Ast 101 Chapter 24.docx

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Astronomy & Astrophysics
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Ian Shelton

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Chapter 24: Life in the Universe 24.1 Life on Earth When did life arise on earth? The Geological Time Scale - We learn about the history of life on earth through study of fossils, relics of organisms that lived and died long ago - Formation of sedimentary rock. Each layer represents a particular time and place in earth’s history and is characterized by fossils of organisms that lived in that time and place o Rives carry sediment to the oceans. sedimentary rocks containing fossils form on the ocean floor o Over time, more layers are added, containing fossils from each time period o Tectonic stresses level changes push the seafloor upward, exposing sedimentary rocks. Erosion by rives reveals layers, deeper layers contain older fossils Fossil Evidence for the Early Origin of Life - Increasingly difficult to find fossils as we look deeper into earth’s history, for 3 major reasons o First, older rocks are much rarer than younger rocks because most of earth’s surface is geologically young o Second, even when we find very old rocks they often turn out to have subject to transformations (caused by heat and pressure) that would have destroyed any fossil evidence they may have contained o Third, all life prior to a few hundred million years ago was microscopic and microscopic fossils are difficult to identify - Life on earth 3.5 billion years ago o Evidence comes from rocks called stromatolites which look strikingly similar to large bacterial mats found in some locations today o Carbon isotope evidence suggests that life was present more than 3.85 billion years ago How did life arise on Earth? The Theory of Evolution - Evolution simply means “change with time” o Fact 1: overproduction and struggle for survival. Any localized population of a species has the potential to produce far more offspring than the local environment can support with resources such as food and shelter. His overproduction leads to a competition for survival among the individuals of the population o Fact 2: individual variation. Individuals in a population of any species vary in many heritable traits (traits passed from parents to offspring). No two individuals are exactly alike and some individuals possess traits that make them better able to compete for food and other vital resources o the inescapable conclusion: unequal reproductive success. In the struggle for survival, those individuals whose traits best enable them to survive and reproduce will, on average, leave the largest number of offspring that in turn survive to reproduce. Therefore, in any local environment, heritable traits that enhance survival and successful reproduction will become progressively more common un succeeding generations - unequal reproductive success that Darwin called natural selection: over time, advantageous genetic traits will naturally win out (be “selected”) over less advantageous traits because they are more likely to be passed down through many generations. This process explains how species can change in response to their environment –by favoring traits that improve adaptation and it is the primary mechanisms of evolution The Mechanism of Evolution - a piece of DNA molecule, which looks like a zipper twisted into a spiral. Hereditary information is contained in the teeth linking the strands. These teeth are the DNA bases. Only four DNA bases are used and they can link together only in specific ways: o T attaches only to A o C attaches only to G - Any change in an organism’s DNA is called a mutation The First Living Organisms - Biologist can determine the evolutionary relationships among living species by comparing the sequences of bases in their DNA - Life on earth is divided into 3 major groupings or domains, called Bacteria, Archae and Eukarya, and that all three domains share a common ancestor - Organisms located close to the root of the tree must contain DNA that is evolutionarily older, suggesting that they more closely resemble the organisms that lives early in earth’s history - DNA evidence suggests that the ancestor of all life on earth resembled organisms that live today in hot water near volcanic vents or hot springs The Transition from Chemistry to Biology - A summary of the steps by which chemistry on the early Earth may have led to the origin of life o 1. Organic precursor molecules appear o 2. RNA molecules become self-replicating o 3. Membrane enclosed pre cells arise o 4. True cells with RNA genome appear o 5. Modern cells with DNA genome evolve Could Life Have Migrated to Earth? - The idea that life could travel through space to land on earth, sometimes called panspermia, once seemed outlandish - However, the presence of organic molecules in meteorites and once tells us that the building blocks of life can survive in space and tests have shown that some microbes can survive in space for years A Brief History of Life on Earth - Nearly all the oxygen in our atmosphere was originally released through photosynthesis by single celled organism known as cyanobacteria - Oxygen was probably poisonous to most organisms living before about 2 billion years ago and remains a poison to many microbes still living today What are necessities of Life? - The discovery of life in “extreme” environments –such as in the hot water near black smokers and hot springs –shows that any microbes (often caked extremophiles) can survive in a much wider range of condition - Life on earth, we find that life as a whole has only three basic requirements o 1. A source of nutrients (elements and molecules) o 2. Energy, whether rom sunlight, from chemical reactions or from the heat of earth itself o 3. Liquid water 24.2 Life in the Solar System - Habitable worlds –worlds that contain the basic necessities for life as we know it, including liquid water - Mars and a few of the large moon orbiting jovian planets, most notably Europa, are 2 major possibilities besides earth for habitability in our solar system Could there by life on Mars? - No civilization have ever existed - Percival Lowell and his discovery of canals was a mirage of real Martian features and his own imagination - Liquid water flowed once in mars and mars today has a subsurface of ice which is conceivable that life could still survive near sources of volcanic heat where pocket of liquid water persists underground. - Mars mission- two Viking Landers 1976 (looking for signs of life) o All three experiments gave results initially seemed consistent with life o Fourth experiment analyzed the content of the soil and found there is no measureable organic molecules - the opposite of what we would expect if life were present - Methane on Mars o Discovery of methane CH4in the Martian atmosphere o Methane is quickly destroyed by ultraviolet light from the sun so this means it is continually released from the surface o Methane varies across season and varies regionally o Is methane being produced by life? It’s possible that methane could also be produced by nonbiological processes ex. Volcanoes release methane so a low level of volcanic activity could be responsible for the methane on mars (however scientist have not detected other gases if it is volcanic activity producing methane) or chemical reactions taking place beneath the surface - The Debate over Martian Meteorites o A different approach to search for life involves on studies of meteorites whose chemical composition suggests they came from mars o Meteorite designated ALH84001 thought to contain the evidence of past life on Mars.  Found on the Antarctic ice in 1984 13000 years ago, following a 16 million journey through space after an impact blasted it from mars.  The rock itself dates to 4.5 billion years ago which means that it solidified shortly after mars formed  The rock contained layered carbonate minerals and complex organic molecules (polycyclic aromatic hydrocarbons PAHs) that are associated with life when found in earth rock as well as microscopic chains of magnetic crystals quite similar to chains made in earth rocks by living bacteria o Chemical and geological processes can produce structures very similar to those found in the Martian meteorite o Terrestrial bacteria have been found living inside the meteorite indicating that it was contaminated by earth life during the 13000 years it resided in Antarctic o Most scientist now doubt that the Martian meteorite shows true evidence of Martian life Could there be life on Europa or other jovian moons? - After mars the next candidate for life are some of the jovian planets moons - Europa, Ganymede, Callisto, Titan, and Enceladus - Europa is the strongest candidate because it contains a deep ocean beneath its icy crust and its internal heating (due to tidal heating) is strong enough to power volcanic vents on the sea bottom. o The ocean floor look much like black smokers on Earth o The possibility of life on Europa would not necessarily have to be microscopic, unlike mars o However the potential energy source for life on Europa is limited mainly because sunlight could not fuel photosynthesis in the subsurface ocean o Thus scientists believe that any life on Europa would be quite small and primitive - Jupiter’s moons Ganymede and Callisto may also have subsurface oceans however, they have less energy than Europa o Life would be small and primitive - Titan’s surface is too cold for liquid water but it has lakes and rivers of liquid methane and ethane ? o The liquid could support life as water does on earth o Might have liquid water or a colder ammonia/ water mixture deep underground - On Enceladus they discovered iced fountains are powered by subsurface liquid water or an ammonia/water mixture 24.3 Life around Other Stars Where might we find habitable planets? - Nearly all the extrasolar planets found to date are much more massive than the earth; they are more like the jovian planets than terrestrial - Such planets are unlikely candidates for life though of them could conceivable allow the
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