Current consensus envisions the genesis of a planetary system as a natural, perhaps frequent, outgrowth of the birth of a star. The condensation model can generally account for each of the 7, earlier-noted properties characterizing our Solar System today. But exactly how those atoms of gas and grains of dust managed to coalesce into the present planets and moons remains one of the great riddles of modern science. Frankly, we may never know the precise details, given the role played by chance. Not that chance dominated events in the early Solar System, for determinism in science was also functioning. But chance is an essential factor in all evolutionary events, and the birth and development of our planetary system were no exception.

Role of Chance Chance, contingency, catastrophe, and the like are real factors in modern science, especially complexity science. They are evermore acknowledged these days, if not well understood, early in the 21st century. All such stochastic events function as agents of imperfection, helping us appreciate, at least in general terms, the many deviations from Nature’s otherwise well-ordered scheme of things. Variations from “perfect” categories abound among both the living and non-living. But, aside from the quantum realm of elementary particles, chance doesn’t usually rule our world or our lives—probably not ever. Contrary to popular opinion, chance isn’t in control of Nature’s multifarious phenomena. Not everything we see around us is an accident; far from it.

Astrophysicists build grand theoretical models for the origins of systems such as the planets described here, all the while guided by the known principles of deterministic science. Yet, and admittedly, embedded in that determinism that does derive from the laws and formulae of predictable, mechanistic physics is an ingredient of chance. The two go hand in hand like a dance: Chance flirts with necessity, randomness with determinism. To be sure, it is from this interchange that novelty and creativity arise in Nature, thereby yielding unique forms and novel structures. Much as for the galaxies and stars of earlier epochs along the arrow of time, chance did indeed play some role in the origin of our planetary system—and continuing now with the ongoing evolution of Earth itself. Chance and necessity are twin features that weave in and out of the cosmic-evolutionary tapestry. We shall return to them frequently.

Regarding the early Solar System, the role of chance would have manifest itself not only as planetesimals collided randomly to build protoplanets in the formative stages, but also as the established planets and moons were later bombarded with incoming debris left over well after those early stages. The effects of such collisions can still be seen today in many parts of the Solar System, not least on the marred surface of our own Moon. And it’s those chancy, catastrophic effects of collision that likely explain many of the planetary anomalies noted earlier, among them: Two large bodies probably collided and merged to form Venus, yielding its abnormally slow, in fact retrograde, spin. Uranus was likely affected by a grazing encounter with a massive body (possibly as large as Earth), tilting that planet almost completely over on its side. And many of the moons would have been nearly destroyed in random collisional events, accounting for some of their truly bizarre surface terrain. While impossible to test these assertions directly—for these events are long gone and done—we can reasonably suppose that some of the decidedly odd aspects of our Solar System, especially those deviating from its well-ordered architecture, can be explained by means of untraceable, chancy events during and after its formative stages. Sadly, we shall never know with any certainty the specific incidents triggered by chance, for most of this collisional activity must have occurred in the first billion years or so of our system’s history when the formative system was still a mess. These are not only events ancient and over but also events almost hopelessly confused with more recent evolutionary changes that have continued to sculpt our home in space.

Despite the catastrophic implications they carry, comets and asteroids are not entirely “vermin of the skies,” as these interplanetary wanderers were often termed only a half-century ago. Ironically, for they caused much lasting damage, these celestial vagabonds also provide perhaps the most useful information available about our home’s origins. Much of this debris that has survived to this day has preserved within it specks of solid and gaseous matter from eons past. Comets, in particular, may harbor material of a pristine, unevolved nature and thus have much to teach us about our local beginnings. Traveling in highly elliptical orbits and sometimes closely encountering the Sun, comets are often seen as faint and fuzzy patches of light, their tails beautifully sweeping across the nighttime sky. Yet, these “dirty snowballs” are more than spectacular sights of sublimating ice and rock, indeed more than mere inspiration for poets. Each time a comet appears in the heavens, think of it as a harbinger of news from an outer, ancestral reservoir—even if it is the merely postulated and yet unsighted Oort Cloud about a light-year away. Each comet that graces our skies potentially brings us a little more of the story of our Solar System’s origins.

Meteorites that land on Earth’s surface are especially telling about the original state of matter in the solar neighborhood. The smaller ones, <3 cm across, are mostly liberated swarms of cometary debris; the larger ones are more likely stray asteroids from the famous belt between Mars and Jupiter. Either type of meteorite extends our knowledge by bringing us novel information about extraterrestrial matter close to home yet well beyond Earth. The blackest, most primitive ones, known as carbonaceous chondrites that are rich in organic molecules, are of special interest to the cosmic-evolutionist since their clues may harbor data not only about planets and moons but also perhaps about life itself. We are left wondering if rocks on museum shelves, or even some of those in our backyards, might contain vital signs about the origins of home and selves.

We shall return in the BIOLOGICAL EPOCH to debate the relevance to life of these alien intruders of our Solar System, especially the meteoritic chunks that survived the plunge to Earth’s surface. These are the extraterrestrial bodies of rock and ice that have altered the geology of our planet throughout the course of history, as well as impacted—literally—the biology on planet Earth. And like mutations among life forms, not all impacts have been negative. During periods of bombardment, the motor of evolution often accelerates, granting the potential for change, diversity, death, and rebirth among Nature’s complex systems. The effects of impacting bodies on the evolution of life have rich implications for the interface between astronomy and biology—the crux of the newly emerging interdisciplinary subject of astrobiology.

Origin of Our Moon Earth’s Moon may well be the foremost example of a cosmic catastrophe in the local realm, the intrusion of chance into an otherwise straightforward condensation of gas and dust underway ~5 billion years ago. The origin of our Moon is surprisingly uncertain, although its age implies an event contemporaneous with the formation of Earth itself. The 4.5-billion-year-old age of the oldest Moon rocks, collected in the lunar highlands by manned American and robotic Russian missions of the 1970s, agrees closely with the age of all meteorites, as well as with the implied ages of both the Sun and the Earth. Apparently, the Moon partook of a grand event in our cosmic neighborhood that spawned our Solar System eons ago. However, none of the several theories advanced in the 20th century to account for the Moon has proved entirely satisfactory.

One theory holds that the Moon condensed as a separate object near the Earth, and in much the same way as did our planet. The two objects would have then essentially formed as a binary-object system, each revolving about the other. Though favored by many planetologists, this idea suffers from a major flaw: The Moon has only about half the density and a different composition than Earth’s, making it hard to understand how both could have originated from the same protoplanetary blob.

A second possibility has it that the Moon originated far from Earth and was later captured by it. In this way, the density and composition of the two objects need not be similar, for the Moon and Earth would have presumably formed in unrelated regions of the early Solar System. However, the concern here is that the Moon’s capture would not have been easy; it might well have been impossible. Why? Because the mass of our Moon relative to that of Earth (~1%) is larger than for any other moon of any other known planet. It’s not that our Moon is the largest natural satellite in the Solar System; but it is unusually big compared to its parent planet around which it revolves. Mathematical modeling implies that it would have been highly unlikely for Earth’s gravity to have captured the Moon in just the right way to avoid either crashing into Earth or skipping off into deep space.

A third idea maintains that the Moon materialized out of Earth itself. The Pacific Basin has often been mentioned as the place from which protolunar matter might have been torn, the result of centrifugal forces on a young, molten, and rapidly rotating Earth. As absurd as this idea may seem, the early findings of the Apollo Program seemed to favor it. Both the lunar composition and density were found to mimic those of Earth’s mantle, that region just below the crust. However, recent, more exacting studies of our Moon’s makeup show significant dissimilarities to Earth’s underbelly. What’s more, there remains the fundamental mystery of how the Earth could possibly have ejected into a stable orbit an object as large as the Moon.

Clearly, none of these theories is compelling. Each suffers from a major flaw or two. Yet, it would seem that one of them, or some version of them, must be correct. In fact, astronomers now favor a hybrid model combining the best features of each of the above ideas. The most popular model today postulates a vast, ancient collision between a young, molten Earth and a large, Mars-sized object. Impacts were undoubtedly common in the early Solar System, although one of this magnitude would have been nearly catastrophic; perhaps it was more of a glancing blow than a head-on collision. Matter dislodged from our planet, as well as parts of the impacting object itself, presumably then aggregated to form the Moon. Computer simulations do show that, for a collision at an oblique angle, debris having largely the composition of Earth’s mantle could have been ejected into a stable orbit nearly halfway to where the Moon resides today. It probably would have happened quickly, with the far-flung material reassembling into a single clump within a few weeks and forming a spherical rock resembling today’s Moon within a year.

Those computer models, like that shown in Figure 4.9, go on to show that, as Earth aged by billions of years and its spin slowed largely due to lunar tides, the Moon then naturally receded to its current distance, thereby preserving the total angular momentum of the Earth-Moon duo, as required by physical law. This dynamical scenario is supported by tidal layers deposited in rare rocks formed along prehistoric shorelines that show Earth’s day to have lasted ~18 hours and its year to have equaled nearly 500 days roughly a billion years ago. Furthermore, laser measurements (that time light’s travel back and forth) of the Moon’s distance have proved that it continues to move away from Earth at a rate of ~4 cm/year. Such a huge wallop might also explain not only how Earth spins so rapidly on its axis, but also how the tilt of its axis (which causes our seasons) is so large at 23o relative to the plane of its orbit. Not everyone agrees with this hypothesis, however, as other modeling suggests that, for typical collisions, the whole of Earth most likely would have shattered into pieces if hit with an object as large as Mars.

FIGURE 4.9 FIGURE 4.9 – This sequence shows a simulated collision between Earth and an object the size of Mars. The sequence proceeds bottom to top and zooms out dramatically. The arrow in the final frame shows the newly formed Moon. Red and blue regions represent rocky and metallic regions, respectively, showing how the Moon ends up with mostly rock. (W. Benz)

One of humankind’s most ancient questions seems still up for grabs. The origin of the closest celestial body to us, indeed one that hovers above us while reflecting brilliant splendor most evenings, is not well understood. Perhaps the formation of our Moon was the product of circumstances so rare that we shall never be able to unravel the details of its birth. Some researchers have been so perplexed by the origin of Earth’s Moon that they have felt forced, in desperation, to argue that the Moon cannot possibly exist!

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