Prehistoric Supersonic Monster Tides

It's a fact: In the early days of Earth when the Moon was newly formed, the two bodies were much closer and orbited each other much faster, spinning like ice dancers pulled tight. Gradually they've been slowing and pulling farther apart, leading to the distance we measure today, and the relatively mellow tides we enjoy. The Moon's gravity is what drives the tides on Earth, so you may have heard — either on television or the Internet or your other pop culture data spigot of choice — that in those earliest days the tides were enormous, manifesting as a titanic wall of crashing whitewater kilometers high, an endless sweep of destruction roaring across the continents at supersonic speeds, obliterating everything every few hours. It's an image that puts anything in Dante's Inferno to shame. It's an image of such intensity, such drama, that you may wonder if such cataclysmic violence was actually possible and actually real. Today we're going to deconstruct the components of this geologic and oceanographic melee, and see if we can determine just what the real conditions were in comparison to this popularly depicted cataclysm.

Let us go back in time some four and a half billion years ago. Earth and the other planets coalesced from the dust of which the Solar System was formed, the ejected material left over when a previous dying star exploded. Gravity and angular momentum drew this material together into a rotating disc, which gradually separated into bands and then into planets, with most of the material coming together at the center to form our Sun — heavier and heavier elements in each new generation.

When we discuss the origin of the Moon, there are unknowns but there is also some sound data supporting the standard model of its formation. A lot of what we do know comes from isotopic analysis of Earth and the Moon and other parts of the Solar System that we observe today. We can see spectroscopically where various isotopes of certain elements abound in the inner and outer parts of the Solar System; and this helps us determine where the Moon came from. This standard model will necessarily always remain incomplete; but every new detail we learn as technology has improved has bolstered, confirmed, and strengthened our theory.

This standard model tells us that the primordial Earth accreted from the circumstellar disc about 4.5 billion years ago. In those days we didn't even call it Earth yet; its name was Gaia. The angular momentum of the disc was preserved and translated into Gaia's rotation, which was a lot faster than today's Earth — a day was only a few hours long — however, something else soon happened that reset that rotation speed. To see what, let's pull back and observe another object that also accreted along the same orbit as Gaia.

Its name was Theia, another planetesimal. Theia, only about the size of Mars, was in Gaia's L4 or L5 Lagrange point — gravitationally stable points about 60° ahead of and behind Gaia in its orbit around the sun — and there Theia and Gaia would have stayed, happily enjoying the same orbit forever. Except they didn't. Venus, still seeking its own stable orbit, swung close enough to wobble Theia out of that position. Thrown off of its Lagrange point, Theia rolled inevitably toward Gaia, drawn by its gravity, and they collided at about 4 km/s.

Theia plunged into Gaia, sending a tremendous hot mess of ejecta into space. This new combined planet became the Earth. The ejecta formed into a ring, which gradually accreted into the new Moon. This process was completed by 4.4 billion years ago. Earth was rotating very fast, and the Moon was much closer and orbiting Earth much faster than today. Because of the conservation of angular momentum, everything rotates in the same direction: Looking "down" from "north" "above" the Solar System, Earth orbits the sun counter-clockwise; the Moon orbits Earth counter-clockwise; and both Earth and the Moon rotate counter-clockwise. Temperatures on Earth were extreme after the impact — Earth's earliest atmosphere consisted mainly of lava vapor and its earliest ocean was a planet-wide sea of liquid lava. Consequently, those earliest tides were of liquid and gaseous lava.

(We would be remiss not to acknowledge that there are competing hypotheses for what took place prior to the event that left us with today's Earth and Moon, however the standard model just described remains the one with the most evidence and support.)

To study early tides, it's necessary to understand the history of how Earth and the Moon interact. They follow the same laws of celestial mechanics as all bodies in space. The basic change to the movements undergone by all planets and moons that rotate in the same direction is a process called tidal acceleration. Tidal acceleration governs how everything in such a system slows down. Although it seems like a slowing-down process shouldn't be called acceleration, it's called that because when a planet rotates faster than its moon orbits, that rotation moves the tidal bulge just ahead of the orbiting moon, which gives it a tiny little gravitational tug and accelerates it, throwing it out to a farther orbit. The net effect on the whole system is that both bodies rotate more slowly, and the moon orbits more slowly and farther away.

Since tides cause planets to bulge (which we call gravitational kneading) and things like oceans and continents to move, such movements require energy, and the energy has to come from somewhere. This energy is taken out of the bodies' rotation speeds. Eventually they become tidally locked, meaning the same sides face one another. Our Moon is already tidally locked with Earth, and eventually Earth will also become tidally locked with the Moon.

Physics equations describe tidal acceleration, so you'd think that we should be able to plug in the numbers, do the math, and get an exact answer for how close and how fast the Moon was orbiting 4.4 billion years ago. However, some of the variables in these equations have to do with the viscosity of the bodies involved: squishy planets slow down a lot faster than rigid planets. Earth today, with its solid crust and firm mantle, is much more rigid than it was after the impact when its surface was all liquid magma. So the calculations become ferociously complex, and many of these variables for both Earth and the Moon are estimates. So the further back we go, the greater the range of uncertainty. Thus, today we still don't know exactly how far away from the new Earth the Moon accreted from its disc of ejecta.

However, there are some helpful points of reference that come from the geologic record here on Earth. These points of reference over the last couple billion years allow us to stick pins in the graph of the Moon's outward movement over time, thus narrowing our estimates for variables like Earth's viscosity at those points in time. These points of reference come in the form of rhythmites, a geological term for layers in sedimentary rock that record rhythmic events, including tidal cycles in some places. They don't record individual daily tides, but can show data like annual tidal intensity. How they're used gets complicated, beyond the scope of this show; but when we combine the tidal acceleration equations with these geologic observations, it becomes possible to determine the Earth-Moon distance at points in time with great accuracy, and thus also the tidal periods and intensities.

And so — drum roll please — here is what we've learned. We have a strong reference point for about 600 million years ago when the Earth-Moon distance was 96.5% what it is today, and another at 2.5 billion years ago when that distance was 90.6% what it is today, and some others too. We can complete the curve, and what we calculate is that by the time Earth had liquid oceans, the Moon was about 80% as far away as it is now. This indicates a tidal force of only about 40% greater than today's. Tides vary a lot on Earth, from practically nothing in places like the Mediterranean to 17 meters at the most extreme in the Bay of Fundy. A 40% increase in tides is less than today's normal range, so while the tides were indeed greater overall, in most places they'd still have been less than the biggest swings we have in some places today. So the short answer to our question is no, there was never any such thing as vast walls of whitewater crashing across the planet. In fact, if you could go back a few billion years to the earliest oceans and have a look, you probably wouldn't even notice the increase.

While that's disappointing — as we all love dramatic awesomeness — it's also interesting; because extending the computations from multiple lines of evidence reveals that in those first few million years, before there were water oceans, the Moon was indeed close enough to create monster tides. So what exactly happened?

It turns out that the Moon moved away from Earth very quickly, getting most of the way out there before Earth was cool enough for liquid oceans. What drove this sudden change in the rate of tidal acceleration was Earth's changing viscosity. And what drove that change in viscosity was — surprisingly — Earth's climate, a climate so insanely hot that only a planetary collision could account for it. For the first 10 million years after Earth and the Moon were established by the impact, Earth was liquid lava covered by an incredibly hot, dense, and heavy atmosphere of rock vapor thick with carbon dioxide — and it was kept molten-rock hot by that impenetrable atmosphere. Gravitational kneading on this fluid body was very effective at speeding the tidal acceleration, so the Moon moved away quickly. What put the brakes on was a relatively sudden "firming up" of Earth. As continental plates began to form above the roiling, convecting magma, there was rapid planet-wide subduction; and carbon-rich minerals formed in that atmospheric inferno were sucked into Earth's interior. This multi-million year process sharply reduced the atmospheric carbon dioxide; the temperatures dropped rapidly; continental plates firmed up and allowed the mantle to stabilize; and Earth became rigid enough that the tidal acceleration slowed. It was this point, some 10 million years into Earth's earliest history, that the Moon was about 80% as far away as it is now, and the planet became cool enough for its basins to fill with today's big blue oceans, thanks to all that carbon dioxide subducting into the interior. The Moon spent the next 4.4 billion years moving slowly out to its current orbit, and tides became correspondingly less.

Somewhere there is a Hollywood producer grumbling at this finding, as it's hard to imagine anything more cinematically spectacular than a kilometer-high supersonic wall of whitewater demolishing the planet a dozen times a day. But it's probably for the best, for if that had indeed been the case for a billion or so years of Earth's history, our advanced form of life likely wouldn't be here yet, or perhaps ever. At least for now, the surf is — thankfully — not up.

By Brian Dunning
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