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Third Rock from the Sun


The DVD of the Universe grinds slowly forward. Our sun, named Sol (did you know that?), settles down into a nice class G2 star around which orbits several planets and their moons, some asteroids and planetary objects with names like Ooort clouds and trans-Neptunian objects. Cleverly enough, this collection of planets and planetary bodies surrounding Sol are called the Solar System.



At some moment approximately 4.55 ± 1% billion years ago (based on precise radiometric dating of meteorites, a space traveler in our neck of the woods would observe a solar system quite different from that which we gaze upon today. The sun was considerably weaker, the Earth smaller, the moon—not there. The surface of the Earth may have been molten or maybe not. Its internal fires were undoubtedly ignited but perhaps not fully unleashed.

Modern-day clues to Earth’s origin reside inside, upon and outside Earth and in extraterrestrial objects brought to Earth in the form of meteorites. Scientists can infer a great deal about the origin of our planet through observations of rocks and their chemical constituents, through seismic imaging and by use of computer models, among other tools.

But before we analyze the evidence by which scientists piece together the story of Earth’s origin, let’s spend a moment getting to know our planetary neighbors. (It’s amazing what you can tell from the “neighbors” you keep.)

Our solar system boasts eight or nine planets, depending on whether you want to include Pluto as a planet or a trans-Neptunian object. Much of what we now surmise about Earth comes from our study of these planets, so we’ll take a moment here to mention a few of their more interesting features.

The four inner planets—Mercury, Venus, Earth and Mars—are known as the terrestrial or rocky planets because they are primarily composed of silicate rocks with an iron/nickel core. The outer four planets—Jupiter, Saturn, Neptune, Uranus—are known as the jovian or gas planets because they are primarily composed of helium and hydrogen-based gases, much like our sun. (In fact, Jupiter is thought to be a star that failed to ignite.) Pluto stands alone (in many ways), which is one reason some people prefer to think of it as a large “asteroid” (a trans-Neptunian object, technically). I’ll defer to Interplanet Janet here and stick with its original designation as one among planets.

Between the inner and outer planets lies the asteroid belt, a rag-tag bunch of oddly shaped rocks whose combined mass is no greater than the moon. Asteroids are thought to represent fragments of planetesimals, the rocky material from which the planets were built. As such, they act like a kind of “junkyard” from which much can be learned about the formation of our early solar system.

The cratered surfaces of asteroids remind us of a violent past—a cosmic bowling alley—during which collisions between planetesimals and planets were a frequent occurrence. Indeed, a few frames of the game remain unfinished, as several of these objects trace paths that come perilously close to Earth. Having pummeled our planet in the past, the potential for another collision exists, although the odds are remote. Nonetheless, scientists have launched a program called NEAT, Near Earth Asteroid Tracking, to keep an eye on the tens of thousands of asteroids and near-earth objects (NEOs) in proximity to Earth.

Our brief visit with our neighbors and especially the asteroids conveniently leads us to a more detailed look at ideas concerning the formation of Earth. The evidence is inconclusive and hotly debated, but it gives you a far greater picture of the passionate nature of this science than the glib, one-paragraph description you get in most textbooks. So here goes.

For decades, scientists have debated two basic models by which Earth formed, what has come to be known as the homogeneous versus heterogeneous accretion debate. Both of these models present different solutions and challenges for interpreting available data and understanding the source of Earth’s ocean.

The first model, the homogeneous accretion model, states that planetesimals with a similar and homogeneous composition accreted (stuck together) and subsequently differentiated (separated into layers) to form the layered Earth we see today. The opposing model, the heterogeneous accretion model, states that Earth formed in a step-wise fashion from planetestimals that were not uniform in composition (i.e. they were heterogeneous).

The principal evidence used to evaluate these models includes:

  • measurements of extinct radioisotopes in meteorites and their inclusions
  • measurements of extinct radioisotopes in moon rocks brought back by the manned Apollo lunar missions
  • measurements of stable isotopes in meteorites and moon rocks
  • spectroscopic measurements of comets
  • measurements of radioactive and stable isotopes in modern rocks, mid-ocean ridge basalts (MORBs), lavas and volcanic gases
  • various measurements of interplanetary dust collected in Earth’s atmosphere
  • simulations by computer models

The principle difference between these models concerns the nature and timing of Earth-forming processes, such as core formation, planetary differentiation (layering of Earth), and formation of the lithosphere, atmosphere and hydrosphere. A rapid timeline for these processes—favored in the heterogeneous models—stretches the time over which life may have formed because conditions on Earth favorable to living matter would have been established more quickly.

Since the early 1980s, many researchers have favored the heterogeneous model, according to Michael Drake and others. According to William F. McDonough, Department of Earth and Planetary Sciences, Harvard University:

Given the likely event of the outer portion of the mantle experiencing significant global melting, one would expect that the mantle would have also experienced some degree of differentiation (crystal-liquid separation). However, there is no geochemical and/or isotopic evidence, based on a wide spectrum of crustal and mantle rocks (including peridotites and komatiites), in support of this global differentiation process.

(Chapter 1 of Earthquake Thermodynamics and Phase Transformations in the Earth’s Interior, 2000)

He does concede that the evidence could have been erased as a result of convective mixing in the mantle (much like what happens in a pot of boiling pasta sauce). Despite these statements (and others), the homogeneous model has its proponents and this model dominates modern textbooks, especially ocean science textbooks. For those of you that favor a knock-out punch, I’ll have to disappoint you here. There may be considerable evidence from a variety of sources for heterogeneous accretion but new ideas concerning our interpretation of that data (especially the chemistry of processes in the interior of the Earth) may sway scientific opinion back towards the homogeneous model.

Whatever the specific details of its formation, some important milestones in Earth’s early history bear mentioning. Stein Jacobsen at the Department of Earth and Planetary Sciences at Harvard University summarized key events in the timeline of Earth’s formation (How Old is the Earth?, June 6 2003 Science) that help us understand exactly what we mean by the age of the Earth (especially as you hear it expressed by instructors and in various textbooks and documentaries). Jacobsen asks, “how old is the Earth” not in a rhetorical sense, but because it behooves us to define exactly what we mean by “Earth” before we get specific about its age. This awareness lets us better understand and appreciate the processes that led to the formation and evolution of our planet.

Theese numbers are based on his article.

  • 4.5647 + 0.6 Ga: the first solid bits of raw planetary materials formed from the solar nebula
  • 4.5646 Ga, about 100,000 years later, the embryonic protoplanets were distinguishable as objects accreting faster than surrounding objects
  • 4.5547 Ga, about 10 million years after formation of the solar system, at least 64% of Earth’s mass was accreted (also known as the “mean” age)
  • 4.54 – 4.44 Ga, about 30 million years after the solar system formed, the Moon solidified following the impact of a large, possibly Mar-sized or larger, “meteorite” with Earth

Truth is, you could use any of the above milestones to officially declare “Earth formed” as long as you make clear your assumption of what Earth is (i.e., the first bits of planetary materials or the first hint of protoplanet or the average age of these materials or the Earth-moon event). I like to use the age of 4.56 Ga (reducing the significant digits to two) because the data on which this age is based (dating of meteorites) are the most robust, meaning they are a direct measurement and subject to the least amount of interpretation.

That's quite a lot to digest...game for more.

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