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Extraterrestrial Life: Lecture #7 Next Homework Extraterrestrial Life: Lecture #7 Is the climate stable over geological time? Next homework: Thursday Feedback: if the surface temperature rises, does the concentration of greenhouse gases in the atmosphere: To first approximation: distance from star and luminosity of star determine the surface temperature and possibility • increase, possibly raising the temperature further? for liquid water on surface Positive feedback • decrease, offsetting the rise? Atmosphere (`greenhouse effect’) also plays an important Negative feedback role - warms Earth by ~20 degrees Celsius • water vapor Empirically, expect negative feedback to prevail on the increased concentration • carbon dioxide Earth - but may be different for other planets… warms the planet • methane Particulate matter (dust from volcanoes) cools Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Short term feedback processes Extent of ice cover also affects the climate via changes Short term feedbacks are mostly positive - destabilizing to the mean albedo e.g. water vapor: increased temperature leads to more If increased temperature leads evaporation from the oceans, increasing the atmospheric to less snow and ice, fraction concentration of water vapor of Solar energy that will be absorbed increases Water is a greenhouse gas, so this is positive feedback Positive feedback Role of clouds? Timescale: 1000s of years Timescale: essentially instantaneous Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Long term feedback processes Volcanic activity can liberate CO2 from rocks and release it into the atmosphere On longer timescales, geological processes dominate Magma can be Tiny fraction of the Earth’s total CO2 reservoir is in the few % by mass atmosphere water • atmosphere: CO2 ~381 parts per million Large eruptions release km3 to 8 x 1011 tonnes: 800 GigaTonnes ~1000 km3 of • ocean: rock 4 x 1013 tonnes: 38,000 GigaTonnes • rocks 5 x 1016 tonnes Mt Pinatubo Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 1 Carbon sinks Estimate for the current volcanoes to atmospheric CO2: In the atmosphere: 1.3 x 108 tonnes / year = 0.13 GigaTonnes / yr water + CO2 -> carbonic acid (H2CO3) Timescale for significant changes to the atmosphere: Weak acid in rain weathers silicate rocks: 8 $1011 tonnes " # 8 = 6000 yr • yields bicarbonate 1.3$10 tonnes / yr • becomes locked up in calcium carbonate (shells) from biological activity in the Expect volcanic activity to be important on timescales oceans 4 of the order of 10 years • carbonates becomes locked up in the crust ! Same process can occur without biological activity Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Carbonate-silicate cycle This is a negative (stabilizing) feedback loop: Atmospheric CO2 • warmer temperatures: more moisture in the atmosphere, more rain, greater rate of rain - fails if volcanism: depends removal of CO2 from the atmosphere temperature is on age, size of • cooler temperatures: less rain, longer residence too high planet etc… time for CO2 in the atmosphere Very long term changes in climate are driven by: Oceanic carbon • Sun’s changing luminosity • cyclical changes in the Earth’s orbit / rotation Carbon in forms rocks, which are • different arrangements of the continents mantle rock subducted via plate • changes in volcanism tectonics • life… Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Other planets? Despite these variations, appears that the natural carbonate-silicate cycle has been sufficient to roughly Volcanoes on surface of stabilize the Earth’s surface temperature for several Venus - probably still billion years (but, ice ages, `snowball Earth’ episodes…) volcanically active planet But, water has all been What are the limits to this stabilizing influence? evaporated by high surface temperature and lost into space Magellan radar image of Venus Weathering aspect of the carbonate-silicate cycle does not operate - large concentration of CO2 in the atmosphere Venus’ surface temperature is 464 Celsius Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 2 Mars certainly had active Mars: active volcanism and plate tectonics appears to have volcanism ceased in distant past during the planet’s Small size of planet (10% mass of Earth) has allowed Mars history to cool and plate tectonics has ceased Olympus Mons: Volcanic outgassing 27km high aspect of the cycle has been broken Summit of Olympus Mons on Mars: Mars Express Extraterrestrial Life: Spring 2008 Extraterrestrial Life: Spring 2008 Implications for extrasolar habitable worlds? Small planets, without long term plate tectonics, may not be able to maintain a habitable climate - `habitable zone’ for such planets may be much smaller / non-existent What is the typical mass of terrestrial planets? • probably depends upon the mass of solid material: less favors more, smaller planets • Solar System have two `Earth-mass’ planets • around lower mass stars may be quite different Extraterrestrial Life: Spring 2008 3.
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