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habitable planets.docx

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Life Sciences
Luc Bernier

January 24 , 2013 Life Sci 2H03: Environmental Life Sciences Habitable Planets Only Earth? - until now the only example of a know habitable planet - earth orbits the Sun within a region called the habitable zone - what is the habitable zone: region where an Earth analog planet can maintain liquid water on its surface, certain concentration of greenhouse gases that maintain idea temperature, right mass to have developed plate tectonics Liquid Water Important for Habitability - the best solvent for life to emerge and evolve in - several key characteristics: large dipole moment, ability to form hydrogen bonds, hydrophilic/hydrophobic fashion - water is found in a multitude of places in the universe - water: abundant in our galaxy - can be found in different environments: stellar atmospheres (gaseous form) The Habitable Zone (HZ) - inner edge of the HZ:  runaway greenhouse conditions  gaseous, water vapour  Venus, Mercury - outer edge of the HZ:  greenhouse effect fails to keep the planet above the freezing point  condensed liquid or solid  no greenhouse gases that can influence temperature  Neptune, Saturn - Earth is at the ideal distance from the Sun - Depending on the mass of the star, the habitable zone increases with a higher mass Basic Habitability Requirements for Life - main factors:  long time span: conditions must be stable enough for evolution to take place  liquid water in contact with building blocks  environmental conditions that allow liquid water to exist - from Achaean to closest era: the overall conditions were relatively stable - Atmospheric Requirements - dense enough that can stabilize the surface temperature - greenhouse effect: caused by compounds that are very efficient absorbers in the infrared - -15 C, -16 C Earth’s Radiation Budget - all the planets receive solar radiation - absorbed by the surface, a fraction is reflected - in return Earth will also receive radiation (in the infrared) - sun radiation (short wave radiation) - greenhouse gases trap radiation and reflect - leads to moderated temperature need the surface - what leads to average temperature of 15 C due to greenhouse gases ability to back radiate Venus and the Limits of Planetary Habitability - Venus: what happens when surface temperature exceeds a certain limit - Surface pressure: 9000kPa - Surface temperature: 480 C o - And atmosphere but little water - Why so little water: run-away greenhouse gas effect (favourite hypothesis)? Or because water was loss to space due to the activity of the Sun? Greenhouse Effect on Venus - on earth most of the carbon is in rocks - thicker layer on Venus - lot more radiation escaping from Earth in the form of infrared than Venus - explains why the temperatures are this high on the surface of the planet Structure of Venus’s Atmosphere - sulphuric acid - inner edge of habitable zone Mars and the Limi
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