Tori M. HoehlerNASA Ames Research Center

The Drake Equation: N = R* fp ne fl fi fc L

N = The number of communicative civilizations

R* = The rate of formation of suitable stars

fp = The fraction of those stars with planets.

ne = The number of Earth-like worlds per planetary system

fl = The fraction of Earth-like planets where life actually develops

fi = The fraction of life sites where intelligence develops

fc = The fraction of communicative planets (those on which

electromagnetic communications technology develops)

L = The "lifetime" of communicating civilizations

Once the origin of life occurs, how resilient is a biosphere to changes that occur over a planet’s lifetime?

Adaptability

Challenges

Our single example suggests that life can be resilient on time scales of at least 1/3 the age of the solar system

Any of the factors we identified as “extremes” could constitute a challenge to the

long-term stability of life

Harsh conditions for biomolecules(temperature extremes, radiation, pH, unsuitable chemistry)

Resource Limitation(energy, materials, solvent)

Stars Evolve – as they do, their temperature,

light emission, and even size change

Planets Evolve . . .

(for one thing, they start hot and cool off)

Just Right?

Too HotToo Cold

The Importance of Heat Flow

Heat flow → volcanism, crustal turn-over

Volcanoes Bring Mantle Chemistry to the Surface

A chemically differentiated planetis like a battery . . .

=

(but the battery is only tapped when volcanoes and vents operate)

Climate Fluctuates, Sometimes Dramatically

Mars Through Time?

Saltier

Ultimately No Light

Colder More Radiation?

More Acidic?

“Stuff” Happens

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 . . .

Temperature (°C)

Dep

th (

km)

2

0

200

1

1000

Geotherm

al Gradient

Surface-Sterilizing Impacts

(Sleep & Zahnle, 1998)

Habitable

Heat-Sterilized

Impact Heating

Life Alters its own Environment

Resource Recycling

Energy BudgetChemistry

Climate

Energy Balance(Used solar radiation to “charge up” the Earth’s chemical battery (by creating very oxidizing conditions at the Earth’s surface)

Oxygen Production(Toxic for some, great for others – shifted the “balance of power”)

Climate(Consumed CO2 and may have altered the production of other greenhouse gases (e.g., from methanogens, who are sensitive to O2) – this must have affected greenhouse warming and climate)

Radiation Budget(Produce ozone (from O2), which created a shield for UV – less radiation = clement conditions for a greater variety of organisms)

How can life survive (thrive!), in the face of all these potential challenges, on time scales comparable to the lifetime of a solar system?

At an individual level, versatility is important

Tolerance to Extremes

Metabolism

*These factors may sometimes be at odds

At the level of the whole biosphere, diversity is key

Technological Innovation (?)

Sufficient Rates of Evolution

Diversity of Niches, Into Which Organisms Can Evolve

(these have worked on Earth for 3+ billion years)

Questions?