Astronomy Today 9Ed. Ch. 7 - The Earth R&D Questions 1-15 odd. ANSWERS Answers in [ brackets ] were not assigned or found only in the 7th edition. 1. The density of water is 1000 kg/m^3 and the density of rock is typically 3000 kg/m^3. Since the average density of the Earth is 5500 kg/m^3, the interior of the Earth must be made up of high-density material, such as iron and nickel. [7thEd.: 2. Rayleigh scattering is a kind of deflection of light rays by particles in a way that depends strongly on the wavelength of the light. For air molecules, the blue and UV wavelength's scatter the most strongly. The result is a blue sky, and red sunsets. ] [ 2. When waves from earthquakes travel through the Earth's interior, properties of those waves, such as their intensity and velocity, are changed by the materials they pass through. By seeing exactly how those properties change, geologists can determine the nature of he material in the interior. Some seismic waves (S-waves) cannot pass through liquids, so the Earth's liquid core is revealed by the way it blocks these waves, but not P-waves. P-waves (or primary waves) are longitudinal seismic waves, while S-waves (or secondary) a slower, transverse seismic waves. Both are produced by earthquakes, but the P-waves arrive first at a distant seismograph and how much sooner can tell you the distance. The S-waves cannot pass through the liquid core so the liquid (outer) core casts a "shadow" in S-waves, but not P-waves. ] 3. The greenhouse effect is a process that allows the Earth's atmosphere to trap heat. Sunlight passes through the atmosphere and is absorbed by the Earth's surface where it turns into "heat". The hot surface emits infrared radiation. However, infrared radiation cannot easily pass through the atmosphere and back into space because it is absorbed by "greenhouse gases" such as carbon dioxide and water vapor. The infrared gets reradiated many times until it can escape into space by random chance. [ 4. Geologists think that the outer core is liquid because of the way seismic S-waves are shielded by something inside the Earth. Modelling and solid state physics also suggest that the phase of an iron nickel mixture should be liquid at that pressure and temperature. Finally, the presence of a magnetic field in planets is explained as a consequence of a rotating, liquid, conducting interior. ] 5. The differentiation of the Earth's interior is a clue that it went through an early period (estimated at 700 million yrs) of being mostly liquid magma. The heating was presumably caused by formation, meteoroid impacts, and radioactive decay. While liquid, the densist atoms (like iron) can sink towards the core. [ 6. Convection is the transfer of heat through the bulk (large-scale) motion of a fluid. a) It is important in the atmosphere for carrying heat from the surface up to the sky and creating clouds. b) It is important in the Earth's interior because the best explanation for the driving force for plate techtonics is the convection of the semi-liquid mantle. ] 7. How did radioactive decay heat Earth early in its history? When did this heating end? Radioactive elements were mixed in with the more common elements when the Earth formed. These radioactive elements were dispersed throughout the Earth (although differentiation should have brought the denser ones closer to the core). As these underwent nuclear fission, they heated the surrounding rock and that heat could not easily escape. Initially, the Earth could not cool fast enough (through IR radiation from its surface) to keep up with the heating by radioactivity and impacts. This kept it molten for about 700 million yrs, at which time both radioactive decay and impacts had decreased enough to allow the Earth's temperature to drop. In truth, radioactive heating continues to this day, but at a diminished rate.] [ Old ? : Radioactivity (along with chemistry) can reveal the presence and abundance of certain radioactive elements in Earth rocks. Some of these elements decay into "daughter" products (which are sometimes also radioactive but with a different half-life, or time to decrease in number by 1/2). By measuring the relative amounts of the daughter and parent nuclei, the age of the parent can be determined. For example, if there are equal quantities of a parent nuclei and its (stable) daughter, then 1/2 of the original parent nuclei remain and so one half-life has elapsed. The oldest rocks set a minimum age for the Earth. ] [8. Plate tectonics is the cause of mountains, trenches, and most other surface features of the Earth. Mountains are caused by plates colliding with each other, pushing up parts of one plate into mountains. When plates pull apart, the form trenches that allow new crustal material to rise. ] 9. The dynamo process requires both a conductive fluid (like liquid metal) inside the planet, and rapid rotation of the planet. The liquid outer core of the Earth is likely to be responsible for a dynamo process which creates the Earth's magnetic field. This magnetic field has the effect of protecting life on the surface of the Earth from charged particles (e.g., cosmic rays). [10. The magnetosphere is a region above a planet's atmosphere in which the planets magnetic field controls the dynamics of charged particles in space. The Earth's field lines form a dipole structure (leaving the South pole and entering at the North pole). Since charged particles (e.g. solar wind) are deflected into helical paths by magnetic field lines, they tend to collect in regions like the "van allen belts". The magnetosphere was discovered by early satellites that passed through these belts. ] The magnetosphere deflects or traps fast, charged particles mostly electrons and protons, given off by the Sun, distant stars and AGN. If not for Earth's magnetic field these high-speed particles would be able to enter deeper into the Earth's atmosphere and threaten life on Earth. ] 11. How do we know that Earth's magnetic field has undergone reversals in the past? How do you think this influenced life? When liquid magma emerges from a mid-oceanic rift, it solidifies into crustal rocks which are magnetized in the direction of the Earth's magnetic field. The crust is progressively older at greater distances from mid-oceanic ridges. So as we measure the magnetization of the crust at greater distances from the ridge, we are measuring the Earth's magnetic field further into the past. It appears that reversals of the Earth's magnetic field occur every 0.5-1.0 million yrs. These reversals may effect the evolution of life if there are periods in which the magnetic field is so reduced that we loose our magnetosphere and charged particles can reach the Earth's surface. The bombardment of particles could lead to more reproductive mutations and even extinctions. [12. The Moon's gravity decreases with distance from the Moon. The side of the Earth closest to the Moon feels a stronger pull than the center of the Earth, and the side of the Earth farthest from the Moon feels a weaker pull than the Center. Consequently, there are two tidal bulges on the Earth. The oceans bulge out more than the solid crust, so the water level rises near the shorelines. The sides of the Earth near and far from the Moon experience high tides, while the other sides experience low tides. ] 13. Since the Moon doesn't rotate relative to the Earth, the near and far sides of the Moon would be at "high tide" almost permanently. The tidal force would vary a little bit with a period of 1 sidereal month, because of libration (the part of the Moon that faces us oscillates back and forth a little) and because of the varying distance between the Earth and Moon (tides strongest during perigee). So changes in water level should occur with a ~27 day period. The strength of the tidal force on the Moon would be about (ME/Mm)(RM/RE)=(80)*(1/4)=20x that on Earth because of Earth's larger mass. The magnitude of the distortion also depends on the strength of materials (water flexes more than land) and the surface gravity of the Moon (~1/7 that on Earth), so that makes a larger distortion on the Moon. It is not visible in images, just as images of the Earth from space don't resolve its ~3-20 ft tides. [14. Even without the Moon, the Earth would still experience tides from the Sun. Since the Earth is small compared to its distance from the Sun, the solar tides are half of the lunar tides, or one third of the total tides experienced now. So, even without the Moon we would know about the tides, although they would be much smaller and weaker. ] 15. Some greenhouse effect is definitely needed to sustain life as we know it. Without any, the average surface temperature would be about 40 degrees C cooler. If the amount of greenhouse effect increases and causes global warming, that could be disasterous. Ocean levels would rise, flooding coastal cities, areas would suffer desertification, some animal and plant ecosystems would shift and in some cases disappear (e.g., coral reefs), the food chain would be influenced in complex ways, and current farmlands would no longer be able to sustain the same crops.