21st C ENTURY A STRONOMY T HIRD E DITION Hester | Smith | Blumenthal | Kay | Voss Chapter 8 Lecture Outline Atmospheres of the Terrestrial Planets

©2010 W.W. Norton & Company, Inc. Primary Atmospheres New planets formed primary atmospheres by sweeping up gas in the accretion disk. All terrestrial bodies lost their primary atmospheres. Planets could not hold onto the gas.

©2010 W.W. Norton & Company, Inc. Secondary Atmospheres Secondary atmospheres were acquired later by: Accretion (but locked in the interior) Volcanism (releasing the gas from the interior) Comet impacts

©2010 W.W. Norton & Company, Inc. Planetary Atmospheres Mercury and the Moon have basically no atmosphere. Venus, Earth, and Mars were able to obtain significant secondary atmospheres.

©2010 W.W. Norton & Company, Inc. Venus, Earth, and Mars All three were at some point volcanically active, and should have been hit by comets. The resulting secondary atmospheres are very different.

©2010 W.W. Norton & Company, Inc. Venus, Earth, and Mars These planets differ in an important way: their surface gravity. Venus and Earth were able to keep more gasses than Mars. Venus has 2,500 times more atmospheric mass than Mars.

©2010 W.W. Norton & Company, Inc. Greenhouse Effect Incoming sunlight heats the planet. Outgoing infrared radiation cools the planet. Some gases, especially CO 2 and water vapor, block some infrared radiation, preventing the planet from cooling.

©2010 W.W. Norton & Company, Inc. Greenhouse Effect This greenhouse effect means the planet is at a higher temperature due to the extra energy needed to be in equilibrium with the sunlight. Earth would freeze without this! While Earth is 35 K warmer because of it, a runaway effect makes Venus 400 K.

©2010 W.W. Norton & Company, Inc. The Atmospheres Evolved Differently Mars and Venus both have mainly CO 2 atmospheres. Mars’ atmosphere is much less dense than Venus’, so no greenhouse effect. Venus: larger planet, more volcanoes, bigger atmosphere, more greenhouse effect, higher escape velocity.

©2010 W.W. Norton & Company, Inc. The Atmospheres Evolved Differently Venus: hot early, so no water on surface. Earth: water removes CO 2 from atmosphere, forming limestone. Earth: life further removed CO 2. Two of the most effective greenhouse gasses, water and CO 2, were therefore contained on Earth; on Venus, they were not, which meant a runaway greenhouse effect.

©2010 W.W. Norton & Company, Inc. Earth’s Atmosphere Mostly nitrogen and oxygen. Other planets do not have oxygen (O 2 ) in their atmospheres. Life is responsible for the oxygen in Earth’s atmosphere.

©2010 W.W. Norton & Company, Inc. Earth’s Atmosphere Sun’s ultraviolet radiation produces ozone (O 3 ) by breaking O 2 into 2 Os 1 O can then combine with another O 2. Ozone blocks ultraviolet light, which is harmful to life.

©2010 W.W. Norton & Company, Inc. Layers in the Earth’s Atmosphere Troposphere (surface to 10–15 km altitude): Temperature and pressure decline with altitude. Water vapor mainly here.

©2010 W.W. Norton & Company, Inc. Layers in the Earth’s Atmosphere Tropopause: Temperature stops declining with altitude. Stratosphere (15–50 km): Temperature rises with altitude. Ozone absorbs light, heats stratosphere.

©2010 W.W. Norton & Company, Inc. Layers in the Earth’s Atmosphere Mesosphere (50–90 km) No ozone, temperature declines with altitude. Upper mesosphere is coldest part of atmosphere.

©2010 W.W. Norton & Company, Inc. Layers in the Earth’s Atmosphere Thermosphere (> 90 km) Ultraviolet radiation and solar wind can ionize atoms. Solar wind = flow of particles from the Sun.

©2010 W.W. Norton & Company, Inc. Earth’s Magnetic Field Earth’s magnetic field, the magnetosphere, extends out into space. Blocks much of the solar wind.

©2010 W.W. Norton & Company, Inc. Earth’s Magnetic Field Particles from the solar wind collide with the atmosphere. This creates the northern and southern lights (auroras).

©2010 W.W. Norton & Company, Inc. Winds and Circulation Parts of the Earth are heated differently. Vertical circulation of air (convection) distributes surface heating. Global winds carry heat from hot to cool regions.

©2010 W.W. Norton & Company, Inc. Winds and Circulation On Earth, Venus, and Mars, the circulation depends on heating pattern and rotation period. The Coriolis effect breaks the larger convection cells into smaller ones

©2010 W.W. Norton & Company, Inc. Weather Convection creates violent weather patterns on Earth. Thunderstorms Tornadoes Hurricanes

©2010 W.W. Norton & Company, Inc. Weather Dust Devils can occur on both Mars and Earth. Lightning has been observed on Earth, Venus, and all four giant planets

©2010 W.W. Norton & Company, Inc. Venus Hot, dense atmosphere, completely cloud-covered. Surface pressure 92 times that on Earth. Mainly CO 2, strong greenhouse effect.

©2010 W.W. Norton & Company, Inc. Venus Surface temperature about 737 K. Thick atmosphere means nearly uniform temperatures over the entire planet. Rotates on its axis very slowly, and in the opposite direction

©2010 W.W. Norton & Company, Inc. Mars Cold, thin atmosphere. No oxygen, no ozone. Thin atmosphere = extreme temperature variations.

©2010 W.W. Norton & Company, Inc. Mars Equator: up to 293 K (20 C). Pole: down to -150 C. Consequently large winds, which can make big dust storms.

©2010 W.W. Norton & Company, Inc. The Moon and Mercury Almost totally airless. Combination of temperatures and low escape velocity means any atmosphere is lost. No erosion from wind or other weathering, so old, cratered surfaces are retained.

©2010 W.W. Norton & Company, Inc. Concept Quiz – The Moon Is Airless The Moon and Earth have approximately the same average temperature. Why does the Moon lack an atmosphere? A.Comets, which deliver water and air, collide only with Earth. B.The Moon has no life. Life produces the atmosphere. C.The Moon never had any volcanism. D.The Moon has a low escape velocity.

©2010 W.W. Norton & Company, Inc. Concept Quiz – Global Warming Burning oil and coal raises the CO 2 content of our atmosphere. By 2100, Earth’s average temperature should be 2 to 5 K higher than now. Which will be larger? A.The rate at which Earth absorbs sunlight. B.The rate at which Earth emits infrared light. C.Both rates will be equal.

©2010 W.W. Norton & Company, Inc. Concept Quiz – Earth’s Carbon Earth started with about as much carbon as Venus has. In Venus, the carbon is mainly in the atmosphere. Where is it on Earth? A.In rocks, the ocean, and in life. B.It was lost when the primary atmosphere escaped. C.It evaporated from Earth’s atmosphere while other gasses were retained.

©2010 W.W. Norton & Company, Inc. Atmospheres: Formation and Escape Click the above picture to launch the animation

©2010 W.W. Norton & Company, Inc. Greenhouse Effect Click the above picture to launch the animation

©2010 W.W. Norton & Company, Inc. This concludes the Lecture PowerPoint presentation for Chapter 8 ©2010 W.W. Norton & Company, Inc. For more learning resources, please visit the StudySpace website for 21st Century Astronomy at 21st C ENTURY A STRONOMY T HIRD E DITION Hester | Smith | Blumenthal | Kay | Voss