Transcript oct29
How atmospheres are created 1. 2. 3. 4. Direct capture from solar nebula Outgassing Evaporation/sublimation Bombardment– meteors/solar wind How atmospheric gasses are lost 1. 2. 3. 4. 5. Thermal escape Stripping via solar wind/cosmic rays Atmospheric cratering Condensation Chemical reactions Thermal escape 1. Escape velocity 2. Temperature 3. The mass of the gas particles The Moon’s atmosphere Because the Moon has no global magnetic field, solar wind particles constantly strike the daylight side of the Moon, causing bits of the surface to fly off. These particles can travel 10’s of kilometers and come to rest when they get to the night side of the Moon. The Moon may contain a large deposit of frozen water The deepest excavation on the Moon is the Aitken basin which is over 12 km deep and lies near the south pole. Because the Sun never shines in this basin it remains perpetually cold and may contain 6 billion tons of ice, delivered from impacts with comets. Mercury’s atmosphere Mercury is too hot and has too little surface gravity to hold much of an atmosphere. But there are trace amounts of oxygen, sodium, and helium. These must be constantly replenished. Mercury Moon Discussion What does the clumpy nature of Mercury’s atmosphere tell you about its origin? Mercury may also contain water ice? Mercury has radar bright areas near the poles which are very similar to reflections from Jupiter’s moons and the polar ice caps on Mars. Mercury’s rotation axis is nearly perpendicular to its orbit. Thus within 6.5 degrees of the poles, crater floors are deep enough that the Sun never shines there allowing temperatures to stay below freezing. Because some of the atmosphere is constantly escaping into space, atmospheres have no edge. They just get thinner and thinner. Discussion Can the Earth hold hydrogen and helium gas in its atmosphere? Explain how you know? Discussion Is it possible that any of the other terrestrial planets or the Moon can hold hydrogen or helium gas in their atmospheres? Explain why. Early atmospheres All of the planets started out with atmospheres captured directly from the solar nebula containing primarily hydrogen and helium. Early H and He atmsphere lost into space by thermal escape. Each of the terrestrial planets then acquired an atmosphere through a combination of bombardment and volcanic outgassing. In the case of Mercury and the Moon, this atmosphere was also lost into space over time. Discussion If the terrestrial planets’ atmospheres came from volcanic outgassing, why is Earth’s so different from the atmospheres of Venus and Mars? Where did all the nitrogen and oxygen in Earth’s atmosphere come from? The origin of life on Earth The oldest surviving rocks on Earth date from 3.8 billion years ago and show that life in the form of blue-green bacteria already existed on Earth. These bacteria used photosynthesis to convert CO2 into O2. Chemical reactions remove O2 from atmosphere Bacteria extract oxygen from nitrates in the soil and emits N2 into atmosphere. N2 is combined with O2 by lightning to form nitrogen oxides. The nitrogen oxides are then absorbed by rain water and re-deposited in the soil. Oxygen buildup Approximately 2 billion years ago, oxygen could no longer be absorbed chemically and started to build up in atmosphere. Structure of Earth’s atmosphere Discussion Suppose you are on a rotating merry-goround sitting near the center and you roll a ball toward the edge. Describe how the ball moves on the merry-goround as it rolls outward. If you sat at the outer edge and rolled the ball toward the center how would it move? The Coriolis Effect Discussion How does the rotation of Venus differ from that of the Earth? How would you expect the circulation of heat to differ on Venus? Hadley cells on Venus It “snows” on Venus The tops of high mountains on Venus are coated with a shiny substance, much like high peaks on Earth covered with snow. On Venus, which is too hot for water snow, it “snows” lead and magnesium sulfides which collect on high mountain peaks which are cooler than the surface. Discussion In general, temperature falls with height. Why does the temperature rise in Earth’s stratosphere and thermosphere? Discussion Why doesn’t the temperature rise in the stratosphere of Venus, (or more precisely why doesn’t Venus have a stratosphere)? Discussion Why are the pictures from Verona 13 red? Discussion Why is Venus so hot? Although Venus is closer to the Sun than Earth, its cloud cover reflects nearly 60% of the light that falls on it. The surface therefore receives less energy from the Sun that the Earth’s surface does. The greenhouse effect Carbon dioxide (as well as water vapor and methane) absorb infrared radiation. Sunlight can pass to the surface of Venus and heat it. But the infrared radiation it emits can not escape back into space. Greenhouse temperature increase Venus – 400 degrees C. Earth – 36 degrees C Mars – 6 degrees C Tan is opaque White is transparent Discussion Explain why the previous plot looks the way it does. Discussion Venus is too hot to have liquid water on the surface. Earth has just the right temperature to liquid water. Mars is too cold to have liquid water. Explain why. The carbon-silicate cycle Rain water dissolves carbon dioxide, and sulfur dioxide, forming carbonic acid and sulfuric acid. This acid rain erodes rocks that contain calciumsilicate minerals. Carbonic acid reacts with the rock and forms calcium and bicarbonate ions, which are carried into the oceans. There these ions are deposited on the sea floor as limestone or used by plankton and other organisms for calcium carbonate shells, which are deposited on the ocean floor when the organisms die. Discussion What happens to the limestone on the ocean floor? Does it just stay there for the rest of eternity? Spreading of the sea floor causes these deposits to be cycled into the Earth’s interior where it reacts with quartz to produce new silicate rock and carbon dioxide. Discussion What would happen if the Earth grew colder so that the oceans completely froze over? Discussion What happens as the average global temperature of the Earth goes up? The faint Sun problem Sun was only 70% as bright as it is today, but liquid water still existed on Earth As the Sun brightened, CO2 removed from atmosphere to regulate temperature. Discussion Why is the Sun getting brighter over time?