Prepared by Lisa M. Will, San Diego City College

Lecture Slides CHAPTER 7: Atmospheres of Venus, Earth, and Mars Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College

Atmospheres of Venus, Earth, and Mars For these three terrestrial worlds, understand: • Primary versus secondary atmospheres. • Compositions. • Greenhouse effect.

Origin of Atmospheres • New planets formed with primary atmospheres (mostly hydrogen) • All terrestrial bodies lost their primary atmospheres. • Planets could not hold onto the gas. • Combination of high gas speeds and low planetary gravity. • Secondary atmospheres were acquired later by: • Accretion (but locked in the interior). • Volcanism (releasing the gas from the interior). • Comet impacts.

Origin of Atmospheres

Origin of Atmospheres: Atmospheres of Planets • Mercury and the Moon have basically no atmosphere. • Venus, Earth, and Mars were able to obtain significant secondary atmospheres. • Similar geologic histories. • Should have had similar secondary atmospheres

Class Question The Moon and Earth have approximately the same average temperature. Why does the Moon lack an atmosphere? • Comets, which deliver water and air, collide only with Earth. • The Moon has no life. Life produces the atmosphere. • The Moon never had any volcanism. • The Moon has a low gravity.

Compositions • Earth: Primarily nitrogen and oxygen. • Venus and Mars: Mostly carbon dioxide. • Venus and Earth have much more atmosphere than Mars. • Mars has a much lower surface gravity than either Earth or Venus.

Greenhouse Effect Incoming sunlight heats the planet. Outgoing infrared radiation cools the planet.

Greenhouse Effect: Greenhouse Gases • Some gases, especially carbon dioxide and water vapor, block some infrared radiation, preventing the planet from cooling. • These are examples of greenhouse gases.

Greenhouse Effect: Equilibrium • The heated atmosphere also warms the ground. • This greenhouse effect means the planet’s surface reaches a higher temperature. Equilibrium is attained between absorbed sunlight and thermal energy radiated away by the planet.

Greenhouse Effect: Temperature of Planets • The average temperature of the Earth would be below freezing without the greenhouse effect! (some global warming is a good thing!) • Due to the greenhouse effect, Venus is hotter than Mercury, although about twice as far from the Sun.

Greenhouse Effect • Venus: Hot early in its history, so no water could remain on surface. • Earth: Mild temperature early in its history allowed water to remove carbon dioxide from atmosphere (by being absorbed into the oceans) and life further removed carbon dioxide. • Two effective greenhouse gases, water and carbon dioxide, were therefore constrained on Earth; on Venus, they were not, which led to a runaway greenhouse effect.

Compositions • Earth’s atmosphere is mostly nitrogen and oxygen. • Life is responsible for the oxygen’s presence. • Other planets do not have substantial amounts of oxygen in their atmospheres.

Why does Earth have a substantial amount of oxygen in its atmosphere while the other planets do not? The Earth is the only planet with life. The oceans emit oxygen. The oxygen in the primary atmosphere escaped. Class Question

Layers: Troposphere • Troposphere (surface to 10–15 km altitude): • Temperature and pressure decline with altitude. • Water vapor mainly is located here. • Weather happens here!

Layers: Stratosphere • Tropopause: Temperature stops declining with altitude. • Stratosphere (15–50 km): • Temperature rises with altitude. • Ozone absorbs ultraviolet light, heats stratosphere.

Layers: Mesosphere • Mesosphere (50–90 km): • Temperature declines with altitude. • Above ozone layer • Upper mesosphere is coldest part of atmosphere.

Layers: Thermosphere and Ionosphere • Thermosphere (> 90 km): • Ultraviolet radiation and solar wind can ionize atoms. • Solar wind = flow of particles from the Sun. • The ionosphere overlaps the thermosphere but is mostly in the upper portions of it, extending up to 1000km.

Layers • Earth’s magnetic field makes a region surrounding the Earth called the magnetosphere, which extends out into space. • It blocks much of the solar wind.

Layers: Auroras • The magnetosphere can trap charged solar wind particles. • The particles collide with the atmosphere, heating it up and causing it to glow. • This creates the aurora.

Layers: Auroras (Cont.)

Layers: Hadley 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.

Layers: Coriolis Effect The circulation depends on heating pattern and rotation period. The Coriolis effect, due to rotation, breaks the circulation into zonal winds.

Layers: Coriolis Effect (Cont.)

Venus • Venus: Hot, dense atmosphere, completely cloud-covered. • 96% carbon dioxide • Runaway greenhouse effect. • Surface temperature about 737 K, hotter than Mercury, and hot enough to melt lead! • Clouds are nasty: made of sulfuric acid!

Venus: Atmosphere • Thick atmosphere • Uniform temperature over entire planet • Opaque in visible light • Atmospheric pressure at surface is almost 100x times the pressure at Earth’s surface! • Strong winds in upper atmosphere • Surface imaged by radar. • Rotates on its axis very slowly, and in the opposite direction as rest of solar system.

Mars • Mars: Cold, thin atmosphere, composed mostly of carbon dioxide • Thin atmosphere => extreme temperature variations.

Mars: Atmosphere • Daily, seasonal, and latitudinal temperature variations create large winds. => Planet-covering dust storms!

Class Question Why is Mars so cold compared to Venus if both of their atmospheres are made mostly of carbon dioxide, a greenhouse gas? Mars is farther from the Sun. Mars has lower gravity. The atmosphere on Mars is much thinner All of the above.

Greenhouse Effect • Earth temperature measurements show a steady increase over the past 130 years. • More greenhouse gases increase the greenhouse effect. • Earth’s climate is a complex system. • ` • Why care about the greenhouse effect on Earth? => We do not want a runaway greenhouse effect, like Venus!

Earth started with about as much carbon dioxide as Venus has. In Venus, the carbon dioxide is mainly in the atmosphere. Where is it on Earth? In rocks, the ocean, and in life. It was lost when the primary atmosphere escaped. It evaporated from Earth’s atmosphere while other gases were retained. Class Question

Chapter Summary • The atmospheres of Venus, Earth and Mars have evolved since they first formed. • The atmospheres of the three planets differ in temperature, pressure and composition. • All three planets have greenhouse gases in their atmospheres. • The conditions of the atmospheres of Venus, Earth and Mars can be explained by the greenhouse effect.

Astronomy in Action Changing Equilibrium Click the image to launch the Astronomy in Action Video(Requires an active Internet connection)

Astronomy in Action Charged Particles and Magnetic Forces Click the image to launch the Astronomy in Action Video(Requires an active Internet connection)

AstroTour Click the image to launch the AstroTour Animation(Requires an active Internet connection) Atmospheres Formation and Escape

AstroTour Click the image to launch the AstroTour Animation(Requires an active Internet connection) Greenhouse Effect

Nebraska Applet Gas Retention Plot Click the image to launch the Nebraska Applet(Requires an active Internet connection)

Nebraska Applet Gas Retention Simulator Click the image to launch the Nebraska Applet(Requires an active Internet connection)

Understanding Our Universe SECOND EDITION Stacy Palen, Laura Kay, Brad Smith, and George Blumenthal Prepared by Lisa M. Will, San Diego City College This concludes the Lecture slides for CHAPTER 7: Atmospheres of Venus, Earth, and Mars wwnpag.es/uou2

Prepared by Lisa M. Will, San Diego City College