Earth’s Moist And Mysterious Moon
By Judith E Braffman-Miller
Earth’s Moon is our planet’s closest neighbor in space, but it is remarkable how even neighbors can keep secrets from each other. For years, astronomers thought that Earth’s Moon was barren of water and other volatile compounds, but this notion began to change in 2008, when a team of planetary scientists announced that they had discovered small quantities of water imprisoned within volcanic glass beads, that astronauts had carried back to Earth from the Apollo 13 and 17 missions to our Moon. In 2011, additional research revealed extremely tiny crystalline formations within those beads–indicating that they contained quantities of water similar to some basalts on Earth.
The prevailing theory of lunar formation–the Giant Impact hypothesis–proposes that our Moon was born as the result of a disastrous collision between our still-forming proto-Earth and a doomed Mars-sized body named Theia–and this impact is thought to have created a partially vaporized, extremely hot disk of material that swirled around our infant planet. Eventually, this primordial disk cooled off, and ultimately accreted to form our Moon. In February 2018, a team of astronomers announced that their ongoing research is revealing that Earth’s Moon may be wetter than initially thought, which raises important questions about some aspects of this origin story.
For a long time, planetary scientists thought that in the aftermath of the Moon-forming collision, hydrogen dissociated from water molecules. According to this scenario, both water and other elements that have low boiling temperatures (volatile elements), escaped from the disk and were lost forever to space. This model would form a volatile-element-depleted and bone-dry Moon. At the time, this scenario seemed to be consistent with earlier analyses of lunar samples.
But ongoing studies about lunar chemistry are showing that it may be much wetter than planetary scientists initially hypothesized. In fact, these wetter conditions conflict with some aspects of the Giant Impact theory.
“This is still very much an area of active research, so there is much that scientists including our Department of Terrestrial Magnetism staff scientist Erik Hauri, as well as many other Carnegie colleagues and alumni, are figuring out about how much water exists on the Moon. This is a highly important and challenging question to answer given that we have limited knowledge on the history and distribution of lunar water,” explained Dr. Miki Nakajima in a February 26, 2018 Carnegie Institution Press Release. Dr. Nakajima, who is of the Carnegie Institution of Washington (D.C.), along with California Institute of Technology’s (Caltech’s) Dr. David Stevenson, set out to determine whether prevailing lunar formation models need to be adjusted to explain more recent higher estimates of the quantity of water on Earth’s Moon. Caltech is in Pasadena.
Earth’s Nearest Neighbor
In July 2017, a team of astronomers announced that they had used satellite data to find–for the first time–signs of widespread water hidden beneath ancient volcanic material on Earth’s Moon. The scientists’ discovery suggests that the interior of our Moon holds large quantities of indigenous water. This plentiful, but well-hidden water, reveals its secret presence in numerous volcanic deposits, that had been explosively distributed across our Moon’s surface when ancient lunar volcanoes erupted. These primordial deposits contain unusually large amounts of imprisoned water compared with nearby terrains. The detection of water within these lunar deposits, is believed to be made up of glass beads that formed as a result of the explosive fiery eruption of magma, hurled out from deep within our Moon. This finding supports the theory that the lunar mantle is surprisingly soggy.
Earth’s Moon is the fifth largest moon in our entire Solar System, as well as the only world beyond our planet that we have visited. Our lunar companion is the largest and brightest object in the sky at night, and many astronomers think that it was born when the tragedy that was the pulverized Theia blasted into ancient Earth billions of years ago. There are other theories, however, that seek to explain how our Moon came to be. Nevertheless, the Giant Impact theory stands its ground as the most credible explanation for lunar birth. When the doomed, destroyed Theia impacted Earth, it shot debris above our planet. This abundant debris eventually coalesced to form our Moon.
Even though Theia came to a tragic end, it did not die in vain. This is because the ill-fated Theia made the emergence of life possible on Earth. Our lunar companion is responsible for creating a welcoming abode for living things on our planet, because it moderates Earth’s wobble on its axis–thus creating a stable, life-friendly climate. Earth’s Moon also is the source of ocean tides that form a rhythm that has guided human beings for thousands of years.
Until 1610, when Galileo Galilei discovered the quartet of large Galilean moons orbiting Jupiter–Io, Europa, Ganymede, and Callisto–Earth’s Moon was the Moon, because it was the only moon known to exist. Now, we know differently. There are over 100 known moons in our Solar System alone, and probably many, many more, circling distant alien planets belonging to the families of stars beyond our Sun. Most of the moons in our own Solar System are relatively small, icy worldlets that contain only small amounts of rocky material. The faraway multitude of sparkling, frozen moons that inhabit our Sun’s family are mostly found circling the quartet of outer gaseous giant planets–Jupiter, Saturn, Uranus, and Neptune. In this dimly lit region, far from our Star’s heat and light, these tiny icy moons perform a strange and lovely ballet around their large, gaseous host planets. The quartet of giant gaseous planets, that inhabit our Solar System’s outer suburbs, are enshrouded by heavy atmospheres of gas, and they are accompanied in their travels around our Sun, by their own orbiting entourage of moons and moonlets.
In dramatic contrast, the inner region of our Solar System, where our Earth dwells–along with Mercury, Venus, and Mars–is relatively barren of moons. Mercury and Venus have no moons, and Mars is orbited by a small duo of deformed moons, Phobos and Deimos, that are probably asteroids that escaped from the Main Asteroid Belt between Mars and Jupiter billions of years ago–only to be captured by the Red Planet’s powerful gravitational embrace. Our Earth is the only inner planet that possesses an impressively large, spherical Moon.
Moons are natural satellites that orbit another body that, in turn, circles its parent-star. A moon is held in place by both its own gravity and the gravitational grip of its host planet. Some planets have moons; some do not. Several asteroids in our Solar System also are orbited by very small moons–and some dwarf planets, such as Pluto, also have moons. One of Pluto’s five moons, Charon, is almost 50% the size of Pluto. For this reason, the two frozen worlds inhabiting our Solar System’s remote twilight zone, are sometimes classified as a double-planet.
Our Moon is Earth’s only permanent natural satellite. It is also the largest moon in our Solar System relative to the size of its host planet. Second only to Jupiter’s volcanic Galilean moon, Io, our Moon is the densiest natural satellite among those whose densities have been determined.
Born approximately 4.51 billion years ago, Earth’s companion world formed soon after our own planet’s birth in the primordial Solar System. The average separation between Earth and Moon is about 238,000 miles (1.28 light-seconds), and it is locked in synchronous rotation with Earth–meaning that it always shows us the same face. The near-side of our Moon is known for its bewitching dark volcanic maria (Latin for seas) that are located between large impact craters, as well as for its very ancient, bright crustal highlands. The lunar surface is really extremely dark–even though it appears to be very bright in the night sky above our planet–with a reflectance only a bit higher than that of old asphalt. The prominent position of our lunar companion in the dark midnight sky, as well as its rhythmic and regular cycle of phases, made our Moon an important influence on human culture ever since ancient times–especially in mythology, art, language, and on calendars.
Earth’s Moon completes one orbit around our planet every 27 days, and it rotates (spins) at the same rate. Because Earth is also moving–rotating on its axis as it circles our Star–from our perspective our lunar companion appears to orbit us every 29 days.
In addition to the Giant Impact theory, there are several other models that have been proposed to explain how our Moon was born. One alternative model to the Giant Impact scenario suggests that Earth’s Moon was once a part of our planet that simply budded off when our Solar System was in its infancy–approximately 4.5 billion years ago. According to this model, the Pacific Ocean basin would be the most likely cradle for lunar birth. A second model proposes that our Moon was really born elsewhere in our Solar System and, like the duo of tiny potato-shaped Martian moons, was eventually snared by the gravitational tug of a major planet. A third theory postulates that both Earth and Moon were born at about the same time from the same protoplanetary accretion disk, composed of gas and dust, from which our Sun’s family of planets, moons, and smaller objects ultimately emerged.
A fourth, more recent model, is based on the existence of a synestia. A synestia is a doughnut-shaped cloud composed of vaporized molten rock. This recently discovered inhabitant of the Universe is believed to take shape when planet-sized bodies catastrophically blast into one another with both high energy and angular momentum. Soon after the discovery of these puffy celestial “doughnuts” in 2017, planetary scientists came to the realization that they may have a new way to explain Moon-birth. The ancient collisions, that create a synestia, are so violent that the objects that form from these cosmic crash-ups melt and partially vaporize. Ultimately, after having cooled off sufficiently to solidify, they create (almost) spherical planets, such as those inhabiting our own Solar System.
Earth’s Moon Has A Soggy Secret
The February 2018 study, conducted by Carnegie and JPL astronomers, created detailed scenarios in order to determine whether existing theories about the catastrophic Giant Impact theory could explain a wet Moon that is still depleted in other volatile elements like sodium and potassium.
The scientists modeled different temperatures and water abundances that may have been present in the Moon-birthing disk. At higher temperatures, their disk was primarily composed of silicate vapor, which formed as a result of evaporation of the mantles of both the proto-Earth and the doomed Theia. The disk at these higher temperatures also contained a relatively small quantity of hydrogen dissociated from water. In contrast, at lower temperatures, their disk was primarily composed of water, from which hydrogen did not dissociate under this cooler temperature range–thus making its escape mechanism very inefficient.
“The good news is that our models show that observations of a wet Moon are not incompatible with a Giant Impact origin,” Dr. Nakajima explained in the February 26 2018 Carnegie Institution Press Release.
However, the new findings also mean that scientists must come up with other explanations for why Earth’s Moon is depleted of potassium, sodium, and other volatile elements. Additional explanations for this exist. One possible alternative explanation is that the volatile elements contained within the disk showered down onto the proto-Earth instead of escaping, or being part of lunar formation. Another explanation is that these volatile elements were part of our Moon when it first accreted from the post-collision disk but were later lost.
The new study is published by Earth and Planetary Science Letters.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. 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 some of the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.
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