The Origin of the Moon

A Digital Supplement to Astronomy Insights Astronomy Magazine © 2019 Kalmbach Media THE ORIGIN OF THE MOON December 2019 • Astronomy.com The Moon’s violent origin Apollo astronauts returned 842 pounds of Moon rocks to Earth. These precious samples have revealed a lot about the Moon’s past. BY ROBIN CANUP T HE MOON HAS ALWAYS BECKONED. Long before our ancestors realized “wandering stars” were actually planets sharing the solar system with Earth, they recognized the Moon was a sort of sibling to our planet. And surely one of the first big questions to arise was: How did the Moon come to be? Fifty years ago, humans accomplished one of our greatest feats of exploration when we set foot on the Moon. The Apollo program has been recognized as a political and technological triumph, but less widely appre- ciated is the scientific windfall brought by the valuable lunar samples Apollo astronauts returned to Earth. These relics have ultimately proven vital to answering the age-old question of how the Moon formed. Apollo rocks reveal the Moon’s past Our planet has largely erased the record of its ancient past, thanks to a continual reshap- ing of its surface through geological activity. But the Moon is essentially dormant, so its heavily cratered surface preserves a record of solar system events going back billions of years. Thus, the Moon also opens a window into our planet’s primordial history. The Giant Impact Hypothesis posits the Moon formed after a Harrison Schmitt collects lunar samples from the surface of the collision between early Moon during Apollo 17 using a specialized scoop. Note how dirty Earth and a planet-sized Schmitt’s suit is, thanks to the cohesive nature of lunar regolith. impactor, as shown in NASA this artist’s concept. NASA/JPL-CALTECH 2 ASTRONOMY INSIGHTS • DECEMBER 2019 The Moon’s violent origin Apollo astronauts returned 842 pounds of Moon rocks to Earth. These precious samples have revealed a lot about the Moon’s past. BY ROBIN CANUP T HE MOON HAS ALWAYS BECKONED. Long before our ancestors realized “wandering stars” were actually planets sharing the solar system with Earth, they recognized the Moon was a sort of sibling to our planet. And surely one of the first big questions to arise was: How did the Moon come to be? Fifty years ago, humans accomplished one of our greatest feats of exploration when we set foot on the Moon. The Apollo program has been recognized as a political and technological triumph, but less widely appre- ciated is the scientific windfall brought by the valuable lunar samples Apollo astronauts returned to Earth. These relics have ultimately proven vital to answering the age-old question of how the Moon formed. Apollo rocks reveal the Moon’s past Our planet has largely erased the record of its ancient past, thanks to a continual reshap- ing of its surface through geological activity. But the Moon is essentially dormant, so its heavily cratered surface preserves a record of solar system events going back billions of years. Thus, the Moon also opens a window into our planet’s primordial history. The Giant Impact Hypothesis posits the Moon formed after a Harrison Schmitt collects lunar samples from the surface of the collision between early Moon during Apollo 17 using a specialized scoop. Note how dirty Earth and a planet-sized Schmitt’s suit is, thanks to the cohesive nature of lunar regolith. impactor, as shown in NASA this artist’s concept. NASA/JPL-CALTECH WWW.ASTRONOMY.COM 3 One of the main goals of the Apollo Lunar samples GIANT IMPACT collected during the program was to distinguish among the Apollo missions, like HYPOTHESIS then-leading theories about how the those seen here, help reveal the Moon’s Moon formed: capture, co-formation, mysterious origin. and fission. The capture theory posited The small cubes the Moon formed independently from measure 0.4 inch Theia (1 cm) on a side and Earth Earth, only to be caught later by our are included for planet during a fortuitous close flyby. scale. NASA/DAVID KRING The co-formation theory, however, envisioned the Moon grew alongside Earth, with the pair accumulating mass from Earth to spin more slowly. This implies the same time, work on the competing enter into a stable orbit around the the Moon, while simultaneously leading the same source of material. The third the Moon formed much closer to Earth models proved increasingly unsatisfying. planet. However, a large enough impact to a five-hour day for Earth. Then, over model, fission, proposed ancient Earth than it is now. Precise measurements of The energy dissipation needed to cap- — one by a body roughly the size of the billions of years, tidal interactions trans- rotated so rapidly that it became unsta- the Moon’s position using surface reflec- ture the Moon intact during a close flyby target itself — distorts the shape of the ferred angular momentum to the Moon, ble, developing a bloated midsection and tors placed during the Apollo program seemed implausible, if not impossible. impacted planet, altering its gravitational which gradually pushed the Moon out- shedding material from its equator that subsequently confirmed this, verifying Models of the Moon’s co-formation interactions with the ejecta. ward and slowed Earth’s spin. This fits would eventually become the Moon. the Moon’s orbit expands by about alongside Earth struggled to explain why Additionally, researchers knew par- well both with Earth’s current 24-hour Apollo’s cache of lunar samples and 1.5 inches (3.8 centimeters) each year. the Moon would have a vastly different tially vaporized material can get its own day, as well as the present orbital distance Impact data introduced researchers to tantaliz- proportion of iron. Additionally, the cur- acceleration with the help of escaping of the Moon. ing new clues and constraints for these Giant Impact Hypothesis rent angular momentum of the Earth- gas, which changes the material’s trajec- three models. For instance, measuring As is not uncommon in science, the Moon system is too low to be explained tory. However, assessing the effects of Lingering questions the age of the oldest Apollo samples Apollo data, which was originally by a rotationally unstable Earth that this scenario required a new generation If the Moon were like other astronomi- showed that the Moon must have formed intended to test existing theories, instead flung off enough material to form the of computer simulations at a scale never cal bodies, for which we typically have some 4.5 billion years ago, only inspired a new one. In the mid- Moon. Although researchers carried out before modeled. With then-available only remote observations, at this point 60 million years or so after the 1970s, researchers pro- little quantitative work on the giant technology, such simulations were we would have likely declared the origin first grains in our solar sys- posed the Giant Impact impact model at first, it eventually extremely challenging, but researchers story of the Moon solved. In this case, tem condensed. This The Apollo Hypothesis. The novel emerged as the most promising theory eventually showed that giant impacts however, we have physical samples from means the Moon came data, which was scenario envisioned during a mid-1980s conference on lunar could produce orbiting material that both the Moon and Earth that we can to be during the same that at the end of its origin, largely due to the weaknesses of might assemble itself into the Moon. compare. Explaining the chemical rela- early epoch that saw originally intended to formation, Earth col- competing theories. Thanks to vast computational tionship of those samples has proven the birth of the planets. test existing theories, lided with another But could a giant impact really pro- improvements, researchers had identified to be the biggest challenge to the Giant From remote mea- planet-sized body. duce the Moon? The answer to this by the early 2000s what would later Impact Hypothesis, inspiring a flurry of surements of the instead inspired This produced a great question was not obvious at the time. become known as the “canonical” impact work over the past decade investigating Moon’s mass and radius, a new one. deal of debris in Earth’s From basic physics, scientists knew that theory: a low-velocity collision at about a how exactly the Moon came to be. researchers also know its orbit, which in turn ejecta launched from a spherical planet 45-degree angle by Theia, which had a The conundrum is this: In most giant, Debris density is anomalously low, coalesced into the Moon. should either escape entirely into space mass similar to that of Mars. Such an disk-forming impacts like those disk indicating it lacks iron. While The impacting planet would or plummet back to the planet’s surface. impact would produce an iron-depleted described above, it is mainly material about 30 percent of Earth’s mass is later be named Theia, in honor of the The ejected material should not simply disk of material massive enough to form from the outer portions of Theia that are trapped in its iron-rich core, the Moon’s Greek goddess who was the mother of slung into Earth orbit. But we cannot core accounts for only a few percent of its the Moon. know for certain what Theia’s composi- total mass. Despite this substantial dif- The new impact theory seemed to rec- tion was when it impacted Earth. If ference in iron, Apollo samples later oncile multiple lines of evidence. If the METEORIC CONFLICT Theia, like Mars or main-belt asteroids, revealed that mantle rocks from the material that formed the Moon origi- 3.5 Meteorites from Mars were made of different material than Debris coalesces and Vesta have different Moon and Earth have remarkably similar nated from the outer layers of Earth and compositions than the Earth, then ejecta coming from Theia Moon concentrations of oxygen.

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