ASTR/GEOL 3300: ET Life

1. ASTR/GEOL 3300: ET Life • Logistics: • Pick up old homeworks please (Big HW due in 2 weeks.) • MT2 is a week from next Monday • Paper proposals returned to you now, revised proposals due NEXT MONDAY (Nov. 1). • Plan for Today: • Finish Venus • Habitable zones • Next week: • Europa! And Icy Moons

2. Mojzsis’ pet peeves in writing (of many) • “martian” is always lower case • “Earth” is always upper case, unless you are referring to dirt/soil • A galaxy is a collection of hundreds of millions of stars • The Universe is EVERYTHING that is and ever will be, not just our Milky Way Galaxy • Our Solar System is one of many within the Milky Way Galaxy, and there are billions and billions of galaxies • Never start a sentence with a conjunction (and, but, or) • Avoid gerunds when you can (words ending in “-ing”) • YES, I know, your textbooks break these rules too!

3. Venus

4. Varied volcanic styles: Some extremely runny lava. Some more stiff. Lowland plains volcanism: Basaltic (?) Covers much of surface. Volcanism on Venus

5. surface lithosphere mantle

7. Earth Venus Venus: Why No Magnetic Field? • “Dynamo” magnetic field requires: • Internal conductor (e.g. molten Fe). • Rotation (Venus is slow, but okay). • Core convection (not in Venus?!?). • Earth: • Plate tectonics cools mantle. • Temp gradient across core drives core convection, powering magnetic field. • Venus: • No plate tectonics. • Interior more insulated: stays very hot. • Insufficient core temperature gradient to drive core convection. • No protective magnetic field!

9. Atmospheric History of Venus • Outgassing: lots of H2O and CO2 into Venus' atmosphere early on, dust & comets brought SOME H2O: • Venus should have similar water supply as Earth! • ~30% dimmer early Sun means Venus may have had oceans! • Because of Venus' distance from Sun, it experienced a runaway greenhouse: • Closer to Sun, early Venus was hotter than early Earth. • So, more H2O in the early Venus atmosphere than early Earth's (more evaporation; & more water can be held in a warmer atmosphere). • More atmospheric H2O meant Venus had a stronger greenhouse effect. • More greenhouse warming put even more water into Venus atmosphere.... AND SO ON: POSITIVE FEEDBACK!

10. Highway to Hell • With H2O in atmosphere, most then lost from Venus: • By atmospheric stripping(no magnetic field!). • By thermal escape(following UV dissociation). • Venus has no CO2 cycle: • Most of Earth's CO2 is in carbonate rocks! • Most of Venus' CO2 remains in its atmosphere!

11. Let’s discussVenus Atmospheric Evolution • What is the best evidence that Venus has lost significant amounts of water over time? • Large amounts of hydrogen are observed flowing away from the planet today. • The ratio of deuterium to hydrogen in the atmosphere of Venus is abnormally high. • Comets are routinely observed to impact onto Venus today. • Ancient shorelines are observed in radar images of Venus. • Sinuous channels observed in radar images of Venus were probably carved by liquid water.

12. Venus Atmospheric Evolution • What is the best evidence that Venus has lost significant amounts of water over time? • Large amounts of hydrogen are observed flowing away from the planet today. • The ratio of deuterium to hydrogen in the atmosphere of Venus is abnormally high. • Comets are routinely observed to impact onto Venus today. • Ancient shorelines are observed in radar images of Venus. • Sinuous channels observed in radar images of Venus were probably carved by liquid water.

13. Runaway Greenhouse: Could It Happen Here? As Sun continues to brighten: Probably some day! Could human activity trigger this? We'll see!

14. Life on Venus?!? • Could life have existed on Venus early on? • Sun was cooler: Venus may have had oceans. • Meteorites travel between planets: maybe carry life? • Why not life today? • Too hot (organics destroyed). • No water (solvent needed). • No ozone to protect from UV (organics destroyed). • Can life possibly overcome these problems?

15. Artemis Corona, Venus: Stalled Plate Tectonics?

16. Life on Venus?!? • Could life have existed on Venus early on? • Sun was cooler: Venus may have had oceans. • Meteorites travel between planets: maybe carry life? • Why not life today? • Too hot (organics destroyed). • No water (solvent needed). • No ozone to protect from UV (organics destroyed). • Can life possibly overcome these problems? • Life might have adapted to cooler high atmosphere. • Water is slightly enriched at ~50 km altitude. • UV protective coating (sulfur “sunscreen”) is possible.

17. The Goldilocks Paradox • Mars: Too cold. • Venus: Too hot. • Earth: Just right! Why??

18. Factors Controlling Planetary Habitability • Distance from Sun. • Planet size. • Greenhouse atmosphere (& loss).

19. Distance from Sun • Venus: • Too hot for liquid water  • Runaway greenhouse. • Earth: • Liquid water oceans  • CO2 cycle regulates climate. • Mars: • Too cold for liquid water (±greenhouse)  • Ground ice instead.

20. Size Matters • Larger planets retain heat longer, so are active longer. • Activity implies volcanism, and chemical energy. • Mars lost mag field as interior cooled, so atmosphere was stripped. • Plate tectonics depends on size and water.

21. Effects of an Atmosphere:Greenhouse Warming • Venus, Earth, Mars all have some greenhouse warming: • Venus: very strong • Earth: moderate • Mars: weak • Greenhouse atm. can be critical to habitability: • Atmospheric retention is key.

22. Why is Earth Habitable? • Geology: plate tectonics • Water: abundant & liquid • Atmosphere: oxygen, ozone, some CO2 • Stable climate (unlike Mars & Venus) • Life: astonishing and planet-altering These unique characteristics are interrelated. How rare are habitable worlds?

23. The Habitable Zone • Habitable Zone (HZ): • Temperatures allow liquid water to be stable on a planet’s surface. • Based on 1 example (our solar system) & atmospheric models: • Just inside Earth orbit, to about Mars orbit.

24. Boundaries of the Habitable Zone • Outer edge (today): • Depends on strength of greenhouse atm. • 1.4 - 1.7 AU • Mars = 1.5 AU • Inner edge (today): • Runaway greenhouse. • 0.84 - 0.95 AU • Venus = 0.7 AU

25. Continuously Habitable Zone • Continuously Habitable Zone (CHZ): • Region located within habitable zone long enough for life to thrive. • Sun brightness has increased, so HZ has evolved outward. • Earth leaves HZ 0.5 to 3 Byr from now!

26. Europa & the Galilean Satellites

27. The Galilean Satellites

28. Io: The Volcanic Moon • Solar System's most volcanically active world! • Why so colorful? • Red & yellow sulfur. • Dark volcanic flows. • Bright SO2 frost. • No impact craters have been found: • Very active and youthful surface. • Very warm interior.

29. Io: The Volcanic Moon • Volcanic plumes! • Volcanic flows!

30. Io: The Volcanic Moon • Extreme geology in action!

31. Why So Hot? Tidal Heating! • Tidal heating keeps Io hot! • Jupiter's gravity stretches Io, creating tidal bulges. • During Io's eccentric orbit about Jupiter, the tidal bulges grow when Io is closer, and shrink when Io is farther. • Tidal bulges also "nod" from side-to-side. • This flexing generates heat. • Tidal heating generates enough heat energy to melt rock and power Io's volcanoes. not to scale