1. Where might we find life in the Solar System? Temperatures of planets

2. 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

3. Where might we find life? Without going into details of biology, life as were familiar with it requires the presence of liquid WATER What do we need to find liquid water? –Hydrogen and oxygen (H 2 0) –Temperature where water is liquid Between 0 and 100 C (32 and 212 F) 273 and 373K Plenty of H 2 O throughout the solar system, so where are temperatures right?

4. 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?

5. Imagine youre 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.youll eventually heat up to 2000 K because the energy just keeps coming B.Youll heat up, but eventually youll reach some stable temperature that is cooler than 2000K C.Youll feel warmer, but your body wont actually heat up at all

6. The stable temperature that you eventually arrive at depends on A.How far away you are from the fire, but not on what youre 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

7. 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

8. Does this work? PLANETBLACKBODY Temp reflectivityreal material Temp Observed temp Mercury107 C0.12165 C425 (day) -175 (night) Venus5.3 C0.75-39 C470 C Earth-36 C0.29-14 C15 C Mars-81 C0.16-54 C-50 C

9. What else can affect temperature? If you are cold and want to warm up, what can you do?

10. 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

11. 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!

12. 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

14. 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 dont have any emission, they only reflect B.Ultraviolet light C.Visible light D.Infrared light E.Radio light

15. 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 doesnt let out infrared, the planet will get warmer! Some gases in planetary atmospheres have exactly this property! –Carbon dioxide (CO 2 ), methane, several others –Water vapor to a lesser effect

16. 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

17. 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 Earths atmosphere! –Most of atmosphere is nitrogen and oxygen, but these are not greenhouse gases Issue: carbon dioxide content is rising in Earths atmosphere!

18. The greenhouse effect is best described as A.The temperature of a planet depends on what color it is B.The temperature of a planet can be higher if it has an atmosphere because the atmosphere can absorb more heat/light from the Sun C.The temperature of a planet can be higher if it has an atmosphere because the atmosphere can trap more heat/light from the planet D.The temperature of a planet can be affected by heat coming from plants

19. You will get a greenhouse effect A.On any planet B.On any planet with an atmosphere C.On any planet with an atmosphere if the atmosphere has gases that transmit visible but block infrared light D.On any planet with an atmosphere if the atmosphere has gases that block visible but transmit infrared light E.On any planet with an atmosphere if the atmosphere has gases than block both visible and infrared light

20. Carbon dioxide in Earths atmosphere Recent record of carbon dioxide –http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mio.ht mlhttp://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.jpghttp://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

21. Earth and climate change We know that the greenhouse effect is real, Venus makes this obvious Key question: is increased CO 2 in Earths 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

22. 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 CO 2 with temperature change is particularly worrisome! –Models suggest that it is the increasing CO 2 that is responsible for the increasing temperatures Most (but not every!) climate scientists think that climate change is occurring and is human caused

23. Climate change: the future If temperatures are rising because of increasing CO 2, 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 youre 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

24. ….. But back to potential locations for life on other planets …..! From external heating alone, Earth seems to be about the only place for liquid water now But there are other heat sources ….

25. Internal heat sources of planets External heating from the Sun is from the outside in Some heating can come from the inside out One source of internal heat: heat from planetary formation –When planets form, they are hot, because gravity has accelerated particles to make them form –Amount of internal heating varies from planet to planet, probably both from initial heat and from different cooling rates

26. If you put several potatoes of different sizes in the oven, let them all heat up, then take them out A.They will all cool at the same rate B.The inner parts of the smaller potatoes will stay hotter for longer than the inner parts of the bigger potatoes C.The inner parts of the bigger potatoes will stay hotter for longer than the inner parts of the smaller potatotes D.They will all stay hot forever

27. Internal heating of planets Bigger planets expected to stay hotter for longer Earth is still hot in the center, evidence: –volcanoes –Magnetic field –Plate tectonics Moon is not hot internally –Surface is geologically dead Mercury is not hot internally Mars seems pretty dead, but perhaps not for so long

28. Internal heating Based on this, expect that objects smaller than Mars (roughly) are probably cooled in their centers Surprise! Voyager flies by Jupiter, takes pictures of moons, and sees volcanoes on innermost large moon, Io!

29. Tides and tidal heating Another mechanism for creating internal heat: tides Tides come when you have two objects orbiting each other –Pull of gravity is stronger on closer side than on middle, so closer side is pulled a little more –Pull of gravity is stronger on middle than on far side, so far side is pulled a little less –Result: planet is stretched in the direction of the other object, with near side towards the object and far side away from it

30. Tides on Earth arise because the Earth is stretched by the gravitational pull of the Moon. How often do you expect a high tide? A.Once per day B.Twice per day C.Once per month D.Twice per month

31. Tides and tidal heating On Earth, water flows to accommodate this stretching --> tides on a two times per day cycle On Jupiter moons, tides are really strong –Jupiter has a lot of mass, hence a strong gravitational pull –Moons are relatively close, so theres a big change of gravity from one side to the other –stretching of the solid moons creates internal friction that heats them up!

32. Solar System life sites Earth Venus too hot because of greenhouse effect Mars seems too cold now, but perhaps warmer in the past? –Evidence of past water, present water?present water? –Evidence of past geological activity –Perhaps more greenhouse effect in past? –Evidence of life? ALH84001 –Lots of current Mars exploration!

33. Solar System life sites Moons of Jupiter –Io too hot –Europa surface looks like ice Europa also has induced magnetic field