Earth. Astronomy 1-1 Lecture 10-1

Transcription

1 Earth Astronomy 1-1 Lecture 10-1

2 Vital Statistics Mean Distance from Sun x 10 8 km Mean Orbital Speed 29.8 km/sec Sidereal Period days Rotation Period hours Inclination of Axis 23º 27' Inclination of Orbit 0º Mass x kg Mean Density 5.52 g/cm 3 Albedo 0.39 Astronomy 1-1 Lecture 10-2

3 Composition of Atmosphere Atmosphere is considerably different from that of Venus and Mars. Composition is Nitrogen 78% Oxygen 21 Carbon Dioxide 0.033% Plus trace amounts of other gases The mixture of the gases remains uniform vertically. The nitrogen and oxygen in the Earth's atmosphere are due processes involving life forms. Astronomy 1-1 Lecture 10-3

4 Structure of Atmosphere Five regions to the Earth's atmosphere Troposphere 0-11 km This is the weather zone Half the atmosphere in this layer Temperature drops from 20C to -60C Stratosphere km Ozone layer is at the top Temperature increases to ~ 15C Mesosphere km Meteorites primarily burn up here Here the temperature drops to ~ -80C Thermosphere 80 km 500 km Ionosphere located here Temperature increases to a constant Aurora occur in this region and up Shuttle flies in this region Exosphere 500 km and up Atmosphere merges into space Astronomy 1-1 Lecture 10-4

5 Atmospheric Circulation Atmosphere is in constant motion due to Differential heating Causing convection cells Rotation of the Earth Breaks up the convection cells causing the atmosphere to rotate. There are two types of rotation cells in the atmosphere: High Pressure Cells These contain more than usual amounts of air Low Pressure Cells These contain less than usual amounts of air Astronomy 1-1 Lecture 10-5

6 Atmospheric Circulation Gravity causes air to flow from high pressure areas to low pressure areas The air is deflected from directly flowing from the high to low pressure areas by the Coriolis effect Earth is rotating underneath atmosphere Which way a high or low pressure cell is rotating, clockwise or counterclockwise, is dependent upon which hemisphere, northern or southern, it is in. Clockwise Counter Clockwise Northern Hemisphere High Pressure Low Pressure Southern Hemisphere Low Pressure High Pressure Astronomy 1-1 Lecture 10-6

7 Earth's Structure The Earth can be divided into three basic regions Mantle Extends from the surface to 2900 km below the surface There are three distinct regions to it. Outer Core Extends from 2900 km to 5000 km It is liquid Inner Core Extends from 5000 km to the center of the Earth at 6370 km It is solid. Astronomy 1-1 Lecture 10-7

8 Earth's Structure Astronomy 1-1 Lecture 10-8

9 Determining Earth s Structure Structure was determined by looking at how the seismic waves that travel around and through the Earth There are three types of seismic waves L waves These are rolling waves that travel only along the surface of the Earth They do not provide much information about the internal structure of the Earth S waves These are transverse waves Can only travel in material that is somewhat rigid P waves These are longitudinal waves Can travel through material that is either rigid or liquid Astronomy 1-1 Lecture 10-9

10 S and P Waves S waves P waves Astronomy 1-1 Lecture 10-10

11 Seismic Waves P waves travel twice as fast as S waves Both P and S waves are refracted as they travel through the Earth because of the varying densities in the Earth Astronomy 1-1 Lecture 10-11

12 Seismic Waves When an earthquake occurs, both P and S waves are detected at sites that are near where the earthquake occurred At points on the opposite side of the Earth, only P waves are detected Since S waves are transverse waves and they can only travel in solid material, the fact that only P waves were detected in certain areas, was interpreted as being evidence for there being a liquid portion to the core of the Earth. There are also certain regions, called the shadow zone, where no waves of any type are detected This is due to how the P waves are refracted By measuring the size of the shadow zone, the size of the core region could be determined. Astronomy 1-1 Lecture 10-12

13 Liquid Core Region How is it that the Earth has a liquid region sandwiched between two solid regions? This is because of an interesting balance between the pressure, temperature curves, and material composition as a function of depth. The temperature increases continuously as a function of depth. At the boundary between the mantle and core region there is a change in the composition. The melting point of the material in the outer core region is lower than the temperature at that point. So the material is liquid. As the depth increases, the pressure increases to the point where the melting point of the material is again higher than the surrounding temperatures. Astronomy 1-1 Lecture 10-13

14 How is Earth s Mass Distributed Earth's average density ~ 5.5 g/cm 3 Average density of rock in crust < 3 g/cm 3 on average Implies a denser material inside the Earth Mantle largely composed of minerals containing iron and magnesium Outer and inner core contain primarily pure iron with a small amount of nickel Astronomy 1-1 Lecture 10-14

15 Three distinct regions Lithosphere Includes the crust Extends to 100 km The Mantle Asthenosphere Layer is plasticized Near melting point Partly molten and structurally weak This allows the lithosphere to move Extends to about 200 km Mesosphere It is rigid and bottom part of mantle Extends remaining distance to outer core Astronomy 1-1 Lecture 10-15

16 Lithosphere Upper part of the lithosphere consists of two types of material distinct in composition and structure The oceanic crust - 55% of surface area Thin < 6 km Composed of basalts Rich in Fe, Mg, Poor in Si, Al The continental crust - 45% of surface area Thicker ~ km Composed of granites Rich in Si, Al, Poor in Fe, Mg Oceanic crust is denser (3.2 gm/cm 3 ) than the continental crust (2.7 gm/cm 3 ) Astronomy 1-1 Lecture 10-16

17 Plate Tectonics Lithosphere is not one unit but segmented into plates Composed of ~ 7 major plates and ~ two dozen smaller plates Astronomy 1-1 Lecture 10-17

18 Plate Tectonics The plates of the lithosphere ride on the asthenosphere which is a partially molten, plasticized, layer Within the asthenosphere there are convection currents which bring up heat from the core regions of the planet Where the convection currents rise to the surface, new material is added to the lithosphere pushing the plates apart Where the convection currents sink below the surface, the plates are pulled downwards Astronomy 1-1 Lecture 10-18

19 At the Boundaries of Plates Crustal material is either being created or destroyed Three types of interactions which occur near plate boundaries Constructive Rift Zones, Mid-Oceanic Ridges Destructive Subduction Zones Conservative Transform Faults Astronomy 1-1 Lecture 10-19

20 Rift Zones Upwelling convection currents from asthenosphere cause overlaying crust to dome up and stretch eventually cracking With the cracking, domed regions collapses to form valleys Magma from underlying asthenosphere fills the crack between the diverging plates Astronomy 1-1 Lecture 10-20

21 Mid-Oceanic Ridges These form basically the same way as do rift zones They are undersea mountains more than 12,000 feet high and 1,200 miles wide These ridges releases much of the Earth s internal heat Mid-Atlantic and Indian Ocean Ridges Astronomy 1-1 Lecture 10-21

22 Subduction Zones Three types of Subduction zones Continental Collisions Ocean Continent Collisions Ocean Ocean Collisions Astronomy 1-1 Lecture 10-22

23 Continental Collisions Two plates containing large areas of continental crust are pushed together The results of such collisions can be found in the Himalayan Mountains Asian and Indian Plates Alps and Atlas Mountains European and African Plates During these collisions, fold mountains are generated Astronomy 1-1 Lecture 10-23

24 Ocean Continent Collisions Oceanic plates collide with continental plates Oceanic plates being denser are forced downwards where the crust is melted This area is known as a subduction zone Oceanic trenches, mountain ranges, volcanism and earthquakes are often found near subduction zones Best seen along west coast of South America Astronomy 1-1 Lecture 10-24

25 Ocean Ocean Collisions Here two oceanic plates collide with each other Volcanic activity at the boundary of two oceanic plates creates island arcs Astronomy 1-1 Lecture 10-25

26 Transform Faults Transform faults neither create or destroy crustal material Plates slip past each other, though not smoothly resulting in periodic earthquakes Most transform faults are found on the sea floor though do occur on land also San Andreas Fault Astronomy 1-1 Lecture 10-26

27 Hot Spot Volcanism Hot spot is a localized sources of high heat energy Plate moves over a deep, stationary hotspot in the mantle Heat produces a persistent source of magma Magma rises through the mantle and crust to erupt onto the seafloor, forming an active seamount Countless eruptions cause the seamount to grow until it finally emerges above sea level to form an island volcano Astronomy 1-1 Lecture 10-27

28 Continental Drift Plate tectonics implies continents are not fixed in position but move Astronomy 1-1 Lecture 10-28

29 Evidence for Drift Topographic Fit Tectonic Fit Climatic Evidence Astronomy 1-1 Lecture 10-29

30 Earth s Magnetic Field Earth has a magnetic field surrounding it Magnetic field extends thousands of kilometers into space Magnetic field is like that of a bar magnet Axis of magnetic field does not coincide with that of the rotation axis but differs by 11 o Also center of bar magnet is displaced with respect to center of Earth Magnetic poles have wandered ~680 miles during 20 th century Astronomy 1-1 Lecture 10-30

31 Earth s Magnetic Field Mechanism responsible for magnetic field is not well understood Magnetic dynamo hypothesis The magnetic field is due to the rotational movement of the liquid outer core Possibility that some of the iron and nickel atoms are ionized giving rise to a current Flows of the molten metals not steady over time Earth s magnetic field has reversed itself over time Evidence found along mid-oceanic ridges Current field has been weakening since ~1850 Astronomy 1-1 Lecture 10-31

32 Earth s Magnetic Field Magnetic field about the Earth is not symmetric as would be expected for a bar magnet Sun s solar wind distorts the shape of the magnetic field Side facing the Sun takes on the form of a shock wave Total volume encompassed by the magnetic field is called the magnetosphere Majority of the solar wind is deflected by the magnetic field Astronomy 1-1 Lecture 10-32

33 Trapped Particles Some of the particles are trapped into two rings surrounding the Earth - the Van Allen Radiation Belts First observed in 1958 Inner belt contains both protons and electrons and is centered about 3,700 mi (6,000 km) above Earth's surface Outer belt primarily contains electrons and is centered about 12,500 mi (20,000 km) above Earth's surface Astronomy 1-1 Lecture 10-33

34 Auroras Northern (aurora borealis) and Southern Lights (aurora australis) Astronomy 1-1 Lecture 10-34

35 Auroras The three most important factors for the occurrence of Auroras are 1) The Earth's atmosphere, which consists of Nitrogen, Oxygen, Helium, Hydrogen and small amounts of other gases 2) The plasma around the Earth, which contains charged particles 3) The Earth's magnetic field, which is the strongest at the poles Caused by the collision of charged particles (ions (+) (-) ) found in the magnetosphere, with atoms in the Earth's upper atmosphere Astronomy 1-1 Lecture 10-35

36 Colors of the Auroras High altitude oxygen in ionosphere red auroras Oxygen at lower altitudes causes yellow-green auroras Ionized nitrogen molecules cause lightly shining blue aurora Neutral nitrogen glows in a weak tint of red Astronomy 1-1 Lecture 10-36

37 Astronomy 1-1 Lecture 10-37