The Moon

To this day, the Moon remains the only celestial body that has been directly studied by humans, in addition to numerous spacecraft and landers. At present, it is the only body besides Earth where seismic activity and heat flow measurements have been conducted on the surface, thus significantly expanding the knowledge about the interior of Earth’s satellite. Between 1969 and 1972, 12 astronauts visited the Moon as part of the US Apollo missions and returned approximately 382 kilograms of samples of various rocks to Earth. The Moon samples are mostly between three and over four billion years old. Their very closely studied chemical and mineralogical composition allows researchers to look back into the early days of the Solar System and the evolution of the Earth-Moon system. They are important for gaining a deeper understanding of the formation of the Solar System, particularly the four Earth-like planets and the large asteroids.

At the same time, lunar research lays the foundations for a better understanding of the young Earth and its evolution. The Moon may have played an important role in the evolution of life on Earth, as its gravitational force has been stabilising Earth’s axis for at least three and perhaps more than four billion years. In addition, the tides caused by the Moon ensure that along the coastlines, dynamic water movement frees essential minerals from rocks, thus enriching ocean waters.

The Moon orbits Earth in 27 days, 7 hours and 43.7 minutes relative to the fixed stars, in the same direction of rotation as that of the Earth moving around the Sun. The Moon takes almost exactly as long to rotate once about its own axis. This is referred to as synchronous rotation and is a consequence of the gravitational effect of Earth on the Moon, resulting in ‘tidal locking’. Synchronous rotation means that the same side of the Moon always faces Earth. This is why it is referred to as the near side of the Moon. The far side of the Moon can never be observed from Earth. It was first imaged by the Soviet spacecraft Luna 3 in 1959. Nevertheless, 59 percent of the Moon’s surface can be seen from Earth due to librations (small, apparent movements of the Moon as it travels on its slightly-elliptical orbit around Earth).

Earth’s moon is the smallest of the ‘terrestrial’ bodies in the inner Solar System (in addition to Mercury, Venus, Earth and Mars). In terms of comparative planetology, the Moon is considered to be an Earth-like planet due to its size and composition. With a diameter of approximately 3476 kilometres, it has a surface area of almost 38 million square kilometres, just under a quarter of the area of all of Earth’s continents. Due to its relatively small size, the Moon does not have sufficient mass (only one eightieth of that of Earth) to retain an atmosphere. It is surrounded by an extremely thin ‘exosphere’, which consists of atoms and ions of volatile elements with an overall mass estimated at just 10 tonnes; this is very close to a perfect vacuum. This exosphere contains sodium and potassium atoms that the solar wind has driven from the regolith, dust from the lunar surface, together with helium (4He), which forms part of the solar wind. Isotopes of argon (40Ar), radon (222Ra) and polonium (210Po) have also been detected. Polonium is formed during radioactive decay in the Moon’s crust and mantle; it is then released into the exosphere in gaseous form.

Two distinctly different areas are apparent when looking at the Moon – the pale highlands, which cover over 80 percent of the surface, and the dark lunar maria, plains that make up almost 20 percent. The highlands consist of feldspars that are rich in calcium and aluminium, a mineral group that is also widespread on Earth. The plains are made of dark volcanic rock. The highlands are also pockmarked with significantly more impact craters, allowing scientists to infer that these surfaces are generally older, dating back more than 3.8 billion years.

The mineralogical and geochemical composition of the Moon’s entire crust can be determined by means of spectral measurements performed from lunar orbit and using Earth-based telescopes. The lunar maria are of volcanic origin and are found mainly on the near side of the Moon. They are younger than the highlands and mainly fill the large circular basins caused by asteroid impacts. There are more large impact basins filled with basaltic lava on the near side of the Moon than on the far side due to the fact that the Moon’s crust is significantly thicker on the far side of the Moon, making the rise and exit of magma from the Moon’s mantle more difficult.

It is thought that Earth’s satellite was formed 4.5 billion years ago when a planetary body approximately the size of Mars collided with Earth, which at that time was just 50 million years old but already differentiated into crust, mantle and core. As a result, a large part of Earth’s mantle melted and vaporised, and was ejected into space. This material recondensed and collected in a ring around Earth’s equator. Through accretion (the gravitation-related accumulation of material), the particles in this disc of dust and rock particles came together to form the Moon, which grew to its current size over the course of several million years.

The geological formation of the Moon was essentially complete at a relatively early stage. As the young Moon was continually bombarded with asteroids and comets, and due to the decay of heat-producing radioactive elements in its interior, a vast global ocean of magma several hundred kilometres thick formed during this early stage of the Moon’s development. Once the bombardment and radioactive decay declined, the cooling of this magma ocean led to the crystallisation of a number of rock-forming minerals. At first, metallic iron sank into the depths, forming a small core measuring only a few hundred kilometres across. After that, heavy magnesium- and iron-rich minerals crystallised, again sinking and forming a mantle. Finally, a crust formed from light, calcium- and aluminium-rich silicates. Due to their low density, these anorthositic feldspars drifted to the surface in this now almost completely solidified magma ocean and formed the primary crust, the lunar highlands. Elements such as potassium, uranium, thorium, phosphorus and some rare earths that struggled to work their way into the crystal lattices of rock-forming minerals gathered at the base of the feldspar-rich crust. A partial remelting of the mantle ultimately led to the rise of iron- and magnesium-rich silicate magma, which now cover the lunar maria in the form of basalt rock.

Overall, the Moon is a fully differentiated and an only slightly more primitive body than the planets of the inner Solar System since, like these, it has a core, mantle and crust. It is not yet clear why the crust on the far side of the Moon is much thicker than the one on the near side. Over the period around 4.3 to 3.8 billion years ago, frequent and extremely violent meteorite and asteroid impacts formed the face of the Moon. The biggest impacts penetrated so deep into the crust that they facilitated the rise of basaltic lava, particularly on the near side of the Moon, allowing it to escape on the surface. Over several hundred million years, the enormous impact basins filled with darker basalt. Observers on Earth suspected that these dark areas were seas filled with water, explaining why the basalt-filled areas of the impact basins are known to this day as ‘seas’ (Latin ‘Mare’), such as the Mare Imbrium, the Mare Serenitatis or the Oceanus Procellarum.

Volcanic development had more or less completed approximately three billion years ago, with magma reaching the Moon’s surface only sporadically up to between one and one-and-a-half billion years ago. Since then, the Moon has been an almost inactive body in geological terms, at least on the surface, but was unable to bind an atmosphere made of volatile gas molecules to it due to its low gravitation attraction. It does not have any large amounts of water. However, data from the Lunar Prospector probe, which orbited the Moon from 1997 to 1999, reveal that there is a not inconsiderable amount of ice in the lunar soil in the deep, permanently-shadowed craters at the north and south poles. A detailed study of these possible instances of water ice has also been the objective of several international lunar missions in recent years.

Spectrometers on board the Indian mission Chandrayaan-1 and NASA’s Lunar Reconnaissance Orbiter (LRO) and Lunar Crater Observation and Sensing Satellite (LCROSS) were not only able to confirm the presence of this ice in the deep polar craters, but they also found ‘water’ distributed across the entire the Moon as hydroxyl (OH) ions in minerals and the regolith, albeit at very low concentrations.

In recent times, the Moon has again become one of the most important targets of planetary exploration. After a long hiatus in lunar research, several orbiter missions have been conducted over the last 20 years. Emerging Asian space nations are taking part in lunar research alongside the USA. Important scientific findings have been yielded by the Japanese SELENE / Kaguya mission (2007 to 2009), the US impact experiment LCROSS (2009), and four Chinese craft from the Chang’e series (with an initial landing in December 2013 and a second landing on the far side of the Moon in January 2019). In addition, the Moon has been studied by the Indian orbiter Chandrayaan-1 (2008 to 2009), the dual satellites of NASA’s GRAIL mission to survey the gravitational field (2011 to 2012), and, most importantly, the US Lunar Reconnaissance Orbiter, which has been in a low polar orbit since June 2009 and is imaging the Moon at high resolution and surveying its topography.