KNOWLEDGE GET FROM TODAY S CLASS MEETING Class Meeting #15, Monday, February 22 nd, 2016 1) Earth s internal composition and structure Text Page 197, Chapter 7; Text Pages 234-239 (Chapter 9) 2) What does the term DIFFERENTIATED mean in the context of Earth s internal structure? Pages 235-238 3) Earth s magnetic field: how is it generated? What does its presence indicate about the Earth s interior? Page 240 (Chapter 9) You should be finishing CHAPTER 6 (telescopes) in the text and also reading CHAPTER 9 in the text (the Earth sections)!!
HOMEWORK #2 was distributed Monday, Feb 8 th With HW #2, you are obtaining OBSERVATIONS of the evening sky / MOON every-other evening for 4 more days Your set of OBSERVATIONS, and also your answers to QUESTIONS distributed Wed, Feb 17 th, are due to be handed in here in BX102 at the start of class on THIS CMING FRIDAY, February 26 th Additonal Observation Sheets are available online within the class web page HOMEWORK folder You should already have 7 or 6 observations
This Week s Lab will occur in WH 232 You will conduct a lab focused upon SEASONS available at: http://astronomy.nmsu.edu/murphy/astr105g-m040506-spring2016/lab-manual QUESTIONS?
EXAM #1 has been graded and will be returned here in class Wednesday, Feb 24 th
Thus far this semester we have focused upon some fundamental physical ideas& processes that are responsible for some of the basic observed characteristics of what we see in the Solar System (or beyond) Starting today, we will use these fundamental ideas to learn about and understand specific objects (individual planets, the Sun) or types of objects (moons, asteroids, comets, etc.) within our Solar System BEGINNING TODAY, our own EARTH will be the focus of our attention We will thereafter compare solar system objects to Earth
Earth as viewed by the Galileo Spacecraft in the 1990 s (..Galileo was traveling to Jupiter but passed back by Earth on the way )
Determining EARTH's composition from a distance. Imagine you are a MARTIAN (living on Mars), and you point your telescope toward Earth What characteristics of Earth do you see? Let's list some characteristics that might help us determine Earth's composition.we are trying to determine Earth s density, since density provides information about composition (materials the planet is composed of)..
EARTH: Size (6378 km radius Volume) Surface covered by water (75%) and 'rock' (25%) Presence of an orbiting Moon (but we'll ignore this to start..) From this information, what do you determine Earth's density (and thus composition ) to be?
Well,. since 75% of the Earth s surface is WATER, and 25% of the Earth s surface is rock, I ll guess that Earth s total composition is 75% WATER and 25% ROCK and I ll use my knowledge of the densities of WATER (1 gram per cubic centimeter) and ROCK (~2.5 grams per cubic centimeter) to ESTIMATE Earth s total density
Determining Earth s Density (?) Well, the: Density of Water = 1 gram per cubic centimeter and the Density of Rock = 2.5 grams per cubic centimeter So: 75% (0.75) x 1 g/cm 3 + (0.25) x 2.5 g/cm 3 = 1.375 g/cm 3 25% Is this value representative of Earth's density? NO it is not representative of Earth s density! We know that Earth's density is 5.5 g/cm 3.. you did a similar calculation for the DENSITY Lab..
What went wrong in our estimation? We made the error of assuming that: 'what we see on the surface is what we get'.. which is not necessarily true!!! Earth's interior composition is NOT well represented by its surface materials [This incorrect density would result in an incorrect determination of Earth s mass..]
If you Martians DID NOT know that Earth's density is 5.5 grams per cubic centimeter, how could you, using the complete Earthinformation you obtained with their telescopes (including the Moon's presence and its orbit characteristics mentioned 2 weeks ago..) better determine the Earth's mass? USE NEWTON's modified form of KEPLER's 3 rd LAW:
NEWTON's modified form of KEPLER's 3 rd LAW: (M Earth /M Sun ) P Moon2 = OSA Moon 3 with OSA Moon in AU (0.00256 AU) and P Moon in Earth Years (0.0747 Earth years per Moon orbit, corresponding to 27.3 Earth days) This would show that M Earth = M SUN / 333,000 or M Earth = 6 x 10 24 kilograms This value for Earth s mass is CORRECT!!
With knowledge of Earth's VOLUME (from its observed Radius, with VOLUME=4/3 π Radius 3 ) and its MASS as we calculated on the previous page: DENSITY = MASS/VOLUME = 5.5 grams/cm 3 From our knowledge of reasonable planet materials, we can use this density to determine that Earth is composed of ROCK and METAL 32 % of Earth's mass is composed of IRON (a metal)
Is 32% of Earth's surface composed of IRON? NO!! Then, WHERE IS THE IRON? THE IRON IS CONTAINED WITHIN THE EARTH (near Earth's center.why there?) Within a strong gravitational field (such as that produced by the mass of the Earth), which substance sinks to the bottom (or center), the less dense or more dense substance?
Imagine a mixture of metal spheres, plastic golf balls, and water ice, mixed randomly.. Let me heat this mixture to a temperature where the ice will melt.. What 'structure' of materials (from bottom to top) will we get after the ice has melted? The INITIAL structure will be random:
The FINAL vertical structure will be: PLASTIC GOLF BALLS (least dense) at the top WATER (intermediate density) in the middle METAL SPHERES (most dense) at the bottom
This structure, with most dense material at the bottom and least dense at the top is said to be: "DIFFERENTIATED"..layers of different density materials.. Earth's internal structure is DIFFERENTIATED
Within the DIFFERENTIATED Earth: The DENSE metal is concentrated in the Earth's central CORE MEDIUM DENSITY rock is in the MANTLE above the core And LEAST DENSE ROCK is in the CRUST that surrounds the MANTLE and in fact forms the surface of the Earth (both water covered and dry-land surface)
Earth's Internal Structure MANTLE CORE CRUST
Earth's Internal Structure Outer Core CRUST Solid Inner Core Molten Iron MANTLE The stiff rock of the crust and upper mantle are the LITHOSPHERE
The Internal Structure of the Earth CRUST (5-70 kilometers thick, on top of the MANTLE) Rocky-type materials, too, but a bit less dense than the mantle materials MANTLE (3400-6200 kilometers from the center) Rocky type materials CORE (0-3400 km from the center) Composed of iron (with some nickel, sulfur)
The fact that Earth is differentiated is a clear indication that as the Earth was forming 4.5 billion years ago, it was at one point in time a melted mass of material. As materials (dust grains) began to 'stick' together, they were cold (not melted); as more and more materials became stuck together, and Earth's gravitational pull increased (as its mass increased), infalling material arrived at faster and faster speeds, with the collisions releasing large quantities of energy which was sufficient to melt the materials
Once melted, the denser materials could SINK to the CORE with moderate density material ending up in the MANTLE (the middle) and finally the least dense materials 'floated' to the top (the CRUST) If melting had never occurred, the Earth would not exhibit this internal density structure (the dense materials would have had trouble migrating down through the less dense material if everything had remained solid
The Internal Structure of the Earth CRUST (5-70 kilometers thick, on top of the MANTLE) Rocky-type materials, too, but a bit less dense than the mantle materials MANTLE (3400-6200 kilometers from the center) Rocky type materials CORE (0-3400 km from the center) Composed of iron (with some nickel, sulfur)
The fact that Earth is differentiated is a clear indication that as the Earth was forming 4.5 billion years ago, it was at one point in time a melted mass of material. As materials (dust grains) began to 'stick' together, they were cold (not melted); as more and more materials became stuck together, and Earth's gravitational pull increased (as its mass increased), infalling material arrived at faster and faster speeds, with the collisions releasing large quantities of energy which was sufficient to melt the materials
Once melted, the denser materials could SINK to the CORE with moderate density material ending up in the MANTLE (the middle) and finally the least dense materials 'floated' to the top (the CRUST) If melting had never occurred, the Earth would not exhibit this internal density structure (the dense materials would have had trouble migrating down through the less dense material if everything had remained solid) now, let s talk about one consequence of Earth having an Iron core
PLANETARY MAGNETIC FIELD http://www.phy6.org/earthmag/reversal.htm EARTH (and several planets and moons, and the Sun) acts as if it is a giant BAR MAGNET with a NORTH magnetic pole and a SOUTH magnetic pole (if you could find a large enough refrigerator, you could 'stick' the Earth to it!!.. ) [the abovestatement is meant as an idea Earth s magnetic field that we detect here at Earth s surface is weak. but does have importance]
Magnetic Field Lines: diverge from the South (S) Magnetic Pole and converge on the North (N) Magnetic Pole S N S
North Pole (rotation) North Magnetic Pole 80 N, 110 W South Magnetic Pole South Pole (rotation) http://csep10.phys.utk.edu/astr161/lect/earth/magnetic.html NOTE: Earth s MAGNETIC POLES do not correspond to Earth s ROTATION POLES
WHAT PHYSICAL PROCESS PRODUCES MAGNETISM? In a piece of MAGNETIZED metal: The ELECTRONS all orbit their NUCLEI in the same direction (this is a mental model.) + This is a magnet! Nucleus Electron orbit
If the electron orbits are NOT all in the same direction, than there is no magnetism In a piece of NOT MAGNETIZED metal: The electrons DO NOT all orbit their nuclei in the same direction This is not a magnet! Nucleus Electron orbit
A flowing ELECTRIC CURRENT (electrons moving along a path, like a wire) also generates a MAGNETIC FIELD The MAGNETIC FIELD spirals around the direction of the FLOWING CURRENT Current flowing to the right in a wire...
The Earth's MAGNETIC FIELD can be considered as the result of Earth's ROTATION and the MOLTEN IRON in its outer core A flowing ELECTRIC CURRENT produces an accompanying MAGNETIC FIELD SO, because Earth has MOLTEN IRON in its outer core, and Earth ROTATES (the motion that causes day and night) at a fast enough rate to cause this MOLTEN IRON and its free electrons to flow like an electric current, Earth generates a MAGNETIC FIELD (this is a simplified description of a DYNAMO)
Earth's Magnetic Field exists BECAUSE Earth is Differentiated!!! and MOLTEN IRON is present and because Earth rotates relatively rapidly As we descend down in to the Earth, temperatures increase, and pressures increase, and we experience denser and denser material
At the bottom of the MANTLE, as we enter the OUTER CORE, temperatures are hot enough (and pressures are not yet too high) to melt iron [as we descend deeper toward the inner core, temperatures are still hot, BUT the pressure increases and this pressure increase 'squeezes' the iron into a solid form] We can use this Magnetic Field to determine the direction of NORTH... animals do, also
BUT The MAGNETIC North Pole is NOT in the same location as the ROTATION North Pole (and the magnetic north pole position wanders..the circled numbers represent years) ROTATION NORTH POLE
and the SOUTH MAGNETIC POLE location has also changed, or WANDERED, through time
THERE IS CLEAR EVIDENCE THAT THE POLARITY (which hemisphere is positive, which negative) OF EARTH S MAGNETIC FIELD HAS FLIPPED IN THE PAST (~ every 500,000 years...) AND The current locations of the magnetic poles wander
Magnetic Rocks at the bottom of the Atlantic Ocean indicate that they have solidified during both normal (current) and opposite magnetic field orientations We will return to this topic after we have discussed PLATE TECTONICS
ANOTHER PICTURE OF THE DAY The Aurora Borealis ( Northern Lights ) (charged particles from the Sun interacting with Earth s magnetic field and atmospheric atoms)
..and yet more Pictures of the Day.. The charged solar particle / magnetic field / atmosphere interaction lights can also be seen from ABOVE the Earth looking down, and can also be seen on other planets (the example here is Saturn) which also possess magnetic fields..
Earth's Magnetic Field exists BECAUSE Earth is Differentiated!!! And MOLTEN IRON is present and because Earth rotates relatively rapidly