Our Moon is the largest object in Earth‘s night sky, brightly gleaming as it reflects the light of our Star. But how did Earth‘s lovely and bewitching Moon come to be? The most widely accepted theory of lunar formation proposes that our Moon was born when a Mars-sized primordial protoplanet, named Theia, crashed into our ancient planet billions of years ago–and was pulverized. This catastrophic collision shot debris screeching into orbit around our young planet. The debris, composed of both material from the doomed Theia and our badly battered ancient Earth, eventually coagulated to create a single body–Earth‘s Moon. In January 2019, a team of astronomers reported their new findings that the bulk of Earth‘s essential life-forming elements–including most of the carbon and nitrogen in our bodies–most likely came from another planet, the doomed primordial protoplanet that was the Moon-birthing Theia. The paper describing this new study is published in the January 23, 2019 issue of the journal Science Advances.
According to the new study, conducted by Rice University (Houston, Texas) petrologists, the collision occurred about 4.4 billion years ago, when our 4.56 billion year old Solar System was young.
“From the study of primitive meteorites, scientists have long known that Earth and other rocky planets in the inner Solar System are volatile-depleted. But the timing and mechanism of volatile delivery has been hotly debated. Ours is the first scenario that can explain the timing and delivery in a way that is consistent with all of the geochemical evidence,” study co-author Dr. Rajdeep Dasgupta commented in a January 23, 2019 Rice University Press Release.
Earth‘s Moon has long been the source of imaginative and magical myths and legends. It has also been the stuff of poetry, as well as an ancient symbol for that which is feminine. Indeed, Earth‘s companion world has inspired wonderful tales of both romantic love and madness–the word “lunatic” is derived from the word lunar. There are fascinating tales and delightful childrens’ stories that speak of a “Man In The Moon” and a “Moon Rabbit” that appear to be etched on the lunar surface. Lovely myths and tales aside, Earth‘s Moon is a real object in space, and it has been with our planet almost from the very beginning. It is the only world beyond Earth that human beings have walked upon, leaving footprints in the Moon-dust.
The Lunatic, The Lover, And The Poet
There are over 100 moons in orbit around the eight major planets of our Sun’s family. Most of our Solar System’s many moons are icy, relatively small objects, that contain small quantities of rocky material, and circle the quartet of giant gaseous planets in the colder outer region of our Solar System. The giant planets– Jupiter, Saturn, Uranus and Neptune–are enshrouded by layers and layers of gas, and are orbited by myriad moons and sparkling icy moonlets. In dramatic contrast the solid quartet of small inner Solar System planets–Mercury, Venus, Earth and Mars–are almost entirely barren of moons. Mercury and Venus have none, and Mars is orbited by a duo of small, shapeless little moons named Phobos and Deimos, that are likely asteroids that escaped from the Main Asteroid Belt situated between Mars and Jupiter. The wandering potato-shaped duo, during their long journey through interplanetaary space, traveled too close to the gravitational pull of Mars, and thus experienced a sea-change from migrating asteroids to the moons of a major planet.
In the warm and well-lit inner region of our Solar System, only Earth is orbited by a large Moon–and it is the fifth largest moon in our Sun’s family.
A moon is a natural satellite circling another body that itself is in orbit around its Star. A moon is kept in its place both by its host’s gravitational pull, as well as by its own gravity. Some planets have moons; some do not. Several asteroids are now known to be orbited by tiny moons of their own, and some dwarf planets–such as Pluto–are also circled by moons. One of Pluto’s five moons, Charon, is approximately 50% the size of Pluto. It has been proposed that Charon may really be a large chunk of Pluto itself that was ripped off as the result of a violent collision with another migrating object long ago. Because Charon is about half the size of Pluto, the two small worlds are sometimes classified as a double planet.
Several theories have been proposed over the years that attempt to explain how Earth‘s Moon was born. One theory suggests that our Moon was once actually part of Earth, and that it budded off about 4.5 billion years ago. According to this scenario, the Pacific Ocean basin is the most likely place for Moon-birth to have occurred. A second theory proposes that the Earth and Moon were both born at about the same time from the original protoplanetary accretion disk, made up of gas and dust, from which our Sun and its family of familiar objects emerged. The third model suggests that Earth‘s Moon was born elsewhere in our Solar System, and was ultimately snared by Earth‘s gravitational embrace when it passed too close to our ancient planet. The fourth theory states that the interactions of Earth-orbiting and Sun-orbiting planetesimals (ancient planetary building blocks) in the early days of our Solar System caused them to fragment. According to this theory, Earth‘s Moon eventually coalesced out of the pulverized debris of the shattered ancient planetesimals.
However, the Giant Impact Theory is considered to be the most probable explanation for the birth of Earth‘s large Moon. When the tragedy that was the Mars-sized Theia crashed into Earth billions of years ago, the blast resulted in part of the ancient Earth‘s crust to be launched into into space. This primeval catastrophe hurled myriad tiny moonlets screaming into the sky above our ancient planet. Some of this material was ultimately captured into Earth-orbit approximately 4.5 billion years ago–where it was finally pulled together by gravity to evolve into a single large Moon–Earth‘s Moon.
Until Galileo Galilei discovered the four Galilean moons of Jupiter in 1610, it was thought that Earth‘s large Moon was the Moon, because it was the only one known to exist. The discovery of the quartet of Galilean moons—Io, Europa, Ganymede and Callisto put the matter into its proper perspective. Earth‘s Moon is not alone in our Solar System. In addition, there is also evidence that exomoons orbit some of the exoplanets that circle stars beyond our own Sun.
However, Earth‘s Moon is the largest moon in our Solar System relative to the size of its host planet. For this reason, Earth and its Moon are sometimes considered to be a double planet–in a way similar to Pluto and its largest moon Charon. Earth‘s Moon is also one of the densest natural satellites in our Sun’s family–second only to Jupiter’s innermost Galilean moon, Io.
As the fifth largest moon in our Solar System, only Ganymede (Jupiter), Titan (Saturn), Callisto (Jupiter) and Io (Jupiter) are bigger than Earth‘s lunar companion.
Even though Theia came to a violent end, it did not die in vain. It’s been recognized for years that the doomed Theia made the emergence of life possible on our planet. This is because it is responsible for creating a comfortable abode for living creatures. The Moon–born from the wreckage of Theia–moderates Earth‘s wobble on its axis, thus creating a stable climate. Earth‘s Moon is also the source of ocean tides which form a rhythm that has guided human beings since ancient times.
Theia Did Not Die In Vain
The team of Rice University petrologists compiled their evidence from a combination of high-temperature and high-pressure experiments conducted in Dr. Dasgupta’s lab, which specializes in studying geochemical reactions that occur under the conditions of extreme heat and pressure that take place deep within a planet.
In a series of experiments, study lead author and graduate student Damanveer Grewal collected data to test a theory that Earth‘s volatiles were delivered to our ancient planet as the result of a primordial smash-up with an embryonic protoplanet that possessed a sulfur-rich core. The sulfur contained in the donor planet’s core is important.This is because it would explain the mysterious array of experimental evidence concerning the nitrogen, carbon and sulfur that exist in all parts of our planet–except for its core.
“The core doesn’t interact with the rest of Earth, but everything above it, the mantle, the crust, the hydrosphere and atmosphere, are all connected. Material cycles between them,” Grewal explained in the January 23, 2019 Rice University Press Release.
One model explaining how Earth received its volatiles is called the “late veneer” theory. According to this model, volatile-rich relic fragments of primordial material from the outer Solar System arrived after Earth‘s core had already formed. While the isotopic signatures of our planet’s volatiles match these ancient primitive objects–known as carbonaceous chrondrites–the elemental ratio of carbon to nitrogen does not. Earth‘s iron core, which geologists refer to as the bulk silicate Earth, is approximately 40 parts carbon to each part nitrogen. This amounts to about twice the 20 to one ration seen in carbonaceous chondrites.
Simulations of the high-pressures and temperatures occurring during core formation, as modeled in Greval’s experiments, tested the theory that a sulfur-rich planetary core might exclude carbon or nitrogen–or both. This scenario would leave much greater percentages of those elements in the bulk silicate as compared to Earth. In a series of tests at a range of temperatures and pressures, Grewal studied the amount of carbon and nitrogen that may have made it into Earth‘s core in a trio of scenarios: no sulfur, 10 percent sulfur and 25 percent sulfur.
“Nitrogen was largely unaffected. It remained soluble in the alloys relative to silicates, and only began to be excluded from the core under the highest sulfur concentration,” Grewal explained in the January 23, 2019 Rice University Press Release.
In contrast, carbon was considerably less soluble in alloys with intermediate sulfur concentrations, and sulfur-rich alloys took up approximately 10 times less carbon by weight than sulfur-free alloys.
Using this information, in combination with the known ratios and concentrations of elements both on our planet and in non-terrestrial bodies, Dasgupta, Grewal and Rice University postdoctoral researcher Chenguang Sun created a supercomputer simulation to discover the most probable scenario that produced our planet’s volatiles. Discovering the answer to this question involved varying the beginning conditions, running about 1 billion scenarios and comparing them against the known conditions in our Solar System today.
“What we found is that all the evidence–isotopic signatures, the carbon-nitrogen ratio and the overall amounts of carbon, nitrogen and sulfur in the bulk silicate Earth–are consistent with a Moon-forming impact involving a volatile-bearing, Mars-sized planet with a sulfur-rich core,” Grewal commented in the January 23, 2019 Rice University Press Release.
Dasgupta is the principal investigator on a NASA-funded project dubbed CLEVER Planets. This project is studying how life-essential elements could combine on distant rocky alien worlds. Dasgupta explained in the same Rice University Press Release that an improved understanding of the origin of our planet’s life-essential elements has important implications beyond our own Solar System.
“This study suggests that a rocky, Earth-like planet gets more chances to acquire life-essential elements if it forms and grows from giant impacts with planets that have sampled different building blocks, perhaps from different parts of a protoplanetary disk,” Dasgupta added.
“This removes some boundary conditions. It shows that life-essential volatiles can arrive at the surface layers of a planet, even if they were produced on planetary bodies that underwent core formation under very different conditions,” Dasgupta continued to explain.
Dasgupta further noted that it does not appear our own planet’s bulk silicate, on its own, could have attained the life-essential volatile budgets that formed Earth‘s atmosphere, biosphere, and hydrosphere.
“That means we can broaden our search for pathways that lead to volatile elements coming together on a planet to support life as we know it,” he continued to comment.
CLEVER Planets is part of the Nexus for Exoplanet System Science, or NExSS, a NASA astrobiology research coordination network studying planetary habitability.