Habitability

1. Habitability Bonnie Meinke January 27, 2009

2. Introduction • Define Habitability • The Habitable Zone • Environment of early Earth

3. Defining Habitability Defining Habitability • What do we mean when we say habitable? • Earth-like animal life: specific requirements • Microbial life: broader set of conditions

4. Defining Habitability Defining Habitability • What do we mean when we say habitable? • Earth-like animal life: specific requirements (oxygen, water, dry land, temperature range) • Microbial life: broader set of conditions (more extreme conditions ok)

5. Defining Habitability Common basic requirements for life • Water • Stable climate

6. Defining Habitability What stabilizes the climate? • Size - long-term heat source • Stellar evolution - incoming solar energy • Impact rate - could result in climate change • Presence of large, natural satellite - prevents large swings in obliquity • Oceans - regulate global temperatures

7. The Habitable Zone Habitable Zones • Why is Earth the only (as far as we know) habitable planet in our solar system? • 2 main properties: • Abundant liquid water • Environmental conditions that maintain liquid water

8. The Habitable Zone Liquid Water • Required temperature: 273-373 K • Use this as simple requirement for identifying possibly habitable planets • Where do planets in this temperature range orbit?

9. The Habitable Zone Liquid Water • Where do planets in this temperature range orbit? • Called the Habitable Zone • Let’s work it out…

10. The Habitable Zone How does star type affect HZ? • Different sized stars have different luminosities • T L1/4 • Brighter stars have HZs farther out

11. The Habitable Zone How does star type affect HZ? • Main sequence (MS) stars have different luminosities throughout their lifetimes • Continuously Habitable Zone: maintains conditions suitable for life throughout MS lifetime of star

12. Albedo, a Atmosphere - what part of spectrum can pass through Moves HZ inwards Moves HZ outwards The Habitable Zone Is it that simple?

13. Kasting proposed the Carbon Dioxide Thermostat Extends to HZ for Earth-like planets Keeps off temperature extremes Carbon sources: Volcanic outgassing Decarbonation Organic carbon Carbon sinks: Calcium carbonate formation Photosynthesis The Habitable Zone Role of the Carbon Cycle

14. The Habitable Zone Role of the Carbon Cycle

15. Inner edge: 0.95 AU Outer edge: 1.15 AU Were other planets habitable in the past? Will other planets be habitable in the future? The Habitable Zone Continuously Habitable Zone

16. Early Mars Evidence of large amounts of flowing liquid water Warmer temperatures: Heat from interior would have been higher Warm climate from greenhouse gases or CO2 clouds Current Mars Gullies may be due to underground water Carbon cycle not as active as on Earth The Habitable Zone Mars: Once Habitable? Still Habitable?

17. The Habitable Zone Characteristics that make a habitable planet • Size of planet • Internal heat comes from • Accretional heat • Differentiation • Radiogenic decay • Allows for plate tectonics • Mars cooled quickly, so no plate tectonics at present • Other Heat sources to sustain liquid water • Geothermal • Iceland • Tidal • Europa

18. Star Type: stable luminous stars necessary Sufficiently long lifetime for life to evolve Large enough so planets don’t tidally lock The Habitable Zone Characteristics that make a habitable system • Star system • Single star: allows for stable orbit • Binary system: • Fewer stable orbits exist • HZ calculated on individual basis

19. Galactic Habitable Zone Area of high metallicity (elements w/ Z>2) Outer region of galaxy Lower stellar density Lower radiation levels The Habitable Zone Characteristics that make a habitable neighborhood

20. Early Earth Astr 3300 September 16, 2009

21. Early Earth Environment of early Earth • Evidence of a habitable planet 3.8 Ga • Geological evidence near Isua, Greenland • Limestone and sandstone • We can infer presence of liquid water • Earth must have had temperatures similar to today’s

22. Early Earth Liquid water 3.8 Ga? • Faint young Sun • Sun was 25-30% less luminous • Simple energy balance shows Earth’s surface temperature would have been below 273 K • Other heat sources • Geological activity • More internal heat from radioactive decay and primordial heat • Plate tectonics release CO2 - greenhouse traps heat

23. Early Earth Snowball Earth • Global glaciations brought on by disruptions in the carbon cycle • Up to 4 occurred between 750 Ma and 580 Ma ago • Geological record shows layered deposits in tropics attributable to glacial erosion • CO2 sinks would cease, but sources would continue. 350 times current CO2 levels would accumulate to create a severe greenhouse, causing the ice to melt w/in a few hundred years. • All eukaryotes today are from the survivors of snowball earth

24. Early Earth Early Hydrosphere How did Earth get all it’s water?

25. Early Earth Origin of Earth’s Water • Delivered by comet impact • D/H ratios of comets are not the same as on earth • This is unlikely the delivery mechanism • Solar nebula • Unlikely because relative abundance of other volatiles are higher in the solar nebula than in planetary atmospheres • From un-degassed interiors of planetary embryos • Most likely scenario • Hydrated minerals could form around 1 AU

26. Early Earth Origin of Earth’s Atmosphere • Only trace amounts of oxygen for the first 1 billion years • O2 resulted from breakdown of water vapor by UV radiation • Current atmosphere is oxygen-rich, so where did it come from? • PHOTOSYNTHESIS! • First developed in cyanobacteria 3.8-2.5 Ga ago (Archaean era)

27. Early Earth Banded Iron Formations • Geologic evidence for appearance of free oxygen are Banded Iron Formations (BIFs) • BIFs provide clues as to the oxidation state of ocean and atmosphere at time of formation • Usually formed in shallow seas - oxygen available here

28. Early Earth Role of hydrothermal systems • Seawater flowing through hydrothermal vents dissolved iron • Injected iron into deep ocean through vents • Deep ocean too oxygen-poor to oxidize iron, so it cycled through system to be deposited in shallow seas. • Possible iron was consumed by bacteria near vents and transported in drifts of large colonies

29. Early Earth Evidence for early life on Earth • Stromatolites • Oldest known is 3.46 Ga-old • Formed from cyanobacteria and blue-green algae • Organisms for gelatinous mat and precipitate calcium carbonate, so it looks like stack of pancakes w/ alternating layers

30. Can be used as indicators of biological processes 12C and 13C are stable isotopes Ratio is affected by physical processes More energy efficient to make or break 12C bonds 12C is preferentially incorporated into products of chemical reactions Early Earth Carbon Isotopes

31. A negative value can be used as a biomarker, indicating fractionation is due to photosynthesis Isotopic fractionation let’s work it out Early Earth Carbon Isotopes

32. Ediacaran fauna show distinctive changes in size ~670 Ma ago) Life started small (maximum of a few mm in size) In the last 600 Ma, evolution of more larger, more complex organisms has occurred Early Earth Evolving Complexity

33. Early Earth Evolving Complexity • Ediacaran fauna show distinctive changes in size ~670 Ma ago) • Tubular, frond-like, radially symmetric • cm-m in size

34. Early Earth Increase in size and diversity • Subsequently, after 500 Ma ago, sizes increased 2 orders of magnitude • Dinosaurs • Larger mammals

35. Early Earth Major extinctions • Marked periods of Earth’s biological history • Reduce diversity • Most recent • Possibly due to comet or asteroid impact • Die out of the dinosaurs (65 Ma ago) • Demonstrates how important “environmental stability” is for a habitable planet

36. Banded-Iron Formations (BIFs) • Most formed 3Ga-1.8Ga • Amount of Oxygen locked in BIFs is ~20 times the volume in the modern atmosphere

37. Banded-Iron Formations (BIFs) • Formation: • Oxygen produced by cyanobacteria combined with iron in the ocean (early ocean was acidic and iron-rich) • Oxidized iron then deposits in a layer • Process is cyclical due to oscillating availability of free oxygen • Eventually, photosynthesis caught on, the oceans because well-oxygenated, and the available iron in the Earth's oceans was precipitated out as iron oxides

38. Banded-Iron Formations (BIFs) • Snowball Earth cycles may have been the cause of bands • During snowball periods, free oxygen not available and iron • Followed by oxidizing periods of melt • Metal-rich brines may also be responsible • Carry iron from the deep ocean (near hydrothermal vents) • Deposited in shallow seas where it has access to free oxygen

39. Carbon Isotopes • 12C and 13C are stable isotopes • More energy efficient to make 12C bonds • 12C is preferentially incorporated into products of chemical reactions (like photosynthesis!) • Ratio of the two isotopes can be used as an indicator of biological processes

40. Carbon Isotopes • If 12C has preferentially been incorporated, 13C/12C will be smaller than the standard • If sample < standard, 13C is negative • A negative value can be used as a biomarker, indicating fractionation is due to photosynthesis

41. Extreme Environments ASTR/GEOL 3300 September 18, 2009

42. Overview • Extreme Conditions • Other Worlds

43. Extreme conditions • Conditions on early earth may have been “extreme” compared to present-day • Extremophiles - organisms that thrive in exteme environments • Heat/Cold • Acids/alkalines • High pressures • dessication

44. Extreme Conditions Temperature • Majority of organisms on Earth thrive in the temperature range 20-45 °C (mesophiles) • Usual response to extreme temperatures: • Cold: • Formation of ice crystals in the body • Hot: • Structural breakdown of biological molecules (proteins and nucleic acids) • Disruption of cells’ structural integrity due to increased membrane fluidity

45. Extreme Conditions Temperature

46. Extreme Conditions Thermophiles • Thermophiles live between 50 and 80 °C • Example: Thermoplasma • Archaea • Lives in volcanic hot springs • Hyperthermophiles live between 80 and 115 °C • Example: Sulfolobus • No multicellular plants or animals can tolerate >50 °C • No microbial eukarya can tolerate >60 °C

47. Extreme Conditions Thermophiles • First true thermophile discovered in Yellowstone National Park in 1960s • > 50 hyperthermophiles have been isolated to date • Many live in or near deep-sea hydrothermal systems (black smokers)

48. Extreme Conditions Thermophiles: how they cope • Since high temperatures change membrane fluidity, adaptation is change of membrane composition • Evolution of proteins to better cope w/ high temps

49. Extreme Conditions Psychrophiles • Supported in frozen environments of Earth • Lowest recorded temperature for active microbial communities: -18 °C • Found in all 3 domains of life

50. Extreme Conditions Psychrophiles: how they cope • Low temps mean decrease in membrane fluidity, so adaptation is adjustment of ratios of lipids in their membranes • Prevent water from freezing with soluble compounds that lower freezing temp of water (e.g. thermal hysteresis proteins)