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Life: Definitions(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 Life • Based on Carbon-Hydrogen bonds • Commonly include Oxygen and Nitrogen • Sometimes phosphorous • Silicon? • Water also appears to be important.
Early 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 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 acids • Amino acids are molecules from which proteins are built up • e.g. : as produced in the original Miller-Urey experiment • In a less primitive atmosphere, HCN is easily produced, and this can also produce (e.g.) Glycine
Extraterrestrial 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? • 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 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
Extremophiles • 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 life Modern • 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. Precambrian
Atmospheric 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
Requirements for (complex) life to start? • Presence of amino acids (seems to be fairly easy) • Liquid water? • Stability • Moderate temperatures? • Moderate pressures?
Habitable 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 Equation • A guess at the number of civilizations in our galaxy, with which we might hope to communicate 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 • fiis 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 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 landers • Tested soil for organic material and found it completely sterile
Allan 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 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) • 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
SETI • 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