Luna, the Moon, is the only natural satellite of our planet. It is the 5th largest satellite in our Solar System and the largest satellite relative to the size of the planetary body it orbits with 27% the diameter and 60% the density of Earth. But how was such a massive orbiting body formed?
The Birth of the Solar System
Our Solar System was born from the death of a giant rotating cloud of gas and dust known as the Pre-solar nebula. The solar nebula collapsed due to the gravitational attraction of its own particles to one another, creating an immense orbiting disk of matter formed around a central mass.
The central mass, a proto-Star, consisted of most of the material in the early Solar System. It became increasingly hot and dense, eventually igniting thermonuclear fusion in its core, forming our star (a yellow dwarf), called Sol.
It is hypothesized that in its earliest years the Solar System was populated by a vast number of planetesimals which formed in a process known as accretion, whereby each planetesimal began as dust grains in orbit around the central protostar.
The solar nebula disk separated into layers based on the density of materials which accounts for the various compositions of the planets we know today. Gas giants formed towards the outer edges of the Solar System, while the inner planets were created from denser materials.
The Collision of Young Earth and Theia
Some 7 billion years ago, the first Earth was created as material orbiting Sol coalesced over time. The ancestor planet grew as more material was pulled in by its gravity, but it was still smaller than the Earth we know today; roughly 90% of its current size.
Around 4.6 billion years ago, a celestial body the size of Mars, called Theia, collided with the proto-Earth, tearing away large parts of early Earth’s surface, reducing them to steam from heat released during the impact.
Newly made gases material from the outer layers of both bodies was thrown into space, forming a vast cloud which orbited the young Earth. Gravity eventually caused the cloud to flatten out in its orbit around our planet, forming a ring. The ring of material was absorbed by the earth and the remainder which lay outside the Earth’s gravitational pull gradually coalesced to form the Moon.
The moon was later shaped by a number of impacts with various debris around the Solar System. These formed the various craters and geographic features that we recognize on the Moon’s surface today.
The Giant Impact Hypothesis
The giant impact hypothesis, was first proposed in its modern form at a conference in 1975, and is the currently favored scientific hypothesis for the formation of the Moon. This theory explains many features of the Moon, such as the lack of volatile substances in its rocks (due to the intense heat generated by the impact) and its lack of an iron core (since it formed from rocky surface material).
Supporting evidence includes:
- The identical direction of the Earth’s spin and the Moon’s orbit
- Moon samples that indicate the surface of the Moon was once molten
- The Moon’s relatively small iron core, lower density compared to the Earth
- Evidence of similar collisions in other star systems (that result in debris disks), and that giant collisions are consistent with the leading theories of the formation of the Solar System.
Washington University in St. Louis planetary scientist Frédéric Moynier has discovered evidence to support the Giant Impact Theory: His team’s analysis of lunar rocks (brought back by the Apollo missions) shows they have heavier forms of zinc —a telltale sign of the impact billions of years ago.4
Researchers analysed 20 samples of Moon rocks including ones from the Apollo 11, 12, 15 and 17 missions and one lunar meteorite. Compared to terrestrial or Martian rocks, the lunar rocks have much lower concentrations of zinc but are enriched in the heavy isotopes of zinc. These heavier zinc atoms probably condensed out of the roiling cloud of vaporized rock created by a catastrophic collision quicker than other, lighter zinc atoms, and the remaining vapor escaped before it could condense, later coalescing to form the Moon.
Scientists have been looking for this kind of sorting by mass, called isotopic fractionation, since the Apollo missions first brought Moon rocks to Earth in the 1970s, and planetary scientist Frédéric Moynier and his team are the first to find it.
Our planet exists in a kind of planetary symbiosis with the Moon, and without it, life on Earth may never have existed.
The Moon’s gravitational pull caused massive tides to ebb and flow every few hours and was originally much stronger because it orbited much closer to us than it does now. The tides resulted in a dramatic fluctuations in salinity around coastlines, which is believed to have driven the evolution of primitive DNA-like biomolecules, which in turn evolved over millennia into more advanced lifeforms, including humans.
1. Space.com, Proof of Moon’s Birth in Giant Impact Found in Zinc
2. BBC Earth, The Moon may have been formed by a mighty crash
3. Science Daily, Moon Was Created in Giant Smashup; Vaporization of Impactor Left Signature in Tiny Excess of Heavier Form of Zinc
4. The Indian Express, Fresh evidence that Moon was created in a giant smashup