Habitability: Making a habitable planet 26 January 2016

Introduction  Define Habitability  The Habitable Zone  Environment of early Earth  Define Habitability  The Habitable Zone  Environment of early Earth

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

Common basic requirements for life  Water  Stable climate  Water  Stable 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  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 Defining Habitability

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  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 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?  Required temperature: K  Use this as simple requirement for identifying possibly habitable planets  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  Where do planets in this temperature range orbit?  Called the Habitable Zone The Habitable Zone

How does star type affect HZ?  Different sized stars have different luminosities  T goes as L 1/4  Brighter stars have HZs farther out  Different sized stars have different luminosities  T goes as L 1/4  Brighter stars have HZs farther out 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  Main sequence stars have different luminosities throughout their lifetimes  Continuously Habitable Zone: maintains conditions suitable for life throughout the lifetime of star The Habitable Zone

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

Role of the Carbon Cycle  Kasting proposed the Carbon Dioxide Thermostat  Extends to HZ for Earth-like planets  Keeps off temperature extremes  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

How the CO2 cycle buffers the planet’s temperature

Continuously Habitable Zone  Inner edge: 0.95 AU  Outer edge: 1.15 AU  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

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 CO 2 clouds  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 CO 2 clouds  Current Mars  Gullies may be due to underground water  Carbon cycle not as active as on Earth The Habitable Zone

Characteristics that make a habitable planet The Habitable Zone Other Heat sources to sustain liquid water Geothermal Iceland Tidal Europa Other Heat sources to sustain liquid water Geothermal Iceland Tidal Europa 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 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

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

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 The Habitable Zone

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

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 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 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 CO 2 - greenhouse traps heat 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 CO 2 - greenhouse traps heat 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 CO 2 sinks would cease, but sources would continue. 350 times current CO 2 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 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 CO 2 sinks would cease, but sources would continue. 350 times current CO 2 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 Early Earth

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  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