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Extraterrestrial Life: Lecture #6 Liquid water is important because: What are the requirements for the Earth (or another planet) • solvent for organic molecules to be habitable? • allows transport of chemicals within cells • involved in many biologically important • liquid water on surface chemical reactions • atmosphere • plate tectonics / volcanism Other solvents (ammonia, methane etc) exist in liquid • magnetic field form on planets but are much less promising for life • … Normal atmospheric pressure: liquid water requires: 0o C (273K) < T < 100o C (373K) …require planets with surface temperatures in this range ! Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 What determines the Earth’s surface temperature? Solar constant The flux of energy is the amount of energy that passes fraction of 2 incident radiation through unit area (1 m ) in one second is reflected Measured in units of watts / m2 incident Solar Solar flux declines with distance as 1 / d2: radiation L flux = remainder is 2 absorbed by 4"d the Earth and then reradiated …where d is the Sun - planet distance and L is the total If the Earth is not heating up or cooling down, the total luminosity of the Sun (watts) of incoming and outgoing radiation must balance ! Are there other sources of energy for a planet? Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Solar constant The fraction of the incident flux that is reflected is called Solar luminosity is 3.9 x 1026 watts the albedo of the planet: 0 < A < 1 The fraction that is absorbed is (1-A) Earth-Sun distance is d = 1.5 x 1011 m (1 AU) 26 The albedo varies greatly depending 3.9 "10 watts on the surface terrain Solar flux = 2 4# " 1.5 "1011 m ( ) For the Earth, a global average 2 =1380 watts / m value is about A ~ 0.3 This is the Solar flux that would be measured above the How does A change as the Earth Earth’s atmosphere rotates? ! Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 1 First consider the reflected component of sunlight How much energy is absorbed? d P flux R2 (1 A) RE = " # E " $ 2 =1380 " # " (6.4 "106 ) " (1$ 0.3) watts Fraction of total Solar luminosity that is reflected is: =1.24 "1017 watts flux " area of Earth as seen from Sun " A f = c.f. total world electricity consumption ~ 5 x 1012 watts solar luminosity 2 2 1380 watts / m " #RE " A ! = Note: total forcing due to greenhouse gases is about 3.9 "1026 watts 2 watts / m2 - i.e. a few tenths of a percent of the total 6 Earth radius is RE = 6.4 x 10 m Solar flux… this is why climate change is a complex scientific problem ! f = 1.4 x 10-10 Seen from another star, Earth is ~10 billion times dimmer than the Sun Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 The absorbed radiation is reradiated as thermal radiation, Thermal radiation emitted by the Earth is: mostly in the infra-red part of the spectrum 2 4 Pthermal = 4"RE #$T IR As T increases, the peak area of the Earth’s power (watts / m2) emitted of thermal radiation moves surface in m2 by thermal radiation at a to shorter wavelengths, and ! temperature T the total emission increases 4 rapidly as ~T σ is a constant called the Stefan-Boltzmann constant, it equals 5.67 x 10-8 watts per m2 per K4 Sun: 6000K (visible) Earth: 300K (IR) Setting the emission equal to the energy absorbed from sunlight determines the equilibrium temperature of the Earth Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Find that predict T ~ 260K - a bit too cold! But we have Habitable Zone ignored the influence of the atmosphere in blocking some of the outgoing radiation… Earth orbit What does the surface temperature depend on: Define the habitable zone as the range of distances L(1# A) from the Sun for which a T 4 " d 2 planet can have liquid water on its surface • distance to the star Empirically: Venus is inside • luminosity of the star the habitable zone and Mars •! p roperties of the atmosphere and surface outside for the Solar System But… calculating the exact boundaries is hard - depends upon the nature of the planet and its atmosphere Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 2 Additional complication: Solar luminosity changes with The continuously habitable zone is the range of radii time (slowly)… Sun was less luminous in the past and for which liquid water is possible throughout a planet’s is slowly getting more luminous lifetime Faint Sun problem: initial Solar luminosity is predicted Obviously narrower than the instantaneous habitable to be ~70% of the current luminosity… but no evidence zone - possibly much narrower… that temperature on the early Earth was much colder Means that stars whose luminosity changes relatively Thought to be an atmospheric effect quickly are unpromising hosts for life-bearing planets Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 What about planets on elliptical orbits that dip in and out of the habitable zone? • surface temperature adjusts to the Solar forcing on a timescale << 1 year (e.g. seasons!) • temperature underground, or in the oceans, adjusts much more slowly • planets with non-circular orbits can’t be ruled out immediately Extraterrestrial Life: Spring 2008 3.
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