Life and Extremes
Tori Hoehler
NASA-Ames Research Center
Seeks to Understand
the Origins, Evolution,
Distribution,
and Destiny
of Life in the
Universe.
What is Life?
Requirements and Limitations
“Extremes”
Think Globally, Act Locally . . .
In Astrobiology:
Think as broadly as possible about our key questions
(or risk missing something important)
Be practical about where to look and what to look for
(that’s the only way to design discerning missions)
So what is life, anyway?
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)
Capable of Darwinian evolution (mutation
and natural selection)
So what is life, anyway?
Life According to Erwin Schrödinger (1944):
Does Something
Keeps on Doing
Something (Longer than
if it were Not Alive)
Doing anything
requiring speed or
specificity often
requires molecules
that are very complex
To keep doing that
something requires
plenty of energy
The Factory
Analogy for Life
(cells are little factories that
make more little factories)
To build a new factory, we require:
Raw Materials
A Blueprint
Energy & Work
Tools & Machinery
Life Requirements: Chemical Stuff, Energy,
Conditions Appropriate for Complex Molecules,
Solvent for Chemistry
Raw Materials
(remember, we are talking about
atomic materials)
A basic building block that
can be assembled into large,
complex backbones
Some interesting decorations
to hang along the chain
A way to connect the pieces
together
A lesson from Earth . . .
Phosphorus
& Nitrogen
(backbone &
decoration)
Carbon
(the backbone)
Sulfur, Oxygen
(interesting decorations)
Hydrogen
(mostly filler)
These building blocks are
connected together by
chemical bonds
Chemical bonds are made
from electrons, of which life
requires some source
To do something, molecules need to interact
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
Energy
Radiation
Chemical
Heat
Mechanical
(as visible light)
Bottom Line
Requirements for (our) Life
Source of Energy
Water
Source of Carbon
Nutrients
Source of Electrons
Microbiologists classify organisms
based on how they fulfill these needs
What conditions threaten
the integrity of big,
complex biomolecules?
Heat
Radiation
Strong Acid/Base
Harsh Chemicals
Any environment in which access to basic
requirements is sketchy, or in which
conditions threaten the stability of
biomolecules, could be considered extreme
Some extremes are absolute
(universal to life), some are relative
(specific to a particular kind of life)
How can we define the limits for 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
Genetic
Diversity
Metabolic Diversity
Macroscopic World
Aerobic (O2-based):
Microbial World
Light
Inorganic Chemicals
Organic Matter
Plants
Anaerobic:
Light
Inorganic Chemicals
Organic Matter
Animals
Microbial Mat
Tolerance of Extremes . . .
Hydrothermal Vent (T = 115 ºC)
(Photo: NOAA)
Halite-Saturated Ponds, SF Bay
(Photo: NASA)
Acid Drainage (pH -0.7), Iron Mountain, CA
(Photo: C. Alpers & D. Nordstrom, USGS)
Some extremes that we will see
High Temperature
Low Temperature
Highly Salty
High / Low pH
Desiccation / High Radiation
Chemical Toxicity
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?