Planetary Differentiation on Earth and Its Implications: From The Solar Nebula to TodayGuest Scientist: Kevin Wheeler Originally Presented 10 Feb 2007
Culmination of a Quest The next few months mark the culmination of Kevin Wheeler’s five year quest to discover more about Earth’s interior than anyone has ever known—in short, he will soon defend his Doctoral dissertation! Today, Kevin shares with use some of his investigations during this period. We wish him “Good Luck”!
Today’s Program • We first consider some theories about the origin of the Earth, especially those that explain possible origins and changes into the complex, multi-layered planet it is today. • Next, we’ll consider in general terms what we know about Earth’s interior • Finally, we’ll review some aspects of “Radioactivity,” key to heating the interior
How, putting it briefly, did the Solar System come into existence? • The Solar System formed when a cold, slowly-rotating cloud of gas and dust collapsed because of its own gravity about 4.5 billion years ago. As the Sun grew hot enough to ignite the nuclear reactions which sustain it today, it vaporized the cold ices and frozen gasses in the inner solar system, leaving behind the rocky dust and metals which form the inner planets. The outer Solar System remained cold, and the ices and gas there collected into the giant outer planets. • The problem with this scenario is that we now have observations of planetary systems around other stars -- and few if any of them resemble our Solar System. http://www.ifa.hawaii.edu/faculty/barnes/ast110_01/fotss.html
This videoclip summarizes current theories about our origins Origin of the Solar System(3.4 Mbyte avi animation)http://www.ifa.hawaii.edu/faculty/barnes/ast110_01/fotss.html
Planetary formation from the solar nebula One generally accepted theory is that the nebula from which our Solar System formed was composed of gas and dust. Somehow in that cloud, the Sun formed in the center and the planets formed around it. The inner, rocky planets formed by accretion--they accumulated dust and rocks to become planets. Studying the physics of planet formation and countless computer simulations reveal three stages in the accretion of the planets. During the first stage, dust grains stuck to each other until objects were large enough to begin to attract material with their gravity fields, producing objects the size of asteroids (up to a few hundred kilometers in diameter). This discussion comes from http://www.psrd.hawaii.edu/Dec98/OriginEarthMoon.html
During the second stage, a period of runaway growth took place, leading to tens of objects much larger than the Moon. Most of the mass of the inner Solar System was contained within these planetary embryos. It may have taken only about a million years from the end of stage 1 to the end of stage 2.
During the final stage, these huge objects whacked into each other, creating larger planets, but a smaller number of them. The entire process was dominated by large impacts, making the formation of the Moon by a giant impact a natural consequence of planet formation. Simulations indicate that the third stage took 100 to 200 million years, about the time estimated from isotopic data on rocks from Earth, the Moon, and meteorites.
Impact events, like the ones that formed Meteor Crater about 50,000 years ago in Arizona and the Manicouagan impact structure about 210 million years ago in Quebec, represent the dominant process of planetary accretion (growth) and surface restructuring. Planets without significant tectonic reworking, weathering or erosion of their surfaces have old surfaces that reflect numerous impacts during their early growth stages. Although the rate of impacting has diminished over the past 4.5 billion years, these events still happen periodically, occasionally with enough energy to cause massive destruction. http://wapi.isu.edu/Geo_Pgt/Mod03_PlanetaryEvo/mod3_pt1.htm
Another View of the Creation of Our Planet Earth passed through four distinct phases from its formation to the present. They are fairly typical phases for all terrestrial planets to have gone through. The better we understand Earth, the more we can infer to other planets such as Mars. http://www.windows.ucar.edu/tour/link=/earth/earth.html Much of the following discussion comes from: http://starryskies.com/solar_system/Earth_html/under_the_surface.html
First stage: “Differentiation” As the materials that became Earth gathered together, they underwent a separation according to density. The most dense (iron and nickel, for example) settled downward towards the center of the planet, and lighter materials (such as oxygen, hydrogen gas, argon) stayed near the surface and in the atmosphere. The densest minerals formed the core of the Earth, while the lighter silicate minerals formed the crust.
Second stage: “Heavy Cratering” As Earth solidified, impacts from objects left the typical cratering marks we can still see on the Moon, Mars, and Mercury. During the early history of the solar system, there much have been a great deal of debris left over from planet-making which floated around the planets. The early Earth's surface much have resembled the Moon's as we see it today, heavily cratered with craters on top of craters. As the debris began to clear, cratering slowed down.
Third stage: Flooding As radioactive decay of some elements heated up the Earth's interior, lava began upwelling through fissures in the Earth's crust. Lava flooded crater impacts and other basins. Outgassing of water and cooling of the atmosphere caused rain to condense. These first flooding rains began to fill the primordial oceans and lakes.
Fourth/current stage: Surface Evolution This process began perhaps 3.5 billion years ago soon after rain began to fall. Crustal movements result in uplifting to produce folded mountains in some areas, fault-block and volcanic mountains in others. Running water, wind, and ice continually weather and erode the surface. Over most continents, a thin veneer of sediment covers igneous and metamorphic rocks — “The Rock Cycle.”
Differentiation from a homogenous body into a heterogeneous one Heat buildup inside Earth reached a maximum early in the Earth's history and declined significantly since. Greater heat due to: • greater abundance of radioactive elements, • greater number of impacts, and • early gravitational crowding. This and the next slide are based on http://www.geology.sdsu.edu/how_volcanoes_work/Heat.html
Initial accretion of particles resulted in a rather homogeneous sphere composed of a loose amalgam of metallic fragments (iron meteorites), rocky fragments (stony meteorites), and icy fragments (comets). Increased heat content of the early Earth resulted in melting of the Earth's interior, so that the young planet became density stratified with the heavier (metallic) materials sinking to the center of the earth, and the lighter (rocky) materials floating upward toward the surface of the earth. Very lightest volatile materials (derived from comets) were easily melted or vaporized, rising beyond the earth's rocky surface to form the early oceans and the atmosphere.
How Can We Interpret the Interior? Technology does not allow us to “dig a hole to China.” So how can we understand Earth’s interior? One of the first clues came from working out Earth’s magnitude. Earth’s average density is 5.52 g/cm3, but most surface rocks range from 2.7 – 3.0 g/cm3. The best explanation was a core composed of dense elements, such as Fe and Ni.
Evidence from Outer Space Meteorites generally belong to one of three types: “Stony,” similar to surficial rocks; “Stony-Iron”; and “Metallic,” mostly Fe with some Ni and other heavy elements. We now speculate that many meteorites come from the Asteroid Belt between Mars and Jupiter, and are the remains of a planet that went through differentiation and then broke apart.
Evidence from Seismic Waves Studying records of P- and S-waves moving through Earth’s interior led to theories that—on a very broad level—there are three layers beneath the crust: mantle, outer core, and inner core. http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html
Our “usual” image of Earth • The crust, on which we live, is very thin • The mantle has denser materials interacting with the crust in plate tectonics • The outer core is liquid Fe and other elements • The inner core is solid Fe and Ni http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html
We now know that Earth’s interior is subdivided into many zones. Kevin will present some of the evidence to support this understanding. http://www.windows.ucar.edu/tour/link=/earth/images/earthint_image.html&edu=mid
Today’s models are much more detailed and sophisticated Red blobs are warmer plumes of less dense material, rising principally into the ocean-ridge spreading centers. A huge plume seems to be feeding spreading at the East Pacific Rise directly from the core. Most of the heat being released from the Earth's interior emerges at the fast-spreading East Pacific Rise http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html
What Are Conditions within Earth Interior? Density, pressure, and temperature within Earth’s interior will obviously have major impacts on how our planet behaves and even the materials at various depths. In general, tremendous pressures and very high temperatures control what minerals or other substances will exist.
NYS Earth Science students and teachers are familiar with this image representing conditions within Earth’s Interior http://emsc32.nysed.gov/osa/reftable/esp10-16.pdf
Radioactive Decay One final concept we must consider in our introductory overview is “Radioactivity.” As most of you know, broadly speaking, this involves the transformation of an atom of one chemical element into an atom of another, with the release of energy. This energy release has played a major role in the interior heating of our planet.
Alpha Decay “Parent Atom” emits an “alpha particle” (like He nucleus– 2 P and 2 N) Energy released Atomic Number drops by 2 Mass Number drops by 4 U-238 Th-234 + α + energy Beta Decay “Parent element” emits a “beta particle” (like an electron) Energy released Atomic Number increases by 1 Mass Number remains the sameTh-234 Pa-234 + β + energy C-14 N-14 + β + energy Two Basic Types of Radioactive Decay
With this general background, we may be ready to greet Kevin Wheeler and listen to his discussion about: Planetary Differentiation on Earth and Its Implications: From The Solar Nebula to Today