Habitability: Making a habitable planet 2 September 2016

Introduction 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 Water, oxygen, dry land, Energy source (chemical, light, CO2)

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)

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

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

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

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

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

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

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

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

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

How the CO2 cycle buffers the planet’s temperature

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

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

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

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

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

Discussion Topics: 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

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

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. 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

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