Lec20 2D

Keys to Solar System Formation: 

Keys to Solar System Formation Any theory for the formation of the Solar System must explain The flatness of the Solar System, and orbital similarities The separation of Terrestrial and Jovian planets The decrease in planet densities with distance from the Sun Bode’s Law

Star Formation and Interstellar Matter: 

Star Formation and Interstellar Matter Stars are made from the gas and dust in the interstellar medium. The gas and dust in the interstellar medium comes from stars. Material is constantly being recycled.

Molecular Clouds: 

Molecular Clouds At any time, there is just as much material in the interstellar medium as there is in stars. Much of this matter is far, far from any star. It is therefore very cold. In these cold regions, atoms can stick together to form molecules, such as H2. A Giant Molecular Cloud may contain over 100,000,000 M of material!

The Beginning of Star Formation: 

The Beginning of Star Formation Where there is gas, there is also dust, which absorbs and scatters light. Dust in space can be seen in silhouette, as it blocks out the light from more distant stars.

Cold Clouds: 

Cold Clouds Since dust blocks the light, the temperatures within these clouds can be just a few degrees above absolute zero!

Cloud Collapse: 

Cloud Collapse Since the temperature is so low inside these clouds, gas pressure is almost non-existent. There is nothing to stop gravity from condensing the cloud. The cloud will get smaller and increase in density.

Initial Collapse: 

Initial Collapse Each fragment is a protostar. Also, since the clouds are lumpy to begin with, the collapse process causes the clouds to fragment. Dark clouds are much denser in their center than on the outside, so their inner regions collapse first.

Formation of the Solar Nebula: 

Formation of the Solar Nebula Self gravity begins to collapse the cloud As the cloud gets smaller, it begins to rotate faster, due to conservation of angular momentum. Centripetal force prevents gas from collapsing in the plane of rotation Gas falling from the top collides with gas falling from the bottom and sticks together in the ecliptic plane In a large, slowly rotating cloud of cold gas

Formation of the Solar Nebula: 

Formation of the Solar Nebula The densest region (the center) becomes the Sun. Friction in the disk causes the Sun to accrete matter and grow in mass. Eventually, fusion occurs. Atoms orbiting in the disk bump together and form molecules, such as water. Droplets of these molecules stick together to form planetesimals. In the flat solar nebula

Formation of the Solar Nebula: 

Formation of the Solar Nebula Over time, the planetesimals grow as more molecules condense out of the nebula Differential rotation (due to Kepler’s laws) cause particles in similar orbits to meet up. They stick together forming a bigger body. The bigger the body, the greater its gravity, and the more attraction it has for other bodies. Protoplanets form. Planetesimals grow …

Formation of the Solar Nebula: 

Formation of the Solar Nebula While protoplanets are forming, the Sun’s luminosity is growing, first due to gravitational contraction, then due to nuclear ignition. Regions of the nebula close to the Sun will get hot; the outer regions will stay cool. In the hot regions, light elements will evaporate; only heavy elements will condense out of the nebula Material begins to evaporate

Temperature of the Solar Nebula: 

Temperature of the Solar Nebula Inside the orbit of the Earth, only metals can condense out of the solar nebula. Rocky (silicates) can condense near Mars. In the outer solar system, water and ammonia ice can survive.

Radiation Pressure and the Solar Wind: 

Radiation Pressure and the Solar Wind Two other processes are also important for driving light gases from the inner part of the solar system. Radiation pressure: Photons act like particles and push whatever particles and dust they run into. Solar wind: The Sun constantly ejects (a little) hydrogen and helium into space. This solar wind pushes whatever gas and dust it runs into.

The Pre-Main Sequence Sun: 

The Pre-Main Sequence Sun As the Sun is formed, it generated its energy via gravitational contraction. During this time, it was a lot brighter than it is today. The radiation pressure in the inner solar system was greater. In addition, due to conservation of angular momentum, the young Sun was also spinning faster than it is today. This caused the solar wind to be stronger.

The Pre-Main Sequence Sun: 

The Pre-Main Sequence Sun As the Sun is form, it generated its energy via gravitational contraction. During this time, it was a lot brighter than it is today. The radiation pressure in the inner solar system was greater. In addition, due to conservation of angular momentum, the young Sun was also spinning faster than it is today. This caused the solar wind to be stronger. Radiation pressure and the solar wind blew out the light material from the inner part of the solar system.

Accretion : 

Accretion Once the major bodies of the solar system were formed, most of the remaining debris was either ejected out of the solar system or accreted onto other bodies by gravitational encounters.

Formation of the Solar System: 

Formation of the Solar System From interstellar cloud to planetary system

Star Forming Regions in the Infrared: 

Star Forming Regions in the Infrared As a star’s mass grows due to accretion, its core grows hotter. At this time, since the star is still surrounded by dust, it is invisible in the optical. But the heat from the star causes the dust to glow in the infrared.

Star Forming Regions in the Infrared: 

optical infrared Far infrared observations can not only see the warm dust, but the protostars as well. Star Forming Regions in the Infrared

Stellar Winds: 

Stellar Winds Later, when the proto-star ignites hydrogen in its core, the excess energy greatly increases the radiation pressure and the stellar wind. Material surrounding the star is blown away.

Slide21: 

Stellar Winds Later, when the proto-star ignites hydrogen in its core, the excess energy greatly increases the radiation pressure and the stellar wind. Material surrounding the star is blown away.

Reddening and Scattering: 

Reddening and Scattering Stars behind large piles of dust will be reddened. Other regions will appear blue, due to the scattering by dust. This is just like the daytime sky.

Observations of Protostellar Disks : 

Observations of Protostellar Disks Our technology is just beginning to be able to resolve the proto-planetary disks around stars.

Observations of Protostellar Disks : 

Observations of Protostellar Disks Our technology is just beginning to be able to resolve the proto-planetary disks around stars.

Evolution of Terrestrial Planets : 

Evolution of Terrestrial Planets After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.) Differentiation – in a molten planet, heavy materials sink

Differentiation : 

Differentiation Early in the history of the solar system, planets would be molten due to Continuous accretion of left over material from the solar system formation. Energy from the fission of radioactive isotopes.

Evolution of Terrestrial Planets : 

Evolution of Terrestrial Planets After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.) Differentiation – in a molten planet, heavy materials sink Cratering – left over bodies impact the planet’s surface

Evolution of Terrestrial Planets : 

Evolution of Terrestrial Planets After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.) Differentiation – in a molten planet, heavy materials sink Cratering – left over bodies impact the planet’s surface Flooding – water, lava, and gases trapped inside the planet come to the surface and cover the terrain.

Evolution of Terrestrial Planets : 

Evolution of Terrestrial Planets Erosion – surface features are destroyed due to running water, atmosphere, plate tectonics, and geologic motions After the condensation and accretion phases of planet formation, terrestrial bodies can go through 4 different stages of evolution. (The rates of evolution can vary greatly.) Differentiation – in a molten planet, heavy materials sink Cratering – left over bodies impact the planet’s surface Flooding – water, lava, and gases trapped inside the planet come to the surface and cover the terrain.

Slide30: 

Planetary Atmospheres

The Atmospheres of the Solar System: 

The Atmospheres of the Solar System

The Atmospheres of the Solar System: 

The Atmospheres of the Solar System

The Atmospheres of the Solar System: 

The Atmospheres of the Solar System

The Atmospheres of the Solar System: 

The Atmospheres of the Solar System

The Atmospheres of the Solar System: 

The Atmospheres of the Solar System

The Atmospheres of the Solar System: 

The Atmospheres of the Solar System

The Atmospheres of the Solar System: 

The Atmospheres of the Solar System

The Atmosphere of Jovian Planets: 

The Atmosphere of Jovian Planets Jovian planets are similar to the Sun. Due to their smaller mass, their central pressures and temperatures are not great enough to fuse hydrogen. They are thus cooler, so hydrogen can react with other atoms to form molecules. H + H  H2 4H + C  CH4 3H + N  NH3 Since the inside of Jupiter is hot (due to the pressure), while the cloud tops are cool, the composition of the atmosphere changes with depth. The fast rotation rate of the Jovian planets also drives strong currents and storms, similar to the trade winds and hurricanes on Earth.

The Structure of Jupiter’s Atmosphere: 

The Structure of Jupiter’s Atmosphere The inside of Jupiter is extremely hot and, in fact, Jupiter shines (in the infrared) by gravitational contraction.

Jupiter’s Trade Winds: 

Jupiter’s Trade Winds Jupiter’s equator is moving faster than the poles (it has farther to go in a day). This drives a network of very strong winds and storms.

Formation of an Atmosphere: 

Formation of an Atmosphere When a terrestrial planet enters its flooding stage, gases trapped inside the planet during formation (or created as a result of radioactive decay) will be outgassed. These gases include H2, He, H2O, N2, CO2, and probably CH4 and NH3 As the planet cools, water vapor condenses out of the atmosphere and falls as rain. Oceans form. But will the planet be able to keep this atmosphere?

Temperature versus Gravity: 

Temperature versus Gravity Escape velocity: the speed a particle must have to escape the gravity of a body and not come back Temperature: the average kinetic energy of an atom or molecule The kinetic energy of an atom or molecule depends both on its speed, and its mass. Light particles move quickly; heavy particles move slowly. It’s easier for a body to hold onto heavy gases than light gases.

The Masses of Gases: 

The Masses of Gases

Mercury versus Titan: 

Mercury versus Titan Mercury and Titan are both low-mass bodies. But … Mercury is close to the Sun, so it is hot. Its gravity is not strong enough to keep its gases from escaping into space. Titan is in the outer solar system and is cold. The molecules are moving slowly, so the moon can hang onto its atmosphere (except for the lightest gases of H2 and He).

The Atmosphere of Mars: 

The Atmosphere of Mars The composition of Mars’ atmosphere is determined by The mass of the planet. Since Mars is only about 0.1 M, it does not have the gravity to hold onto H2 and He. It can barely hold onto N2. Proximity to the Sun. Gases such as CH4 and NH3 are destroyed by ultraviolet light. Mars’ atmosphere is not thick enough to shield itself from ultraviolet photons. Chemistry. Oxygen (O2) reacts with almost anything (i.e., minerals in rocks), so it cannot stay free. Consequently, Mars’ atmosphere is primarily CO2 with a little bit of N2.

Carbon-Dioxide and Mars: 

Carbon-Dioxide and Mars Mars’ pole is tipped 24° from the ecliptic. It therefore undergoes seasons, just like the Earth. In winter at the pole, CO2 freezes out and becomes dry ice. In summer, this ice evaporates and becomes part of the atmosphere. This cycle produces strong winds and dust storms.

Carbon-Dioxide and Mars: 

Carbon-Dioxide and Mars Mars’ pole is tipped 24° from the ecliptic. It therefore undergoes seasons, just like the earth. In winter at the pole, CO2 freezes out and becomes dry ice. In summer, this ice evaporates and becomes part of the atmosphere. This cycle produces strong winds and dust storms.

Venus and Earth: 

Venus and Earth The similarities: The planets have similar masses (0.82 M versus 1.0 M) The planets have similar densities (4.2 versus 5.5) The planets’ distances from the Sun are similar (0.72 A.U. versus 1.0 A.U.) Neither planet can hold onto light gases (H2 and He) Neither planet can keep large amounts of CH4 and NH3 in its atmosphere (due to ultraviolet light from the Sun) The main difference: The Earth’s temperature is between -50° C and +50° C, while Venus’ temperature is +470° C

Properties of Carbon-Dioxide: 

Properties of Carbon-Dioxide CO2 has two interesting properties: CO2 dissolves into liquid water (H2O) to create H2CO3 (carbonic acid). Carbonic acid (i.e., the fizz in soda) then reacts with any number of minerals. For instance H2CO3 + Ca  H2 + CaCO3 (limestone) The result is that, if liquid water is around, CO2 will be removed from the air, and locked up in rocks. CO2 is a greenhouse gas. It is transparent to optical light, but it absorbs infrared light. So sunlight can make it through CO2, but the heat it brings cannot get out.

Runaway Greenhouse Effect: 

Runaway Greenhouse Effect Venus and Earth both started out with similar atmospheres. But because Venus is slightly closer to the Sun … Venus was a bit warmer, and had a bit less liquid water With less liquid water, less CO2 dissolved away With more CO2 in the atmosphere, the greenhouse effect was more effective The warmer temperature caused more water to evaporate With even less liquid water, even less CO2 dissolved away As all the water evaporated, and the temperature increased, outgassing of greenhouse gases (CO2 and CH4) became easier, CO2 was “baked out” of the rocks Ultraviolet light destroyed the CH4, NH3, and H2O in the atmosphere, leaving a thick atmosphere of CO2

The Atmosphere of Earth: 

The Atmosphere of Earth Venus and Earth both started out with similar atmospheres. But because the Earth is slightly farther away from the Sun … Earth was a bit cooler, and had a bit more liquid water With more liquid water, more CO2 dissolved away With more CO2 in the atmosphere, the greenhouse effect was less effective With more liquid water and a comfortable environment, photosynthetic life developed Photosynthesis removed even more CO2 from the atmosphere, replacing it with O2 Lightning plus atmospheric O2 created ozone, which shielded the Earth from ultraviolet light. Water molecules in the atmosphere survived longer (along with life)

Today on Earth and Venus: 

Today on Earth and Venus A small change in the conditions now can lead to large changes later on!