The Origin of the Earth: Part III
Okay, here is Part III of my Origin of the Earth paper. Part IV will follow tomorrow. I don’t even think I stole any pictures from the internet for this section. Rather, I poorly scanned a couple of images from my book on meteorites. Hey, I’m a busy grad student, so you’ll have to take what you can get. Anyway, enjoy!
The Origin of the Earth
Part III: Rocks from Space and the Accretion of the Earth
A meteorite is a rock from space that survives passage through Earthâ€™s atmosphere and lands on Earthâ€™s surface. Objects that become meteorites on Earthâ€™s surface are called meteoroids while still in space and are referred to as meteors when falling through Earthâ€™s atmosphere (Norton, 1994). Meteorites originate from various places in the solar system. Primarily, meteorites are fragments of asteroids from the asteroid belt between Mars and Jupiter. Potentially, some meteorites are also fragments from further asteroid belts or from asteroid belts that no longer exist in the solar system. Some meteorites have also been identified to originate from the moon and other planets in our solar system. Currently, there are 34 meteorites identified as originating from Mars (NASAâ€™s Mars Meteorites webpage). Most meteorites from Mars probably were chipped off the planetâ€™s surface during a meteorite impact and flung out of the planetâ€™s gravitational influence into space heading towards Earth (NASAâ€™s Mars Meteorites webpage). However, the parent bodies of most meteorites are unknown and may no longer even exist, having been broken into fragments. Regardless, families of meteorites can be identified based on chemical composition. Oxygen isotopes are especially useful in defining families of meteorites (Norton, 1994). Several thousand meteorites have been discovered on Earth, and many more are discovered each year, mostly in deserts or in Antarctica, places where there are few terrestrial rocks and where there are active searches for meteorites.
There are three general categories of meteorites: iron meteorites, stony meteorites, and stony-iron meteorites. Iron meteorites are primarily iron and nickel and are believed to represent the iron cores of differentiated asteroids (Norton, 1994). Since iron meteorites come from the cores of asteroids, the parents of this group of meteorites must have been broken up thoroughly and likely no longer exist. There are two categories of stony meteorites: chondrites and achondrites. Chondrites, which will be described in detail below, are believed to be the most primitive material in the solar system (Carlson and Lagmair, 2000). That is, chondrite meteorites represent undifferentiated asteroids. Achondrite meteorites, on the other hand, are believed to represent igneous material, such as basalt, originating on a parent body with a core and differentiated upper section. Lastly, stony-iron meteorites are believed to come from the core-mantle boundary of a differentiated asteroid. These unusual meteorites are composed of crystals, often olivine, in an iron-nickel matrix. Figure 8 shows where various meteorites are thought to originate in an asteroid and compares these locations with a larger, differentiated planet.
Figure 8: Terrestrial layers and asteroid layers, showing the origin of various meteorite types (Norton, 1994)
The most important meteorites to study for the origin of the Earth are chondrite meteorites. These meteorites are believed to represent very primitive asteroids that have undergone very little differentiation, heating, or metamorphism since they formed at the beginning of the solar system. More than 85% of known meteorites are chondrites, which suggests that most of the solar systemâ€™s rocky matter is undifferentiated and that differentiated bodies such as the Earth are relatively rare (Norton, 1994). Chondrites often contain chondrules, which are small, spherical, silicate inclusions. There is some debate about how chondrules form, but most likely they formed from the initial heating of early solar system dust and thus represent very old material (Norton, 1994; Carlson and Lugmair, 2000). Many chondrites also contain inclusions called CAIs which stands for calcium-aluminum inclusions. CAIs are large, irregular, white inclusions that likely represent the first material that condensed from the solar nebula (Carlson and Lugmair, 2000).
Chondrites are generally classified based on their texture and chemical composition (see Figure 9), both of which are related to the degree of alteration and thermal metamorphism of the meteorite. The least altered and metamorphosed chondrites are carbonaceous chondrites. Carbonaceous chondrites are rich in organic compounds and water-bearing minerals, which can only exist in a meteorite that has not been heated significantly (Norton, 1994; Carlson and Lugmair, 2000). The most primitive of the carbonaceous chondrites are the CI chondrites of which there are only five samples (Norton, 1994). CI chondrites are believed to have not been heated about 50Âº C and contain significant water (up to 20%) and organic molecules such as hydrocarbons and amino acids (Norton, 1994). These CI chondrites likely grew in the cooler, outer solar nebula and represent the most primitive solar system material ever found. Thus, chondrites and CI chondrites in particular are believed to represent the original, undifferentiated composition of the Earth.
Figure 9: Classification of Chondrite Meteorites (Norton, 1994)
An initial chondritic composition for the Earth is assumed in many geochemical and geophysical models of the formation and differentiation of the Earth. That Earth accreted from chondritic planetesimals is thus an important assumption made in all of these models. However, just how valid is this assumption of the chondritic Earth? There are several potential problems with this assumption. First of all, chondritesâ€”even carbonaceous chondritesâ€” have varying compositions. The assumed chondritic compositon of the early Earth is actually the average composition of many individual chondrite meteorites. There is some question as to just how representative this average composition may be since it is based upon chemical analysis of the few chondrites which have fallen to Earth; there are only five known CI chondrites, for instance. If there were to be another big meteorite fall, there is a possibility that the â€œaverageâ€ composition of chondrites could change.
Furthermore, the oxygen isotopic composition of chondrites indicate that there is considerable variation in the parent bodies of these meteorites (Carlson and Lugmair, 2000). Thus, averaging chondritic compositions may not be fair as Earth may have accreted from a specific family of chondrite planetesimals. That chondrites have varying compositions is no surprise as, very likely, the solar nebula was heterogeneous. Indeed, spectrographic observations of other nebula confirm that nebula are, in general, heterogeneous.
Another concern is the difference in ages of CAI and chondrule inclusions in chondrites. CAIs are up to 2 million years younger than chondrules and thus indicate that the solid material of the solar system had a composition which was not only heterogeneous but also evolved over time (Norton, 1994). Lastly, what if the Earth accreted partly from differentiated planetesimals, as some recent studies have suggested (Halliday, 2006)? If so, then one cannot fairly assume a homogeneous, chondritic composition for the early Earth.
However, in the following sections a chondritic Earth will be assumed as this is the best estimate we have, currently, of early Earth’s composition. One just must realize that this assumption of a chondritic early Earth is not without the concerns mentioned above.
The Accretion of the Earth:
Starting around 4.567 billion years ago (the age of chondrites), the Earth began accreting from planetesimals that most likely had a composition similar to CI carbonaceous chondrites (Taylor and Norman, 1990). Computer models suggest that most of Earthâ€™s mass was accreted within ten million years of the formation of the solar system but there was significant accretion up until about 100 million years after the formation of the solar system (Canup and Agnor, 2000). Actually, Earth is still accreting as 10^6-10^7 kg of meteorites are added to Earth each year (Norman, 1994). However, compared to the Earthâ€™s current mass of 5.9736 * 10^24, this yearly accretion from meteorites is very small. However, in the early solar system meteorites played a significant role in Earthâ€™s accretion as asteroids and planetesimals were much more abundant and had not yet settled into the current asteroid belts. Thus, collisions between these bodies were common in the early solar system. Also, Earth was not always so largeâ€”the planet grew over time from the accretion of meteorites.
After the solar nebula was formed, there were four main stages that led to the accretion of the Earth (e.g. Chambers, 2004; Boss, 1990):
1. The settling of circumstellar dust to the mid-plane of the protoplanetary disk. This stage occurred in thousands of years after the formation of the solar system.
2. The growth of planetesimals ~1 km in diamater through gravitational collapse of dense regions of the early solar nebula and through collisions of smaller particles.
3. Oligarchic (runaway) growth producing planetesimals ~1000 km in diamater. Note that this runaway growth can produce up to Mars-sized planetesimals but cannot produce an Earth-sized planet. Stages 2 and 3 together occur within a few hundred thousand years of the formation of the solar system.
4. Collisions between large, moon to Mars-sized planetesimals that lead to the formation of larger (i.e. Earth-sized) planets. The timing of this final stage of Earth accretion is debated, but Earth accretion likely occurred within 100 million years of the formation of the solar system, with most accretion occurring by 10 or even as little as 5 million years (e.g. Chambers, 2004; Hayashi et al., 1985).
There is significant evidence in the solar system that the final accretion of the Earth involved collisions between large impactors. First, asteroids are likely leftover planetesimals that were never accreted into a larger planet. The large asteroid belt between Mars and Jupiter likely formed because the strong gravitational pull of Jupiter kept these asteroids from accreting with the terrestrial planets (Norton, 1994). The presence of impact craters on planets and moons, especially on our own moon, also suggests that there were significant numbers of collisions between planetesimals in the early solar system. Modern-day asteroids still crash into planets, such as the famous impacts on Jupiter in 1994 when the Shoemaker-Levy 9 comet collided with the planet, and there were many more asteroids in the early solar system. Also, the offsets in the tilts of planets relative to their axes of rotation suggests that late in their accretion planets were hit by large impactors. That this offset exists supports a few collisions between large planetesimals late in planetary accretion. The affect of many small planetesimals colliding with accreting planets would not have an overall affect on the tilt of the planets as the effects of all the small impactors hitting the planet from all different directions would tend to cancel out (Taylor and Norman, 1990).
Very nice summation of the basic theories of planetary formation.
Nice qualifier too, about the chondritic early-Earth model. A reminder of how science works: here's a theory that best explains what we know so far, but here are also problems with that theory that could lead to revisions.
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