At the last Tompkins County Friends of the Library Used Book Sale, I picked up a copy of Donald Yeomans's fascinating book titled Near-Earth Objects (which has the rather alarming subtitle, Finding Them Before They Find Us). Yeomans has impeccable credentials -- senior fellow with NASA's Jet Propulsion Laboratory, manager/supervisor of the Near-Earth Object Program Office and Solar System Dynamics Group, and researcher with the Deep Impact Project that investigates the composition, origins, and trajectories of comets. His book is about the potential for a significant asteroid or comet strike on Earth -- and, more importantly, how we might find potentially hazardous orbiting objects soon enough to have a chance to avert the collision.
As Canadian astronaut Chris Hadfield put it, "The dinosaurs went extinct because they didn't have a space program."
One of the topics in Yeomans's book is the history of impacts, including the famous one that ended the Mesozoic Era. But his timeline goes back a great deal further than that; one of the sections is devoted to a period called the Late Heavy Bombardment -- on the order of four billion years ago -- during which it is thought that the Earth got absolutely pummeled.
What caused this barrage? Well, first of all, it must be stated that not all scientists even think it happened. The geological processes on the Earth's surface have erased most of the evidence. Studies of cratering on the Moon (which presumably would also have gotten clobbered during the same period) have yielded conflicting results; Patrick Boehnke and Mark Harrison, of the University of California, wrote a paper back in 2016 suggesting that the radioisotope dating of rocks from the Moon supported a uniformly decreasing impact rate over its history (i.e., no sudden spike about four billion years ago).
Other researchers disagree. Three of the largest impact basins on the Moon, the Mare Imbrium, Mare Serenitatis, and Mare Nectaris, all appear to date from right around the time of the hypothesized bombardment. If the same happened on Earth, it was cataclysmic -- turning large areas of the Earth's crust into molten lava, and vaporizing huge volumes of water in the early oceans.
Where it gets interesting is the explanation for why the Late Heavy Bombardment happened -- if it did. The whole thing hinges on a bit of physics that falls into the "stuff that I theoretically knew, but never really thought about" department.
The orbital path of a planet (or asteroid, or comet, or whatever) remains stable as long as nothing adds or removes energy from it. If something subtracts energy, the orbit becomes smaller; if something adds energy, the orbit gets bigger. Enough added energy, and it achieves escape velocity and is ejected from the system altogether. But what would itself have enough energy to interact with something the size of a planet in such a way as to make any difference?
Back in the early history of the Solar System, there was a clutter of debris left over from its formation. We still have three major bands of it left -- the Asteroid Belt between Mars and Jupiter, the Kuiper Belt beyond the orbit of Neptune, and the Oort Cloud way out past the orbit of Pluto. There are few asteroids left in the vicinity of the planets, because any that were there were swept up gravitationally. In fact, that's one of the requirements for an object to be classified as a planet; that it clear the space near it of asteroids. (This is the characteristic that caused Pluto to get demoted.)
But four billion years ago, there was a great deal more debris around. Any large-ish asteroids that got near a planet resulted in their giving a gravitational yank on each other; if the asteroid was ahead of the planet, it had a bit of its energy stolen by the planet (making the planet's orbital axis get bigger); if it passed behind the planet, the reverse happened (making the planet's orbital axis shrink). Well, according to the models described by Yeomans, eventually the pushing and pulling by all of the asteroids added up, and a curious thing happened.
The two largest planets, Jupiter and Saturn, had their orbits altered until they were in a highly stable configuration called a 2:1 orbital resonance.
What this means is that they were in a pattern where Saturn's orbital period was exactly twice Jupiter's. (They're still close to that; Saturn orbits the Sun once every 29.4 years, and Jupiter once every 11.9 years.) But when they were in perfect 2:1 resonance, they reinforced each other's gravitational influence on the outer planets, Uranus and Neptune, giving them a kick every time they lined up -- a little like a kid on a playground swing kicking off every time they pass the ground.
This did two things. First, it gave energy to Uranus and Neptune, making their orbits bigger, moving them outwards. Second, it subtracted energy from Jupiter and Saturn, making their orbits smaller (and eventually destroying the resonance). But the important one here is Neptune, because the increase of its orbit moved it out into a region of space that hadn't been cleared of debris. When Neptune slipped outward into the inner Kuiper Belt, around four billion years ago, this had the effect of slingshotting a great deal of that debris into the inner Solar System...
... turning Earth into a gigantic bullseye for meteor strikes.
So it's fascinating that if the Late Heavy Bombardment actually did occur, there's a good model for what might have caused it.
The good news is that now that Jupiter and Saturn are no longer in resonance, Neptune is more or less staying put, so any further target practice is unlikely. Doesn't mean we're out of the woods completely, of course. Yeomans's whole book is about the possibility of asteroid strikes.
But at least it looks like the barrage is a thing of the past.
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