Origin and History of Our Origin and History of Our Solar SystemSolar System

TEK Objective 5: Earth in space and time. The student understands the solar nebular accretionary disk model. The student is expected to:

a) 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?Where did Earth come from?The origin of the planets in our Solar System has

been debated since 1755 when Immanuel

Kant proposed the first theory

Scientists debated whether the origin of our Solar System was usual, or a common consequence of star formation

During the 1900s, many believed Earth was not a unique occurrence because of the recent discovery of other planetary systems around other stars

Two Main TheoriesTwo Main Theories

The Nebular The Nebular HypothesisHypothesis The The

Protoplanet Protoplanet HypothesisHypothesis

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 are star “nurseries”, where stars are born. This nebular photograph was taken by the Hubble Space

Telescope

Sun-like stars usually take around 100 million years to form.

A nebula is the product of a supernova event. The death of one system, may well be the birth of another!

The Nebular Hypothesis 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/PL3YNQK9

60Y

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 Nebular Hypothesis: How Does Accretion Occur?Accretion Occur?

Protostar forms with opaque coreProtostar 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

Accretion – gradual growth of planets by the accumulation of other smaller bodiesHeavy bombardment

period on Mercury (4.1-3.8 bya)

1. What are the two main theories about the origins of our solar system?

2. What is a nebula?

3. How do nebulas form?

4. 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 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

The solar system begins to form as a rotating cloud, or nebulae, collapses

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.

The Protoplanet HypothesisThe Protoplanet Hypothesis

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.

https://youtu.be/Uhy1fucSRQI

As the nebula cools1) The inner zone stays warm (>100 ° C) and only high temperature condensates form - giving the terrestrial planets high density.

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.

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?

13.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. 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.

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

Silicates include minerals that contain both oxygen and silicon, and compose the vast majority of the Earth’s crust.

Over time, differentiation occurred based on temperature and density.

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.

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?