Looking for life in the Universe

1. Looking for life in the Universe Solar System Sites Attempts at Communication Probability of Life Detection of Planets

2. Recap • Final assignments: • Lab credit this week: Canvas evaluations, due before Monday morning • Homework this week: Canvas post-course assessment, due by final exam time • Final: Wednesday 12/11, 10:30-12:30 • Potential life sites in the solar system • Look for locations where temperature allows for liquid water • What affects temperatures of planets? • Heating from Sun: balance of heat in = heat out • temperature depends on distance from Sun and composition/reflectivity of planets • But this doesn’t seem to give the observed temperatures for some planets, especially Venus, which is WAY hotter • The Greenhouse effect: planetary atmospheres as blankets that can increase the temperature • Greenhouse effect and Earth: natural, man-made greenhouse effect, and climate change

3. You will get a greenhouse effect • On any planet • On any planet with an atmosphere • On any planet with an atmosphere if the atmosphere has gases that transmit visible but block infrared light • On any planet with an atmosphere if the atmosphere has gases that block visible but transmit infrared light • On any planet with an atmosphere if the atmosphere has gases than block both visible and infrared light

4. ….. But back to potential locations for life on other planets …..! From external heating + Greenhouse, Earth seems to be about the only place for liquid water now • Venus too hot from Greenhouse • Mars Greenhouse isn’t enough to make it globally hot enough, although there are locations on Mars that come close… But there are other heat sources ….

5. 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

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

7. 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

8. 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!

9. Tides and tidal heating • Another mechanism for creating internal heat: tides • Tides come when you have an orbiting object • 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

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

11. 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 to Jupiter, so there’s a big change of gravity from one side to the other • “stretching” of the solid moons creates internal friction that heats them up!

12. 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? • Evidence of past geological activity • Perhaps more greenhouse effect in past? • Evidence of life? ALH84001 • Lots of current Mars exploration!

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

14. Life outside the Solar System • For life outside the solar system, we can’t find it by exploration • Look for intelligent life by communication • Look for observable tracers of life • So far, nobody has contacted us, nor have we received any signals!

15. Attempts at communication • SETI project (also note SETI at home) • 1974 Arecibo message to M13 • Pioneer plaque • Voyager golden record • Communication time scales are very long! • Review: light years and distances in the Milky Way