Origin and History of Our Solar System

Earth and Space Science Objectives and Expectations: TEK Objective 5: Earth in space and time. The student understands the solar nebular accretionary disk model. The student is expected to: analyze how gravitational condensation of solar nebular gas and dust can lead to the accretion of planetesimals and protoplanets; b)  investigate thermal energy sources, including kinetic heat of impact accretion, gravitational compression, and radioactive decay, which are thought to allow protoplanet differentiation into layers;

Where did Earth come from? Scientists debated whether the origin of our Solar System was usual, or a common consequence of star formation The origin of the planets in our Solar System has been debated since 1755 when Immanuel Kant proposed the first theory During the 1900s, many believed Earth was not a unique occurrence because of the recent discovery of other planetary systems around other stars

The Protoplanet Hypothesis Two Main Theories The Nebular Hypothesis The Protoplanet Hypothesis Most generally accepted evolutionary model for the origins of solar systems The current working model for the formation of the Solar System, it incorporates many of the components of the nebular hypothesis, but adds some new aspects.

Nebula Sun-like stars usually take around 100 million years to form. Nebula are star “nurseries”, where stars are born. This nebular photograph was taken by the Hubble Space Telescope The Eagle Nebula is an emission nebula famous for its gaseous star making regions, like the the Pillars of Creation shown here. Next, see the Orion Nebula. The Orion nebula is a tapestry of star formation, from the dense pillars of gas to the hot, young, massive stars that have emerged from their gas-and-dust cocoons. The next nebula looks like the head of an animal. Probably the most recognizable nebula to most people, The Horsehead Nebula is a dark, easily visible nebula in the constellation Orion. This is the blue lagoon nebula. You can see where it gets its name! The crab nebula is named after the cluster it is found near, the Cancer constellation. A nebula is the product of a supernova event. The death of one system, may well be the birth of another!

The Nebular Hypothesis Kant believed a nebula began to collapse due to gravity, and slowly began to rotate. The rotation turned the nebula into a flattened disk of debris. At the center, the Sun formed as all the densest materials fell in, and over time, it heated up and ignited. This ignition blew the debris into rings orbiting the star. Laplace stated that these rings coalesced into planets. https://youtu.be/PL3YNQK960Y

Evidence Supporting… These are protoplanetary discs imaged by the Hubble Space Telescope, from the Orion Nebula. Scientists suspect these are the early stages of planetary systems forming, some 1,500 light years away! 28.5 x 1015 km

Nebular Hypothesis: How Does Accretion Occur? Accretion – gradual growth of planets by the accumulation of other smaller bodies Heavy bombardment period on Mercury (4.1-3.8 bya) Protostar forms with opaque core Energy is given off by protostar causing a cooling Cooling causes gas to condense into tiny specs of metal, rock, & ice “Stellar Debris” Stellar Debris begins to stick together to form Planetesimals Accrection of Planetesimals forms Protoplanets Some Planetsimals will form into asteroids, comets, and moons Planetesimal is a “baby” planet. A protoplanet is a small body that could later become a planet. In the meantime, the heat and radiation released by the protostar and the gas flow vaporize the dust grains of the cloud. The protostar begins its evolution into a star, accreting the gas. The disk starts cooling through energy radiation. Depending on the amount and distribution of gas, it can be gravitationally stable or unstable and form one or more new protostars. In such a way, a binary or multiple stellar system is formed. Far from the star, the gas is cold enough so that part of the gas condenses into dust and ice. The dust grains merge due to collisions until they form small pieces of rock called planetesimals. Planetesimals merge and form protoplanets. The maximum size of protoplanets depend on their distance from the star and on the chemical composition of the primordial nebula. It is much smaller in the inner regions than in the outer regions since the protostar tends to disrupt and vaporize dust. The later evolution of the planetary system is governed by the collisions between its constituent bodies. Impacts of meteorites and planetesimals upon protoplanets and satellites, generate craters on their surfaces. Many of craters from this time are still visible today on planets within our own Solar System. A good example of this can be found on Mercury. When impacts are especially violent, they can even move the bodies out of their original orbit. Our Solar System went through this phase roughly 4-4.5 billion years ago. This period is known as Heavy Bombardment. Ev idence of it is seen on Mercury.

What are the two main theories about the origins of our solar system? What is a nebula? How do nebulas form? Describe Kant’s Nebular Hypothesis in a nutshell. 5. Describe how planetesimals and protoplanets are different. 6. Describe the process of accretion, and how it allows planets to grow larger. 7. Describe what the “heavy bombardment” stage of solar system development was, and when it happened.

The Protoplanet Hypothesis Because scientists weren’t completely happy with the nebular hypothesis, other explanations of planet formation were sought. During the mid 1900s, astronomers called their new version of nebular hypothesis the Protoplanet Hypothesis Encounter theory:

The Protoplanet Hypothesis The solar system begins to form as a rotating cloud, or nebulae, collapses The Protoplanet Hypothesis Instabilities in the nebulae cause dust particles to stick together and accrete into billions of planetesimals with diameters of about 10 meters. The planetesimals then collide and form protoplanets. Meanwhile, the protosun in the center of the nebular disk becomes massive and hot enough to "turn on" by fusing hydrogen.

https://youtu.be/Uhy1fucSRQI The Sun begins to radiate energy and vaporize dust in the inner part of the Solar System. The remaining gas is blown away by solar winds (or T Tauri winds), to join the outer planetary gas giants. The main difference between nebular theory, and protoplanet theory is that in the nebular theory, the Sun forms before the planets, and in the protoplanet theory, the Sun forms concurrently with the planets, and right alongside them. The planets continue building, accreting debris and increasing their mass and gravitation. Then, the Sun ignites, and blows away all the rest of the dust.

Differentiation The forces operating during the formation of the Solar System were responsible for the diversity of matter in the Solar System and also responsible for diversity of planetary internal-structures. Low temperature condensates burn off in the inner solar system, including those bodies that form beyond the “frost” line, where frozen hydrogen compounds can exist. Remember, this marks the point in our solar system where the frozen gas giants can exist. As the nebula cools 1) The inner zone stays warm (>100 ° C) and only high temperature condensates form - giving the terrestrial planets high density. Five major elements; Fe, Mg, Si, O, and S; comprise at least 95% of the mass of each of the terrestrial planets. These elements are high temperature condensates. 2) Outer zone cools more, so low-T materials condense into the outer - low density planets with lots of, ice, and frozen gases like CH4, CO2 etc...

8. What is the main difference between Nebular Hypothesis of solar system formation, and Protoplanet Hypothesis? What version of solar system origins is accepted today? 10. What are the five major high temperature condensates that compose the terrestrial planets? 11. Describe briefly why planets in our solar system formed where they did.

This model shows planetesimal accretion in our solar system This model shows planetesimal accretion in our solar system. The total time frame for the process is about 441 million years. There were as many as 11 inner planetesimals after 79 million years, and six after 151 million years. Suggestions are that the Earth accreted in about 100 million years. The terrestrial planets (inner rocky planets) formed close to the sun, because nothing else would accrete there. The gases all vaporized because of the temperature. Gases have the chance to freeze past the frost line (between Mars and Jupiter), and the outer planets are composed largely of frozen gases as a result.

Kinetic Heat of impact accretion Planets can differentiate by mass and density, the same way that solar systems do. As you might remember: Potential energy is stored, while kinetic energy is possessed by objects in motion. Early differentiation of the Earth involved the separation of Fe-Ni rich (heavy) from silicate material (light) to form the core and mantle.   High temperatures were necessary and differentiation likely occurred in response to large-scale melting, induced by high-energy impacts. (kinetic heat of impact accretion) Kinetic energy from these impacts caused the melting. "Earth's accretion history was dominated by multiple high-energy collisions with Moon- to Mars-sized bodies Early Earth heats up due to radioactive decay, compression, and impacts. Over time the temperature of the planet interior rises towards the Fe-melting point. The iron "drops" follow gravity and accumulate towards the core.  Lighter materials, such as silicate minerals, migrate upwards in exchange towards the crust and mantle. Over time, differentiation occurred based on temperature and density. Silicates include minerals that contain both oxygen and silicon, and compose the vast majority of the Earth’s crust.

Gravitational Compression High-density materials tend to sink through lighter materials. Iron, the most common element to form a very dense molten metal phase, tends to congregate towards planetary interiors. The main zones in the solid Earth are the very dense iron-rich metallic core, the less dense magnesium-silicate-rich mantle and the relatively thin, light crust composed mainly of silicates of aluminum, sodium, calcium and potassium. Even lighter still are the watery liquid hydrosphere and the gaseous, nitrogen-rich atmosphere. low-density silicate rocks, such as granite, are well known and abundant in the Earth's upper crust.

Internal Structure and Thermal Energy Temperature within the Earth increases with depth. The Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%). The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K (Water freezes at 273 Kelvins)

12. Explain how kinetic heat of impact accretion is how early Earth began differentiating. 13. Why does it make sense that Earth’s hydrosphere and atmosphere formed where they did? 14. How does the thermal structure of the Earth change from crust to core? 15. Other than impact accretion, what other force accounts for the internal temperature of the Earth? 16. What are the major heat-producing isotopes in the Earth?