Earth as a Planet Mass M = 6 x g. Mass M = 6 x g. Mass M = 6 x g Mass M = 6 x g Radius R = 6371 km. Radius R = 6371 km. Radius R = 6371 km Radius R = 6371 km Mean density = M/(4/3  R 3 ) = 5.5 g/cm 3 Mean density = M/(4/3  R 3 ) = 5.5 g/cm 3 Moment of inertia I of the Earth: Moment of inertia I of the Earth: I =  r 2 dmI =  r 2 dm I/(MR 2 ) = I/(MR 2 ) = for a uniform sphere I/(MR 2 ) = 0.4.for a uniform sphere I/(MR 2 ) = 0.4.

Differentiation early in Earth’s history

Interior of the Earth Crust: variable thickness with an average value of 35 km in the continents and 7-8 km in the oceanic regions. Volume ~10 19 m 3 Mass 2.8 x kg. Crust: variable thickness with an average value of 35 km in the continents and 7-8 km in the oceanic regions. Volume ~10 19 m 3 Mass 2.8 x kg. Mantle : between the Moho discontinuity (crust- mantle) and the core-mantle boundary (R = 3480 km). Volume 9 x m 3 Mass 4 x kg. Mantle : between the Moho discontinuity (crust- mantle) and the core-mantle boundary (R = 3480 km). Volume 9 x m 3 Mass 4 x kg. Core : from the center of the Earth to the core-mantle boundary. Volume 1.77 x m 3 Mass 1.94 x kg. Core : from the center of the Earth to the core-mantle boundary. Volume 1.77 x m 3 Mass 1.94 x kg.

More Details about Layering…

What is Earth made of? Why do we need to look outside the Earth to learn about what is inside the Earth? Why do we need to look outside the Earth to learn about what is inside the Earth? We know the Earth is layered and that what we can sample on the outside is not typical.We know the Earth is layered and that what we can sample on the outside is not typical. By studying members of the Solar System, it is possible to estimate its original composition and the physical and chemical processes that have led to its present state.By studying members of the Solar System, it is possible to estimate its original composition and the physical and chemical processes that have led to its present state.

The Solar System: A highly diverse zoo!

Overview of the Solar System Sun Sun Sun Mercury Mercury Mercury Venus Venus Venus Earth Earth Earth MoonMoonMoon Mars Mars Mars Asteroids Asteroids Asteroids Jupiter Jupiter Jupiter Saturn Saturn Saturn Uranus Uranus Uranus Neptune Neptune Neptune Pluto Pluto Pluto

The Origin of the Solar System Frank Crary, CU Boulder A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance could be, for example, the shock wave from a nearby supernova. A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed and collapses under its own gravity. The disturbance could be, for example, the shock wave from a nearby supernova. As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust to vaporize. The initial collapse is supposed to take less than 100,000 years. As the cloud collapses, it heats up and compresses in the center. It heats enough for the dust to vaporize. The initial collapse is supposed to take less than 100,000 years. The center compresses enough to become a protostar and the rest of the gas orbits/flows around it. Most of that gas flows inward and adds to the mass of the forming star, but the gas is rotating. The centrifugal force from that prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away its energy and cools off. The center compresses enough to become a protostar and the rest of the gas orbits/flows around it. Most of that gas flows inward and adds to the mass of the forming star, but the gas is rotating. The centrifugal force from that prevents some of the gas from reaching the forming star. Instead, it forms an "accretion disk" around the star. The disk radiates away its energy and cools off. Here is a brief outline of the current theory of the events in the early history of the solar system:

The Origin of the Solar System Frank Crary, CU Boulder First break point. Depending on the details, the gas orbiting star/protostar may be unstable and start to compress under its own gravity. That produces a double star. If it doesn't... First break point. Depending on the details, the gas orbiting star/protostar may be unstable and start to compress under its own gravity. That produces a double star. If it doesn't... The gas cools off enough for the metal, rock and (far enough from the forming star) ice to condense out into tiny particles. (i.e. some of the gas turns back into dust). The metals condense almost as soon as the accretion disk forms ( billion years ago according to isotope measurements of certain meteors); the rock condenses a bit later (between 4.4 and 4.55 billion years ago). The gas cools off enough for the metal, rock and (far enough from the forming star) ice to condense out into tiny particles. (i.e. some of the gas turns back into dust). The metals condense almost as soon as the accretion disk forms ( billion years ago according to isotope measurements of certain meteors); the rock condenses a bit later (between 4.4 and 4.55 billion years ago). The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of boulders or small asteroids. The dust particles collide with each other and form into larger particles. This goes on until the particles get to the size of boulders or small asteroids.asteroids

The Origin of the Solar System Frank Crary, CU Boulder Run away growth. Once the larger of these particles get big enough to have a nontrivial gravity, their growth accelerates. Their gravity (even if it's very small) gives them an edge over smaller particles; it pulls in more, smaller particles, and very quickly, the large objects have accumulated all of the solid matter close to their own orbit. How big they get depends on their distance from the star and the density and composition of the protoplanetary nebula. In the solar system, the theories say that this is large asteroid to lunar size in the inner solar system, and one to fifteen times the Earth's size in the outer solar system. There would have been a big jump in size somewhere between the current orbits of Mars and Jupiter: the energy from the Sun would have kept ice a vapor at closer distances, so the solid, accretable matter would become much more common beyond a critical distance from the Sun. The accretion of these "planetesimals" is believed to take a few hundred thousand to about twenty million years, with the outermost taking the longest to form. Run away growth. Once the larger of these particles get big enough to have a nontrivial gravity, their growth accelerates. Their gravity (even if it's very small) gives them an edge over smaller particles; it pulls in more, smaller particles, and very quickly, the large objects have accumulated all of the solid matter close to their own orbit. How big they get depends on their distance from the star and the density and composition of the protoplanetary nebula. In the solar system, the theories say that this is large asteroid to lunar size in the inner solar system, and one to fifteen times the Earth's size in the outer solar system. There would have been a big jump in size somewhere between the current orbits of Mars and Jupiter: the energy from the Sun would have kept ice a vapor at closer distances, so the solid, accretable matter would become much more common beyond a critical distance from the Sun. The accretion of these "planetesimals" is believed to take a few hundred thousand to about twenty million years, with the outermost taking the longest to form.lunar EarthMarsJupiterSunlunar EarthMarsJupiterSun

The Origin of the Solar System Frank Crary, CU Boulder Two things and the second break point. How big were those protoplanets and how quickly did they form? At about this time, about 1 million years after the nebula cooled, the star would generate a very strong solar wind, which would sweep away all of the gas left in the protoplanetary nebula. If a protoplanet was large enough, soon enough, its gravity would pull in the nebular gas, and it would become a gas giant. If not, it would remain a rocky or icy body. Two things and the second break point. How big were those protoplanets and how quickly did they form? At about this time, about 1 million years after the nebula cooled, the star would generate a very strong solar wind, which would sweep away all of the gas left in the protoplanetary nebula. If a protoplanet was large enough, soon enough, its gravity would pull in the nebular gas, and it would become a gas giant. If not, it would remain a rocky or icy body. At this point, the solar system is composed only of solid, protoplanetary bodies and gas giants. The "planetesimals" would slowly collide with each other and become more massive. At this point, the solar system is composed only of solid, protoplanetary bodies and gas giants. The "planetesimals" would slowly collide with each other and become more massive.

The Origin of the Solar System Frank Crary, CU Boulder Eventually, after ten to a hundred million years, you end up with ten or so planets, in stable orbits, and that's a solar system. These planets and their surfaces may be heavily modified by the last, big collision they experience (e.g. the largely metal composition of Mercury or the Moon). Eventually, after ten to a hundred million years, you end up with ten or so planets, in stable orbits, and that's a solar system. These planets and their surfaces may be heavily modified by the last, big collision they experience (e.g. the largely metal composition of Mercury or the Moon).MercuryMoonMercuryMoon Note: this was the theory of planetary formation as it stood before the discovery of extrasolar planets. The discoveries don't match what the theory predicted. That could be an observational bias (odd solar systems may be easier to detect from Earth) or problems with the theory (probably with subtle points, not the basic outline.) Note: this was the theory of planetary formation as it stood before the discovery of extrasolar planets. The discoveries don't match what the theory predicted. That could be an observational bias (odd solar systems may be easier to detect from Earth) or problems with the theory (probably with subtle points, not the basic outline.)extrasolar planetsextrasolar planets

The Big Questions What is the origin of the solar system? It is generally agreed that it condensed from a nebula of dust and gas. But the details are far from clear. What is the origin of the solar system? It is generally agreed that it condensed from a nebula of dust and gas. But the details are far from clear. How common are planetary systems around other stars? There is now good evidence of Jupiter-sized objects orbiting several nearby stars. What conditions allow the formation of terrestrial planets? It seems unlikely that the Earth is totally unique but we still have no direct evidence one way or the other. How common are planetary systems around other stars? There is now good evidence of Jupiter-sized objects orbiting several nearby stars. What conditions allow the formation of terrestrial planets? It seems unlikely that the Earth is totally unique but we still have no direct evidence one way or the other.good evidencegood evidence Is there life elsewhere in the solar system? If not, why is Earth special? Is there life elsewhere in the solar system? If not, why is Earth special? Is there life beyond the solar system? Intelligent life? Is there life beyond the solar system? Intelligent life? Is life a rare and unusual or even unique event in the evolution of the universe or is it adaptable, widespread and common? Is life a rare and unusual or even unique event in the evolution of the universe or is it adaptable, widespread and common?

Solar abundance of elements Determined from spectral absorption lines Determined from spectral absorption lines Light from visible surface of sun passing through cooler gases above the surfaceLight from visible surface of sun passing through cooler gases above the surface This is thought to represent total solar abundance because nuclear reactions powering the star take place deep inside and there is little convection there to mix modified material up with original material. This is thought to represent total solar abundance because nuclear reactions powering the star take place deep inside and there is little convection there to mix modified material up with original material.

Meteorites Meteorites Meteorites

Meteorites: Summary The fabric of chondrites is quite unlike that of any terrestrial rock and required very different conditions in which to form. These are identified with early stages in the development of the Solar nebula. The fabric of chondrites is quite unlike that of any terrestrial rock and required very different conditions in which to form. These are identified with early stages in the development of the Solar nebula. The carbonaceous chondrites are a close approximation to the material of the Solar Nebula, having lost only the most volatile elements. It is, therefore, plausible to regard them as a starting point from which the composition of the Earth has evolved. This leads to the Chondritic Earth Model. The carbonaceous chondrites are a close approximation to the material of the Solar Nebula, having lost only the most volatile elements. It is, therefore, plausible to regard them as a starting point from which the composition of the Earth has evolved. This leads to the Chondritic Earth Model. The meteorites derive from the asteroids by collision. The meteorites derive from the asteroids by collision. The differentiated meteorites were formed within minor planets, or asteroids, which heated sufficiently to segregate into layers, forming an iron core, silicate mantle and transitional region between. Subsequent break-up due to collisions produced iron, achondrite, and stony-iron meteorites. The differentiated meteorites were formed within minor planets, or asteroids, which heated sufficiently to segregate into layers, forming an iron core, silicate mantle and transitional region between. Subsequent break-up due to collisions produced iron, achondrite, and stony-iron meteorites.

Next Question: What is the origin of the distribution of elements in the Solar System? Hydrogen, the simplest element, is the basic building block.

Hydrogen burning – 4 protons become alpha particle (helium nucleus) Helium burning - 3 alpha particles become 12 C (which can absorb another to become 16 O Carbon burning and oxygen burning produce 28 Si (very stable), 24 Mg, 32 S, and other elements Each of these requires more heat than the fusion reaction before it.

Silicon burning involves breaking of pieces of other nuclei and adding them to others. This produces many stable nuclei heavier than Si. As temperature rises, this becomes the equilibrium e-process which is like shaking and breaking up all the existing nuclei and recombining them randomly to make all possible stable nuclei up to the iron group elements. Everything bigger than the iron group is less stable and the e-process would rearrange them into iron group elements.

Neutron capture becomes the method that builds larger nuclei. Slow-neutron or the s-process. Add a neutron to a nucleus. If nucleus becomes unstable because its neutron/proton ratio is too high, it has time to “fix” itself before another neutron arrives. It “fixes” things by a beta decay. A neutron converts to a proton and an electron is emitted. The nucleus has moved one element up the periodic table. The s-process can build elements up to 209 Bi. At that point there is no neutron/proton ratio stable enough to allow the one by one conversion of neutrons to protons.

The rapid-neutron or r-process involves adding neutrons to a nucleus faster than things can be “fixed.” Much heavier nuclei can be built up. Once the bombardment is over, the neutron-rich nucleus will undergo repeated beta decays to produce nuclei that are relatively more stable but which, in turn, are unstable to alpha decay and so break down into lighter nuclei. These include 238 U, 235 U, and 232 Th which have half- lives comparable to the age of the Earth, and so have not yet decayed to negligible amounts.

The s-process also helps fill in gaps between some of the lighter elements such as between 12 C and 16 O. This mechanism can only produce neutron-rich nuclei, so other processes must account for known nuclei with lower than average neutron/proton ratios. The p-process resolves this by adding protons to nuclei. Light elements Li, Be, and B are not produced by any of the above processes. In fact, they are destroyed at the temperatures required for hydrogen burning. They are probably formed as fragments when heavy nuclei in interstellar dust are struck by cosmic rays. This is a very slow process, but interstellar dust spends a lot of time in space!

The most likely place for these reactions to take place is in the interior of a large star. The Sun is not large enough to ever get beyond hydrogen burning, and therefore will not generate the distribution of elements found in the Sun or meteorites. One or many larger stars were needed to produce these elements that formed the nebula that became the Solar System. The matter from these stars would have been disseminated by supernova explosions at the end of their existence as stars. There is time between the formation of our galaxy, 15x10 9 years ago and the formation of the Solar System 4.6x10 9 years ago for many generations of stars to form, explode and slowly enrich the interstellar medium. This heavy material is the 2% of the Solar System.

One additional observation suggests that the last of these supernovae must have occurred just 2-3 million years before the initiation of the formation of the Solar System. It must have occurred close to the dust cloud that became the Solar System. There is evidence that certain elements like 26 Al with very short half-lives were present in the material that formed the Solar System. If these had been formed gradually by many stars, these elements would have decayed away. They could only have been formed and disseminated by a very recent supernova. It is possible that this supernova not only contributed material to the cloud, but also initiated its collapse.