Life and Extremes

Tori HoehlerNASA-Ames Research Center

Seeks to Understand the Origins, Evolution,

Distribution, and Destiny of Life in the Universe.

“Extremes”

Requirements and Limitations

What is Life?

In Astrobiology:

Think Globally, Act Locally . . .

Be practical about where to look and what to look for(that’s the only way to design discerning missions)

Think as broadly as possible about our key questions(or risk missing something important)

So what is life, anyway?So what is life, anyway?

Capable of Darwinian evolution (mutation and natural selection)

Some Commonly Cited Attributes:

Capable of reproducing itself

(Can carry out chemical reactions and synthesis)

(Can harness energy from the environment to drive these chemical processes)

So what is life, anyway?So what is life, anyway?Life According to Erwin Schrödinger (1944):

Does Something

Keeps on Doing Something (Longer than if it were Not Alive)

To keep doing that something requires plenty of energy

Doing anything requiring speed or specificity often requires molecules that are very complex

The Factory Analogy for Life

(cells are little factories that make more little factories)

To build a new factory, we require:

Raw Materials

Tools & MachineryEnergy & Work

A Blueprint

Energy,Life Requirements: Chemical Stuff,Conditions Appropriate for Complex Molecules, Solvent for Chemistry

Raw Materials

A way to connect the pieces together

A basic building block that can be assembled into large, complex backbones

Some interesting decorations to hang along the chain

(remember, we are talking about atomic materials)

A lesson from Earth . . .

Hydrogen(mostly filler)

Carbon(the backbone)

Sulfur, Oxygen(interesting decorations)

Phosphorus & Nitrogen

(backbone & decoration)

Chemical bonds are made from electrons, of which life requires some source

These building blocks are connected together by chemical bonds

This won’t happen (much) in the solid phase, because the molecules can’t come together across a significant distance. It could happen as a gas, but

complex molecules are so big that they usually break down before they vaporize. So the molecules

of life need to be dissolved in something.

For Earth life, water is the solvent

To do something, molecules need to interact

Energy

Chemical

Heat

Mechanical

(as visible light)Radiation

Bottom LineRequirements for (our) Life

Source of Carbon

Source of Electrons

Nutrients

Source of Energy Water

Microbiologists classify organisms based on how they fulfill these needs

What conditions threaten the integrity of big,

complex biomolecules?

Radiation

Strong Acid/Base

Harsh Chemicals

Heat

Any environment in which access to basic requirements is sketchy, or in which conditions threaten the stability of

biomolecules, could be considered extreme

How can we define the limits for life?

Some extremes are absolute (universal to life), some are relative (specific to a particular kind of life)

Use the only life we know – life on Earth – as a guide to understanding the prospects for, and how to seek, life elsewhere in

the universe.

How valid is the Earth-analog approach?

It’s the best we’ve got, so far . . .

Demands a focus on common traits, avoidance of highly specific circumstances

If we seek to broadly define life's capabilities and limits,

microbes are the place to look

GeneticDiversity

Aerobic (O2-based):

LightInorganic Chemicals

Organic Matter

Anaerobic:

LightInorganic Chemicals

Organic Matter

Microbial World

Microbial Mat

Macroscopic World

Plants

Animals

Metabolic Diversity

Tolerance of Extremes . . .

Acid Drainage (pH -0.7), Iron Mountain, CA(Photo: C. Alpers & D. Nordstrom, USGS)

Halite-Saturated Ponds, SF Bay(Photo: NASA)

Hydrothermal Vent (T = 115 ºC)(Photo: NOAA)

Some extremes that we will see

Low Temperature

Highly Salty

Chemical Toxicity

High Temperature

High / Low pH

Desiccation / High Radiation

Back to the Big Picture . . .

Understanding extremes on Earth, especially with the broad example of microbes, helps us to define “habitability”. In a theoretical sense, this tells us how common life could be. In a practical sense, it tells us where and how to

focus a search for life on other worlds.

Questions?