The unique Moon. How we happened to have the natural satellite

We are so used to the presence of the Moon in our night (and sometimes day) sky that we hardly think about how lucky we are to have such a “space neighbor”. The Earth and its satellite are a unique phenomenon in the Solar System, and the latest studies show that such combinations are not so common in the universe as a whole. How did this strange pair appear and what did scientists need to know to answer this question?

The first of the known satellites

The Moon is probably the only body in the Solar System that scientists correctly “put in place” at the early stages of the development of astronomy: our ancestors long ago realized that it revolves around the Earth. However, they were a little confused by the almost unchanged appearance of our satellite, so it was easy for them to imagine that it is “pinned” to a transparent crystal sphere (which, in fact, revolves around us). Also, in ancient times, it was noticed that the Moon, like the Sun and the planets known at that time, moves across the sky near the ecliptic, the plane of the Earth’s orbit. But the unusualness of this fact was realized much later, when they realized that not only the Earth has natural satellites. This revolutionary discovery was made by Galileo Galilei in 1610, when he managed to see the four largest moons of Jupiter through one of his first telescopes.

Ptolemy’s geocentric system (scheme from “Heavenly Atlas” by Johann Doppelmayer). The closest body orbiting the Earth is the Moon

Later, natural satellites of Saturn, Uranus and Neptune were discovered (and for the last two it happened almost simultaneously with the discovery of the planets themselves). In the end, this list was supplemented with Mars, when astronomers managed to discern its two tiny moons named Phobos and Deimos (in 1877). So, it turned out that of all the “non-lonely” large planets of the Solar System, the Earth is the closest to the Sun, and this is one of the manifestations of its uniqueness. Among other things we might mention the fact that our Moon rotates close to the plane of the ecliptic, and not the Earth’s equator. The vast majority of other large satellites really “gravitate” to the equatorial plane of their planets.

Back in the 2nd century AD the ancient Greek astronomer Ptolemy made fairly accurate estimates of the size of the Moon and the distance to it (of course, not in kilometers, but in terms of the Earth’s radius). Almost 1,700 years later, mankind managed to measure other large satellites of the planets, and it became clear that our Moon belongs to the seven largest of them, and in this “big seven” it is not even in the last place: it is inferior in size to Europa (the smallest of the four Galilean satellites of Jupiter) and the Neptunian satellite Triton. It was more difficult to determine the mass of these objects, and here scientists were in for a new surprise: it turned out that the Moon is only 81.3 times lighter than the Earth. This is the smallest ratio between the masses of a planet and its satellite in the Solar System, followed by Titan, which is 4,200 times lighter than Saturn.

The largest satellites of the planets of the solar system. Mercury and the dwarf planet Pluto are also listed for comparison

Finally, another feature of the Moon is its average density (3344 kg/m3), which is 60% of the density of the Earth. In all large satellites of gas giants, this indicator is greater than in the mother planets. Only the Martian satellites Phobos and Deimos turned out to be less dense than the body around which they revolve. We can name only one feature common to all large satellites: the equality of the periods of orbital and axial rotation (that is, all of them constantly face their planets with the same side).

Secrets of birth

The listed “lunar anomalies” required some kind of explanation, and the scientific community gradually began to lean toward the conclusion that they were based on the mechanism of the Moon’s origin. Back in the 18th century, the Swedish theologian Emanuel Swedenborg, the German philosopher Immanuel Kant, and the French mathematician Pierre-Simon Laplace formulated the so-called “nebular hypothesis” of the origin of the solar system, which has survived up to our time with some refinements and is considered basic. It explains the main physical features of the planets: they all revolve around the central luminary in the same direction, their orbits lie approximately in the same plane, and their orbital eccentricities are not significantly different from zero (the two smallest planets, Mars and Mercury, are the “record holders” here).

The essence of the hypothesis is that initially there was a gas-dust nebula in the place of the Solar System, composed mostly of hydrogen, helium and interstellar dust — small particles of ice, carbon compounds, as well as oxides of silicon, aluminum and heavier metals. Almost 99% percent of its matter at the first stage of evolution was “drawn” to its center under the influence of its own gravity, forming the Sun, and the rest was gradually drawn into orbital motion and formed a protoplanetary disk. It had its own local concentrations of mass — the “germs” of future planets. Around the most massive of them their own “vortices” formed, which gave rise to satellite systems. Scientists assumed that a similar disk existed on the site of the proto-Earth, and the Moon later arose as a result of consolidating of matter from its outer regions. Two important lunar characteristics contradicted this assumption: large mass and relatively low density.

After the main pool of the mass of the gas-dust cloud had concentrated to form the proto-Sun, its remnants formed a protoplanetary disk, where planets began to “grow” from individual knots of matter.

Then an alternative hypothesis appeared. It argued that the Moon formed further from the Sun than our planet (for example, in the Main Asteroid Belt, where the average density of some bodies is really close to that of the Moon), and then, due to a collision with another asteroid or gravitational disturbances, changed its orbit, came closer to the Earth and was captured by its gravity. The problem with this hypothesis was that the mechanism of such “capture” works well enough in the case of gas giants, but for less massive bodies it requires the presence of a large number of additional favorable circumstances, which is statistically very unlikely. However, this option was considered quite acceptable until the 60s of the last century — more precisely, up to the time when the first samples of the lunar substance brought by the astronauts of the Apollo missions appeared in the laboratories.

Here it is worth going back to the beginning of the 20th century, when the English chemist Frederick Soddy, who was engaged in the study of radioactive decay products, suggested the existence of varieties of atoms of the same chemical element with different atomic masses. Four years later, his guess was confirmed, and the term “isotopes” appeared in scientific circulation. The vast majority of elements in nature occur in the form of several isotopes in a certain ratio. Studying extraterrestrial matter that falls on the surface of our planet in the form of meteorites, scientists noticed that for them this ratio turned out to be different and depended on the area of ​​the Solar System where the “space visitor” was formed. Later, it was possible to discover the characteristic “isotopic signatures” of Mars, Mercury, the Asteroid Belt… Some bodies, although they had sure signs of having traveled in outer space, practically did not differ in composition from samples of terrestrial rocks. Some experts have suggested that these are “moon rocks”. The first expeditions to the Moon finally confirmed: yes, it has the same isotopic composition as the Earth. And therefore, it was formed in the same region of space and, most likely, almost at the same time.

According to modern ideas, the Moon arose as a result of the collision of the proto-Earth with a body about half its size, which was named “Theia”

However, there was also a completely exotic explanation to the peculiarities of the Moon’s chemical composition and its orbit. It could have formed as a result of Earth’s collision with another huge protoplanetary object, about ten times less massive and half the size. This must have happened after our planet underwent a partial gravitational differentiation of matter: heavier matter sank to the center and formed the core, while lighter ones concentrated on the surface. The collision disrupted part of these surface minerals, which first formed a ring around the Earth (some of them were scattered throughout the Solar System), and then the Moon was formed from them. The main objection to this assumption (the so-called impact hypothesis) was that after such a catastrophe, the shape of the Earth’s orbit would differ from the “right circle” much more than it does now. And, of course, if the impactor body arrived to us from another region of the solar system, geologists should have found its remains in the Earth’s crust — areas with unusual “isotope signatures”. But we don’t see anything like that.

Lagrange comes to the rescue

And now let’s return briefly to the 18th century. In 1772, the French mathematician of Italian origin Joseph-Louis Lagrange published an article with the analytical solution of the so-called three-body problem — that is, the possibility of calculating the relative position of three objects that influence each other gravitationally, at any moment in time (a similar problem for two bodies was successfully solved by Newton a hundred years earlier). The scientist came to the conclusion that solving such a problem by mathematical methods is possible only under a few very rigid restrictions: the first body of the system must be much more massive than the second one, which, in turn, must move in a circular orbit. Then there are five points in such a system where the third body—much lighter than the second—can remain virtually stationary relative to the first two for a long time.

Lagrange points of the Sun-Earth system (the sizes of celestial bodies and distances are not shown true to scale)

Three of these points are located on a straight line passing through the centers of the two more massive bodies, and they are unstable: if an object caught in them is subjected to some additional external force, it will leave its position and never return to it (if it is not returned there forcibly).

But the last two points — now they are customary denoted with the indices L4 and L5 — turned out to be more interesting. They are located in the orbit of the second mass body of the system at 60° in front and behind relative to its orbital motion — and they are the positions of stable equilibrium. Once there, an object of relatively low mass will remain there for a long time, and a sufficiently powerful external influence will be needed to knock it out of there. With regard to the Solar System, these calculations remained purely theoretical until 1906, when the first asteroids were found at points L4 and L5 in the orbit of Jupiter, which later received the name “Trojans” (they were assigned the names of the mythical heroes of the Trojan War). Now more than eight thousand such objects are known. Several dozen “Lagrangian asteroids” were discovered in the orbit of Neptune; four — in the orbit of Mars; Uranus, Earth and Venus have one each; only Mercury and Saturn are deprived of such “orbital companions”, but astronomers are sure that their discovery is only a matter of time.

In 1975, the American planetary scientist William Hartmann and astronomer Donald Davis suggested that in the early stages of the evolution of the Solar System, the density of dust in the protoplanetary disk was sufficient to form a kind of “gravitational trap” to accumulate enough dust particles to form another smaller planet-like body. It continued to “collect” the surrounding matter, and when its mass became about ten times less than the mass of the proto-Earth, the equilibrium of the system collapsed — a collision with our planet became almost inevitable. This collision was not “head-on”, but took place tangentially, spinning the Earth to a high speed (it still rotates around its axis faster than all the other planets of the terrestrial group). As a result our planet was deprived of a part of the light silicate crust which burst into space, and then our natural satellite was formed out of this debris.

The Moon against the Earth, photographed by the Deep Space Climate Observatory spacecraft operating at the L1 point of the Earth-Sun system. The ratio of the apparent sizes of the planet and its satellite practically corresponds to the ratio of their diameters.

This concept explained almost all the phenomena of the Earth-Moon pair, from the isotopic composition to the relatively small eccentricity of the Earth’s orbit and the lack of light elements on the Moon (they almost completely evaporated at the stage of its formation and escaped into outer space or to the more massive central body). The hypothesis became more and more popular in the scientific community. In 2000, Alexander Halliday suggested naming the object presumably formed at the Lagrange point after the titaness Theia (Θεία) from ancient Greek mythology, the daughter of the Earth goddess Gaia and the sky god Uranus, the mother of the moon goddess Selena.

In recent years, representatives of various sciences have found more and more evidence of the impact hypothesis (regardless of the origin of the body which collided the Earth). For example, seismic studies have shown that the Pacific Ocean basin, together with the volcanic “ring of fire” that surrounds it, has slightly different physical characteristics, and these differences get almost as deep as the edge of the Earth’s core. It seems that this depression is a huge “scar” left on Earth after a great cosmic clash.

A gift from heaven

No matter where the Moon came from, we must admit that we are very fortunate to have it. Due to its large mass, it creates powerful tides in the Earth’s body, which contribute to tectonic processes and maintaining the outer layers of the Earth’s core in a liquid state. This, in turn, helps our planet maintain a magnetic field that protects the atmosphere from being “blown away” by the solar wind. Lunar gravity stabilizes the position of the Earth’s axis, preventing our equator from deviating too much from the plane of the ecliptic, and this contributes to the stabilization of climatic conditions. And almost every year, our satellite puts on a magnificent celestial show — a total solar eclipse (although it can be observed only in a rather narrow band on the ground).

The Moon’s gravity stabilizes the position of the Earth’s axis, “not allowing” it to deviate too much from the perpendicular to the plane of the orbit

Such uniqueness of the Moon has led some scientists to express opinions about the uniqueness of the Earth itself, in particular as a place where living organisms were able to have originated and evolved. The point is that the probability of the appearance of an Earth-Moon pair in the zone where the amount of energy received from the central star is just sufficient for the existence of liquid water on the Earth’s surface is actually very small, and it is not a fact that there will be one more pair of such a kind. Well, if the presence of a large satellite is an critical condition for existence of life on a planet — it is quite possible that our efforts to find “brothers in mind” are in vain. One way or another, most of the scientific community and amateur searchers for extraterrestrial civilizations keep optimistic, and employees of observatories and ground control groups for space telescopes are inventing ways to register satellites of exoplanets. Only by discovering a sufficient number of such objects will we be able to find out, how unique our Moon really is.