1. Habitability: Making a habitable planet
2 September 2016

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

3. Defining Habitability
What do we mean when we say habitable?
Earth-like animal life: specific requirements
Microbial life - broader set of conditions
Water, oxygen, dry land,
Energy source (chemical, light, CO2)

4. 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. Common basic requirements for life
Defining Habitability
Common basic requirements for life
Water, biogenic elements, energy (but elements and energy are common, so liquid water is the key)
Stable climate, perhaps for long periods

6. What stabilizes the climate?
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. Liquid Water is the Key! Required temperature: 273-373 K
The Habitable Zone
Liquid Water is the Key!
Required temperature: K
Use this as simple requirement for identifying possibly habitable planets
Where do planets in this temperature range orbit?

9. Liquid Water Where do planets in this temperature range orbit?
The Habitable Zone
Liquid Water
Where do planets in this temperature range orbit?
Called the Habitable Zone

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

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

12. Is it that simple? Albedo, a
The Habitable Zone
Is it that simple?
Albedo, a
Atmosphere greenhouse – chemical absorbers act like a warm blanket
Moves HZ inwards
Moves HZ outwards

13. Role of the Carbon Cycle
The Habitable Zone
Role of the Carbon Cycle
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
Decarbonation - losing carbon through chemical reactions - breaking bonds

14. How the CO2 cycle buffers the planet’s temperature

15. Continuously Habitable Zone
The Habitable Zone
Continuously Habitable Zone
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?
Earth is the only one in our system in the CHZ
Venus was once in the HZ, but obviously not suitable for life now

16. Mars: Once Habitable? Still Habitable?
The Habitable Zone
Mars: Once Habitable? Still Habitable?
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

17. Characteristics that make a habitable planet
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
Size - heat escapes quickly from a small planet. Volume: SA ratio important. Larger for bigger planets so they don’t radiate away energy as quickly

18. Characteristics that make a habitable system
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
Star Type: stable luminous stars necessary
Sufficiently long lifetime for life to evolve
Large enough so planets don’t tidally lock

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

20. Discussion Topics: How common are habitable zones?
What does this say about the likelihood of life?
What stars are the best to search?

21. 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. Liquid water 3.8 Ga? Faint young Sun Other heat sources Early Earth
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. Snowball Earth Early 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 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. Summary
Habitable Zone: area around a star where liquid water can exist on the surface
Continuously Habitable Zone: the region where water would exist over the entire star lifetime