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Formation of the Solar System © Sierra College Astronomy Department 1 Midterm! Part I (Take home exam, including 10 points from Mastering Astronomy, 50 pts) is available, due October 26th, noon Next week, Part II (in class exam, 50 pts.) – Taken in 3rd hour (week of 10/22 to 10/25) – Bring SCANTRON (882 form) and #2 pencil – Based on “Review Questions” handout, available now! Also: 10 of the 25 extra credit points are due by October 26th, noon. Lecture 8a: The Formation of the Solar system The Formation of the Solar System What properties must a planetary formation theory explain? It must explain the patterns of motion of the present solar system (last week). 2. It must explain why planets form into 2 groups. 3. It must explain the huge existence of asteroids and comets. 4. It must allow for possible exceptions to the rules. The theory may be able to be used on other solar systems in the Galaxy 1. © Sierra College Astronomy Department 3 Lecture 8a: The Formation of the Solar system The Formation of the Solar System Evolutionary Theories All evolutionary theories have their start with Descartes’s whirlpool or vortex theory proposed in 1644. Using Newtonian mechanics, Kant (in 1755) and then Laplace (around 1795) modified Descartes’s vortex to a rotating cloud of gas contracting under gravity into a disk. The Solar Nebular Hypothesis is an example of an evolutionary theory. © Sierra College Astronomy Department Solar Nebula 4 Lecture 8a: The Formation of the Solar system The Formation of the Solar System Catastrophic Theories Catastrophic theory is a theory of the formation of the solar system that involves an unusual incident such as the collision of the Sun with another star. The first catastrophic theory - that a comet pulled material from the Sun to form the planets - was proposed by Buffon in 1745. Other close encounter hypotheses have been proposed too. Catastrophic origins for solar systems would be quite rare (relative to evolutionary origins) due to the unusual nature of the catastrophic incident. © Sierra College Astronomy Department 5 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis Origin of the Solar Nebula Galactic recycling Galactic recycling – Most of the universe started as Hydrogen and Helium. All other heavy elements (loosely called “metals” by astronomers) were formed in stars – When stars die they release much of the content into space – While this has been going on for 4.6 billion years, only 2% of all the have been converted to “metals” Evidence from other gas clouds – All new systems that we can observed formed within interstellar clouds, such as the Orion Nebula © Sierra College Astronomy Department Orion Nebula 6 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis Towards a Solar Nebula Hypothesis A supernovae shock wave likely triggered the events which led to the birth of our solar system The nebular cloud collapsed due the force of gravityCloud on the cloud. But the cloud does not end up collapse2 spherical (like the sun) because there are other processes going on: – Heating – The cloud increases in temperature, converting gravitational potential energy to kinetic energy. The sun would form in the center where temperatures and densities were the greatest – Spinning – as the cloud shrunk in size, the rotation of the disk increases (from the conservation of angular momentum). – Flattening – as cloud starting to spin, collisions flattened the shape of the disk in the plane perpendicular to the spin axis © Sierra College Astronomy Department 7 Lecture 8a: The Formation of the Solar system Testing the Model If the theory is correct, then we should see disks around young stars Dust disks, such as discovered around beta-Pictoris or AU Microscopii, provide evidence that conditions for planet formation exist around many Sun-like stars. AU Mircoscopii © Sierra College Astronomy Department HD 141569A 8 Disks around other stars Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis categories The Formation of Planets As the solar nebula cooled and flattened into a disk some 200 AU in diameter, materials began to “freeze” out in a process called condensation (changing from a gas to a solid or liquid). The ingredients of the solar system consist of 4 categories (with % abundance): – – – – Hydrogen and Helium gas (98%) Hydrogen compounds, such as water, ammonia, and methane (1.4%) Rock (0.4%) Metals (0.2%) Since it is too cool for H and He to condense, a vast majority of the solar nebula did not condense Hydrogen compounds could only condense into ices beyond the frost line, which lay between the present-day orbits of Mars and Jupiter Frost line © Sierra College Astronomy Department 10 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis Building the Terrestrial Planets In the 1940s, Weizsächer showed that eddies would form in a rotating gas cloud and that the eddies nearer the center would be smaller. Eddies condense to form particles that grow over time in a process called accretion. Materials such and rock and metalplanetesimals (categories #3 and #4). These accreted materials became planetesimals which in turn sweep up smaller particles through collision and gravitational attraction. Mixed rock These planetesimals suffered gravitational encounters which meteorite altered their orbits caused them to both coalesce and fragment. Only the largest planetesimals grew to be full-fledged planets. Verification of this models is difficult and comes in the form of theoretical evidence and computer simulations. © Sierra College Astronomy Department Jovian planetesimals 11 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis Building the Jovian Planets Planetesimals should have also grown in the outer solar system, but would have been made of ice as well as metal and rock. But Jovian planets are made mostly of H and He gas… The gas presumably was captured by these ice/rock/metal planetesimals and grew into the Jovian planets of today. © Sierra College Astronomy Department 12 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis A Stellar wind is the flow of nuclear particles from a star. Some young stars exhibit strong stellar winds. If the early Sun went through such a period, the resulting intense solar wind would have swept the inner solar system clear of volatile (low density) elements, molecules and compounds. The giant planets of the outer solar system would then have collected these outflowing gases. © Sierra College Astronomy Department 13 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis An object shrinking under the force of gravity heats up. High temperatures near the newly formed Sun (protosun) will prevent the condensation of more volatile (low density) elements. Planets forming there will thus be made of nonvolatile, dense material. Planet Building Farther out, the eddies are larger and the temperatures cooler so large planets can form that are composed of volatile elements (light gases). © Sierra College Astronomy Department 14 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis Problem: The total angular momentum of the planets is known to be greater than that of the Sun, which should not occur according to conservation laws (i.e. the present Sun is spinning too slowly). Solution: As the young Sun heated up, it ionized the gas of the inner solar system. – The Sun’s magnetic field then swept through the ions in the inner solar system, causing ions to speed up. – As per Newton’s third law, this transfer of energy to the ions caused the Sun to slow its rate of rotation. © Sierra College Astronomy Department 15 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis Explaining Other Clues Over millions of years the remaining planetesimals fell onto the moons and planets causing the cratering we see today. This was the period of heavy bombardment. Comets are thought to be material that coalesced in the outer solar system from the remnants of small eddies. © Sierra College Astronomy Department 16 Lecture 8a: The Formation of the Solar system Solar Nebula Hypothesis The formation of Jovian planets and its moons must have resembled the formation of the solar system. Jupiter specifically: – Moons close to Jupiter are denser and contain fewer light elements; – Moons farther out decrease in density and increase in heavier elements. © Sierra College Astronomy Department 17 Lecture 8a: The Formation of the Solar system The Exceptions to the Rule Phobos Deimos Captured Moons – satellites which go the opposite way were likely captured. Most of these moons are small are lie far away from the planet. Giant impact Moon Giant impacts – may have helped form the Moon and explain the high density of Mercury and the Pluto-Charon system. Furthermore, the unusual tilts of Uranus and Venus can also be explained by giant impacts. Solar Nebula Theory Summary © Sierra College Astronomy Department 18 Lecture 8a: The Formation of the Solar system Solar System Destiny The nebular hypothesis accounts for all major features in the solar system It does not account for everything, however It probably took about a few tens of million of years, about 1% of the current age of the solar system The solar system was probably not completely predestined from the collapse of the solar nebula, though the initial were orderly and inevitable The final stage of accretion and giant impacts were fairly random in nature and made our solar system unique © Sierra College Astronomy Department 19 Lecture 8a: The Formation of the Solar system Radioactivity Radioactivity Periodic Table Certain isotopes (elements which contain differing number of neutrons) are not stable and will decay into two or more lighter elements The time it takes for half of a given isotope to Half-life K-40 decay is called the half-life By noting what percentage a rock (or human body) has left of a radioactive element can enable us to estimate the age of that object. This process is called radioactive dating. See Mathematical Insight 8.1 Half-life 2 © Sierra College Astronomy Department 20 Lecture 8a: The Formation of the Solar system Radioactivity Radioactivity - examples Potassium-40 decays into Argon-40 with a half-life of 1.25 billion years Half-life K-40 – Since Argon-40 is an inert gas, it is very unlikely to have formed inside a rock as the solar nebula condensed, so it must have formed via decay Uranium-238, after a series of decays, turns into Lead206 with half-life of 4.5 billion years Periodic – Lead and Uranium have very different chemical behavoirs Table – Some minerals have nearly no lead to begin with, so when uranium is mixed with lead, we can assume that the lead formed via decay © Sierra College Astronomy Department 21 Lecture 8a: The Formation of the Solar system Radioactivity Periodic Radioactivity Table The general formula for the age of a radioactive material is (see Mathematical Insight 8.1): current amount log10 original amount t thalf 1 log10 2 Half-life K-40 Half-life 2 © Sierra College Astronomy Department 22 Lecture 8a: The Formation of the Solar system Radioactivity Half-life 2 Earth rocks, Moon rocks, and meteorites The oldest Earth rock date back to 4 billion years and some small grains go back to 4.4 billion years. Moon rock brought back from the Apollo mission date as far back as 4.4 billion years. – These tell us when the rock solidified, not when the planet formed The oldest meteorites, which likely come form asteroids, are dated at 4.55 billion years, marking the time of the accretion of the solar system © Sierra College Astronomy Department 23 Lecture 8b: Terrestrial Geology Basics Earth’s Structure & Composition EarthQ The Interior of the Earth (overall density = 5.5 g/cm3) Earth’s interior is determined by analyzing travel times of two types of waves generated by earthquakes. Earth’s interior is made up of three layers: Terr. insides – Crust is the thin (<100 km) outermost layer of the Earth and has a density of 2.5–3 g/cm3. – Mantle is the thick (2,900 km), solid layer between the crust and the Earth’s core. Density of the mantle is 3–9 g/cm3. The crust “floats” on top of the mantle. – Core is the central part of the Earth, composed of a solid inner core and a liquid outer core. Density of the core ranges from 9–13 g/cm3 and is probably composed of iron and nickel. Increasing density trend is called differentiation sinking of denser materials toward the center of planets or other objects. © Sierra College Astronomy Department EarthQ2 interior Diff 24 Lecture 8b: Terrestrial Geology Basics Earth’s Structure & Composition Layering by Strength Most of the Earth is not molten and most of the lava from volcanoes rises upward from a narrow region of the mantle which is partially molten. The shape of a planet is determined by the strength and fluidity of the inside as well as the strength of gravity Shapes – Large worlds (> 500 km diameter) are round – Small worlds are irregular in shape The crust and the top part of the mantle is relatively cool region of rock called the lithosphere that floats on the rest of the mantle. © Sierra College Astronomy Department Terr. insides 25 Lecture 8b: Terrestrial Geology Basics Causes of Geological Activity Geological Activity describes how much ongoing change occurs on the surface of a solar system body Interior heat is the primary driver for geological activity But how do interior heat up and cool off? © Sierra College Astronomy Department 26 Lecture 8b: Terrestrial Geology Basics Causes of Geological Activity How planets heat up Heat of accretion – Energy brought from afar from colliding planetesimals – potential energy converted into kinetic energy Heat of differentiation – As the planet redistributes its mass and denser material sinks towards the core gravitational potential energy is converted to thermal energy via friction Heat from radioactive decay – Decay from radioactive materials heats up the interior as some of the Heat nuclear decay energy ( E = mc2 ) gets transferred to thermal energy sources Note: the first of these two tend to happen early in a planet’s history while the last (radioactive decay) happens throughout the history of the planet, but is strongest at the beginning of the formation of the planet. Radioactive decay likely contributes several times more energy over the life of the planet than does accretion and differentiation. © Sierra College Astronomy Department 27 Lecture 8b: Terrestrial Geology Basics Transfer of Energy How Interiors Cool Off: Conduction – Transfers occurs between atoms – Examples: metal rod in fire, Earth’s core and lithosphere Three Types Convection – Warmer (less dense) air rises and carries energy into cooler (denser) regions Demo – Requires large temperature gradient Lava lamp – Examples: Lava lamp, Earth atmosphere and mantle, Sun’s outer layers Radiation – – – – Photons directly transfer energy Less efficient in high density situations Photons take ~ 200,000 years to get of Sun. Examples: Heat lamp, Earth’s surface, Sun’s interior © Sierra College Astronomy Department Earth cooling 28 Lecture 8b: Terrestrial Geology Basics Transfer of Energy How the Earth moves energy from the core to the surface: Convection is the most important process in the Earth’s deep interior – The ongoing process of transferring heat upward creates convection cells – Ongoing mantle convection goes at the rate of 1 cm/year: It would take about 100 million years to move the mantle from the base to the top At the lithosphere, conduction is probably the most important process © Sierra College Astronomy Department Earth cooling 29 Lecture 8b: Terrestrial Geology Basics Planetary Size A small object cools more quickly than a large object So size is the most important factor in planetary cooling This can be seen in the terrestrial worlds: – Earth and Venus: still very active. – Mars: Activity in the past, but mostly dead now. – Moon and Mercury have been dead for 3 billion years or so. © Sierra College Astronomy Department Earth cooling 30 Magnetic field Lecture 8b: Terrestrial Geology Basics Earth’s Magnetosphere Basic Earth Mag field Earth’s Magnetic Field A magnetic field is a region of space where magnetic forces can be detected. The region around a planet is called a magnetosphere Earth’s magnetic poles are not located at its poles of rotation. The location of the magnetic poles changes with time. Demo Dynamo effect is the model that explains the Earth’s and other planets’ magnetic fields as due to currents within a liquid iron core and a rapidly spinning planet. dynamo Earth dynamo © Sierra College Astronomy Department magnetosphere 31 Lecture 8b: Terrestrial Geology Basics Earth’s Magnetosphere The Van Allen belts are doughnut-shaped regions composed of charged particles (protons & electrons) emitted by the Sun & captured by the magnetic field of the Earth. Auroras result from disturbances in the Earth’s magnetic field that cause some of the particles to follow the magnetic field lines down to the atmosphere, where their collisions with atoms of the air cause it to glow. Aurora © Sierra College Astronomy Department 32 Aurora from the Ground Aurora From Space © Sierra College Astronomy Department 34 Lecture 8b: Terrestrial Geology Basics Shaping the Earth There are 4 processes which shape the virtually all features on Earth 1. Impact Cratering Bowl shaped from asteroids or meteors 2. Volcanism Eruption of lava from planet’s interior 3. Tectonics Disruption of planet’s surface by internal forces 4. Erosion Wearing down or building of geological features by wind, water, ice etc… © Sierra College Astronomy Department 35 Lecture 8b: Terrestrial Geology Basics Impact Cratering impact As a general rule the craters made by meteors are 10 times bigger than the impactor and 1020% as deep as the crater is wide. Most impacts happened very early in the history of the solar system The most prominent impact crater on Earth is Meteor Crater near Winslow, Arizona (only Meteor crater 50,000 years ago). Many of the craters on the Earth have been wiped out by erosion processes – Not true for Moon and Mercury © Sierra College Astronomy Department 36 Lecture 8b: Terrestrial Geology Basics Volcanism Volcanism occurs when underground molten rock finds it way through the lithosphere. This is due for 3 reasons: – Molten rock is generally less dense than solid rock – Most of the Earth’s interior is not molten and it requires a chamber of molten rock to be squeezed up the surface – Molten rock often has gas inside of it, leading to dramatic eruption and to outgassing The most common gasses released are water vapor, carbon dioxide, nitrogen, and sulfur gasses (H2S or SO2) © Sierra College Astronomy Department Drift Plates Rift Subduc 37 Lecture 8b: Terrestrial Geology Basics Plate Tectonics Plate Tectonics Alfred Wegener is credited with first developing the idea of continental drift the gradual motion of the continents relative to one another. Rift zone is a place where tectonic plates are being pushed apart, normally by molten material being forced up out of the mantle. Subduction Zone is where two plates are Drift Plates Rift Subduc forced together. © Sierra College Astronomy Department tectonics 38 Lecture 8b: Terrestrial Geology Basics Erosion The surface of the Earth is changed by erosion, the processes that break down or transport rock through the action of ice, liquid, or gas Erosion – Valleys shaped by glaciers – Canyons carved by rivers – Shifting of sand dunes by the air Erosion can pile up sediments into layers called sedimentary rocks (Ex. Grand Canyon) The Earth has the most erosion of any terrestrial planet © Sierra College Astronomy Department 39 Lecture 8b: Terrestrial Geology Basics Age of surfaces The number of craters in a given region can tell one the age of the planet/moon since the last major change on surface – Does not necessarily indicate formation age Erosion from wind, water, and lava will wipe out craters in a given region – This led to determining the development of different parts of the planet/moon © Sierra College Astronomy Department Craters 40 The End © Sierra College Astronomy Department 41