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Global Average Temperatures of Planetary Surfaces and the “Habitable Zone”. Astrobiology Workshop June 27, 2006. Habitability. What Might Make a Planet or Moon “Habitable”? IF life on Earth is a reliable guide, life requires Carbon Chemistry Energy Source to Sustain Metabolism
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Global Average Temperatures of Planetary Surfaces and the“Habitable Zone” Astrobiology Workshop June 27, 2006
Habitability • What Might Make a Planet or Moon “Habitable”? • IF life on Earth is a reliable guide, life requires • Carbon Chemistry • Energy Source to Sustain Metabolism • Liquid Water! • Or some other good liquid medium for carbon chemistry (but water seems best) • IF liquid water really is essential, then • temperatures (and pressures) must permit liquid water to exist • So we are led to ask: • What determines planet temperatures?
Planet Temperatures • What Heats Surfaces of Moons & Planets? • What are the sources of heat for the surfaces of a solid (or liquid) body in a planetary system? • Starlight and Planet Light • Outflow of Internal Heat • Impacts • Latent Heats of Surface or Atmosphere Constituents • Which of these usually dominates the Global Average Temperature Tsurf after a fewx108 years? • Starlight!
Planet Temperatures • Basic Principles for Global Average T’s • Equilibrium • Input of Energy per second to the surface equals the Output of Energy per second • Factors affecting Input? • Luminosity of the star • Distance from the star • Reflectivity (Albedo) of the surface and/or atmosphere • The fraction of the starlight that is scattered or reflected by the surface without being absorbed
Planet Temperatures • Basic Principles for Global Average T’s • What is the principal mechanism that cools the surface of a planet? • Radiation by the surface because it is hot! • Factors affecting Output? • Temperature of the surface • At typical temperatures of planetary surfaces, they radiate in the infrared • Energy/sec. as temperature • Insulation of the surface by a blanket of atmosphere that blocks the infrared • The Greenhouse Effect
The Planet Temperature Calculator • What is It? • The PTC is a Web-based tool that calculates the Global Average Temperature Tsurfassuming • Energy Input Rate = Energy Output Rate • Parameters you provide to the calculator: • Mass of the star Luminosity of the star • Distance of the planet from the star • The Albedo of the planet • The Greenhouse Factor relative to Earth • This is a measure of the column density of Greenhouse gases in the atmosphere relative to the same quantity on Earth
Calculating Tsurf with No Atmosphere • Tsurf Calculation • Equilibrium says • Star Energy/sec. In = Planet Energy/sec. Out • Input Energy/sec. from starlight depends on • Luminosity L = light energy emitted per sec. • Distance D = distance of object from star • Energy/meter2/sec. at the object • Energy/sec. hitting an object of Radius R L 4D2 L 4D2 R2x
L 4D2 (I-A)R2x Calculating Tsurf withNo Atmosphere • Input Energy/sec. absorbed by the object • Albedo A = fraction of light energy scattered or reflected by object • Fraction absorbed is 1-A • Input Energy/sec. from star that is absorbed
Calculating Tsurf withNo Atmosphere • Energy/meter2/sec. emitted by a hot object at Temperature T (Stefan-Boltzmann Law) is • OutputInfrared Energy/sec. from the object • Assume uniform surface temperature Tsurf • Set Input =Output and solve for Tsurf T4 4R2xTsurf4 [(1-A)L]1/4 2()1/4D1/2 Tsurf =
Calculating Tsurfwithan Atmosphere • Greenhouse Effect • The Greenhouse Effect occurs for gases that are • Transparent to visible light but • Opaque to infrared light • Examples of Greenhouse Gases: • H2O, CO2, CH4, Freon • The surface then has to reach a higher Tsurf to force an equilibrium flux of infrared light back up through the atmosphere.
Calculating Tsurfwithan Atmosphere • Greenhouse Effect • The change in Tsurf is greater when the “mean free path” for infrared photons is smaller, which depends on the amount of greenhouse gas in the atmosphere. • Roughly one extra Tsurf4 for each mean free path through the atmosphere • For large amounts of greenhouse gas, if you double the column density of greenhouse gas, then Tsurf increases by 21/4times
Luminosities of Stars? • What about L? For Main Sequence Stars, which burn hydrogen to helium in their centers, it is approximately true that • In this formula, M refers to the star’s mass and the subscript refers the solar value. L = L (M/M)3
The Habitable Zonefor Earth-Like Planets • Simple Definition of the Habitable Zone: • Range of distances from a Star for Tsurfis such that the surface water of an Earth-like planet (or moon) would not either • Freeze or • Boil • Questions for Activities & Discussions: • What is the Habitable Zone for the Sun and for your star? • What would happen to Earth if we moved it to the edges of the HZ? What do Venus and Mars suggest about the edges of the HZ? • Where was the Sun’s Habitable Zone in the past? Where will it be in the future?