Mysterious Moon-Birth Within A Primordial Cloud
By Judith E Braffman-Miller
Bewitching, beguiling, and bewildering, Earth’s Moon gleams like a light-reflecting mirror as it glistens high above us–the largest object in our sky on clear dark nights. Even though our Moon is Earth’s closest companion in space–and the only world beyond Earth that we have walked upon–it still has managed to keep many of its secrets to itself. How was our Moon born, and where did it come from? Although the prevailing theory, known as the Giant Impact Model remains the favored scientific scenario of lunar origin, there are other possibilities that can also explain ancient Moon-birth. In February 2018, planetary scientists offered a new explanation for the birth of Earth’s Moon, suggesting that it formed inside the Earth when our primordial planet was a spinning, searing-hot cloud, called a synestia.
A synestia is a doughnut-shaped cloud made up of vaporized molten rock. This recently discovered inhabitant of the Cosmos is thought to take shape when planet-sized bodies catastrophically crash into each other with both high energy and angular momentum. Soon after the discovery of these celestial “doughnuts” in 2017, planetary scientists realized that they may have opened up a new window, revealing the previously unknown secret of Moon-birth. The primordial collisions, that produce a synestia, are so violent that the bodies born from these cosmic crashes melt and partially vaporize–finally cooling off and solidifying to form (almost) spherical planets, such as those in our own Solar System.
This new model, developed by planetary scientists at the University of California, Davis and Harvard University in Cambridge, Massachusetts, resolves several mysteries of lunar formation. A paper describing this study has been accepted for publication in the Journal of Geophysical Research–Planets, a publication of the American Geophysical Union (AGU).
“The new work explains features of the Moon that are hard to resolve with current ideas,” commented Dr. Sarah Stewart in a February 28, 2018 AGU Geospace report. Dr. Stewart is a professor of Earth and Planetary Sciences at UC Davis.
“The Moon is chemically almost the same as the Earth, but with some differences. This is the first model that can match the pattern of the Moon’s composition,” she added.
Currently, the most favored model of lunar birth proposes that Earth’s Moon formed as a result of a glancing blow between our ancient planet and an ill-fated Mars-sized protoplanet named Theia. According to this theory, the violent collision between the ancient Earth and the catastrophe that was Theia hurled molten rock and metal screaming into orbit around our magma blanketed planet. The fragments left over from the disaster bumped into one another, eventually merging together to evolve into Earth’s Moon.
Man-In-The-Moon, Moon Rabbit, Etc.
Our mysterious, mesmerizing Moon has haunted the collective imagination of humanity for eons. Ancient myths and legends, as well as children’s fairy tales, tell fanciful stories about our Moon. Some see a man’s face etched out on the lunar surface, while others tell strange stories about a “Moon Rabbit”. As an ancient symbol for madness, romantic love, and poetry, Earth’s Moon holds a special fascination for us, appearing after the Sun has begun to set. The soft glow of moonlight in the darkness is both gentle and haunting–and tenderly eerie. The lingering ghosts of dead lovers meet in the moonlight described by poets.
Our 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. We have visited our Moon, and have left our footprints behind in the lunar dust–a silent testimony that human beings once existed on Earth and had the ability and curiosity to explore space.
There are more than 100 moons orbiting the eight major planets of our Sun’s family. Most of them are icy and small worldlets that contain relatively small quantities of rock. This distant frozen multitude of icy moons and moonlets are mostly found in orbit around the quartet of giant, gaseous major planets that inhabit our Solar System’s outer kingdom. Here, in this dimly-lit, frigid region of our Sun’s domain, these sparkling, icy moon-worlds perform a lovely dance around their majestic parent planets. The four gasous giant planets–Jupiter, Saturn, Uranus, and Neptune–are heavily blanketed by dense envelopes of gas, and they are accompanied in their orbits around our Star by their retinue of numerous moons and glistening little moonlets.
The inner, toasty, and well-lit region of our Solar System–where our Earth dwells–is almost completely barren of moons. Of the quartet of solid, inner major planets–Mercury, Venus, our Earth, and Mars–Mercury and Venus are moonless, and Mars is orbited by a tiny, tantalizing duo of deformed moons, dubbed Phobos and Deimos. The two lumpy little potato-shaped Martian moons are likely captured asteroids that escaped from the Main Asteroid Belt between Mars and Jupiter. During their ancient journey, that carried them far from their birthplace, the two little asteroids were finally snared by the gravitational tug of the Red Planet, thus becoming its “adopted” moons.
Earth is the only major inner planet that is orbited by a large Moon–and our lunar companion is a significant world in its own right. The fifth largest moon in our Sun’s family, Earth’s Moon is responsible for making our planet more life-friendly. This is because it moderates our planet’s wobble on its axis, resulting in a relatively stable climate. It also causes tides, thus creating a rhythm that has effected our species for thousands of years. Up until 1610, when the great Italian astronomer Galileo Galilei discovered the four large Galilean moons of the banded behemoth Jupiter, Earth’s natural satellite was simply designated “the Moon” because we didn’t know that other moons existed.
Earth’s Moon boasts a radius of 1,079.6 miles, that makes it less than a third of the width of Earth, and it is farther away from us than we realize. Indeed, our Moon is situated at an average distance of 238,855 miles from us. This means that 30 Earth-sized planets could fit in between Earth and its lunar companion.
Our Moon is slowly traveling away from Earth, wandering an inch farther from us annually. When our Moon was first born, it was much closer to us than it is now. Our lunar companion rotates at the same rate that it revolves around Earth (synchronous rotation) and, for this reason, the same hemisphere faces our planet all the time. Some people call the lunar far side–the hemisphere that we never see from our planet–its “dark side”. However that designation is misleading. This is because, as our Moon orbits Earth, different regions of it are in sunlight or darkness at different times. These alterations of lunar illumination is the reason why, from our vantage point, the Moon seems to go through phases. During a “full moon”, the hemisphere of the Moon that we can observe from our planet is fully illuminated by the Sun. Conversely, when there is a “new moon”, the far side of our lunar companion has full sunlight, and the side facing us is having its night.
The Moon completes an orbit around Earth every 27 days and spins at the same rate, or in that same amount of time. Because our planet is also moving–rotating on its axis as it circles our Star–from our vantage point, our Moon seems to orbit us every 29 days.
Our Moon has a core, mantle, and crust. Its core is proportionally smaller than other terrestrial bodies’ cores. Earth’s Moon has an iron-rich, solid core that is about 149 miles in radius. It is enveloped by a liquid iron shell that is approximately 56 miles thick. A partially molten layer with a thickness of 93 miles encircles the iron core.
Our Moon’s mantle reaches from the top of the partially molten layer to the bottom of the lunar crust. It is probably composed of minerals like pyroxene and olivine, which are themselves made up of magnesium, silicon, oxygen, and iron atoms.
The lunar crust is approximately 43 miles thick on the near-side hemisphere and 93 miles thick on the far-side. It is composed of oxygen, silicon, magnesium, calcium, aluminum, iron, as well as small quantities of titanium, uranium, thorium, potassium and hydrogen.
Very long ago, Earth’s Moon had active volcanoes. Today, however, the lunar volcanoes are dormant and have not erupted for millions of years.
The lunar atmosphere is too thin to protect it from impacting objects from space. For this reason, a continuous storm of falling asteroids, comets, and meteoroids pummels the surface of Earth’s Moon. This steady rain of terror leaves in its wake a multitude of craters on the lunar surface. Tycho Crater is more than 52 miles wide.
As billions of years passed, these continual impacts have managed to grind up the lunar surface into fragments that range in size from enormous boulders to powder. Nearly the entire Moon is coated by a rubble pile of powdery, charcoal-gray dust, as well as rocky debris called the lunar regolith. Hidden beneath the lunar regolith is a layer of fragmented bedrock called the megaregolith.
The light regions of Earth’s Moon are called the highlands. The dark areas are called the maria (Latin for seas). The lunar maria are really impact basins that were once filled with lava between 4.2 and 1.2 billion years ago. These light and dark regions represent rocks of differing ages and compositions, and they provide precious clues about how our primordial Moon’s crust may have crystallized from a global ocean of fiery magma. The craters themselves have been well-preserved for billions of years, and they provide an impact history for Earth’s Moon and other bodies inhabiting the inner kingdom of our Solar System.
The lunar temperature can reach approximately 260 degrees Fahrenheit when in full Sun but, in darkness, the temperature dives down to a truly frigid -280 degrees Fahrenheit.
Where Did Earth’s Moon Come From?
There are several theories that have been proposed to explain how Earth’s Moon was born. However, at this particular time, the Giant Impact Hypothesis is considered to be the most likely explanation for lunar formation. According to this theory, when the tragedy that was the Mars-sized Theia crashed into the primordial Earth billions of years ago, the catastrophic smash-up shot part of our ancient planet’s crust into space. This truly horrific blast is believed to have created myriad moonlets in the sky above our planet. Some of the ejected material was ultimately pulled together by gravity to evolve into our bewitching Moon.
However, there are several other theories that have been proposed to explain Moon-birth. One theory suggets that Earth’s Moon was once a part of Earth that simply budded off when our Solar System was in its infancy, approximately 4.5 billion years ago. According to this particular model, the Pacific Ocean basin would be the most probable cradle for where the Moon was born. A second theory postulates that Earth and its Moon were born at the same time out of the original protoplanetary accretion disk, composed of gas and dust, from which our Star’s family of planets, moons, and smaller objects emerged. A third model proposes that Earth’s Moon was really born somewhere else in our Solar System and, like the tiny Martian moons, was eventually snared by our planet’s gravitational tug when it happened to wander too close.
There is also a fourth theory that suggests that interactions between Earth-orbiting planetesimals in our primeval Solar System caused them to fall apart. Earth’s lunar companion ultimately coalesced out of the pulverized debris of the doomed planetesimals. Planetesimals were the building blocks of the major planets in our ancient Solar System. These objects collided and then merged to form increasingly larger bodies–ultimately becoming the size of major planets. The asteroids and comets of our Sun’s family are the left-overs of this abundant population of primordial objects.
The New Synestia Model
The new theory is based on the existence of a synestia, which is a recently discovered beast inhabiting the celestial zoo. Dr. Sarah Stewart and Simon Lock, a graduate student at Harvard and visiting student at UC Davis, first proposed the existence of these puffy planet-forming doughnuts in 2017. A synestia forms when a catastrophic collision between planet-sized objects occurs in a young solar system. This collision results in a rapidly spinning mass of molten and vaporized rock with part of the body orbiting around itself. The entire object ultimately puffs out into a giant doughnut of vaporized rock–the synestia.
A synestia doesn’t hang around it’s young parent-star for very long. These puffy objects last for only hundreds of years, a very brief moment on cosmological time scales. A synestia is destined to shrink rapidly because it radiates heat. This causes the rock vapor to condense into a liquid. Finally, the forming object collapses into a molten planet.
Simon Lock explained in the February 28, 2018 Geospace report that, “Our model starts with a collision that forms a synestia. The Moon forms inside the vaporized Earth at temperatures of four to six thousand degrees Fahrenheit and pressures of tens of atmospheres.”
One of the advantages of this new scenario, Lock continued to note, is that there are many ways to form a suitable synestia. The formation of a synestia does not merely depend on a devastating collison with the right sized object happening in precisely the right way.
Once the Earth-synestia formed, large blobs of molten rock–that had been shot into orbit during the impact–created the seed that would become Earth’s Moon. Vaporized silicate rock condensed at the surface of the primordial synestia and then poured down onto the proto-Moon, while the Earth-synestia itself slowly shrank. Ultimately, our Moon would have emerged from the clouds of the natal synestia, trailing its own atmosphere composed of vaporized rock. Earth’s Moon inherited its composition from our planet, but because it was born at extremely high temperatures it lost the more easily vaporized elements. This explains our Moon’s distinct composition.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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