# Presentation: 180491

## Astrobiology and The Origin of Life on Earth- A Chemical Perspective -  

- Christopher Glein
- 2005 NASA Glenn Academy
- Mentor: Al Hepp
- Photovoltaics and Space Environments Branch

## Introduction 

- Asks three fundamental questions
- 	1. How does life begin and evolve?
- 	2. Does life exist elsewhere in the universe?
- 	3. What is the future of life on Earth and beyond?

- Astrobiology - the study of the origins, evolution, distribution, and future of life in the universe.

## Astrobiology is Not Science Fiction

- But a new science combining multiple disciplines

## Project Goals

- Investigate possible life environments in the solar system
- Apply thermodynamics to planetary and biological systems to determine life potential
- Search for the beginnings of metabolism
- Establish the connection between metal sulfides and the origin of life on Earth
- Consider Astrobiology on Titan

## What is life? 

- _Definition _
- A chemical system that
- undergoes Darwinian
- evolution and degrades
- high-quality energy from
- its environment in
- metabolism
- _Common Assumptions_
- Requires:
-  Free energy
-  Organic molecules
- Liquid water (maybe!?)

## The First Living Entity

- The most popular hypothesis is that the first life form was a ribozyme – a catalytic RNA molecule
- It can mimic with limited functionality both DNA and proteins

## The Origin of Life 

- RNA molecules are complex molecules (contain 1000’s of atoms)
- Process: Build from simpler precursor molecules that gradually self-assembled
- Question: What molecules were present and how did synthesis occur?
- Consider two approaches
- 	- Thermodynamics
- 	- Experiments

## Thermodynamics

- The study of heat and energy and their relationship to material properties
- Define system of interest using state variables (pressure, temperature, composition)
- Quantitatively describe equilibrium directionality

**→**

**→**

**→**

**→**

- Systems

## The Solar System

- Where is life?

## Extreme Earth

- Terrestrial life thrives in many extreme conditions
- 	- temperature (235.4  ̊F),
- 	- pressure (1300 bar)
- 	- radiation (1000 J m-2)
- 	- desiccation (= dry)
- 	- salinity (5 M NaCl)
- 	- pH (0-10.5)
- 	- oxygen abundance
- 	- heavy metals (lead)
- 	- organic solvents (benzene)

## Venus

- Hothouse world
- Dense CO2 atmosphere → Greenhouse effect
- Surface temperature ~735 K, Surface pressure ~92.1 bar
- Active volcanism on surface
- Clouds might have life (bacteria?)
- Clouds → T ~350 K,    P ~1 bar

- A disequilibrium atmosphere
- But, acid cloud droplets,    pH ~0

## Mars

- Frigid, desert-like environment
- Thin atmosphere: CO2, N2, trace gases
- Surface temperature ~214 K, Surface pressure ~6.36 mbar
- Liquid water not stable at surface
- Lots of big salt deposits
- Maybe hydrothermal life below ground in salty water

## Europa

- A very cold place (T~103 K)
- Hardly has an atmosphere
- Icy crust ~20 km thick covers a salty, basic ocean (ionic strength ~0.3, SW~0.7; pH ~10, SW~8.1)
- Ocean depth ~100 km (Mariana trench ~11 km)
- Seafloor T~260 K, P~1.6 kbar
- Hydrothermal vents could support life

## Titan

- Icebox world
- Thick, mildly reducing atmosphere; N2, CH4, H2, organics
- Surface temperature ~94 K, Surface pressure ~1.5 bar
- Rivers and lakes of liquid methane?
- Subsurface ocean?
- Good planetary lab for assessing prebiotic chemistry

## Other Possible Homes of Life 

- Enceladus

- Triton

- Mercury

- Io

- Ganymede

## A More Quantitative Look at Planetary scale Thermodynamics 

| Planetary Body |  Chemical Potential Energy, ΔG  (J mol-1) |
| --- | --- |
| Venus | 1 |
| Earth  | 11,000 |
| Mars | 105 |
| Jupiter | 0.4 |
| Titan | 2 |

- ΔG = ΔG0 + RT lnQ

- Disequilibrium given by Gibbs free energy
- Life creates disequilibrium
- Photosynthesis
-     CO2 + H2O  CH2O + O2
- Methanogenesis
-     CO2 + 4H2  CH4 + 2H2O
- Combustion
-     CH4 + 2O2  CO2 + 2H2O

## Entropy and Life

- microbe – system of interest
- interior – irreversible reactions in cell
- Q – heat transfer
- matter – matter transfer
- dS(microbe) = dS(from exterior exchange) + dS(interior)
- dS(from exterior exchange) = dQ/T + dS(matter)
- dS(microbe) = dQ/T + dS(matter) + dS(interior)
- dS(microbe) = 0 (assume at steady state)
- -dS(interior) = dQ/T + dS(matter)

## Entropy and Life

- 2nd Law: The total entropy of the universe increases in all processes that happen
- Exterior exchange helps cells stay in a state of low entropy at the expense of its surroundings.
- Life uses free energy from food to produce entropy and gets rid of it via heat (physical entropy) and metabolic byproducts (chemical entropy)

## Planetary Entropy

## Planetary Entropy

- Planets convert low entropy sunlight to high entropy heat
- Jupiter’s atmosphere is in a state of high chemical entropy and also has stable weather patterns
- What’s the connection between physical entropy, chemical entropy, and life? Nobody knows...

## The Prebiotic World

- Iron-sulfur world hypothesis
- Life arose from reduced fluids reacting with metal sulfides
- Pyrite formation produces energy, ΔG0 = -38.4 kJ mol-1
- 	FeS + H2S → FeS2 + H2
- Use this energy for metabolism (make complex molecules)

## Protometablism

- Citric Acid Cycle is everywhere in nature
- Modern metabolism came from an older, simpler version
- Sulfur was used a lot
- Metal sulfides catalyze the reactions
- Mineral structure looks like enzymes (iron-sulfur clusters)

## Literature Work in Metabolism

- Prebiotic catalysts used in primitive bacteria and archea: Iron, Nickel, Tungsten Complexes
- Important metabolic reactions
-     Sulfur metabolism, Methanogensis
- Target intermediates in metabolism
-     Acetate, Pyruvate

## Astrobiology on Titan

- Current area of study with arrival of Cassini
- Liquid methane is Titan’s version of water
- Erosion and weathering (i.e. rain, rivers, lakes)
- What kind of chemistry is going on down there???

## Atmosphere and Surface

- Atmospheric reactions:
- CH4 + N2 → C-N organics
- Surface is dynamic = cryovolcanism
- Mix C-N organics with cryolava, what happens?
- Study Titan to learn more about prebiotic chemistry and the origin of life

## Don’t Get Scared, but....What if there’s life on Titan!?

- Free energy
-     C2H2 + H2 → 2CH4
-     Reaction gives 334 kJ/mol, Life needs ~42 kJ/mol
- Organic molecules
-      C-N organics falling from the sky
- Solvent for life, uh,         ?

## Liquid Methane as a Solvent of Life

- Can a cold, nonpolar solvent work for life?
- Low Temperatures and Concentrations → slow reactions
- Slow reactions not good for life
- Possibly fix it using enzymes
-      - catalysis
-      - molecular cookie cutting

## Origin of Life in Liquid Methane

## Origin of Life in Liquid Methane (cont.)

- Geothermal heat dies down
- Water that life lives in slowly freezes
- If organisms don’t migrate to liquid methane, they freeze to death
- Strong evolutionary pressure to evolve adaptations
- Might be biochemically impossible

- Sorry, but no animals on Titan

## Acknowledgements 

- Dr. Al Hepp
- Dr. Doug Ogrin
- Photovoltaics and Space Environments Branch
- 2005 NASA Glenn Academy
- Dr. M. David Kankam
- Office of University Programs
- Professor David Catling
- Dr. Christopher McKay
- Washington NASA Space Grant
- Thank You!

## Questions