1The Nature and Evolution of HabitabilityA discussion of Bennett and Shostak Chapter 10Dr. H. GellerHNRS 228Spring 2007Chapter Overview• Concept of a Habitable Zone• Venus: An Example in Potential Habitability• Surface Habitability Factors and the Habitable Zone• Future of life on Earth• Global WarmingHabitability: Introduction• Define “habitability”– Anthropocentric perspective– Astrobiological perspective (capable of harboring liquid water)• Key physical and chemical features of habitability– Surface habitability– Temperature– Source of energy– Liquid water (present and past)– Biological macromolecules (e.g., sugars, nucleotides)– Atmosphere and magnetosphereComparative Planetary EvolutionConcept of a Habitability Zone • Definition of habitability zone (HZ)“Region of our solar system in which temperature allows liquid water to exist (past, present and future)”• Phase diagram for H2O• Retrospective analysis of HZ using the terrestrial planets as case study– Mars, Venus and Earth• Prospective analysis of HZLuminosity of the Sun• Definition of luminosity (watts/m2)• Sun’s luminosity has been changing: earlier in its evolution, luminosity was only 70% of what it is today (how could temperature be maintained over geological time)• Future for luminosity – Remember star sequence from lab and lecture– 2-3 BY, luminosity will place Earth outside habitability zone2Distance from the Sun• Terrestrial planets – heat mostly from Sun• Jovian planets – 2/3 of heat from interior (all planets originally had internal heat source due to bombardment)• Heat from Sun is inversely proportional to distance2or heat energy = k*1/(distance)2• Heat falls off rapidly with distanceHabitability Zone of Our Solar System• Exploration of Mars, Venus and Earth provides a framework to establish a HZ in terms of water– Venus (0.7 AU): liquid H2O in the past– Mars (1.5 AU): oceans primordially– Thus, range of habitability around stars like Sun is 0.7 to 1.5 AU• Zone of “continuous habitability versus zone of “habitability” (which is more narrow?)– needs to maintain habitability for billions of yearsContinuous Habitability Zone of Our Solar System• Outer edge of HZ must be less than Mars (1.5 AU) orbit (closer to Earth than to Mars)– Estimate of ~1.15 AU• Inner edge of HZ closer to Earth than Venus because Venus lost its greenhouse of H2O early in its evolution– Estimate of ~0.95 AU• Conclusion: for planet to maintain liquid H2O continuously for 4 BY, HZ is as follows:– >0.95 AU < 1.15 AU– HZ of only 0.2 AU in breadthHabitability Zone in Our Galaxy• Use the range from our solar system as a basis for analysis– In our solar system, 4 rocky planets that orbit the Sun from 0.4 to 1.4 AU and spaced 0.4 AU apart• If typical, likelihood of other solar systems having continuous habitability zone is just width of the zone divided by the typical spacing– 0.2/0.4 = 0.5 – Probability of 50%– Discuss this probabilityHabitability Zones Elsewhere in the GalaxyHabitability Zone in Our Galaxy• Other factors also relevant– Several stars in our galaxy with planets the size of Jupiter within terrestrial zone from their sun– Mass of star• Larger mass, greater luminosity, shorter life• Most abundant stars in galaxy are least luminous and longest-lived (M-dwarfs)3Signatures of Habitability and Life• Distance from sun• Luminosity of sun• Planet size• Atmospheric loss processes• Greenhouse effect and gases in the atmosphere• Source of energy (internal/external)• Presence of water• Presence of carbon biomolecules• BiotaEarth-like planets: Rare or CommonComparative Habitability of Terrestrial Planets• Venus (0.7 AU; radius 0.95E; same density as Earth)– Very hot; evidence of liquid water in the past• Mars (1.5 AU; radius 0.53E)– Very cold; evidence of water today and in the past• Earth (1.0 AU; radius 1.0E)– Temperature moderation; liquid water today and in the past•Keys– greenhouse effect – size of planet– proximity to SunGreenhouse Effect• Factors to consider– light energy (visible wavelengths) from Sun– transfer through a planet’s atmosphere – absorption on the planet’s surface (soil, H2O)– re-radiation of energy as longer wavelengths• i.e., infrared radiation– inability of infrared radiation to escape atmosphere• Conversion of energy from light to heat energy• Analogy to a greenhouse– Glass versus atmosphere as “barrier”Greenhouse Effect In the Terrestrial Planets• Earth’s greenhouse effect– without greenhouse effect: -23oC– with greenhouse effect: 15oC (+Δ 38oC)• Venus’ greenhouse effect– without greenhouse effect: 43oC– with greenhouse effect: 470oC (+Δ 427oC)• Mars’ greenhouse effect– without greenhouse effect: -55oC– with greenhouse effect: -50oC (+Δ 5oC)Principles of theGreenhouse Effect• Primary principle of the Greenhouse Effect– A greenhouse gas is a gas that allows visible light to be transmitted but is opaque to IR (infrared) radiation• Key is trace gases in atmosphere and cycling in the oceans and terrestrial landscapes– Water (H2O)– Carbon dioxide (CO2)Gas Venus (%*) Earth (%) Mars (%)H2O 0.0001 3 0.1CO298 0.03 96Pressure 90 1 0.007(atm)*% is relative abundance of that gas versus the other gases4Greenhouse Effect:H2O• Water: a “runaway” greenhouse gas– Prolonged periods of excessive heat or cold to change temperature at a global scale• Two key chemical properties of H2O– High heat capacity– Decrease in density with freezing (insulation and reflectance) • Temperature scenario on planetary surface as f [H2O]– Cooling of H2O, leading to ice formation, followed by more cooling (albedo)…runaway greenhouse effect– “Positive Feedback”Greenhouse Effect:CO2• Carbon dioxide: “compensatory” greenhouse gas– Need a molecule to compensate for “positive feedback” of H2O, resulting in a “negative feedback”• Key chemical properties of CO2– Importance of atmospheric state (absorbs visible light)– Concentration in atmosphere linked to oceans, geological reactions, and biota (plants) Cycling of CO2on EarthAtmosphereOceansRockdissolutionSedimentation/ bicarbonateplate tectonicsKeys: (i) recycling of CO2(ii) geological time scales (millions to billions of years)(iii) Earth’s long-term thermostat(iv) interplay of CO2and H2O cyclesGreenhouse