"The Moon is a crescent-shaped boat filling up with souls and transporting them to the sun."
-- Persian Sage, 200 AD
The Universe evolved galaxies and multitudes of stars in its first 9.5 billion years of existence, synthesizing the periodic chart of the elements within the hot interiors of stars and spilling these materials out into space in supernovae as the largest of the stars exhausted the nuclear fusion that sustained them against the relentless tendency toward gravitational collapse. Thus, the Universe evolved from purely H and He gas in space to a mix of H, He and varying amounts of heavier elements (the proportions of which are dictated by the fusion reactions in stars and the explosive synthesis of the heaviest elements in supernova shock waves). There was now gas and dust available for gravity to sweep up into clouds, pulling inward and concentrating mass in the center, with spin flattening the cloud into a disk of dust and gas. New stars formed from this debris of earlier explosions of the Big Bang and supernovas now had a new 'flavor' speckled with all elements, and clumps of material that did not fall into the central star could form planets around the star. Some of those stars and planets prove interesting.....
The recognition that stars could suddenly appear or explode and die, rather than all being permanent fixtures of the Heavens, was one of the profound philosophical wake-up calls of the Renaissance. No longer did the 'firmament' appear to firm. This recognition accelerated Human inquiry into the finite existence of our own star and the solar system about it, and systematic thought addressed the issue of how the Solar System formed.
Early hypothese that were advanced for how stars might form, primarily by Mathematicians and Physicists, include:
(1) Condensation: The idea that the Sun and planets cooled from a hot nebula, a cloud of dust and gas in the interstellar medium, with the spin of the nebula being important. Descartes (of the Cartesian System) advanced the original idea in 1644, while Laplace recognized the importance of spin in forming the system in 1796.
(2) Encounter: The idea that the star formed first, and then close passage of another star gravitationally 'ripped' some of the material out, which cooled in place to make the planets. This was originally advanced by de Buffon, 1785, and favored for many years, but is now thought to be untenable since the material in a star is so hot that once pulled into space it would vaporize and spread rather than consolidate into a planet.
Most current ideas involve
(3) Nebular Turbulence: A hybrid of nebular condensation ideas, modified to recognize the great heterogeneity and turbulence in a condensing spinning nebula. This is key to explaining why the planets have the angular momentum of the system.
A few of the Basic Observations that any hypothesis for Solar System Formation must explain:
The common plane of rotation and the circular orbits strongly suggest in-situ formation of the planets simultaneous with formation of the Sun. The Angular Momentum observation is the most challenging to explain, as most nebular theories lead to the Sun spinning much faster as mass collapses into it. How could the angular momentum of the nebula be transferred out to the outer regions to offset this intrinsic effect (which is why Neutron stars spin so fast, for example)?
The Nebular Turbulence models are the current notion of how the Solar System formed: In essence the idea is that there was a nebular cloud of Gas and dust particles in a region of space, enriched by 2% in the heavier elements of the periodic chart. Probably many stars formed close together in time, perhaps with a supernova triggering localized gravitational collapse of regions of the cloud (there were many proto-stars).
Gravity drives the nebular collapse, but conservation of angular momentum cause the nebula to spin faster and faster as mass concentrates toward its center. The balance of gravity and rotation leads to a flattened disk. The central portions of the nebular disk begin to heat up do to increasing collisions of gas and dust particles, and there are turbulent eddies in the hot nebula, which transfer momentum outward from the center.
As the center of the nebula collapses, enough mass to heat up to the 10,000,000 degree temperatures required for fusion of H to He, the star begins to fire-up, and the protosun is surrounded by a spinning hot ring of nebular material with tubulent structure. The mass of the ring may have been 2%-200% of the solar mass, with much material to be driven off.
As the hot gas in the nebula cools, materials begin to freeze out of the gas, assuming solid state at temperatures that vary depending on the particular compound. This is called the Condensation Sequence, and has been replicated in the laboratory. The nebula need only to have been hotter than about 1500 C to have all of the material in gaseous form. As it cooled to 1400 C it began to freeze out Refractory Material. Some of the compounds that form solid particles at different temperatures are:
Refractory Materials Temperature, C Al2O3 1410 CaTiO3 1200 Fe(Ni) 1150 MgSiO3 1100 Volatile Materials FeS 430 Fe3O4 135 H2O, CH4, CO2, H2 <0
This sequence links the composition of different portions of the nebula to the temperature distribution. The main variation is with distance from the Sun, however, the edges of the nebula are cold so there is also a vertical variation across the nebula. The sequence of condensing materials has a gross similarity to the predominant composition of planets with increasing distance from the sun. Hot, close-in terrestrial planets formed from refractory materials, with the more volatile materials still being in a gas state and not being incorporated into the growing planet. Solar radiation would drive the gas outward, particularly in the T-tauri phase of the Sun, when the fusion engine turned on, and strong radiation would have cleared the inner planetary areas of the volatiles. This left the final terrestrial planets enriched in refractories.
The accretion models are either a one-step process or a two-step process. The one-step builds entire planets out of direct aggregation from the condensed mist of granules. The two-step builds them by first accumulating as 'planetisimals', small masses, which then collide to form the larger planets. The two step is favored for several reasons, including: the very low inert gas content of the Earth relative to the expected abundance from that of the nebula (i.e. the solar abundance); models of planetisimal trajectories; and the very tightly constrained ages of the meteorites.
We now think that the process of accretion of solid particles was very rapid, with collisions leading to larger and larger objects which progressively swept up the material around them by gravity. This led to planetisimals, which had a very chaotic collisional history, and these formed the planets. All of the planets were formed within a few hundred million years, and it is estimated that the Earth formed in about 70 million years.
The accretion of the Earth took place about 4.55 billion years ago. This age is based on the dating of lead isotopes in meteorites and inferences from the lead/Uranium history of materials on the Earth. About 1/2 of the Earth accreted in the first million years, and the rate of bombardment was immense. Relative to today's rate of meteorite impacts, there was about a billion times more collisions. Each collision injects heat energy into the Earth, as the kinetic energy of the meteorite is converted to another form of energy: heat. Calculations of the amount of energy available from these impacts indicate that it is easily sufficient to melt the entire surface of the planet, and perhaps the whole interior. Thus, from 4.55-4.5 billion years ago the planet had a huge ocean of magma, or molten rock: a magma ocean.
During this initial phase the Earth's core formed. There are two basic models for accretion of the Earth, but either must result in layered planet that existed by 4.5 billion years ago. We will see why this is necessary in a moment! The models are:
Heterogeneous Accretion: As the nebula cooled, the refractory materials accreted into a single core, and later, less refractory materials added on, in layers, to give an iron rich core overlain by silicates.
Homogeneous Accretion: The above process may have occurred on the scale of planetisimals, but these then accreted into a larger body, such that the initial distribution of materials was fairly uniform through the planet.
The Earth's core developed by either minor segregation of iron into the core under the heterogeneous accretion model, or by draining all iron out of the uniform planet formed by homogeneous accretion. In either case the planet was very hot from impacts, and the very motion of iron to the core releases heat sufficient to melt the rocky material of the planet.
In any case, the interior has to get hot enough to cause the iron to melt. There are two means of increasing the temperature sufficiently to allow the iron to drain toward a growing core. Radioactive heating was long favored by scientists. This hypothesis holds that the radioactive elements, which have a range of half lives many of which are quite long, decay, generating heat which warms up the interior. This would take hundreds of millions of years to accomplish. The other means is by impact heating, which we have already seen effieicntly produces a molten exterior in the form of a magma ocean. In this model, the earth acts as a continuous iron smelter, draining iron to a growing core as the earth grows, while leaving the floating silicate slag behind as what will eventually be called the mantle. The timing of core formation is therefore the most telling discriminator between these two models. Again, it is difficult to find the answer to this internally, and we have looked outward to find clues. They come in the form of meteorites and The Moon. It turns out that meteorites are all about the same age. They come in several flavors, some looking like metallic pieces of cores of broken worlds, others like the mantles of similarly segregated bodies, and yet others as loose aggregations of primitive grains that have never melted. Those coming from segregated worlds are in fact less than 100 million years younger than the oldest primitive bodies, meaning that in these other (long-ago fragmented) worlds segregation into metallic cores had occurred very rapidly. This supports the impact bombardment heated - smelter model.
Having segregated the iron into the core, the Earth was merrily on its way. Ahh, but wait. There were a few extra large planetismals out there still. One of these rare, large planetisimals about the size of Mars impacted the Earth. This sprayed much of the initial Earth's rocky mantle into space with a huge collision, converting the rocky material into liquid and even gas. While some material fell back to Earth and some escaped to space, a significant amount re-condensed and cooled, finally accreting to form the Moon. This hypothesis is the only one for the origin of the Moon which can be reconciled with all of our observations of the Moon. This includes its bulk composition (especially the low iron content), the tilt of the Earth's axis with respect to the plane of the eccliptic (which yields our seasons), and the angular momentum of the Earth-Moon system, and the age of the Moon (How do we know the age of the Moon? We went there and grabbed some rocks, brought them back and dated them!). The simulations of this collision indicate that the Moon was initially much closer to the Earth, and is slowly receding. Measurements using lasrers have shown that this recession rate is now 3 cm per year. This is changing the length of the day; it is thought to have been only 6 hours at first. A similar large, late impactor may have hit Venus, which has a strange rotation, but it did not produce a moon like ours. There are other planets in fact whose spin axes are also inclined. These also are likely the result of very late, very large impacts. While they were very very rare events, they played a huge role in setting the final look of the solar system.
This great impact would have once again melted most of the Earth, and any atmosphere or ocean would have burned away. So, we have accreted the Earth and formed the Moon, but we have to do a lot of home improvements before the planet could accommodate life.
![[return icon]](../dinoheadt.jpg)
Return to the Earth Sciences 80A Lectures Page
Page maintained by Thorne
Lay.
This page was last reviewed on 9/25/02.