Extraterrestrial lifeExtraterrestrial life

Life: DefinitionsLife: Definitions(from Michael’s four primary sources)(from Michael’s four primary sources)

•Google: the organic phenomenon that distinguishes living organisms from nonliving ones

•Wikipedia: “something” that exhibits: Organization Metabolism Growth Adaptation Reponse to stimuli Reproduction

•Hartmann: a series of chemical reactions using carbon-based molecules, by which matter is taken into a system and used to assist the system’s growth and reproduction, with waste products expelled.

•Woody Allen: full of loneliness, and misery, and suffering, and unhappiness – and it’s all over much too quickly.

The Chemistry of LifeThe Chemistry of Life

•Based on Carbon-Hydrogen bonds• Commonly include Oxygen and Nitrogen• Sometimes phosphorous

•Silicon?•Water also appears to be important.

Early EarthEarly Earth

•Solar nebula rich in C, H, O, N and H2O

•Earth forms, with hot interior Volcanic activity releases many gases, including water vapour.

•By 4 Gyr ago there were probably bodies of surface water Atmosphere rich in H, ammonia, methane, water, N2 and CO2.

Miller experimentMiller experiment

•In the 1950s, Miller and Urey conducted an experiment

Gaseous mix of H, ammonia, methane, H2O over a pool of liquid water

Passed electric sparks through the gas

After some time, amino acids appeared in the water

Amino acidsAmino acids

•Amino acids are molecules from which proteins are built up e.g. : as produced in the original Miller-Urey experiment

2252243 522 HNOHCOHCHNH

• In a less primitive atmosphere, HCN is easily produced, and this can also produce (e.g.) Glycine

22252223 HCNNOHCOHHCN

Extraterrestrial Amino acidsExtraterrestrial Amino acids

•Amino acids found in several carbonaceous chondrite meteorites

• The Murchison meteorite (September 28, 1969 over Murchison, Australia).

• High (12%) water content and more than 92 different amino acids Only nineteen of these are found on Earth.

The next step?The next step?

Proteinoids: simple, dry heating of amino acids can produce protein molecules. When water is added, they form a non-living structure very similar to bacteria.

Coacervates: cell-sized clusters, produced spontaneously when proteins are mixed in solution with other complex molecules

Earliest cellsEarliest cells

Prokaryote cells: the simplest and most primitive, they appeared around 3.6-3.7 Gyr ago. Contain DNA, and formed bacteria and blue-green algae (cyanobacteria)

•Include archaebacteria, which may have been among the first life forms to appear, and from which we are descended

May have been better suited to oxygen-poor environment

Can still be found in oxygen-poor, extreme environments

ExtremophilesExtremophiles

•High-temperature, smoker vents on the sea-floor

Mineral-rich columns of hot water are released from geothermal vents

A form of archaebacteria exist there, collecting biological materials from the vents, and not the Sun

•Life may have begun in geothermal environments rather than tidal pools

•Pompeii worm colony, near a hydrothermal vent

First signs of lifeFirst signs of life

Stromatolites: sedimentary growth structures, formed by cyanobacteria

•Some are 2.7-3.5 Gyr old, suggestive of early life

•Some forms of cyanobacteria began to produce oxygen and change the atmosphere.

Modern

Precambrian

Atmospheric developmentAtmospheric development

•Photosynthesis by cyanobacteria, and later by plants, caused the oxygen content to rise dramatically about 2.5 Gyr ago.

Rocks formed and buried more than 2.5 Gyr show signs of an oxygen-poor environment

•Sunlight dissociated O2 and allowed formation of ozone (O3).

This protected surface from UV rays which tends to break up complex molecules

BreakBreak

Requirements for (complex) life to Requirements for (complex) life to start?start?

•Presence of amino acids (seems to be fairly easy)•Liquid water?•Stability•Moderate temperatures?•Moderate pressures?

Habitable zoneHabitable zone

•Stars with 4000<T<7000 K Live long enough for complexity to evolve Emit some UV but not too much Cooler stars may suffice? Important because common.

•Habitable zone should be stable in time•Star content should be rich in heavy elements

Drake EquationDrake Equation

A guess at the number of civilizations in our galaxy, with which we might hope to communicate

LfffnfRN cilep *

where:• Reasonably well-known are:

R* is the rate of star formation in our galaxy fp is the fraction of those stars which have planets

ne is average number of planets which can potentially support life per star that has planets

• Wild guesses are: fl is the fraction of the above which actually go on to develop life

fi is the fraction of the above which actually go on to develop intelligent life

fc is the fraction of the above which are willing and able to communicate L is the expected lifetime of such a civilization

Solar SystemSolar System

•Mars: cannot rule out presence of microbes Possibly hidden at the base of the permafrost Tantalizing evidence for methane, though there could be non-

biological explanations

• Europa: may have liquid water beneath crust of young ice

• Titan: organic molecules are present Geothermal heat sources

Mars: Viking landersMars: Viking landers

•Tested soil for organic material and found it completely sterile

Allan Hills 84001Allan Hills 84001

•Martian rock, formed about 4500 Myr ago•Indications that liquid water percolated

through rock in the past

• Organic molecules (polycyclic aromatic hydrocarbons and amino acids) were found inside

• Peculiar, microbe-like structures found. Not clear if they are microbes Hard to rule out terrestrial contamination

Methane?Methane?

•Methane detected in Mars’ atmosphere (10 parts per billion) Has short half life (few hundred years), so there must be a

source Active volcanism? (but no sulfur dioxide makes this unlikely) UV-driven reactions involving CO2? Meteors? Or biological processes?

•Recent suggestions that the methane is correlated with underground water, strengthening the biological interpretation

Exogenesis (panspermia)Exogenesis (panspermia)

•Hypothesis that life began elswhere in the Universe and was transported to Earth (e.g. via comets)

Conversely, could life from Earth be transported to other worlds?

Calculations show sufficiently large impact on Earth could have propelled material to Titan, and that microbes might survive.

•Hoyle and Wickramasinghe: hypothesize that viral molecules are synthesized in space and transported by comets

•Extremophiles good candidates for making the journey•Could explain why life began so quickly after Earth formed,

following an era of heavy bombardment

SETISETI

•Listening for extraterrestrial radio signals•Where?

Focus on Sun-like stars About 1000 such stars within 100 light years

•At what radio frequency should we listen? 21 cm is an important frequency, because it is emitted by

neutral hydrogen Increases the chance of being detected “by accident”

•Why no contact? Nearest civilization may be too far away Desire to contact other worlds may not be common Synchronization of evolutionary clocks Requires several thousand times the Earth’s current power-

generating capacity to transmit a radio signal in all directions, out to 100 light years

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