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Where might we find life in the Solar System? Temperatures of planets Recap • Final assignments – Class evaluations: counts as 100% for one lab, must be completed before next Monday! – Final homework/quiz: post course assessment – Final exam, next Wednesday – Lab sections this week • How the Sun works and evolution of the Sun – Nuclear reactions – Stellar evolution driven by depletion of nuclear fuel • Sun and lower mass stars convert H to He, then He to C, then end their lives as white dwarfs + planetary nebulae • More massive stars have longer chain of reactions until they reach an iron core, then end lives as supernovae, ejecting heavier elements back into interstellar medium Where might we find life? • Without going into details of biology, life as we’re familiar with it requires the presence of liquid WATER • What do we need to find liquid water? – Hydrogen and oxygen (H20) – Temperature where water is liquid • Between 0 and 100 C (32 and 212 F) • 273 and 373K • Plenty of H2O throughout the solar system, so where are temperatures right? What determines temperatures in the Solar System? • External heat source: the Sun! – Sun has a surface temperature of 5700 K • But how hot will it get at different locations in the Solar System? Imagine you’re sitting near a fire that is burning at a temperature of 2000 K and thus emitting a lot of heat. If you sit there long enough A. you’ll eventually heat up to 2000 K because the energy just keeps coming B. You’ll heat up, but eventually you’ll reach some stable temperature that is cooler than 2000K C. You’ll feel warmer, but your body won’t actually heat up at all The stable temperature that you eventually arrive at depends on A. How far away you are from the fire, but not on what you’re wearing B. How far away you are from the fire and on what you are wearing C. On what you are wearing but not how far you are from the fire D. The temperature of the fire, but not on how far away you are or what you are wearing Planetary equilibrium temperature • Temperature of planets are determined by a balance between – Energy coming in: depends on distance from the Sun, size of planet, and reflectivity – Energy going out: depends on size of planet, the temperature, and the material • This argument applies to average temperatures, there will be variation around the planet – Locations where Sun is higher in sky will be warmer than locations where Sun is lower – Day side will be warmer than night side • If planet rotates, then some degree of “equal toasting” • If same side towards the Sun, more temperature imbalance Does this work? reflectivity “real” material Temp PLANET BLACKBODY Temp Observed temp Mercury 107 C 0.12 165 C 425 (day) -175 (night) Venus 5.3 C 0.75 -39 C 470 C Earth -36 C 0.29 -14 C 15 C Mars -81 C 0.16 -54 C -50 C What else can affect temperature? • If you are cold and want to warm up, what can you do? If you cover yourself with a blanket, you will A. be warmer because the blanket absorbs more heat from outside B. be warmer because the blanket keeps heat from leaving you C. be the same temperature, but just feel warmer D. get colder, because the blanket prevents heat from reaching you Planetary atmospheres as blankets • Some planets have atmospheres that can act as blankets, preventing heat from getting out • However, a blanket can also prevent heat from getting in! • Planet atmospheres can act as “one-way” blankets, preventing more heat from getting out than blocking heat from getting in – Works because of the nature of continuous/thermal emission! The Sun produces an underlying continuous emission, but not an equal amount at all wavelengths. What is the dominant type of light coming from the surface of the Sun? A. Gamma rays B. Ultraviolet light C. Visible light D. Infrared light E. Radio light The planets are warmed by the Sun to a temperature of a few hundred degrees K and thus, also produces thermal continuous radiation. What is the dominant type of light (emission, not reflection!) coming from the surface of the planets? A. Planets don’t have any emission, they only reflect B. Ultraviolet light C. Visible light D. Infrared light E. Radio light Planetary “Greenhouse” Effect • Energy from the Sun comes in mostly in visible light • Energy from the planets leaves mostly in infrared light • If you have a blanket that lets in visible, but doesn’t let out infrared, the planet will get warmer! • Some gases in planetary atmospheres have exactly this property! – Carbon dioxide (CO2), methane, several others – Water vapor to a lesser effect Planetary Greenhouse Effect • Effect can be huge! Venus is hundreds of degrees warmer than expected – Runaway greenhouse effect: higher temps release more greenhouse gases, which lead to higher temps, which release more greenhouse gases, etc. • Eventutally, process stabilizes at some temperature that is hotter than the equilibrium temperature • Venus is the prime example of the greenhouse effect • However, it exists on other planets – Earth is about 30 degrees C warmer than expected – Mars is about 5 degrees C warmer than expected Greenhouse effect on Earth • Earth has a natural greenhouse effect that makes it a bit warmer than expected based on distance from the Sun • Primary greenhouse gas is carbon dioxide – carbon dioxide makes up only a very small fraction of Earth’s atmosphere! – Most of atmosphere is nitrogen and oxygen, but these are not greenhouse gases • Issue: carbon dioxide content is rising in Earth’s atmosphere! Carbon dioxide in Earth’s atmosphere • Recent record of carbon dioxide – http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mio.ht ml – Not a huge amount (parts per million) but definitely important …. And increasing! • Longer term record – http://www.ipcc.ch/graphics/ar4-wg1/jpg/ts2.jpg • Very likely that human activity is causing the increase – Fossil fuel burning generates greenhouse gases – Deforestation lowers natural absorption of carbon dioxide Earth and climate change • We know that the greenhouse effect is real, Venus makes this obvious • Key question: is increased CO2 in Earth’s atmosphere going to change the climate? • Temperature data is hard to get because there are a lot of natural fluctuations • However, it does seem like the climate is getting warmer – Temperature records – Ocean temperatures – Glacial ice coverage is getting smaller Is climate change natural? • Earth certainly has undergone long term temperature changes: ice ages, etc • Recent rise in temperature appears to be much faster than previous changes • Difficult to understand as part of a natural cycle • Observed dramatic increase in CO2 with temperature change is particularly worrisome! – Models suggest that it is the increasing CO2 that is responsible for the increasing temperatures • Most (but not every!) climate scientists think that climate change is occurring and is human caused Climate change: the future • If temperatures are rising because of increasing CO2, they are predicted to continue to rise • Potential very significant impacts – Ocean levels rise – Ocean temperatures change – Significant effect on human habitation – Dramatic effect on species survival • What to do? – Hope this picture is wrong / spend lots of time arguing about it – Precautionary principle: if potential effect is severe, even if you’re not 100% positive it will happen, you take action to avoid it! – Individual actions definitely help, but issue has to be tackled on a larger level as well