Long ago, when our Solar System was still forming, our lovely lunar companion was much closer to Earth than it is now. Today, Earth’s Moon is the largest object suspended in our sky after sunset, and for eons it has captivated humans with its reflected silver-golden glow and myriad mysteries–singing a strange, beautiful and bewitching sirens’ song to those who stare in wonder at the sky above our planet when all is quiet in the stillness of a star-splattered night. Our Moon has, since very ancient times, been the source of wild tales, poetry, and myths–an eternal symbol for romantic love and for that which is feminine. But our nearest and dearest companion in space has still managed to keep some secrets to itself. In February 2018, astronomers at the University of Colorado in Boulder announced that they have unveiled one such well-kept secret–they have discovered new and fascinating evidence that our Moon’s excessive equatorial bulge is a feature that solidified in place over four billion years ago as our lunar companion gradually distanced itself from Earth.
The new research, conducted by the team of University of Colorado astronomers, sets parameters on how rapidly Earth’s Moon could have receded from our planet. The study suggests that the nascent Earth’s hydrosphere was either non-existent, or still frozen, at that ancient time when our 4.56 billion- year-old Solar System was still experiencing the growing pains of youth. This finding indirectly supports the theory that a weaker, fainter Sun at that time radiated approximately 30 percent less energy than it does today.
Our Moon currently recedes from the Earth at a rate of about 4 centimeters per year according to lunar laser ranging observations derived from NASA’s Apollo missions. This recession is thought to be the result of gravitational or tidal interaction between our Earth and its Moon. This is the same process that causes our planet’s rotation to slow down and the length of a day to increase. However, the rate at which the primordial Moon distanced itself from Earth remains largely unconstrained.
“The Moon’s fossil bulge may contain secrets of Earth’s early evolution that were not recorded anywhere else. Our model captures two time-dependent processes and this is the first time that anyone has been able to put timescale constraints on early lunar recession,” commented Dr. Shijie Zhong in a February 6, 2018 University of Colorado Press Release. Dr. Zhong is a professor in the University of Colorado’s Department of physics and the co-lead author of the new study.
The Lunatic, The Lover, And The Poet
More than 100 moons do their mesmerizing dance around the eight major planets in our Solar System. Most of them are icy, relatively small worlds, that contain only a scant quantity of rocky material. These frozen moon-worlds are mostly found orbiting the four giant, gaseous planets that inhabit the outer region of our Sun’s family. The gigantic quartet of outer planets–Jupiter, Saturn, Uranus, and Neptune–are blanketed with gas, and circled by a multitude of sparkling moons and moonlets. Of the four relatively tiny, solid planetary denizens of the inner Solar System–Mercury, Venus, our Earth, and Mars–Mercury and Venus are barren of moons, and Mars is orbited by two very small, somewhat deformed moons named Phobos and Deimos. Many planetary scientists propose that the duo of potato-shaped Martian moons are really asteroids that escaped long ago from the Main Asteroid Belt located between Mars and Jupiter–only to be captured by the Red Planet’s gravitational pull when our Solar System was young. Earth’s own bewitching Moon is the largest of its kind inhabiting this inner region close to the light and heat of our Star.
A moon is a natural satellite in orbit around another body that, in turn, is orbiting its star. The moon is kept in its position both by its own gravity, as well as that of its host. Some planets have moons; some do not. Several asteroids are known to be circled by very small moons, and some dwarf planets–such as Pluto–also have moons. Charon, Pluto’s largest moon, is approximately half the size of Pluto itself, and is thought to possibly be a large chunk of Pluto that was torn off in a monumental collision with another rampaging object long ago. Because Charon is 50% the size of Pluto, the two bodies are sometimes considered to be a double planet.
Earth’s Moon holds myriad fascinations for observers. Indeed, it has tickled the imaginations of members our species for thousands of years, whispering tantalizing hints of a strange and exotic Wonderland well-hidden in the mysterious night sky above our planet. Long ago, our ancient ancestors spun imaginative yarns, wove colorful myths, and wrote beautiful poetry about Earth’s nearest companion. Indeed, there is a 5,000-year-old rock carving found at Knowth, Ireland, that may show our Moon. If so, this would be the earliest depiction of Earth’s lunar companion yet discovered.
The dramatic contrast between our Moon’s bright highlands and darker maria (Latin for seas) paints patterns on the lunar surface that have been variously interpreted by different cultures. The most familiar of these imaginative interpretations is probably the Man in the Moon. However, some cultures have seen a buffalo or a rabbit etched on the lunar surface. In many prehistoric and ancient cultures, Earth’s Moon became personified as a god or goddess–or other supernatural entity. Astrological views of the lunar surface continue to be propagated even today.
In Proto-Indo-European religion, Earth’s Moon was personified as a male god. The ancient Sumerians imagined that our Moon was a god named Nanna, who was the father of Inanna, the goddess represented as the beautiful morning and evening star that we now recognize as the planet Venus. Nanna came to be known as Sin, and was particularly associated with sorcery and magic. In Greco-Roman mythology, the Sun (Sol) and Moon (Selene) are represented as male and female, respectively. In ancient Greek art, Selene was represented wearing a crescent on her head, in an arrangement that suggested horns.
Earth’s Moon has long inspired creative writers–particularly poets. William Shakespeare wrote in A Midsummer Night’s Dream (Act 5, Scene 1) that The Lunatic, the lover, and the poet. Are of imagination all compact. On the other hand, according to Mark Twain, Everyone is a moon, and has a dark side which he never shows to anybody. The late Beattle John Lennon wrote in his lyrics of Instant Karma, that We all shine on… like the moon and the stars and the sun… we all shine on… come on and on and on…
Our Moon is the only body other than Earth that we have walked upon, leaving our footprints in the lunar dust–a silent testimony in space that we once existed, on our little blue planet, and were curious and adventurous enough to visit worlds beyond our own.
But, where did our Moon come from? Several theories have been proposed that attempt to explain how our lunar companion came to be. One theory proposes that our Moon was once a part of Earth, and that it budded off when our Solar System was very young, about 4.5 billion years ago. According to this scenario, the Pacific Ocean basin is the favored cradle for where Moon-birth occurred. A second theory suggests that our planet and its Moon were born at the same time out of the original protoplanetary accretion disk, made up of dust and gas, from which our Sun and its retinue of planets, moons, and smaller objects formed. The third model, proposes that Earth’s Moon was really born elsewhere in our Solar System, and was ultimately snared by our planet’s gravitational pull when it wandered too close. The fourth theory proposes that interactions between Earth-orbiting planetesimals in our primeval Solar System caused them to fragment. According to this model, Earth’s lunar companion eventually coalesced out of the pulverized debris of the ill-fated planetesimals. Planetesimals were the building blocks of planets in our ancient Solar System. These objects bumped into one another and then merged in the crowded environment of the primordial accretion disk. The asteroids and comets are relics of this abundant population of objects.
However, it is generally thought that the Giant Impact Hypothesis is the most probable explanation for lunar formation. According to this scenario, when a doomed Mars-sized world named Theia crashed into our ancient planet billions of years ago, the enormous and catastrophic blast hurled a myriad of screaming little moonlets into the sky above Earth. Some of this material eventually went into orbit around our planet about 4.5 billion years ago–where it finally was pulled together by gravity to evolve into our Moon.
The Moon Is Distancing Itself From Earth
Our Moon’s rate of rotation causes it to be flattened at the poles, as well as bulge at its equator. About two centuries ago, the French mathematician and physicist Pierre-Simon Laplace calculated that our Moon’s equatorial bulge was too large (by a whopping twenty times) for its one-revolution-per-month rate of rotation. Scientists have since postulated that Earth’s Moon was born hot, and that it rotated very fast after its birth. It also possessed an equatorial bulge that was much greater in size than it is now. This is because the bulge had a tendency to shrink in adjustment to a reduced rotational force that developed as our Moon traveled farther from Earth. This reduced rate of rotation lasted until the Moon cooled and stiffened sufficiently to possess a permanent solidified bulge in its crust. This process created the feature that has come to be known as the fossil bulge.
However, both the necessary conditions and the timing of the fossil bulge formation remained unknown. This is because no physical models were formulated for this process. In order to shed some light on this unknown, Dr. Zhong and his colleagues determined that the process was not a sudden occurrence at all, but was actually very slow. Indeed, the entire process went on for several hundred million years, as our Moon distanced itself from Earth, during what is called the Hadean period approximately 4 billion years ago. But for that to have been the case, Earth’s energy dissipation in response to tidal forces–which for the present-day Earth is primarily controlled by the oceans–would have needed to have been substantially reduced at the time. The Hadean is a geologic eon of the Earth that began with the fiery formation of our planet 4.6 billion years ago, and ended 4 billion years ago.
“Earth’s hydrosphere, if it even existed at the Hadean time, may have been frozen all the way down, which would have all but eliminated tidal dissipation or friction,” Dr. Zhong explained in the February 6, 2018 University of Colorado Press Release. He added that a fainter, weaker young Sun could have rendered such conditions possible, at least in theory.
Indeed, what is termed the “snowball Earth” hypothesis was already suggested for the Neoproterozoic period, about 600 million years ago, based on the geological record. Similar theories were also proposed for the ancient Earth based on the idea of a fainter young Sun. However, direct observational evidence in the geological record is lacking, making it the subject of considerable debate among scientists. The Neoproterozoic period is a unit of geologic time on Earth from 1,000 to 541 million years ago.
Dr. Zhong and his colleagues plan to continue optimizing their model and will try to fill in other knowledge gaps about the Moon and Earth’s primeval era between 3.8 and 4.5 billion years ago.
The study is published in Geophysical Research Letters, a journal of the American Geophysical Union. Co-authors of the study include Dr. Chuan Qin (Harvard University) and Dr. Roger Phillips of Washington University in St. Louis.