Download Lecture 35. Habitable Zones.

Lecture 35. Habitable Zones.
reading: Chapters 9, 10
Goldilocks and the Solar System
Venus
too hot now/not habitable
geologic age of the surface: ~500 Ma
could have been habitable in the past before runaway greenhouse
Earth
liquid water for most or all of geologic history
has always been habitable
carbonate-silicate cycle stabilizes the climate with its negative
feedback loop.
Mars
too cold for liquid water today
geologic evidence of liquid water in the past
could have been habitable in the past
Runaway Greenhouse Effect
Occurs if the T reaches the boiling point of water.
Oceans turned into water vapor.
Water vapor is a greenhouse gas, causing additional warming.
This causes the oceans to evaporate even faster.
This is a positive feedback loop.
Soon, all the water will be in the atmosphere, which will be very hot.
Hot enough (several hundred degrees) to vaporize carbonate rock.
This would turn carbonate rock back into CO2.
The runaway greenhouse is a permanent state - no known way
to escape it.
Concept of the Habitable Zone (HZ)
Focuses on the presence of liquid water.
The HZ is the zone in which temperatures allow
for liquid water to exist on the surface.
(note: this implies Europa is not in the habitable zone Europa is an exception to the definition of the habitable zone)
Key: distance to the Sun and presence of an atmosphere and
magnetic field.
Moon: in the Sun’s habitable zone, but lacks an atmosphere.
Is the Sun’s habitable zone moving in or out with time?
Venus
Venus and Earth likely started out with the same amount of
volatiles.
Evidence of volcanoes/outgassing/active planet on Venus.
But Venus is very dry and hot today.
Where did all the water go?
May have had early oceans.
As Sun got brighter, more water went into the atmosphere.
1. Photochemical reactions break water into hydrogen and oxygen.
Hydrogen is easily lost to space. Oxygen reacts with other gases
in the atmosphere and with rocks on the surface.
2. Water reacts with SO2 to form sulfuric acid.
What Controls Surface Habitability?
1. Distance from the Sun
Venus 0.7 AU
Earth 1.0 AU
Mars 1.5 AU
Distance from the Sun determines how much solar radiation
the planet receives.
Solar radiation drops by 1/r2 - This means that if the distance (radius, r)
from the Sun is doubled, the amount of solar radiation is 1/22, or 1/4).
Solar radiation is important for the greenhouse effect.
What Controls Surface Habitability?
2. Planetary Size
radius, relative to Earth
Venus 0.95
Earth 1
Mars 0.53
Smaller planets:
Lose internal heat rapidly, outgassing ceases.
Can’t replace volatiles that are lost to space or to chemical reactions.
Larger planets:
Greater internal heat, internal heat is retained over time.
Continued outgassing helps to retain the atmosphere.
Plate Tectonics
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Helps to recycle volatiles, trap volatiles in the mantle so that they
aren’t lost to space.
What Controls Surface Habitability?
3. Atmospheric Loss Processes
Mars has also lost a significant part of its atmosphere.
a) lack of magnetic field
solar wind particles strip away the atmosphere
b) low level of volcanism
Early Mars:
thicker atmosphere
stronger greenhouse effect
Inner Boundary of the HZ
Determined by the ability to avoid a runaway greenhouse.
Inner boundary of the HZ lies between Venus and the Earth
If we move Earth to 0.82 AU - Earth would have a runaway
greenhouse
If we move Earth to 0.95 AU - Earth would have a moist greenhouse
where more water is entering the atmosphere
where it can be lost to space.
Outer Boundary of the HZ
Distance from the Sun where a strong greenhouse effect
does not allow the planet to stay warm enough
to keep water from freezing.
Limiting factor determining this boundary: where CO2
condenses into CO2 rain or CO2 ice.
For a large planet with a thick atmosphere, might be ~1.7 AU.
For a smaller planet with a thinner atmosphere, might be ~1.4 AU
(just inside the orbit of Mars).
The HZ of the Solar System
Optimistic boundaries
Conservative boundaries
0.84 AU - 1.7 AU
0.95 AU - 1.4 AU
Venus 0.7 AU
Earth 1.0 AU
Mars 1.5 AU
The evolving HZ: as the Sun becomes brighter, the HZ moves
outward with time.
The Continuously Habitable Zone (CHZ)
Region of the solar system that has been habitable at all times
since the end of heavy bombardment.
.
optimistic
estimate
0.84 - 1.5 AU
Earth and Mars
conservative
estimate:
0.95-1.2 AU
Earth only
Habitability Outside the HZ
Possible liquid water oceans around Europa and Ganymede.
Could be subsurface liquid groundwater on Mars.
So, if you have internal heat sources, this expands and
complicates the definition of the HZ or the CHZ.
Could also have other liquids (methane, ethane).
The HZ is a generalization.
Star Types
Cooler stars:
smaller
burn slowly
have long lifetimes
have narrower habitable zones
Hotter stars:
larger
burn quickly
have short lifetimes
lifetimes may be too short to evolve life
Brown dwarfs:
not large enough to sustain fusion like a star
have no HZ - too cool to heat a planet
may have larger planets with moons that are tidally heated
What Stars Are “Good” For Life?
Spectral Type
% of Stars
Luminosity
Lifetime
O
0.001
1,000,000
500 thousand
B
.1
1,000
50 million
A
1
20
1 billion
F
2
7
2 billion
G
7
1
10 billion
K
15
0.3
20 billion
M
75
.003
600 billion
What Stars Are “Good” For Life?
Type O:
Very short lifetimes
accretion takes 10’s of millions of years
Type B:
Short lifetimes
long enough to form a planet, but Sun dies before heavy bombardment ends
Types A and F:
3% of stars
lifetimes 1-2 Ga
hotter than our Sun, HZ is further out
emit more uv light (breaks down and reacts with organic compounds)
Type G:
7% of stars
What Stars Are “Good” For Life?
Types K and M:
90% of stars
long lifetimes 20-600 Ga
dimmer stars
habitable zones much closer in
frequent bursts of intense light and radiation
K-type stars:
0.25 solar luminosity
HZ at 0.5 AU
M-type stars:
0.01 solar luminosity
HZ 0.1-0.2 AU (inside Mercury’s orbit)
size of the HZ is thinner
Habitable Zones Around Other Stars
Lecture 36. Galactic Habitable Zones.
reading: Chapter 10