Lecture 35. Habitable Zones.

reading: Chapters 9, 10

Goldilocks and the Solar System

Venus too hot now/not habitablegeologic age of the surface: ~500 Macould have been habitable in the past before runaway greenhouse

Earthliquid water for most or all of geologic historyhas always been habitablecarbonate-silicate cycle stabilizes the climate with its negative

feedback loop.

Marstoo cold for liquid water todaygeologic evidence of liquid water in the pastcould 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 wayto escape it.

Concept of the Habitable Zone (HZ)

Focuses on the presence of liquid water.

The HZ is the zone in which temperatures allowfor 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 andmagnetic 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 ofvolatiles.

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 gasesin 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 AUEarth 1.0 AUMars 1.5 AU

Distance from the Sun determines how much solar radiationthe 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 EarthVenus 0.95Earth 1Mars 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 theyaren’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 atmosphereb) low level of volcanism

Early Mars:thicker atmospherestronger 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 runawaygreenhouse

If we move Earth to 0.95 AU - Earth would have a moist greenhousewhere more water is entering the atmospherewhere it can be lost to space.

Outer Boundary of the HZ

Distance from the Sun where a strong greenhouse effectdoes not allow the planet to stay warm enoughto 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 0.84 AU - 1.7 AUConservative boundaries 0.95 AU - 1.4 AU

Venus 0.7 AUEarth 1.0 AUMars 1.5 AU

The evolving HZ: as the Sun becomes brighter, the HZ movesoutward with time.

The Continuously Habitable Zone (CHZ)

Region of the solar system that has been habitable at all timessince the end of heavy bombardment.

.optimisticestimate0.84 - 1.5 AUEarth and Mars

conservativeestimate:0.95-1.2 AUEarth 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 andcomplicates the definition of the HZ or the CHZ.

Could also have other liquids (methane, ethane).

The HZ is a generalization.

Star Types

Cooler stars: smallerburn slowlyhave long lifetimeshave narrower habitable zones

Hotter stars:largerburn quicklyhave short lifetimeslifetimes may be too short to evolve life

Brown dwarfs:not large enough to sustain fusion like a starhave no HZ - too cool to heat a planetmay 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 lifetimesaccretion takes 10’s of millions of years

Type B:Short lifetimeslong enough to form a planet, but Sun dies before heavy bombardment ends

Types A and F:3% of starslifetimes 1-2 Gahotter than our Sun, HZ is further outemit 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 starslong lifetimes 20-600 Gadimmer starshabitable zones much closer infrequent bursts of intense light and radiation

K-type stars:0.25 solar luminosityHZ at 0.5 AU

M-type stars:0.01 solar luminosityHZ 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