Part II: Building a Goldilocks WorldPart II: Building a Goldilocks World

From the Big Bang to Habitable PlanetsFrom the Big Bang to Habitable Planets

Outline:

Formation of the planets

Distribution (and redistribution) of volatiles

Heat production and transport

Radiation budget

The traditional habitable zoneBased on the approach in Jim Kasting’s “How to Find a

Habitable Planet”

Planet Formation

Chambers, 2001

Modeling Accretion

Condensation of Volatiles in the Circumstellar Disk

Jupiter Uranus

Distance from Sun (AU)

Saturn

Tem

pera

ture

(K

)Earth

20

2000

1000

151050

Silicates

Metals

Water Ice

Ammonia Ice

Condensation of Volatiles in the Circumstellar Disk

But not so much the presence of Earth’s oceans . . .

This model does a good job of explaining the distribution of rocky, gas giant, and ice

giant planets

Volatiles are redistributed from the outer to the inner solar system by asteroids and comets (which, recall,

carry more than just volatiles . . . )

Heat

Accretion and Impacts Deliver Energy(much of which becomes heat)

So does radioactive decay . . .

238U → 234Th + α(this is nuclear fission (= energy – remember?))

Consequences of Interior HeatingI. Differentiation

II. Liquid Core = Earth’s Dynamo (Magnetic Field)

III. Volcanism and Plate Tectonics(important for many reasons – we’ll discuss one now, one later)

Volcanic activity connects the terminals

=

A chemically and thermally differentiated planet is like a

battery . . .

Chemical Energy for LifeChemical Energy for Life

Bigger bodies (= higher SA/Vol) cool more slowly, and may have more active or longer lasting volcanism as a result

Nope.

Only about 0.025% of

surface heating comes from

geothermal heat flux.

The rest comes from…

Is this geological heating what

keeps the surface of our planet

warm (and our water in liquid

form)?

Solar Solar RadiatioRadiation Budgetn Budget

Solar Solar RadiatioRadiation Budgetn Budget

(mostly visible)

(mostly infrared)

(NASA Earth Observatory)

Radiation intercepted by planet goes as 1/d2

d

Radiation Budget

Absorbed (Visible) Energy =Radiated (Infrared) Energy

TooHot

TooCold

JustRight

Got Liquid Surface Water?(simple view)

Negative Feedback on Greenhouse Warming

The Carbonate Silicate Cycle

(courtesy Jim Kasting)

Constant source while volcanism is active

Enhanced by higher temperature, more CO2

Puts CO2 back into circulation

Enhanced by biology.

Would still happen without, but with higher CO2 levels

Ocean-atmosphere exchange required to

make this happen

Cautionary Tales for Worlds Aspiring to Habitability

Venus and the Runaway Greenhouse

Mars: The Case of the Missing Greenhouse

Effect

http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-15322/8702_page-2/

The Traditional (Liquid Water) Habitable Zone

Extras

year

century

million yr.

billion yr.

ten thousand yr.

100 millionmillion10,00010010.01

Hiroshima

Tunguska

K/T

TNT equivalent yield (MT)

Global catastrophe

Tsunami danger

(Credit: D. Morrison)

Terrestrial ImpactFrequency

“Armageddon” Impact(Texas-sized!)

“Catastrophic” depends on who you are and where you live . . .

“Catastrophic” depends on who you are and where you live . . .

Temperature (°C)

Dep

th (

km)

2

0

200

1

1000

Geotherm

al Gradient

Surface-Sterilizing Impacts

(Sleep & Zahnle, 1998)

Habitable

Heat-Sterilized

Impact Heating

Effects of Impacts on

Established Life:

Interplanetary Transfer of Life?