L16 : Earth Comparison to Extraterrestial Environments Flashcards
(18 cards)
What are the main factors affecting habitability?
Star luminosity
Plant-star distance
Atmospheric composition
Albedo
Esteroid/comet abundance
Nearby gas giants
Galactic location
What is habitability?
Ability of a terrestrial planet to support life
Dependent on multiple factors
Why is it difficult to define habitability as a single metric?
Because it depends on combination of many spatial and temporal factors that interact in complex ways
What is the circumstellar habitable zone?
Region around a star where liquid water can remain stable on planet surface
What determines the inner boundary of the the CHZ?
Water vapour dynamics
Too hot for stable liquid water due to moist or runway greenhouse limit
What determines the outer boundary of the CHZ?
CO2 condensation
Too cold to support stable liquid water
What is the moist greenhouse limit?
Point where water vapour escapes into upper atmosphere and is lost to space due to photolysis
Explain the inner boundary?
At distances that are close enough to star, increase of:
- Planetary temperatures
- Photolysis of upper atmospheric H20 vapour molecules
What happens at the runaway greenhouse limit?
Positive feedback loop of trapped heat
Leads to desiccation and eventual oxidation of the planet
At what water vapour mixing ratio does the moist greenhouse limit occur?
Around 2 x 10^-3 above cold trap
Surface temperatures near 340 K
What role does CO2 play in the outer boundary?
High CO2 levels condense into clouds/ice
Increase albedo and cool planet beyond habitable range
Where is the CO2 maximum greenhouse limit in our Solar System?
Around 1.67 AU with atmopsheric CO2 pressure of ~8 bars
How do planets around cooler stars differ in CHZ behaviour?
Reduced Rayleigh scattering allows them to maintain higher CO2 levels before condensation
Why is the traditional habitable zone model limited?
Assumes Earth-like atmosphere composition and excludes other gases (CH4, H2)
What do newer habitable zone models include?
- Planets with methane or hydrogen atmospheres
- Ocean or desert worlds
- Those around non-main sequence stars
What is the complex life habitable zone (CLHZ)
Narrower zone defined by the solubility of lipids and other molecules essential for complex life, based on Earth
Who proposed CLHZ concept and when?
Ramirez 2020
Why is CLHZ narrower than traditional HZ?
Complex life needs more than water
Stable environments for lipids and emergence of cell membranes
