Chapter 8 lecture Flashcards
Why is the moon included
It is 90% of Mercury’s diameter and 50% of Mars’ diameter. In fact, it is one of the seven really large satellites in the solar system – the only one in the inner part of the solar system
Earths inner layer
Earth has a core of iron, nickel, and sulfur extending about half way to the surface. This is part solid and part liquid. Above that is the bulk of the Mantle, mostly silicate materials. The Asthenosphere above that is plastic (pliable) as opposed to rigid
- Lithosphere (“lithos” means rocky) and the oceans can “float” around on a non-rigid laye
Impacts
- Micrometeoroid; less than about 10 mm (ten microns = 10-5 m
- Asteroid; greater than one meter
So, a meteoroid is a small body moving through space. If this object enters Earth’s atmosphere at a great rate of speed, the outer layers will burn up because of friction with the air. The resulting streak in the sky (sometimes called a “shooting star”) is a meteor
the chicxulub impact
- about 65 million years ago a large body (the Chicxulub Impactor), perhaps 10 km in diameter, struck on the edge of the Yucat
Explain demise of dinosaurs (what is a consequence of a large impact cratering
The presence of iridium is unusual because this element is rare on Earth, although it is common in stony-iron meteorites. The heat generated by the impact would ignite huge forest fires, and the soot and iridium would be carried by winds world-wide and deposited in the layer. Debris and soot blocked sunlight and prevented photosynthesis as well as making the climate cooler. Large animals like dinosaurs could not adapt, but smaller and more versatile animals like mammals did survive. One of Earth’s “great dying’s”.
collision/impact theory ( formation of the moon)
Early in the history of the solar system, when Earth was still molten, there were many Mars-size planetesimals still in orbit. It is thought that a Mars-size body, which has been named Theia, collided with Earth and that the material torn mostly from Earth’s mantle formed the Moon
evidence of collision/ impact theory
- ## The density of the Moon is similar to the density of Earth’s mantle
other theories of formation of the moon
- with just the right mechanics, it might have been captured by Earth’s gravity while it was moving through space.
- This wide stretch of open water without any continents inspired some people to speculate that Earth was spinning fast enough while it was in its early molten state that some matter was thrown out to form the Moon, leaving the Pacific Basin as the “hole” from which it escaped
what is the problem with the gravity theory
- Suppose the Moon was originally moving through space far away from Earth. it would be in an open or unbound orbit: not bound to Earth (or to any other body).
- is that it has positive energy. It is moving, so it has kinetic energy – and kinetic energy is only positive. It is hardly interacting with anything else when it is far away, so it has negligible potential energy (which can be negative). Now it approaches Earth and is captured by Earth’s gravitational field to go into an orbit around Earth. In this state it is in a bound orbit. It has both (positive) kinetic energy and (negative) potential energy. But, to be in a bound orbit, the potential energy must be greater, and the overall energy must be negative. To be captured, then, it must somehow lose energy. This is most likely to happen
when a third body is involved in the interaction. So, this capture is possible, but not very likel
What discredits the pacific basin theory
the continents drift around very slowly. In other words, that “hole” has not always been there. The location of the continents has changed over geologic history.
Radioactive decay, explain
Radioactive decay involves the disintegration of the nucleus.
what is a Nuclei
Nuclei are made up of protons (positively charged particles) and neutrons (electrically neutral particles). Thus the nucleus itself is electrically positive, and the attraction between the nucleus and the negatively charged electrons (like the gravitational attraction between the Sun and the planets) is what holds the atom together.
what does the number of protons mean
The number of protons in the nucleus defines the chemical element: 1 = hydrogen, 2 = helium, … , 92 = uranium, etc. But the number of neutrons can vary. Consider uranium.
What are isotopes
Protons for an element will remain the same but when they have diffrent neutrons they are isotopes
What are the standard notations of isotopes
U with 235 on top and 92 at the bottom.
isotope of uranium is written as U-235
the one written on top is the total number of nucleons(n+p)
n= neutrons and p= protons
Are all isotopes stable
Not all the isotopes of these hundred plus nuclei are stable. Some decay spontaneously into other nuclei. U-238, for example, emits an alpha particle (which is the same as a helium nucleus 𝐻𝑒24) which has two protons and two neutrons. The result is Thorium-234 with 90 protons and 144 neutrons.
Radioactive decay examples
40 K - 40 Ar
235U - 206Pb
𝐾1940 → 𝐴𝑟1840 1.3 𝑏𝑖𝑙𝑙𝑖𝑜𝑛 𝑦𝑒𝑎𝑟𝑠
When working out an age problem, it is convenient to specify amounts of these isotopes by comparison with a stable isotope which does not decay and is present in a constant amount. In this case we use potassium-39.
Suppose we have a sample, a rock, for example, in which we know the original composition (when the rock was formed) and the present composition to be as follows. (This problem is summarized in Image #17.)
ORIGINALLY NOW
𝐾1940 ∶ 𝐾1939 = 1:10 𝐾1940 ∶ 𝐾1939 = 1:20
𝐴𝑟1840 ∶ 𝐾1939 = 1:20 𝐴𝑟1840 ∶ 𝐾1939 = 1:10
what are the results
Earth
- Oldest rocks on Earth (from the Canadian Shield – the area on both sides of Hudson’s Bay): about 4.0 billion years. (There have been recent reports of rocks in - –
- Western Australia dating from 4.4 billion years ago.)
Moon
Oldest rocks on the Moon: about 4.5 billion years.
Meteorites Oldest meteorites (carbonaceous chondrites): about 4.56 billion years. We believe these date the beginning of the solar system.
Earth density
density = mass/volume = 5.98x1024 kg/1.09x1021 m3 = 5480 kg/m3.
Now, we can measure directly the density of the material on Earth on Earth’s surface. The average is about 3000 kg/m3. So, to produce an overall density of 5480 kg/m3, the material in the interior must have an average density of more than 5480 kg/m3. What could this be? Look at image #20 in the PowerPoint file for Chapter 8. Clearly, the interior must include materials like iron, nickel, and chromium.
Seismic waves
waves are a means of transporting energy from one point to another. Waves involve oscillations, but there are various ways this can happen.
Diffrences between longitudinal and transverse
longitudinal waves are ALONG the direction of the wave velocity whereas the oscillations in transverse waves are ACROSS the direction in which the wave is moving.
One important consequence of this difference is that longitudinal waves compress the material through which they are moving. The compressed material pushes back, helping to sustain the wave motion. -Sound waves are longitudinal waves, and they can travel through solids, liquids, and gases. Transverse waves, however, push sideways on the material through which they move. If this material is a solid, it will push back. But, if it is a liquid or a gas it cannot push back. The motion of a transverse wave cannot be sustained in a liquid or a gas.
Why do planets turn out heating/cooling
Heating:
- Initial collisions – formed molten
- Radioactivity – many radioactive elements with half-lives shorter than the age of the Solar System initially formed. Provided intense heat. Activity now much reduced.
- Friction – from tidal flexing. We discussed this when we studied tidal forces
Cooling
The rate at which an object cools is proportional to its surface area (through which it loses heat). The area is proportional to the linear dimension (radius, for a sphere) squared.
The amount of heat energy an object stores is proportional to its volume, which is proportional to its linear dimension cubed. So, as an object gets bigger, its heat storage capacity (volume) grows faster than the area through which it loses heat. We will see some astronomical consequences of this when we study the Jovian planets where, Jupiter, for example, is still emitting more energy than it receives from the Sun
Bioogical concequences
It doesn’t have much heat-storing volume per unit area. But, if an elephant gets wet, she doesn’t worry. Although her surface area is many times the mouse’s area, her volume is many, many times the mouse’s volume
geometry
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bar magnets
opposite poles attract, like poles repel. See Image #26 and note that the magnetic field lines extend through the magnet, not just at the poles. If you break a magnet, you will get two magnets, not isolated N and S poles.
Since a compass needle points north, that suggests Earth acts like a giant bar magnet. See Figure 8-12 and Image #27.
But, if the north end of a compass needle points to the north, shouldn’t the magnetic pole in the northern hemisphere be a south magnetic pole? You will find many diagrams in which this is done. Your text, however, calls the geographically-north magnetic pole, a north magnetic pole. In this case, that end of the compass needle is sometimes called the north-seeking end of the compass. Anyway, this is just a matter of semantics.
Earths magnetic field, what is needed
planet, this means a rotation around its axis and some liquid electrically conducting material in the interior to provide a current.
Note that the magnetic pole in Earth’s northern hemisphere is not at the North Pole, but in northern Canada – which is why there is a difference between “geographic north” and “magnetic north” on maps.
For another thing, Earth’s magnetic field changes slightly from place to place every day, and experiences a complete reversal of direction every few hundred thousand years
Other planets in terms of magnetic field
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reversals
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magnetosplane
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tectonics
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continents
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Mechanism
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seafloor spreading
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dating reversals
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what is seismology
the study of seismic waves generated by earthquakes and how they move through the planet.
explain atmosphere, biosphere and hydrosphere
The Hydrosphere is the layer containing the oceans, the Atmosphere (which we will study in much more detail in the next chapter), extends above that, and the Biosphere envelops both of these.
What are the mechanics of change for a planets surface
Tectonism – continental drift or tectonic plates
Volcanism – volcanoes and volcanic outflow
Erosion – wind and water
Impact cratering – the main topic of this section
Define micrometeoroid,
meteoroid, meteor and meteorite
1.So, a meteoroid is a small body moving through space.
Meteor; If this object enters Earth’s atmosphere at a great rate of speed, the outer layers will burn up because of friction with the air. The resulting streak in the sky (sometimes called a “shooting star”) is a meteor.
3. parts of a meteoroid might survive to reach Earth’s surface. Such a fragment on the surface is a meteorite.
explain the process of impact cratering
A large meteoroid striking Earth’s surface (Image #5) carries a great deal of energy, and an explosion usually results which ejects material from the resulting crater and also send shock waves which can pulverize subsurface material. The meteoroid itself is often vaporized. “Shock-modified quartz” is a type of mineral found only in impact craters. Tycho (Image #6) is a major crater on the Moon with long streaks of ejecta reaching far across the Moon’s surface.
examples of another atmospheric event
We have seen atmospheric events like this before. Eighteen sixteen was known as “the year without a summer”. It was cold and overcast. Crops failed in the US and Europe. It was later learned that Mount Tambora, still an active volcano in the Lesser Sunda Islands of Indonesia, had erupted the previous year. It is generally believed this eruption to be the greatest volcanic eruption of the past 10,000 years.
State evidence of cratering in the solar system and why some planets do not have as much cratering as the others
Mercury and the Moon are heavily cratered (Images #9 and #6). Venus (Image #10) and Mars (Image #11) are not as heavily cratered. This suggests that the surfaces of these two planets have been refreshed over time, obliterating earlier craters. We believe Venus is still actively volcanic. Mars has been unchanged for billions of years, but it formerly had active volcanoes and liquid oceans which smoothed the surface long ago
Earth’s magnetosphere
Earth’s magnetic field shields us from cosmic rays (Image #32) by diverting many of the charged particles in the solar wind around Earth. Some penetrate to the inner regions of our magnetic field, forming a layer of charged particles called the Van Allen Radiation Belts.
–summarizes our findings for magnetic fields for the Terrestrial planets and the Moon. But it is not a simple problem, and the dynamo theory which is mentioned does not have all the answers. For one thing, the rotation axes for the planets do not always align with the magnetic axes. See Images #30 & #31. Note that the magnetic pole in Earth’s northern hemisphere is not at the North Pole, but in northern Canada – which is why there is a difference between “geographic north” and “magnetic north” on maps.
For another thing, Earth’s magnetic field changes slightly from place to place every day, and experiences a complete reversal of direction every few hundred thousand years. We will see more of this in the next section. And, because the Sun experiences these magnetic field reversals with an average period of only 11 years, we will revisit the problem in Chapter Fourteen.
continents
Many have noted that the shapes of the continents do not seem exactly random. South America and Africa seem to fit together, and Europe might fit into the Gulf of Mexico (Image #36). Newton commented on it in the 1680’s. But, the first to propose that the continents were once all together was Alfred Wegener in 1915. See Image #37. Pangaea was a supercontinent believed to exist about 237 million years ago
convention
Convection is a mechanism for moving energy from one location to another by actually moving material from one location to another.
Convection currents operate in Earth’s atmosphere, among other places. See Image #38. Radiation from the Sun heats Earth’s surface. The air there warms and expands, thus decreasing its density (the volume increases while the mass of the air remains the same). This warm air rises, pushed up by the colder, denser, air above - which is falling because of its greater density. Then the warm air cools and sinks, displacing the surface air which has warmed. In addition to the atmosphere (Image #39), convection operates in your teapot, your refrigerator, the Sun, and in the outer layers of Earth. To explain plate tectonics, we are dealing with the convection currents within Earth’s mantle. See Images #40 and #41 and the pictures on page 212. The Asthenosphere is the upper layer of the mantle, just below the Lithosphere. It is plastic and has low resistance to flow. The lateral convection currents carry the continents. The continents are generally made of granitic rock which is lighter than (and floats upon) the heavier basalts of the seafloor.
seafloor spreading
Seafloor spreading also ties in with the evidence for the magnetic reversals mentioned in the previous section. See Figure 8.14 and Image #44. Molten magma pours out of a rift (or spreading center). The magnetic particles in the magma align with the magnetic field existing at that time – a more complete alignment if the field is strong and a less complete alignment if the field is weak. This magma then solidifies to form basalt, making a permanent record of the magnetic field strength and direction at that time. Later eruptions record the field at later times.
how fast
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measuring magnetic reversals
We have mapped out a system of tectonic plates at the present time. See Figure 8-17 and Image #45. Note that the Mid-Atlantic Rift (or Ridge) – a spreading center- passes right across Iceland. It is actually possible to stand with your back against one wall of the Mid-Atlantic Rift and gaze at the other wall, about three kilometers distant.
volcanisms types
composite and shield.
Composite
Composite volcanoes have steep sides. Mt. Fuji in Japan is a dormant composite volcano (last eruption 1707-08). See Image #50. Composite volcanoes form from uplifts at tectonic plate boundaries. See Images #51 & 52.
You can see from the map in Image #52 that the converging boundaries (subduction zones) of tectonic plate movement are marked by numerous volcanoes. Note particularly the “Ring of Fire” that marks the North Pacific.
Shield;
Shield volcanoes have more gently sloping sides and get their name because they resemble a warrior’s shield. The lava here is not as dense as the lava in composite volcanoes, and thus flows farther. Mauna Loa on Hawaii is a shield volcano (See Image #53). It is the world’s largest active volcano (last eruption, 1984. The 2018 volcanic activity on Hawaii was at Kahauale’a, farther east.
hot spot volanism
Hawaii is in the middle of the Pacific Ocean, far from any plate boundaries. The Hawaiian Island chain is a result of what is called Hot Spot Volcanism. See Images #54 & #55 and Figure 8.21. A hot spot develops in the upper mantle, and the magma finds its way to the surface through some weak channel to form a shield volcano. In the case of the Hawaiian Islands, the whole system sits on the Pacific Plate which moves slowly westward. Thus, as one weak channel is replaced by another, one island after the other is formed. So Kauai, to the west, is older than Hawaii.
olympus mons
The largest volcano in the solar system is Olympus Mons on Mars (Image #56). The caldera is about the size of Nebraska. And it is three
times the height of Mt. Everest. It is so large, in part, because of Mars’ gravity (weak compared to Earth’s) allows structures to rise higher. But the main reason is that Olympus Mons is a hot spot volcano. If there had been plate movement on Mars, this would be part of a chain. But, instead, the hot spot just pumped out lava for millions of years on the same spot
Venus
Venus is similar in size and mass to Earth, so one might expect tectonic plate movement there similar to here. But no evidence has been found. A topographic map of Venus (Image #57) shows continents, shield volcanoes, and (lava) flood plains, but no long chains of mountains (like the Rockies or the Andes) indicating a subduction zone between tectonic plates. So volcanoes on Venus must be hot spot volcanoes. Earth seems to be the only one of the Terrestrial planets to have tectonic plate movement
Moon
The most notable geologic features on the Moon’s surface are craters, almost all of which are of impact origin. The other major features are the maria (“seas”) which are huge impact craters dating from about 4.0 – 3.5 billion years ago. We have seen that the Moon formed about 4.5 billion years ago, and by about 4.0 billion years ago the surface had solidified to form what we now call the highlands. Into this then slammed some large impacts, and perhaps half a billion years later these filled with magma from the Moon’s interior. This solidified to form basalt.
There are no indications of the mountain chains which characterize tectonics. There are signs of a few shield volcanoes dating from perhaps 100 million years ago. There are also rilles – lava channels from old volcanic flows. And there are fault scarps, “wrinkles” which formed when the Moon shrank as it cooled. We have seen these on Mercury as well.
As we have learned, the far side of the Moon cannot be seen directly from Earth. In 1959 the Russian spacecraft Luna 3 photographed the far side of the Moon for the first time. The result was not at all what was expected. First, there are very few maria on the far side. And secondly, it contains the largest impact crater in the solar system. This is the South Pole-Aitken Basin (Image #59) which is about the size of Europe. The reasons for the difference between the two sides of the Moon are complex and not well understood.
