AST101- Chapter 24.docx

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

Life in the Universe [Chapter 24] 24.1 Life on Earth  3 developments in the study of life on earth have made it seem more likely that it might exist elsewhere o Light might also have formed quickly in other worlds with the right conditions (just like we formed early in earths history) o Lab experiments suggest life might have arisen through naturally occurring chemistry- in which case the same chemistry could have given rise to life on many other worlds o Microscopic living organisms that can survive in conditions similar to those on at least some other worlds in our solar system suggest that the necessities of life may be common in the universe When did life arise on earth?  Ancestors of modern life probably could not have arisen much earlier than the end of the heavy bombardment  Last major impacts occurred 4.2 and 3.9 billion years ago The geological time scale  We learn about history of life through fossils- relics of organisms that lived and died long ago o most form when dead organisms fall to bottom of a sea and are gradually buried by layers of sediment which are produced by erosion on land and carried by rivers to the sea o Over millions of years sediments pile up on the seafloor and the weight of the upper layers compresses underlying layers into rock o Erosion or tectonic activity can later expose the fossils  Radiometric dating confirms the relative ages (deeper layers formed earlier and contain more ancient fossils) Fossil evidence for the early origin of life  Harder to detect earlier life because older rocks are much rare than younger rocks because most of earths surface is geologically young  When old rocks are found they have been subject to transformations that would of destroyed fossil evidence they may have contained  All life prior to few hundred million years ago was microscopic and those fossils are hard to detect  Life was already thriving 3.5 billion years ago and evidence comes form rocks called stromatolites: rocks show structure nearly identical to that found in large bacterial mats today and indicates that they must have been living organisms o More primitive organisms must have lived even earlier  Carbon isotope evidence suggests that life was present more than 3.85 billion years ago  Earth arose within no more than few hundred million years How did life arise on earth? The theory of evolution  Overproduction and struggle for survival  Individual variation  The inescapable conclusion: unequal reproductive success o This unequal reproductive success is called natural selection The mechanism of evolution  Strongest support for theory is DNA  Evolution occurs because the passing of genetic info from one generation to the next is not always perfect  Mutations are changes in an organisms DNA (copying errors or by external influences)  Study evolutionary relationships between organisms by comparing their DNA The first living organisms  All life on earth can trace its origins to a common ancestor that lived before 3.85 billion years ago  The tree of life shows that life on earth is divided into 3 major groupings (bacteria, archaea, eukarya)  Organisms on branches located closer to the “root” of the tree must contain DNA that is evolutionarily older, suggesting they more resemble the organisms that lived early in earths history  Modern day organisms appear to be evolutionarily oldest  Early life might of attained energy through chemical pathways in extreme conditions The transition from chemistry and biology  Theory of evolution explains how the earliest organisms evolved into the great diversity of life on earth today  Life could of arisen through natural chemical reactions o Chemical reactions produced all the major molecules of life including amino acids and DNA bases o Many of these molecules are found in meteorites, so some organic molecules may have arrived from space  Given millions of years of chemical reactions occurring all over the earth, RNA might eventually have evolved into DNA, making true living organisms ** Fig 24.11 Could life have migrated to earth?  Possibility that life arose somewhere else first an then came to us through meteorites  Idea that life can travel through space to land on earth- panspermia  Presence of organic molecules in meteorites and comets tell us that life could of survived in space  Life could of originated on either earth, venus, or mars A brief history of life on earth  Earth had oceans 4.3-4.4 billion years ago so early ocean could have led to life  Once heavy bombardment ended, life took over and once it did – evolution rapidly diversified it  Things became to change when oxygen starts building up in the atmosphere o Photosynthesis started as early as 3.5 billion years ago o But O2 did not immediately begin to accumulate because chemical reactions with surface rocks pulled O2 back out of the atmosphere as fast as the cyanobacteria could produce; eventually the surface rock was so saturated by O2 that the rate of O2 removal slowed down (2 billion years ago)  Cambrian period: animal life evolved form tiny and primitive organisms into all the basic body plans that we find on earth today  Earliest humans appeared only a few years ago, after 99.9% of earths history to date has gone by What are the necessities of Life?  We need abundant oxygen in an atmosphere that is otherwise not poisonous, temperature sin a fairly narrow range of conditions and need and abundant/varied food sources o Many microbes can survive in extreme conditions (near hot springs, acidic environments, places with high radiation and even inside rocks)  Life as a whole only has three basic requirements o A source of nutrients(elements and molecules) from which to build living cells o Energy to fuel the activities of life, whether from sunlight, from chemical reactions, or from the heat of Earth itself o Liquid water rd  So far only the 3 criteria is seen to be a constraint o Many worlds are large enough to retain internal heat that could provide energy for life and virtually all worlds have sunlight bathing their surfaces (even though it is less on planets that are further from the Sun), so nutrients/energy should be available to at least some degree on almost every planet and moon 24.2 Life in the Solar System Search for life begins with habitable worlds, worlds that contain the basic necessities for life as we know it, there is a demand for water –only 2 possibilities in our solar system Could there be life on Mars?  Images have shown that no civilizations have ever existed there  Good reason to believe that water believed that liquid water flowed on Mars in the past making it seem possible that life could once have found a home there  Contains subsurface ice, life might still survive near source of volcanic heat, where pockets of liquid water might persist underground Missions to Mars  Only place where we’ve begun an actual search for life o Viking landers have scooped up soil samples which were fed into several on- board robotically controlled experiments o All 3 experiments gave results that initially seemed consistent with life, further study suggested that chemical reactions could have produced the same result  Conclusion is that no life has been present where the Vikings took their sample (4 experiment showed to organic molecules)  Phoenix mission is designed to assess the habitability of the Martian soil  Methane has been seen in the atmosphere, must have a source because it can’t last this long without being transformed into another gas, the source may be biological  Within a decade NASA hopes to launch a mission to Mars that will bring back surface samples; later may send humans to search for living organisms in deep canyons The Debate over Martian Meteorites  Meteorite was found in Antarctic circle, the rock itself dates to 4.5 billion years ago indicating that it solidified shortly after Mars formed o Contains layered carbonate materials and complex organic molecules that are associated with life when found in Earth rock o Also has rod-shaped structures that look like nanobacteria found on Earth  All hints of Martian life can be explained in a non-biological way o Chemical/biological processes can produce structures very similar to those found in the Martian meteorite o Terrestrial bacteria have been found living in the meteorite, indicating that it was contaminated by Earth life  Most scientists now doubt that the Martian meteorite shows evidence of life Could there be life on Europa or other Jovian moons?  After mars the most likely candidates for life our the moons of the Jovian planets  Europa is the strongest candidate because we think it contains a deep ocean beneath its icy crust (ice/rock from which Europa formed undoubtedly included the necessary chemical ingredients for life and Europa’s internal heating is strong enough to power volcanic vents on the sea bottom (areas like where black smokers are on Earth—which have life) o Covered ocean could be hiding large creature swimming within it but the potential energy sources are much more limited than they are on Earth (sunlight could not fuel photosynthesis in the subsurface oceans)  Therefore any life would have to be small and primitive  Ganymede/Callisto have even less energy so organisms would have to be small and primitive  Titan’s oceans of methane/ethane could have life but it is unlikely o Might also have underground oceans of water or cold ammonia/water mixture  Ice fountains on Enceladus could be evidence of life if powered by subsurface liquid water or ammonia/water mixture 24.3 Life Around Other Stars Surface life vs. subsurface life; No foreseeable technology will allow us to find life that is hidden deep underground in other solar system Are Habitable planets likely?  Have not yet found any extrasolar planets that seem to be habitable(may be restriction of our detection techniques)  Existence of Jovian like planets makes it quite reasonable to suppose that terrestrial planets are also common Constraints on Star Systems  1 requirement for a star to have life-bearing worlds is that it be old enough that life could have arisen o We can rule out any stars with a mass few times more than that of our Sun (stars of great mass live shorter lives) nd  2 requirement is that the star allow planets to have stable orbits o We have found a lot of stars in binary/multiple star systems, if we find these types of systems to not be habitable than we can rule out half the stars in our galaxy  3 requirement is the size of a star’s habitable zone (the region in which a terrestrial planet of the right size could have a surface temperature that might allow for liquid water and life) FIG 24.15 o Becomes smaller and closer-in for stars of lower mass and luminosity  All in all it seems that the vast majority of stars are at least potentially capable of having life-bearing planets Finding Habitable Planets  Kepler mission will look for transit of Earth-size planets in front of their stars and will measure orbital properties to tell us whether these planets lie within their star’s habitable zone  We will need images or spectra to determine whethe
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