Bio 1229: Is there life on other planets?

Astrobiology  “Biology of the Stars”  Seeks to understand how to recognise life on other planets Not as easy as it might first appear!  A.k.a. “Exobiology”

The Miller-Urey experiment CH 4, NH 3, H 2 O, H 2 Electrical discharge 1953 Science 117:

What is ‘alive’? (NZ Education system) Movement Respiration Sensitivity Growth Reproduction Excretion Nutrition

What is ‘alive’? Schulze-Makuch & Irwin, 2006  Composed of bounded compartments in thermodynamic disequilibrium with the environment  Transforms energy to maintain a low-entropy state  Encoding and transmission of information for self-replication.

Everywhere we look on Earth, we find life…  Many of these habitats are not so different from the ‘extreme’ environments found elsewhere in the solar system. (Rock fissures 2.8 km deep, >60°C, 28 Ma old!)

Molecules, molecules  Macromolecules and polymers essential for life  On Earth: Carbon Common among the chemical species detected in space

Organic molecules as evidence of life?

Silicon-based life?  Silanes could have many of the properties of carbon macromolecules…  Silicon is not common in space (so far)…

Is there water out there?

Water – is it necessary?  “Life, as we know it, consists of chemical interactions that take place in the liquid state” Schulze-Makuch & Irwin

Properties of a good solvent for life…  Allows some molecular stability  But… enables chemical bonds to break  Dissolves many (but not all) solutes  Dense enough to maintain molecules near each other  Upper and lower limits to liquid state Funnels evolution…  Buffers against environmental fluctuations

Ammonia  Solid denser than liquid  No protection from UV  Reacts with oxygen (metabolism would have to be anaerobic)  Highly basic (pH ~11.6 for a 1.7% solution)  Replace C=O with N=O  Phosphate analogues?  Analogues to hydrolysis etc. mean that ‘proteins’ could be formed

Other solvents…  Organic solvents Protect against UV Possible need for a non-polar biochemistry  Silanes…

What is ‘alive’? Schulze-Makuch & Irwin, 2007  Composed of bounded compartments in thermodynamic disequilibrium with the environment  Transforms energy to maintain a low-entropy state  Encoding and transmission of information for self-replication.

Energy transformation on Earth…  Photosynthesis via chlorophyll Or…  Photosynthesis via Rhodopsin

Energy transformation on Earth…  Chemoautotrophs Hydrogen Sulfide CO 2 +O 2 +4H 2 S → CH 2 O+4S+3H 2 O 6CO 2 +H 2 O+3H 2 S → C 6 H 12 O 6 +3H 2 SO 4 Ammonia Iron (Fe 2+ )

Energy transformation… elsewhere…  Unusual electromagnetic wavelengths (e.g. UV, IR)  Osmotic gradients  Strong thermal gradients  Electromagnetic fields

What is ‘alive’? Schulze-Makuch & Irwin, 2007  Composed of bounded compartments in thermodynamic disequilibrium with the environment  Transforms energy to maintain a low-entropy state  Encoding and transmission of information for self-replication.

Transmission of information  On Earth:  DNA-RNA-Protein (DNAoffspring)  RNADNAmRNAProtein (RNAoffspring)

Transmission of information  RNA world RNA can store & transmit information also has enzyme-like activity  PNA world Peptide nucleic acids Very robust (but not found naturally on earth)  PAH world Polycyclic aromatic hydrocarbon world  All carbon-based… a Silane-nucleic acid world?

Where might there be life?  Earth-like planets Single star Not too hot or cold Not too big Abundant water, carbon 329 Extrasolar planets currently known

Venus  Earth’s ‘Evil Twin’ Runaway greenhouse effect Surface temperatures ~400 °C Highly acidic

Venus Dark streaks are from an ‘unknown UV- absorbent material’… a product of microbial activity?

Venus Life in the Atmosphere?  Large, stable clouds  In chemical disequilibrium COS, H 2 S, O 2, lower than expected CO C, P, N present  Non-spherical particles of comparable size to microbes  At 50 km °C ( K) pH ~0; ~100 kPa  High concentrations (100’s of ppm) in lower clouds

Mars  Water, O 2, CO 2  Past life?  Similar to current Dry Valleys…

Titan  Orbits Saturn  Liquid methane

Europa  Orbits Jupiter  100km layer of water, liquid below surface Thermal vents?  Active (fractures in surface) Heat generated by tidal energy?

Summary  Could there be life outside of Earth?  Would we recognise it?  Would it be bipedal, English- speaking and with funny forehead ridges?