Since ancient times, our bewitching, bewildering, and beautiful Moon has been the enchanting source of wild, weird, and wonderful myths and tales–as well as an inspiration for poetry, a fanciful source of madness, and a symbol for romantic love. Nevertheless, Earth’s Moon is a very real nearby world–the largest object in our sky at night, as well as our closest neighbor in space. But how did our large Moon come to be? Cloaked as our Moon is in its beguiling shroud of mystery, humanity has long attempted to understand how this lovely and fascinating object was born. In January 2017, a team of planetary scientists announced their new theory explaining the primordial origins of our lunar companion–proposing that the Moon that we now observe shining in our sky at night is not our Earth’s first Moon, but is instead the last in a long and lost series of primordial moons that orbited our planet in the distant past when our Solar System was young.
This latest theory of our Moon’s birth was proposed by a team of researchers from the Technion-Israel Institute of Technology and the Weizmann Institute of Science in Israel, and was published on January 9, 2017 in the journal Nature Geoscience.
The new theory, proposed by researchers Dr. Hagai Perets, of the Technion and Weizmann Institutes, Dr. Raluca Rufu (lead author), and Dr. Oded Aharonson, differs from the more widely favored Giant Impact model that suggests Earth’s Moon was born as a single object that formed as the result of a single collision between a Mars-sized protoplanet named Theia and our still-forming Earth.
“Our model suggests that the ancient Earth once hosted a series of moons, each one formed from a different collision with the proto-Earth. It’s likely that such moonlets were later ejected, or collided with the Earth or with each other to form bigger moons,” explained Dr. Perets in a January 9, 2017 Technion Press Release.
In order to understand the conditions necessary to form many primordial moonlets, in orbit around our still-forming proto-Earth, the scientists ran 800 supercomputer simulations of impacts that could have occurred in the primeval Solar System with our planet.
The Lunatic, The Lover, And The Poet
Our lovely, glowing, golden Moon has haunted the collective imagination of our species for eons. Some ancient, traditional myths and stories, as well as classic childrens’ fairy tales, mention the existence of a man’s face etched out on our lunar companion’s shining disc–while others tell beautiful, haunting tales of a “Moon Rabbit.” An ancient symbol for that which is feminine, Earth’s Moon has been with us almost from the very beginning, when our Sun and its family of familiar objects were first forming about 4.56 billion years ago. It is also the only object beyond our Earth that humanity has walked upon, leaving lingering footprints in Moon dust–a legacy telling whatever observers there may be, originating from wherever in Space and Time, that once human beings existed on our planet, and that we had the ability and curiosity to explore space.
There are more than 100 moons in orbit around the eight major planets inhabiting our Solar System. Most of them are small, icy worlds that contain only a relatively tiny amount of rocky material. This distant myriad of icy moons circle the quartet of majestic, gaseous, giant planets that dwell in the outer, cold, and less well-lit regions of our Solar System. In these distant regions, far from our Sun’s brilliant light and melting heat, these frozen worlds twirl in a lovely ballet around their parent-planets. The four magnificent gaseous giants of our Solar System’s outer limits–Jupiter, Saturn, Uranus, and Neptune–are blanketed by dense and heavy envelopes of gas, and are accompanied by their orbiting retinue of many frozen moons and sparkling, dancing, icy moonlets.
In dramatic contrast, the inner region of our Solar System, where our Earth is situated, is almost entirely barren of moons. Of the quartet of relatively small, rocky worlds–Mercury, Venus, Earth, and Mars–Mercury and Venus possess no moon-children, and Mars is circled by two very small, misshapen moons named Phobos and Deimos, that are probably captured asteroids that long ago migrated from their original home in the Main Asteroid Belt between Mars and Jupiter. According to this model, Phobos and Deimos, during their journey away from their place of birth, were snared by the Red Planet’s gravity when our Solar System was still young. In the warm and brilliantly lit inner Solar System, only Earth’s mesmerizing Moon is a large and significant moon-world in its own right.
Moons are natural satellites that circle around another body that, in turn, is in orbit around its parent-star. The moon is kept in its position by both its parent planet’s gravitational hug, as well as by its own gravity. Some planets are orbited by moons, while others are not. Several asteroids are circled by very tiny moons, and some dwarf planets–such as Pluto–also have moons. One of Pluto’s quintet of moons, Charon, is approximately 50% the size of Pluto itself. Many planetary scientists think that Charon is really a large chunk of Pluto that was ripped off in a violent collision with another rampaging object long ago. Because Charon is almost half the size of Pluto, the two small worlds are often considered to be a double planet.
Our Moon is Earth’s only permanent natural satellite. It is also the fifth-largest moon in our Solar System, and the largest among planetary satellites relative to the size of the parent-planet it orbits. After Jupiter’s innermost Galilean moon, Io, Earth’s Moon is the densest satellite among those whose densities have been determined. The Galilean moons of Jupiter–Io, Europa, Ganymede, and Callisto–are relatively large natural satellites that were discovered in 1610 by Galileo Galileo, and were ultimately named in honor of their discoverer.
The average distance of Earth’s Moon from our planet is about 238,900 miles–or 1.28 light-seconds. Our Moon is thought to have been born approximately 4.51 billion years ago, according to a recent study–not very long after our planet’s formation in the primordial Solar System. Our Moon is in synchronous rotation with Earth, always displaying the same face, with the near side notable for its bewitching and beautiful dark volcanic maria (Latin for seas) that extend between the prominent impact craters and the bright ancient crustal highlands. Our Moon’s surface is actually dark, even though compared to Earth’s sky at night it appears to be very bright, with a reflectance just a bit higher than that of old asphalt. Its prominent place in Earth’s sky, as well as its regular cycle of phases, have made our nearest companion in space a valuable cultural influence since ancient times on mythology, art, calendars, and language.
The gravitational influence of the Moon on our planet creates the ocean tides, body tides, and the slight lengthening of Earth’s day. The Moon’s current orbital distance is approximately thirty times the diameter of Earth, with its apparent size in the sky almost the same as that of our Sun. This is the reason why it almost entirely blocks our Sun during a total solar eclipse.
Several theories have been proposed that attempt to explain how Earth’s Moon was born. However, so far, the Giant Impact Hypothesis is generally considered to be the most credible explanation for lunar formation. According to this theory, when the tragedy that was the doomed Mars-sized protoplanet named Theia, crashed into the primordial Earth billions of years ago, the enormous, violent, catastrophic collision resulted in a portion of the ancient Earth’s crust to be shot out into space. This blast in our planet’s past is thought to have sent a multitude of little moonlets screeching like banshees into the sky above Earth–and some of this ejected material was eventually pulled together by gravity to become our bewitching, bewildering, beautiful Moon.
The Giant Impact Hypothesis was first proposed eighteen months prior to an October 1984 conference on lunar origins. Dr. William Hartmann, Dr. Roger Phillips, and Dr. Jeffrey Taylor challenged other planetary scientists thusly: “You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don’t come to our conference unless you have something to say about the Moon’s birth.” At the 1984 conference, held in Kona, Hawaii, the Giant Impact Hypothesis prevailed as the best model.
Indeed, giant impacts are thought to have been common in the ancient days of our Solar System. Computer simulations of a giant impact produced results consistent with the mass of the lunar core and the present angular momentum of the Earth-Moon system.
The Many Moons Of Ancient Earth
Although the Giant Impact Model has long been the most favored explanation for Moon-birth, the new model proposed by the team of Israeli planetary scientists is consistent with the current scientific understanding of the formation of our Earth. In our planet’s final stages of growth, it suffered a large number of giant impacts with other rampaging bodies. Our early Solar System was a violent place, where primordial bodies blasted into one another–sometimes shattering into many pieces as a result of catastrophic collisions; sometimes merging together to create ever larger and larger objects. This chaotic mess of ancient smashups, occurring between storms of Solar System bodies, has inspired some planetary scientists to refer to our ancient, still-forming Solar System as a “cosmic shooting gallery”.
Each of the numerous impacts of ancient objects, that crashed into our newborn planet, contributed more and more of their material to the forming proto-Earth–until it finally reached the size it is today.
“We believe the Earth had many previous moons,” Dr. Peretz commented in the January 9, 2017 Technion Press Release. He went on to explain that “a previously formed moon could therefore exist when another moon-forming giant impact occurs.”
Tidal forces from our Earth could cause ancient moons to slowly travel outwards–just as our current Moon is gradually traveling away at the leisurely pace of approximately 1 centimeter a year. A pre-existing primordial moon could lazily migrate outwards by the time another, newer moon formed. Alas, their mutual gravitational attraction would ultimately–and catastrophically–force the moons to influence one another, thus altering their orbits.
Dr. Rufu noted in the January 9, 2017 Technion Press Release that “It’s likely that small moons formed through the process could cross orbits, collide and merge. A long series of such moon-moon collisions could gradually build-up a bigger moon–the Moon we see today.”
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