When the Earth Had Two Moons

Cannibal Planets, Icy Giants, Dirty Comets, Dreadful Orbits, and the Origins of the Night Sky

Erik Asphaug

• Publisher: Custom House
• Year: 2019
• ISBN:
  9780062657947
• Topic: Physical Sciences
• Subtopic: Space Science

This expert guide to plant formation and the origins of life "makes the solar system an even weirder and more wonderful place than it seemed before" ( Wall Street Journal ). In 1959, the Soviet probe Luna 3 revealed an astonishing truth: the far side of the moon is an enormous mountainous expanse, completely different from the vast lava-plains on the side facing Earth. But why would the two side of the moon be so different? And what might this tell us about our own place in the universe? As it turns out, quite a lot. Fourteen billion years ago, the universe exploded into being, creating galaxies and stars. Planets formed out of the leftover dust and gas that coalesced into larger and larger bodies orbiting around each star. In a sort of heavenly survival of the fittest, planetary bodies smashed into each other until solar systems emerged. Curiously, instead of being relatively similar in terms of composition, the planets in our solar system, and the comets, asteroids, satellites and rings, are bewitchingly distinct. So, too, the halves of our moon. In When the Earth Had Two Moons, esteemed planetary geologist Erik Asphaug takes us on an exhilarating tour through the farthest reaches of time and our galaxy. Beautifully written and provocatively argued, When the Earth Had Two Moons is not only a revealing look at outer space, but a profound inquiry into the nature of life here.

Chapter 1

Ruined Structures

Heaven’s net casts wide.

Its mesh is coarse, but nothing slips through.

—TAO TE CHING

THE STUDY OF PLANETS HAS given rise to hundreds of celebrated philosophers and forgotten sages throughout the world. As with the Big Bang, there was no center to the expansion, but a few great visionaries stand out like signposts on the road that got us to this place.¹ The most transformative epoch in astronomy was in early Greece, so we could start there, or earlier still, in India and China—as if we have to start at all. As if we weren’t just beginning. But for the sake of narrative, and because it’s familiar, let’s go straight to the time of Shakespeare, when the Copernican revolution had spread throughout Europe, and Johannes Kepler was the studious defender of that most dangerous of theorems, De revolutionibus orbium coelestium,² that the Earth goes around the Sun.

Although he would design some of the most versatile telescopes in his day, Kepler’s research was grounded in pre-telescopic observations³ with an emphasis on astrometry, the precise geometry of planets in the sky. Geometry, the measure of the world. The retrograde motion of Mars, going westward through the stars, and then eastward (and brightening), and then westward again, was explained as being due to the relative movements of Earth and Mars, like one horseback rider looking at another against the backdrop of the hills. While both horses gallop in circles, it appears that one is sometimes falling back, sometimes racing ahead, and sometimes sweeping a fast arc as it passes on the inside bend. With the Sun at the center.

This relative motion is the physical geometry of parallax, the same type of geometry that was used by the early Greeks to measure the distance to the Sun. Aristarchus of Samos and others had already figured out in the third century BC that the planets orbited a central fire, that it was many times farther away than the Moon, and that the stars were many times farther still. But empires fall apart; Greek knowledge was all but forgotten, and rediscovered in the West in a way that led to modern physics.

Kepler was bothered by systematic errors in the positions of the planets. These errors would go away, he showed, if the planets orbit the Sun in ellipses rather than circles, and if they move faster when they are closer to the Sun. It was brave to propose such a bold new geometry of the heavens, and the church did not take kindly to it. Only nine years earlier, the Italian philosopher and sometime astronomer Giordano Bruno had been executed for maintaining, before the invention of the telescope, that “innumerable suns exist. Innumerable earths revolve around these suns in a manner similar to the way the seven planets revolve around our Sun. Living beings inhabit these worlds.” With no more evidence to back him up than existed in the age of Thales of Miletus, Bruno stuck to his guns and was burned at the stake in 1600.⁴

Kepler’s own mother had come close to being burned at the stake as a witch, so he was closely aware of the danger of radical ideas. His approach to doing science was more determined and less flamboyant than Bruno’s (although later in his life he would write what many regard as the first science fiction story, Somnium Astronomicum,⁵ which envisions human voyages to the Moon). Why did the errors matter? Kepler’s equations for planetary motion, later known as Kepler’s laws, would be put into a physical framework by Isaac Newton in the form of gravity and momentum, and physics was born.

An engraving of the cosmic boundary by an unknown artist, likely from the 1600s, as reprinted in Camille Flammarion’s 1888 book L’atmosphère: météorologie populaire. The character resembles Giordano Bruno, who was martyred in 1600.

Camille Flammarion, L’atmosphère: météorologie populaire (Paris, 1888), p. 163

Lest you think of Kepler as some sort of driven advocate of facts, his culture was prescientific. (Today we are becoming nonscientific, which is a different and sadder thing.) In the early 1600s there was no coherent theory of physics and no real path forward with a quantitative law. Natural philosophy was revelatory: This is what I see, and this is what I observe to be true.

Kepler discovered a crystalline-cosmological relationship that he would never let go of, that the six planets orbit the Sun according to the distances of spheres circumscribed by the five Platonic solids: the tetrahedron, the cube, the octahedron, the dodecahedron, and the icosahedron,⁶ published in a set of essays and discoveries called Mysterium Cosmographicum, Forerunner of the Cosmological Essays, Which Contains the Secret of the Universe; on the Marvelous Proportion of the Celestial Spheres, and on the True and Particular Causes of the Number, Magnitude, and Periodic Motions of the Heavens; Established by Means of the Five Regular Geometric Solids.

In Shakespeare’s day, one could not imagine there being another planet⁷ any more than one could imagine there being an additional day to the week, so Kepler lived out his days comfortable in his received wisdom. Long after he was gone, this would all be upended when the first new planet since antiquity, Uranus, was discovered. This would ruin the Mysterium Cosmographicum and other theories, and confuse us about the days of the week, but shored up Newton’s general laws.

And here we are today. Never in his wildest dreams would Kepler have imagined a telescope orbiting the Earth in space, bearing his name, one that would discover thousands of new planets of all sizes and orbits, many of them potentially habitable and even Earthlike. Nor would he have dreamed that every one of them would follow the laws that he set forth in Harmonices Mundi. I wonder which idea Kepler had more faith in, the one that he was so wrong about, his beloved Mysterium, or the one that would become the basis for the natural laws and much of modern science?

ALTHOUGH SHE WOULD not have considered herself a planetary scientist, the French-Polish physicist Marie Curie discovered the atomic nature of radioactivity, bringing a reality that would ultimately topple the edifices of nineteenth-century geology and lead to revolutionary ideas about planets and stars. She showed that radioactive atoms break down in decay chains, forming stable daughter products. On a time scale of billions of years, two common isotopes of uranium, ²³⁵U and ²³⁸U, break down,⁸ becoming isotopes of lead (²⁰⁶Pb and ²⁰⁷Pb). Because U is relatively common in rocks, its breakdown skews the lead-lead isotopic ratios inside of crystals, changing in time, so that the ages of rocks can be measured with surprising precision.

Prior to the discovery of radiogenic lead, scientists had been thinking of the Earth and the Sun as tens of millions of years old at most, based on a convergence of arguments that we’ll get into. Maintaining a minority opinion, the Scottish geologist Charles Lyell, close friend of Charles Darwin, was ushering in the quantitative science of sedimentology and arguing for a much older Earth. It would take billions of years to lay down all the layered outcrops he and his followers had begun interpreting as uplifted ocean basins. The science of geochronology advanced, with debates raging over the age of the Earth and its surface evolution, and ideas of biological evolution spearheaded by Darwin’s monumentally important On the Origin of Species, which argued for an immense span of time for life to evolve.

Understanding the atomic nature of radioactivity and the relationship between uranium and lead allowed scientists to reliably calculate the age of the Earth. By the 1930s it was known, based on simple graphs of lead abundance, that some Earth rocks are at least two billion years old. The emerging deep span of geologic time was being discovered just as astronomers were discovering the deep span of cosmic space.

ALMA image of the young star HL Tauri, about 450 light-years from Earth, and its protoplanetary disk. The star is estimated to be only 100,000 years old, yet planet formation seems to be well under way, as emerging giant planets sweep their orbits clear of dust and gas.

ALMA (ESO/NAOJ/NRAO); C. Brogan, B. Saxton (NRAO/AUI/NSF)

In the 1920s the American astronomer Edwin Hubble went on to discover that we are one of a teeming multitude of galaxies distributed throughout the cosmos, the modern view. He also proved that galaxies are receding from us in all directions, and that those farther away are receding the fastest. He deduced from this that the Universe is expanding isotropically, like dots on the surface of a balloon that is inflating. Each dot thinks it’s the middle of an expansion, but in reality none of the dots are special.

He estimated the age of the Universe by calculating the time it would have taken for all the galaxies to get to their current distances, starting from some theoretical critical point when time and space began (the balloon, uninflated). This age (although we don’t understand what it means) also turned out to measure in the billions of years, just like the sedimentary layers, just like the uranium-lead clocks in the rocks, just like the biology. It had become very real, very fast, that creation goes back further than we ever imagined, and goes forward beyond the end of species.

ISAAC NEWTON GREW up in the mid-1600s, familiar with Kepler’s laws. One of his key achievements was to generalize those laws into relationships involving mass and time and space. He invented the inverse square law of gravity, which states that two objects are attracted toward each other in proportion to their masses, divided by the square of how far they are apart. Since its conception, this law of gravity has always proven basically true, and it is so elegant that one gets the feeling it has been there all this time, imprinted into nature, just waiting to be discovered.⁹ But the law of gravity is not completely true, so it was not there waiting, but must be a human creation fitting our preference for simple theories. Neither is Einstein’s general theory of relativity the end; it is another human creation and a perturbation of Newton’s theory, leading to more accurate predictions and a different reality beneath the equations, just as subsequent theories—themselves further perturbations and progressions—will be needed to explain new data and better knowledge. But discussion about physics progressed in mathematical terms using explicitly defined quantities of mass, time, and distance.

The greatest scientific advancement derives from paying attention to small inconsistencies. It had bothered Kepler that circular orbits didn’t quite fit. By the late 1800s, it was similarly troubling that the planet Mercury had an orbit that was precessing too rapidly. Mercury’s orbit is eccentric,¹⁰ and the perihelion point (when it is closest to the Sun) changes location slightly every Mercury year. Its orbit precesses, tracing a pattern like a Spirograph. Precession is mostly due to the tugs from the other planets, but it did not add up; there had to be another significant perturbation not accounted for by Newton’s law. Could there be a substantial “ether,” a substance pervasive through space, dragging on Mercury? No; Mercury would spiral in. Could an unobserved planet Vulcan be influencing Mercury’s orbit? No; Vulcan would have been discovered.

Along came the physicist Albert Einstein, whose 1916 theory of gravity known as general relativity predicted a precession of Mercury’s orbit that accounted for the discrepancy. Astronomers were happy and went on to the next discrepancy. But for physicists it changed their world view forever, adding a new dimension to the Universe. General relativity does not invalidate Newton’s law; it gives a geometrical basis for it: the warped curvature of space-time. Gravity is not a force; it is the gradient of a potential field. For most of us the distinction doesn’t matter—Newton’s gravity holds true to high enough precision to govern the day-to-day movements of the planets and moons, and me in my hammock, and even rockets that reach outer space.

HUMANS AS GIFTED as Newton have been around in every generation since the Stone Age. Fossils show that the cranium doubled in size around a million years ago, and what that was all about, we may never know. We used this new brain to produce stone tools with increasing sophistication—flatter, made out of better materials, with more defined cutting edges and designs. Better tools allowed us to exploit the energy-dense foods needed to pay for these energy-intensive brains. The fabrication of each tool was a challenging puzzle, as was every hunt, every migration to new grounds. We learned the properties of rocks, and the patterns of the Moon and stars and what would become known as planets.

The Enlightenment was a period of global awareness when the smartest humans were able to be brilliant. As Newton wrote in a letter to Robert Hooke, we are “standing on the shoulders of giants.” Science was made possible by the establishment of a connected culture, one that allowed it to advance and to follow every new and detailed observation from near and far—a deep well in Syene, a cliffside in Taihang, the Magellanic Clouds. Also, scientists of the Enlightenment were born into a world that was ready for a new world view, relatively free from the encumbrances of doctrine,¹¹ organized around a system for vetting, debating, and transmitting collaborative knowledge and supported by formal reasoning, especially math, that would allow them to weigh the planets and measure the charge of an electron.

Saturn’s giant moon Titan was discovered in 1655 whe...

Table of contents

• Cover
• Title Page
• Dedication
• Contents
• A Short List of Planets and Moons
• Introduction
• Chapter 1: Ruined Structures
• Chapter 2: Rocks in a Stream
• Chapter 3: Systems Inside Systems
• Chapter 4: Strange Places and Small Things
• Chapter 5: Pebbles and Giant Impacts
• Chapter 6: The Last Ones Standing
• Chapter 7: A Billion Earths
• Conclusion
• Epilogue
• Acknowledgments
• Glossary
• Notes
• Index
• About the Author
• Copyright
• About the Publisher