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Extracts (4)

At the end of the period of planetary accretion, about 4.5 billion years ago, there were more planets than there are now, all slung around by close calls and orbital resonances and pulled together by gravity, so that they sometimes collided. They had been doing it for a billion years to get to this point, and this was the last stage of that process of accretion. During this period every one of the inner planets took at least one very big hit.

A planet called Theia grew at Earth’s L5 point until it was about the size of Mars, then drifted into a collision with Earth. It hit at a forty-five-degree angle, at something less than four kilometers per second—not fast in astronomical terms. Theia’s iron core plunged in and merged with Earth’s core, and Theia’s mantle and some of Earth’s mantle were thrown into orbit. The angular momentum imparted by the hit spun Earth to a five-hour day. Two moons accreted from the ejected material rather quickly; estimates range from a month to a century. Eventually the smaller moon splatted onto the larger one, leaving behind the jagged mountains on the anti-terran side of the resulting moon, Luna.

Around the same time, a small planet about three thousand kilometers in diameter struck Mars and created the Borealis Basin, which is basically Mars’s northern hemisphere, still six kilometers lower than the southern hemisphere.

Venus was struck by a Mars-sized planet, creating a moon like Earth’s, called Neith; ten million years later, another impactor gave Venus its slow retrograde motion. This change in rotation slowed Neith and caused it to plunge back into Venus and merge with it.

Mercury was struck by a protoplanet half its size, at such a speed and angle that Mercury’s mantle was stripped off and cast throughout Mercury’s orbit. Ordinarily Mercury would have swept these pieces back up, but in the four million years this process would have taken, most of the material was pushed outward by solar radiation and thus never made it back to Mercury. About sixteen quadrillion tons of Mercury’s crust eventually ended up on Earth, and more on Venus. In the end only the heaviest 70 percent of Mercury remained, essentially the planet’s core. Thus the Mars g for a diameter smaller than Titan’s.

Somewhat later, the young Jupiter and Saturn fell into a one-to-two orbital resonance, with Jupiter spinning through two of its years for every one of Saturn’s. This created a very powerful combined gravity wave, swinging around the solar system at varying strength, depending on where the two giants were in relation to each other. This new wave at its strongest caught Neptune, which had grown just outside Saturn, and threw it away from the sun! Neptune flew out past Uranus, pulling Uranus outward too, also onto its side. Only at that point did the two smaller gas giants end up in the orbits they now occupy.

Inside Jupiter’s orbit, meanwhile, that same Jupiter-Saturn resonance wave caught asteroids and threw them like pinballs all over the system, in the period called the Late Heavy Bombardment, 3.9 billion years ago. All the inner planets and moons were pummeled with impacts, to the point where the surfaces of these planets often were seas of molten rock.

The Era of Big Hits! The Late Heavy Bombardment! Never let it be said that the great merry-go-round is entirely fixed and regular in its motion—that it doesn’t sometimes resemble more a swirl of bumper cars. Gravity, mysterious gravity, immutably following its own laws, interacts with matter, and somehow the result is complex motion. Invisible waves slinging rocks this way and that.

What if human history has such invisible waves? Because ultimately the same forces apply. What big hits made us what we are? Will some new resonance create a wave and throw us in a new direction? Are we entering our own Late Heavy Bombardment?