Earth_27feb08 - Earth - Basics A Habitable Planet on which...

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1 Earth - Basics A Habitable Planet on which conditions have been sufficiently stable for sufficiently long(at least 3.8 billion years) for simple(e.g.bacterial) and then complex life to evolve – which is an incredible wonder. Distance from The SUN : 1 au from the sun (~150 million km). Radius : 6,378 km. Bulk density : 5.52 g/cm3 Rotation : ~24 hrs.; 23hrs. 56 mins. 4secs. Orbital period around sun : 1 year = 365.26 days. Intrinsic surface magnetic field : dipolar at centre) – 0.35 gauss; Reverses** every ~0.4 myr. Surface temperature : +15 o c. Surface Atmos. Pressure : 1 bar. Satellite(s): 1 moon.
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2 Habitable Zone A stellar habitable zone around any star is where life may develop and exist. Traditionally, this was tightly defined as the zone around any star where liquid water may exist at the surface, which for our solar system is just that restricted band between venus and mars which, of course, includes earth. Concept of habitable zones have now been significantly expanded for microbial organisms by two recent discoveries dating from the 1970’s – 90’s: (i) microbial life can live in extreme environments using chemical energy, not Sunlight e.g. hydrothermal vents 2-4km on the sea floor;~3.5 km. underground, discovered in a Deep gold mine in S. Africa. (ii) Satellites of planets further out in the solar system may have sub-surface oceans beneath ice Surfaces (e.g. Europa,a moon of Jupiter) or deep groundwater (e.g. Mars). Recently, it has even been suggested that Charon, companion of Pluto may contain liquid water in its sub-surface (Science (1999) v.286, p.71). If earth had only been ~5% closer to the sun could it would have suffered a run-away greenhouse, like Venus? Habitability for planets is a delicate balance between several factors.
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3 Salient Features About Earth The oldest preserved rocks are ~ 4.0 billion years. [Acasta Gneiss, N.W. Canada]. Life, and its extraordinary ~3.5 billion year record of evolution are preserved in sedimentary rocks as fossils. Earth’s atmosphere – structure and composition. Surface temperature -- +15 o c (ave.). Large amounts of water on surface (ice and liquid). Plate tectonics and its consequences (e.g., earthquakes, volcanism, mountain belts and recycling of crust) and its cause (convective flow of hot solid material in the earth’s mantle. An Fe-Ni metal core which is partially molten (liquid) and generates a dipolar magnetic field (which reverses polarity). The magnetic field generates a magnetosphere and the two aurora systems – borealis(n.) and australis(s.).
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4 Determination of Ages – Radiometric Dating How are the ages of the earth and its rocks determined so precisely? Mainly from highly accurate measurements of parent and product concentrations of isotopes linked by radioactive decay the decay Decay rate has been measured in the lab. Hence, from measurement of parent/product ratios time can be calculated. The decay rate is expressed by a half-life – defined as the time it takes for the
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Earth_27feb08 - Earth - Basics A Habitable Planet on which...

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