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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 8: Goldilocks and the Three Planets (Habitable Zones)

Key Ideas

Venus was too close to the Sun to be habitable
A planet with a thick atmosphere could be further from the Sun than the Earth and still have liquid water.
We can define the region around the Sun where planets will have stable atmospheres supporting liquid water as the Habitable Zone
Being in the habitable zone does not guarantee habitability
Mars is too small to be habitable (continuously)
Therefore, size is also important for habitability
The Sun gets brighter as it gets older, and therefore the habitable zone moves out and gets wider
Being outside the habitable zone does not necessarily imply a planet is not habitable.

The Habitable Zone

What happens if we move the Earth close to the Sun?
Runaway Greenhouse Effect
Oceans evaporate
No carbon dioxide sink

How close is too close?
Venus (a=0.72) clearly too close.
"Runaway Greenhouse" at a=0.84 AU
"Moist Greenhouse" might operate closer
--Higher temperatures -> water in upper atmosphere
--Water in upper atmosphere gets destroyed by UV rays
--Water is eventually depleted
--No water -> No Carbon Dioxide sink
--More Carbon Dioxide -> stronger greenhouse effect
--Inner edge might be as close as 0.95 AU

Too close = 0.84 AU - 0.95 AU

What happens if we move the Earth further from the Sun?
Eventually water will freeze -- depends on the atmosphere

How far is too far?
Is Mars (a=1.52 AU) too far?
A thick CO2 atmosphere could support liquid water to 1.7 AU!!
Then again, maybe not:
CO2 might 'freeze out'
dry snow!
Therefore, there might be a limit to the amount of CO2 in the atmosphere
Outer edge might be as close as 1.4 AU

Too far = 1.4AU - 1.52 AU

Habitable Zone
Range of distances from the Sun where liquid water can be stable.
a=(0.84-1.7)AU "Optimistic"
a=(0.95-1.4)AU "Conservative"

Other Considerations

Mars is not currently "habitable". Why?
Early Atmospheres of Earth, Venus, and Mars Similar

Nearly the same amount of CO2 on Earth & Venus
--On Earth, CO2 is locked up on rocks
--On Venus, CO2 is in the atmosphere
--Mars probably has much less CO2

Mars probably has much less CO2 and water

Primordial Atmosphere formation:
--Outgassing from primordial volcanoes
--Frozen volatiles delivered from the outer solar system
--Substantial atmospheres of CO2, H2O, and N2
--H & He quickly lost (too warm & gravity too weak)

Water on Mars?
Results from the Mars orbital surveys:
--Evidence of recent, rapid floods of water carving gullies like seen on Earth.
--Layered terrains (e.g., like Grand Canyon)
Mars Exploration Rovers:
--Layered sedimentary rocks with flow patterning
--Salt deposits laid down by evaporating water
--Hydrated minerals like Hematite

What happened on Mars?

Cooling time 53% of Earth's

Escape velocity 46% of Earth's
Thermal velocity 90% of Earth's

Mars is too small
Had a harder time holding on to its atmosphere
--Lost its magnetic field

Atmosphere is gone forever
--Lost its volcanoes
--No outgassing
--No other sources

Both distance and size are important for determining habitability

Continuously Habitable Zone

The Sun is getting brighter
Sun fuses hydrogen into helium
Density of the core increases
Higher density leads to higher temperature
Higher temperatures lead to higher fusion rate
Higher fusion rate leads to higher luminosity

Habitable Zone moves with time

Continuously habitable zone:
Range of distances where a planet can have stable liquid water on its surface for the entire lifetime of the star.

Life Outside the Habitable Zone

Life may exist outside the habitable zone
Other sources of energy
--Latent heat
--Chemical Energy

We must not be too narrow-minded?
Other sources of energy
Other kinds of life
The concept of a habitable zone should guide our thinking, but not restrict it.

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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