to go to the Moon, the National Aeronautics and Space Administration (NASA) developed a series of lunar exploration programs: the Ranger, Lunar Orbiter, Surveyor, and Apollo programs achieved that goal. Ranger provided the first close-up look at the Moon. Lunar Orbiter provided the reconnaissance images to select the landing sites for Surveyor and later, the Apollo spacecrafts. In addition to proving that a lunar soft landing was possible, Surveyor gave scientists the first look at the lunar soil and surface conditions. The six Apollo missions not only returned more than 380 kilograms of lunar rock and soil but also left experiments on the lunar surface to study the Moon’s interior. Later missions, such as Clementine and the Lunar Prospector missions, surveyed the lunar surface for mineral deposits and searched for the presence of water ice. The Russian Luna program also added to the overall understanding of the Moon through the use of a robotic rover and sample-return missions.

The basic geological principles geologists have learned on Earth have been applied to lunar features, yet many of the discoveries made on the Moon have caused geologists to rethink their original theories. In the context of a wealth of lunar data, scientists now see the Moon as a world seemingly similar to Earth yet markedly different. The early processes that created a distinct crust-mantle-core structure for Earth and produced a strong magnetic field never reached completion on the Moon. Earth was able to retain its high internal temperature and remain fluid at specific depths, while the Moon apparently did not and remains only a partially differentiated body. Further studies of the Moon are certainly needed before science can provide a definitive understanding of the Moon’s physical makeup and interior structure. Future lunar missions may answer many of the remaining questions concerning the lunar interior as well as give a better understanding of lunar surface materials and the giant impact processes that have shaped lunar history.

While lunar geologists eagerly await a return to the Moon, analysis of samples returned by the Apollo astronauts continued decades after the last lunar landing of Apollo 17 in December 1972. A rock returned on that mission after being collected by Dr. Harrison Schmitt, the only geologist to land on the Moon during the Apollo program, turned out to be the oldest sample collected that had not been subjected to intense shocks from major bombardments of the Moon occurring after the rock’s formation. This rock possessed a remnant magnetism dating back beyond 4.2 billion years, indicating that in its early history, the Moon had a liquid core that produced magnetism by a dynamo effect, as Earth still does. That lunar field appears to be about one-fiftieth that of Earth, a result that models of lunar core dynamics also predict. This finding adds evidence to the theory that the Moon did not form cold but out of a collision between a Mars-sized object and the early Earth; the material ejected out of the early Earth from this impact is thought to have re-accreted to form the Moon.

In 2017, researchers from Brown University published a new study revealing that the Moon's interior may contain more water than was previously thought. Trace amounts of water had previously been discovered in volcanic glass beads retrieved by the Apollo 15 and Apollo 17 missions, suggesting that water was present in the Moon's mantle and had been trapped in the beads as they formed. Using orbital satellite data, the researchers discovered that the Apollo samples were not unique; similar volcanic deposits were found to be widespread on the Moon's surface, nearly all of which emitted spectroscopic signatures indicating the presence of water. The amount of water present in the deposits suggests that some, if not all, of the lunar mantle may contain as much water as Earth's.

Context

As far back as 400 BCE, ancient Greek philosophers wondered about what lies beneath Earth’s surface. They envisioned a dark, hot, sulfurous underworld populated by demons and the spirits of the dead. This dismal picture was based on a certain amount of truth. The ancients were familiar with volcanoes and sulfurous hot springs and easily made the connection to the underworld.

Scientists now have the ability to study Earth’s interior by means of seismic studies, deep drill holes, and analyses of deep-seated rocks brought up during violent volcanic eruptions. With data like these, it is possible to develop computer models that give a very accurate picture of what lies deep beneath Earth’s crust. Certainly, future lunar missions will not only expand our knowledge of the Moon’s interior but also pave the way for a more comprehensive understanding of the terrestrial planets.

In 2019, NASA announced its Artemis program, a series of missions to orbit the Moon, land on its surface, and build permanent infrastructure. These missions and landings are scheduled to take place in the mid-2020s, though the program is not without challenges and detractors.

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