In the tragicomedy that characterizes human relationships, it has been said that the closer we get to someone, the weirder that person gets. Earth’s Moon is our planet’s closest neighbor in Space–mysterious, bewitching, bothersome, and bewildering, it has successfully hidden many of its secrets from the prying eyes of curious observers. In July 2017, using satellite data, a team of astronomers announced that they have, for the first time, detected widespread water hidden within ancient explosive volcanic material on Earth’s nearest and dearest companion world. This discovery indicates that the interior of Earth’s Moon contains large quantities of indigenous water that has finally been revealed in numerous volcanic deposits distributed across the lunar surface–and these ancient deposits contain unusually high amounts of imprisoned water compared with surrounding terrains. The discovery of water in these ancient lunar deposits, which are believed to be composed of glass beads created in the explosive fiery eruption of magma shooting out from the deep interior of the Moon, strengthens the theory that the lunar mantle is surprisingly water-rich.
Planetary scientists believed for years that Earth’s Moon is depleted of water and other volatile compounds. However, this idea began to change in 2008, when a team of scientists announced that they had detected traces of water in some of the volcanic glass beads carried back to Earth from the Apollo 15 and 17 missions to the Moon. In 2011, additional study of extremely small crystalline formations within those beads revealed that they contain amounts of water that are similar to some basalts on Earth. This indicates that the lunar mantle–at least, part of it–contains as much water as Earth’s.
Earth’s Nearest And Dearest
Earth’s Moon is the fifth largest moon in our Solar System, and the only world beyond our own that we have walked upon, leaving our footprints behind in moon dust as a silent testimony that once we existed, and had been there. Our Moon is both the brightest and largest object in Earth’s night sky, and many astronomers think that our bewitching lunar companion was born as a result of an ancient collision between our planet and an ill-fated Mars-sized protoplanet that has been named Theia. There are other theories that have been devised to explain our Moon’s origin, but the Giant Impact theory is considered to be the best explanation. When the doomed Theia blasted into the primordial Earth, it launched into the sky above our planet the debris resulting from that catastrophic crash. The debris eventually coalesced into Earth’s Moon.
Although Theia made the ultimate sacrifice, it did not die in vain because this unfortunate world’s demise made life possible on Earth. Earth’s Moon makes our planet livable; it moderates Earth’s wobble on its axis, resulting in a relatively stable, life-sustaining climate, and it also causes ocean tides that create a rhythm that has guided humanity for thousands of years.
Earth’s Moon was thought to be The Moon–and the only moon–until Galileo Galilei took his primitive telescope up to the roof of his house in Padua in January 1610. Galileo aimed his telescope up to the clear starlit night sky above his home–one of the first to be used for astronomical purposes–and aimed it at the giant planet Jupiter. As a result, Galileo discovered the four large Jovian Galilean Moons, eventually named in his honor: Io, Europa, Ganymede, and Callisto.
Now we know that there are over 100 moons circling the eight major planets of our Sun’s family. The majority of our Solar System’s moons are icy, small, and frozen worlds that contain only small quantities of rocky material. The distant multitude of sparkling, icy moons in our Solar System are primarily in orbit around the four giant gaseous planets, Here, in this strange, frigid and dimly-lit realm, far from our Star’s melting fires and brilliant light, these tiny frozen moons do their fabulous, lovely dance around their quartet of parent-planets. The giant, gaseous worlds that inhabit our Solar System’s outer suburbs–Jupiter, Saturn, Uranus, and Neptune–are blanketed by heavy atmospheres of gas, and are accompanied, in their travels around our Star, by their orbiting retinue of many moons and sparkling, icy moonlets.
The inner Solar System is dramatically different from the distant realm of the giant planets. The inner region of our Solar System, where our Earth is situated, is almost entirely moon-less. Of the four relatively small, rocky worlds–Mercury, Venus, our Earth, and Mars–Mercury and Venus are barren of moons, and Mars is orbited by two fascinating, but very small, potato-shaped moons named Phobos and Deimos. The duo of Martian moons are often considered to be captured asteroids that long ago escaped from their birthplace in the Main Asteroid Belt between Mars and Jupiter. According to this scenario, Phobos and Deimos, during their dangerous journey from their original home, were snared by the gravity of their adopted Red Planet when our 4.56 billion-year-old Solar System was young. In the warm and well-lit inner Solar System, only Earth’s large Moon is a significant moon-world in its own right.
Moons are natural satellites that circle around another body that, in turn, circles around its parent-star. The moon is held in place by both its own gravity and the gravitational pull of its planet. Some planets have moons, while others do not. Several asteroids are known to be circled by very small moons, and some dwarf planets–such as Pluto–also have moons. One of Pluto’s quintet of moons, Charon, is about half the size of Pluto itself. Some planetary scientists propose that Charon is really a large chunk of Pluto that was torn off in a catastrophic collision with another wandering world long ago. Because Charon is almost 50% the size of Pluto, the two tiny icy bodies are sometimes considered to be a double-planet.
Our Moon is Earth’s only permanent natural satellite. It is also the largest planetary satellite in our Solar System relative to the size of its parent-planet. After Jupiter’s volcanic Galilean moon, Io, Earth’s Moon is the densest natural satellite among those whose densities have been determined.
Earth’s lunar companion is thought to have been born about 4.51 billion years ago, according to a recent study. This means that our Moon was born soon after Earth’s formation in the primeval Solar System. The average distance of Earth’s Moon from our planet is about 238,900 miles–or approximately 1.28 light-seconds–and it is in synchronous rotation with Earth, always showing the same face, with the near side famous for its beautiful bewitching dark volcanic maria (Latin for seas) that are situated between prominent impact craters and the bright, very ancient, crustal highlands. Our Moon’s surface is actually quite dark, even though it appears in the sky at night to be very bright, with a reflectance only a bit higher than that of old asphalt. The prominent position of our Moon in our planet’s night sky, as well as its regular cycle of phases, have made our nearest and dearest celestial companion a valuable cultural influence since ancient times in art, mythology, language, and on calendars.
Our Moon makes a complete orbit around Earth in 27 Earth days and it rotates (spins) at that same rate–meaning, in that same amount of time. Because our planet is also moving–rotating on its axis as it circles our Star–from our viewpoint, our lunar companion appears to circle us ever 29 days.
Earth’s Moon consists of a core, mantle, and crust. The lunar core is proportionally smaller than other terrestrial bodies’ cores. The iron-rich, solid inner core is 149 miles in radius, and it is encased within a liquid iron shell that is about 56 miles thick. A partly molten layer with a thickness of 93 milles surrounds the iron core.
The lunar mantle reaches from the top of the partially molten layer to the bottom of the lunar crust. It is thought to be made up of minerals like pyroxine and olivine–both of which are composed of magnesium, iron, silicon and oxygen atoms.
The crust of Earth’s Moon is 43 miles thick on the near-side hemisphere, and 93 miles on the far-side. It is composed of silicon, magnesium, oxygen, calcium, aluminum, and iron. There are also trace amounts of titanium, uranium, thorium, hydrogen, and potassium.
Very long ago, when our Solar System was young, Earth’s Moon possessed active volcanoes. However, today, the lunar volcanoes are dormant and have not erupted for millions of years.
Because the lunar atmosphere is very thin, it is far too sparse to prevent a steady shower of impacts from tumbling asteroids, comets, and meteoroids. These objects strike the lunar surface, leaving behind numerous crater scars. For example, Tycho Crater is over 52 miles wide.
Over the passage of billions of years, this ceaseless rain of falling objects have pulverized the lunar surface, creating fragments ranging in size from a fine powder to enormous, heavy boulders. Almost the entire lunar surface is literally blanketed by a pile of ground up rubble composed of charcoal gray, powdery dust and rocky debris collectively termed the lunar regolith. Beneath the regolith there is a region composed of shattered bedrock that is called the megaregolith.
The relatively light regions of the Moon are known as the highlands. The dark features, the lunar maria, are impact basins that were later filled with lava between 4.2 and 1.2 million years ago. These light and dark regions were created by rocks of different ages and compositions. This provides evidence for how the ancient crust may have crystallized from a global lunar ocean of magma. The impact craters have been preserved for billions of years, and they provide observers with an impact history for our Moon and other bodies that inhabit the inner Solar System.
Our Moon’s temperature reaches about 260 degrees Fahrenheit when under a full Sun. However, in darkness, the temperature dives down to approximately -280 degrees Fahrenheit.
Earth’s Moon Has A Soggy Secret
“The key question is whether those Apollo samples represent the bulk conditions of the lunar interior or instead represent unusual or perhaps anomalous water-rich regions within an otherwise ‘dry’ mantle. By looking at the orbital data, we can examine the large pyroclastic deposits on the Moon that were never sampled by the Apollo or Luna missions. The fact that nearly all of them exhibit signatures of water suggests that the Apollo samples are not anomalous, so it may be that the bulk interior of the Moon is wet,” explained Dr. Ralph Milliken in a July 24, 2017 Brown University Press Release. Dr. Milliken is the lead author of the new research and an associate professor in Brown University’s Department of Earth, Environmental and Planetary Sciences. Brown University is in Providence, Rhode Island.
The new study, which Dr. Milliken co-authored with Dr. Shuai Li, a postdoctoral researcher at the University of Hawaii and a Brown University graduate, is published in the July 24, 2017 issue of the journal Nature Geoscience. The research was part of Dr. Li’s doctoral thesis.
Discovering the water content of volcanic deposits on our Moon using orbital instruments presents quite a challenge. Planetary scientists use orbital spectrometers to measure the light that skips off of a planetary surface. By determining which electromagnetic wavelengths of light are reflected or absorbed by the surface, the scientists can then get an idea of which minerals and other compounds are present.
However, Earth’s Moon presents a special problem because its surface becomes increasingly hotter and hotter over the course of a day. Alas, this is especially true at latitudes where the pyroclastic deposits are located. This means that in addition to the light reflected from the lunar surface, the spectrometer also winds up measuring heat.
“That thermally emitted radiation happens at the same wavelengths that we need to use to look for water. So in order to say with any confidence that water is present, we first need to account for and remove the thermally emitted component,” Dr. Milliken continued to explain in the July 24, 2017 Brown University Press Release.
In order to do precisely that, Dr. Li and Dr. Milliken used laboratory-based measurements of samples returned from the Apollo missions, combined with a detailed temperature profile of the areas of interest on the lunar surface. Using the new thermal correction, the two astronomers studied the data derived from the Moon Mineralogy Mapper, which is an imaging spectrometer that was carried aboard India’s Chandrayaan-1 lunar orbiter.
The two scientists found clear evidence of water in nearly all of the large pyroclastic deposits that had been mapped earlier across our Moon’s surface, including deposits near the Apollo 15 and 17 landing sites where the water-bearing glass bead samples were collected.
“The distribution of these water-rich deposits is the key thing. They’re spread across the surface, which tells us that the water found in the Apollo samples isn’t a one-off. Lunar pyroclastics seem to be universally water-rich, which suggests the same may be true of the mantle,” Dr. Milliken continued to explain in the Brown University Press Release.
The discovery that the hidden interior of Earth’s Moon contains water raises some interesting questions concerning lunar formation. Most planetary scientists think that our Moon was born from the debris left behind after the catastrophic collision of our primordial planet with the tragedy that was Theia. This collision would have occurred very early in our Solar System’s history. However, one of the reasons scientists had proposed that the lunar interior should be dry is that it is unlikely that the hydrogen necessary to create water could have survived following the ferocious heat of that ancient impact.
“The growing evidence for water inside the Moon suggest that water did somehow survive, or that it was brought in shortly after the impact by asteroids or comets before the Moon had completely solidified,” explained Dr. Li in the July 24, 2017 Brown University Press Release. “The exact origin of water in the lunar interior is still a big question,” he added.
In addition to shedding new light on the lunar water-mystery, the new research could also have important implications for future exploration of Earth’s Moon. The volcanic beads do not harbor a lot of water–only about.05 percent by weight–but the deposits are large, and the water could potentially be extracted.
Dr. Li continued to note that “Other studies have suggested the presence of water ice in shadowed regions at the lunar poles, but the pyroclastic deposits are at locations that may be easier to access. Anything that helps save future lunar explorers from having to bring lots of water from home is a big step forward, and our results suggest a new alternative.”
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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