AST 251 Lecture 8 and 9 (Mar.5 and 12)

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Astronomy & Astrophysics

AST 251 Lecture 8Mars oppositionclosest approach of Mars to Earth happens every 2 yrs and 2 mthsMar 3 2012 o Viewed from Earth Mars opposite Sun tho Mars currently 4 brightest object in sky March 2012after Venus Jupiter and Sirius o Raising around sunset setting around sunrise o Mars orbit eccentricminimum distance in oppositions varies from 54 and 100M km and planet shows retrograde motion o Great oppositions 60M km happen every 15 oppositions eg 2003 Microbial life formed quickly and can exist under a wide variety of circumstances extremophilesComplex life took long to develop and may have depended on many possibly fortuitous circumstanceso CO cycle to regulate temperature 2o Plate tectonics o Generation of O 2o Mass extinctions to open new evolutionary nichesSnowball Earth preceding Cambrian explosionKT event precedes raise of mammalsSun Earth planets formed from same matteralready enriched in heavy elements about 2 in the total mass of the presolar nebula o Old stars have lower heavyelement contentperhaps too low for life thereHeavyelement content might affect planet formation rdtho Most important elements for terrestrial lifeO C N and Hthey are the 3 4 thst6 and 1 most abundant in the universe Of the 92 stable elements known 25 are used by terrestrial lifeSame proportions of elements in the Solar System except for local effects o Condensation in protosolar nebula depends on distance to the Sun rocks metals close to the Sun ices and carbon far from Sun o Loss of hydrogen and helium depends on gravity of bodyelements always gases escapes when gravity is weak o Special role of carbon and waterappear to be crucial for lifeAmounts of CO and HO are just right for terrestrial life 22 Most likely that life on Earth evolved toward perfectly matching C and HO contents 2 Energy from the central star is essential for planets supporting life o Amount of life changes with the inverse square of the distanceEg at 2 AU the amount isof that for Earth o Utilizing chemical energy requires continuous mixing either in an atmosphere or inside a globecan be forced by the unequal illumination rotation plate tectonicsLiquids for life o Substance must be common and must remain liquid over a wide range of temperatures o A wide temperature range for the liquid state permits chemical reactions at different rates at different temperatures o Higher temperatures permit faster chemical reactionsPhase diagrams give properties as temperature and pressure change o Horizontal cut corresponds to a constant pressure and changing temperature o A vertical cut gives changes with the constant temperature and changing pressureWater is liquid at higher temperatures than other possible liquids higher chemical rates remains liquid over a wider temperature rangeWater molecule has slightly negatively and positively charged ends a polar molecule permitting hydrogen bonds to occur o Each molecule can bond to up to 4 other molecules number of bonds decreases at higher temperatures o Under pressure water freezes at T0 CMost substances show an increase of the freezing point temperatureso Water has 14 known solid phases ices o Water ice floatsone of the few substances which expand as they freezeo Ice forms on top of oceans and lakesinsulating the liquid water below o If ice sank lakes and oceans would freeze much more easilyfrom bottom upMore properties of WaterAdvantages Disadvantages Water is an excellent solvent for salts Snow and ice are whitecauses possibly devastating positive feedback snowball Earths Water has unsurpassed ability to store heat Liquid water does not occur in the Universeprotects against swings in temperaturetypical pressures much too low Requirements for habitability o A source of molecules to build living cellscarbonbased organic moleculesFulfilled everywhere certainly in the Solar System everywhere o A source of energy to fuel metabolismneeds disequilibriumIs more constraining on the distance from the central star ok for Marsbut will need other energy sources for Jupiter and beyond o A liquid medium for transporting and exchanging moleculesat least occasionally liquid
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