Artist's image of the Sun as a T Tauri star
Luckily, in the past decade we have learned a vast amount about this history. The mists of time are clearing. It seems we are not alone in passing through a perilous time. The Earth has survived some remarkable disasters. To keep our tale brief, let us focus on four: the Big Splat about 4.55 billion years ago, the Late Heavy Bombardment about 4 billion years ago, the Oxygen Catastrophe roughly 2.5 billion years ago, and the Snowball Earth events about 850 million years ago. The details of these events — and indeed, whether they even happened at all — remain controversial. We shall present some widely accepted theories without dwelling on caveats or alternative scenarios. In every case, there is interesting physics involved in testing these theories.
When the center of this disk became dense enough for its pressure to hold itself up, our Sun was born as a "protostar". This phase lasted a scant 100 thousand years or so; the temperature then rose to the point where an outflow of hot gas prevented the Sun from accreting more material. At this point the Sun became what we call a "T Tauri star", powered only by gravitational energy as it slowly shrank. After about 100 million more years, it became an ordinary main sequence star as the hydrogen at its core began to undergo fusion.
Some dust circling the early Sun got hot and melted, and some of the molten droplets later froze into "chondrules" — millimeter-sized spheres of simple minerals such as pyroxene and olivine, which are mostly made of sodium, calcium, magnesium, aluminum, iron, silicon and oxygen. These chondrules are the main constituent of some of the most primitive objects that still ply their way through the Solar System: stony meteorites called "chondrites".
The dust circling the early Sun started forming lumps called "planetesimals". As these lumps collided, they got bigger and bigger, eventually forming the asteroids and planets we see today. Some lumps melted, letting heavier metals sink to their core while lighter material stayed on top. And some crashed into each other, shattering and forming chondrites and other meteorites: iron-nickel meteorites, and stony meteorites called "achondrites".
By radioactive dating of meteorites, researchers claim a shockingly precise knowledge of when all this happened: sometime between 4.56 and 4.55 billion years ago. So, the Earth was probably formed sometime around then — and our story officially begins at this point.
The Earth's history is divided into four eons: Hadean, Archean, Proterozoic and Phanerozoic. When I was a child, the "Cambrian era" was as far back as my textbooks went, except for the murky "Precambrian". But the Cambrian began just 540 million years ago. The Cambrian marks the start of the current eon, the Phanerozoic, meaning "the age of visible life". This is when multicellular organisms took over the world, leaving fossils we can see. But we will dig much deeper: the Phanerozoic will be end of our story.
Back to the Hadean. As befits its name, this was a time when the Earth was hellishly hot. It began with an event that formed the Moon around 4.53 billion years ago. What made the moon? The current most popular explanation is the "giant impact theory" — sometimes called the "Big Splat" theory.
The idea is that another planet formed at one of the Lagrange points of Earth's orbit. In 1772, Lagrange showed that if you have a planet in a circular orbit about the Sun, a much lighter body will stably orbit the Sun at the same distance if it lies 60 degrees ahead or behind that planet. There are indeed many asteroids located near the Lagrange points of Jupiter, and also some at the Lagrange points of Mars and Neptune.
No asteroids have been found at Earth's Lagrange points. But according to the giant impact theory, a planet did form at one of these points. When it reached about the mass of Mars, it would no longer be stable at this location. It would gradually drift toward Earth, and eventually smack right into us! This collision could have formed the Moon.
It's a dramatic theory, but there is a strong case for it, nicely summarized by Dana Mackenzie's recent book The Big Splat, or How Our Moon Came to Be. For example, tidal friction is making the Moon gradually recede from the Earth. We know it is moving away now, at about 3.8 centimeters per year. Ancient sediments record the tides and show that months have been getting longer at least since the Precambrian. Extrapolating backwards we find a Moon very close to the Earth in the Hadean eon. Could it have been flung off from the Earth by centrifugal force, or formed near the Earth in the first place, or captured by the Earth's gravitational field? All these theories must be considered, but the giant impact theory seems to fit the data best. People take it so seriously that the hypothetical doomed planet that hit Earth even has a name: Theia. In Greek mythology, Theia was a titan who gave birth to the Moon.
In 2004, the astrophysicist Robin Canup, at the Southwest Research Institute in Texas, published some remarkable computer simulations of the Big Splat. To get a moon like ours to form — instead of one too rich in iron, or too small, or wrong in other respects — she had to choose the right initial conditions. She found it best to assume Theia is slightly more massive than Mars: between 10% and 15% of the Earth's mass. It should also start out moving slowly towards the Earth, and strike the Earth at a glancing angle.
The result is a very bad day. Theia hits the Earth and shears off a large chunk, forming a trail of shattered, molten or vaporized rock that arcs off into space. Within an hour, half the Earth's surface is red-hot, and the trail of debris stretches almost 4 Earth radii into space. After 3 to 5 hours, the iron core of Theia and most of the the debris comes crashing back down. The Earth's entire crust and outer mantle melts. At this point, a quarter of Theia has actually vaporized!
After a day, the material that has not fallen back down has formed a ring of debris orbiting the Earth. But such a ring would not be stable: within a century, it would collect to form the Moon we know and love. Meanwhile, Theia's iron core would sink down to the center of the Earth.
The giant impact theory is still much debated, in part because there is little direct evidence left: the oldest known rocks on Earth were formed almost half a billion years later.
After the Moon was formed, the Earth continued to suffer many impacts. Curiously, instead of gradually dropping off over time, these may have spiked during a period called the Late Heavy Bombardment, from 4 to 3.8 billion years ago. A lot of large craters on the Moon date to this period, so probably the Earth got hit too — but here, such old craters would be lost to weathering and geological activity. So, the Moon is our guide.
During the Late Heavy Bombardment, the Moon was hit by 1700 meteors that made craters over 100 kilometers across. The Earth could easily have received 10 times as many impacts of this size, with some being much larger. To get a sense of the intensity of this pummeling, recall the meteor impact that may have killed off the dinosaurs at the end of the Cretaceous period 65 million years ago. This left a crater 180 kilometers across. Impacts of this size would have been routine during the Late Heavy Bombardment.
Why was this era so violent? One theory is that Jupiter and Saturn moved into a 2:1 orbital resonance around this time, causing a big disruption in the original population of asteroids and icy objects orbiting the Sun. In 2005, an international collaboration of planetary physicists including Hal Levison published a paper in Nature on some fascinating computer simulations of a Solar System. As initial conditions, they take all four gas giants to lie in circular orbits more closely spaced than they are now. By interacting with planetesimals, Saturn, Uranus and Neptune gradually migrate outwards. When Saturn reaches the point where it orbits the Sun once for every two orbits of Jupiter, the whole outer Solar System destabilizes. The orbits of Neptune and Uranus become more eccentric, and they throw many planetesimals out of their original orbits. Some are hurled into the inner Solar system, explaining the Late Heavy Bombardment.
It is believed that the Earth's surface cooled enough to form a crust even before the Late Heavy Bombardment. Meanwhile, volcanic activity would have released lots of steam, carbon dioxide, and ammonia. This formed what is called the Earth's "second atmosphere". The Earth's "first atmosphere", mainly hydrogen and helium, was already lost to space. The second atmosphere was mainly carbon dioxide and water vapor, with some nitrogen, but probably not much oxygen. This second atmosphere had about 100 times as much gas as today's "third atmosphere".
As the Earth cooled, oceans formed. They may have boiled away completely during some large impacts, but then reformed. Eventually much of the carbon dioxide in the atmosphere dissolved into the seawater. This later precipitated out as carbonates, starting a new phase in what the geologist Robert Hazen and his coauthors call "mineral evolution". This is not evolution in the Darwinian sense, just the gradual diversification of minerals over the Earth's history. In 2008, a team of geologists led by Hazen estimated that 350 kinds of mineral could be found on Earth during the Hadean Eon. But as the Earth's history proceeds, their count keeps rising. By the end of the Archean it reaches 1500, thanks in part to the formation of oceans — but also thanks to the rise of plate tectonics.
The first step in plate tectonics was the formation of "cratons": ancient tightly-knit pieces of the earth's crust and mantle, dozens of which survive today. For example, southeastern Wales and part of western England lie in the Midlands Craton. While most cratons only finished forming 2.7 billion years ago, nearly all started growing earlier. Cratons are made largely of igneous rocks like granite, which are more sophisticated than basalt. Granite is made in a variety of ways, for example by the remelting of sedimentary rock. Early granite-like rocks were probably simpler.
Cratons fit together to form the larger plates that constitute the Earth's crust today. Indeed, plate tectonics as we know it started about 3 billion years ago. A key aspect of this process is the recycling of the Earth's crust through "subduction": oceanic plates slide under continental plates and get pushed down into the mantle. Another feature is underwater volcanism, leading to hydrothermal vents: fissures in the sea floor that spew out hot water.
It is possible that these vents played a role in the most dramatic of all Archean developments: the origin of life. Since the early Earth lacked free oxygen, the first life must have been anaerobic. Even today, many of the oldest microbes, such as those found in hydrothermal vents, cannot tolerate the presence of oxygen. Such organisms gave rise to an active sulfur cycle and deposits of sulfate ores starting about 3.6 billion years ago. Later they made the atmosphere increasingly rich in methane.
At some point microbes started photosynthesizing and putting oxygen into the atmosphere. It seems likely that the first plants acquired their ability to photosynthesize by symbiosis with such microbes. Indeed, the chloroplasts in plants have their own separate DNA.
It is not very clear when photosynthesis began — estimates range between 3.5 and 2.6 billion years ago. One possible clue: rocks called "banded iron formations", made of thin layers of iron oxides alternating with iron-poor rock, started to appear about this time. They may have formed when oxygen from the first photosynthesizing organisms reacted with iron in seawater. Nobody knows for sure why the periods of iron-rich sediment come and go.
It took a long time for photosynthesis to have a significant effect on the Earth's atmosphere — but when they did, roughly 2.5 billion years ago, the result was dramatic. After all, oxygen is highly reactive in its gaseous form, and most early life could not tolerate it. So, this episode in the Earth's history has been dubbed the Oxygen Catastrophe. Luckily evolution found a way out: now many of us need oxygen.
The Oxygen Catastrophe marks the end of the Archean and the beginning of a new eon, the Proterozoic. The next billion years were dominated by something called the "intermediate ocean": the seawater contained a lot more oxygen than before, but still much less than today.
Since ice reflects sunlight, making the Earth even colder, it's easy to guess how such runaway feedback might happen. The opposite sort of feedback is happening now, as melting ice makes the Earth darker and thus even warmer. The interesting questions are why this instability doesn't keep driving the Earth to extreme temperatures one way or another, why the Snowball Earth events started when they did, and why the Earth didn't stay frozen.
Here's a currently popular answer to the last question. Ice sheets slow down the weathering of rock. This weathering is one of the main long-term processes that use up atmospheric carbon dioxide, by converting it into various carbonate minerals. On the other hand, even on an ice-covered Earth, volcanic activity would keep putting carbon dioxide into the atmosphere. So, eventually carbon dioxide would build up, and the greenhouse effect would warm things up again. When the ice melted, weathering would increase and the amount of carbon dioxide in the atmosphere would drop again. However, this feedback loop is very slow. Indeed, it has been suggested that in the hot phase, as much as 13% of the atmosphere could be carbon dioxide — 350 times what we see today!
By the end of these glacial cycles, it is believed that oxygen had increased from 2% of the atmosphere to 15%. (Now it's 21%.) This may be why multi-celled oxygen-breathing organisms date back to this time. Others argue that the "freeze-fry" cycle imposed tremendous evolutionary pressure on life and led to the rise of multicellular organisms. Both these theories could be true.
The rise of multicellular organisms marks the end of the Proterozoic and the start of the current eon: the Phanerozoic. This is the end of our story — but of course the history of the Earth doesn't end here.
However, studying the history of the Earth will put us in a better position to guess. We cannot run experiments to test the Earth's response to different levels of greenhouse gases. Computer models are essential, but evidence from Snowball Earth and other incidents in the Earth's past are crucial checks on these models. Similarly, studying past mass extinction events, and the Earth's recovery from them, may provide clues about the future of biodiversity on this planet.
Dana Mackenzie, The Big Splat, or How Our Moon Came to Be, Wiley, New York, 2003.
Robert M. Hazen, Dominic Papineau, Wouter Bleeker, Robert T. Downs, John M. Ferry, Timothy J. McCoy, Dmitri A. Sverjensky and Henxiong Yang, Mineral evolution, American Mineralogist 91 (2008), 1693-1720.
Rodney Gomes, Harold F. Levison, Kleomenis Tsiganis and Alessandro Morbidelli, Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets, Nature 435 (2005), 466.
Harold F. Levison, Alessandro Morbidelli, Christa Van Laerhoven, Rodney Gomes and Kleomenis Tsiganis, Origin of the structure of the Kuiper Belt during a dynamical instability in the orbits of Uranus and Neptune, Icarus 196 (2007), 258-273.
William T. Hyde, Thomas J. Crowley, Steven K. Baum and W. Richard Peltier, Neoproterozoic 'snowball Earth' simulations with a coupled climate/ice-sheet model, Nature 405 (2000), 425-429.
Gabrielle Walker, Snowball Earth: The Story of a Maverick Scientist and His Theory of the Global Catastrophe That Spawned Life As We Know It, Three Rivers Press, New York, 2004.
The images come from these sources:
The above article was published by PhysicsWorld, and you can see a PDF version here
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