1. Astronomy190 - Topics in Astronomy Astronomy and Astrobiology Lecture 11 : Earth’s Habitability Ty Robinson

2. Questions of the Day What role does planetary mass have in determining habitability? What is the Carbonate-Silicate Cycle and how has it helped Earth to maintain its habitability? What traits must a planet posses to have a functioning Carbonate-Silicate Cycle?

3. What Factors Affect Planetary Surface Habitability? Parent star – lifetime, UV radiation. Planet’s orbital parameters – distance, eccentricity Planetary mass – tectonics, atmosphere, magnetic field Initial planetary composition Atmospheric composition (albedo, greenhouse) UV shielding Dynamical stability of the planetary system Impacts Biology (CH 4, rise of O 2,albedo)

4. Planetary Mass affects how much internal heat a planet can have internal heat comes from three main sources –accretion impacts that heat the outer layers of the forming Earth –initial “differentiation” the separation of materials in the molten accreted Earth due to their density denser material sinking to the center convert gravitational potential energy into thermal energy Some asteroids are differentiated! Vesta being the largest (530km diameter) –radioactive decay of elements (ongoing)

5. Planetary Mass..and how long it can retain that heat –things that start with more heat energy take longer to cool –long-lived radioactive isotopes keep producing heat Venus (?) and Earth still have molten cores Mars’ core probably solidified less than 1Gy after formation

6. Questions? Which of the following is an argument for why Venus has a molten core? it is of similar mass & composition to Earth it has a thick atmosphere it is slightly less massive than Earth it has a slow rotation rate

7. Planetary Mass sufficient planetary mass (> 0.3 M Earth ) enables –generation of a global magnetic field prevents atmospheric stripping by the solar wind –generation and maintenance of plate tectonics active volcanism can replenish atmospheric gases like CO 2 over geologic time plate tectonics is a crucial component of the cycle which stabilizes the Earth’s surface temperature on long time scales

8. The Obvious Exception….

9. Planetary Atmospheres atmospheric pressure is required to prevent the loss of an ocean provide surface UV shields –Ozone in our atmosphere blocks UV radiation provide greenhouse warming which can raise planetary surface temperature into a habitable range provide buffers against large temperature swings across the planet, and over time

10. The Greenhouse Effect visible light easily reaches the Earth’s surface, where a fraction of it is absorbed the heated ground re-radiates the absorbed energy back towards space as infrared radiation atmospheric gases absorb this radiation, radiating a fraction of it back to the surface the net effect is to make a planet’s surface and lower atmosphere warmer than they would be from the absorption of sunlight alone

11. Surface Temperatures Effective Temperature [ºC] -55-17-43 Surface Temperature [ºC] -5015470 Greenhouse Warming [ºC] 532513

12. Stabilizing Climate The Sun was 30% fainter 4.6Gya, and yet the Earth’s surface temperature has been stable enough to allow life to exist continually for nearly 4Gyrs. This is due to the combined effects of volcanism, plate tectonics and the Earth’s magnetic field.

13. Questions? The Earth’s ability to remain habitable over the last 4Gyrs depended on volcanism, plate tectonics and its magnetic field. Thus, the Earth’s ability to remain habitable boils down to… atmospheric composition rotation rate gravity and temperature its mass and rotation rate

14. Similar starting conditions… Where did all the CO 2 go? Locked into carbonate rocks and dissolved in the ocean.

15. The Carbonate-Silicate Cycle The CO 2 Cycle -or-

16. Stabilizing Climate: The Carbonate-Silicate Cycle this cycle continually moves CO 2 from the atmosphere to the ocean to rock and back again –atmospheric CO 2 dissolves in the ocean and in rainwater, formic carbonic acid –rainfall erodes silicate rocks, forming bicarbonate and carrying dissolved materials it to the oceans –the bicarbonate reacts with dissolved minerals to form carbonate minerals, which fall to the ocean floor –the sea floor carbonates are eventually subducted –the subducted rocks melt to form gaseous CO 2, which is released to the atmosphere in volcanic eruptions

17. The CO 2 Cycle as a Thermostat (-) Surface temperature Rainfall Silicate weathering rate Atmospheric CO 2 Greenhouse effect (Timescale ~ Millions of Years)

18. Example The Sun’s energy output slightly increases… Earth’s mean surface temperature increases… the hydrological cycle speeds up… the rate of weathering rocks increases… CO 2 is pulled from the atmosphere to form carbonate rocks… the greenhouse effect diminishes… Earth’s mean surface temperature decreases, balancing the brighter Sun

19. Questions of the Day What role does planetary mass have in determining habitability? What is the Carbonate-Silicate Cycle and how has it helped Earth to maintain its habitability? What traits must a planet posses to have a functioning Carbonate-Silicate Cycle?

20. Quiz 1 - Sketch this figure and fill in the blanks. 3 - What is one thing you did not understand from today’s lecture? 2 - Given all of the processes going on in this figure, what traits must a world have to posses a functioning Carbonate-Silicate cycle?

21. Summary volcanic outgassing give us atmosphere and oceans planet’s mass and magnetic field help us to maintain the atmosphere greenhouse effect raises the planetary temperature above freezing climate stability is maintained by a carbonate- silicate cycle feedback with surface temperature –oceans, continents, a hydrological cycle and plate tectonics support this process.