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earth sci chap 10.doc

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Western University
Earth Sciences
Earth Sciences 1086F/G

Chapter 10: VENUS - Called the evening/morning star - Not much can be seen from telescope cause looking @ top layer of cloud - About same size as earth (0.95 percent the diameter of earth) - And very similar density - Can peer thru clouds using radar - 23 spacecrafts have flown past - Best data came from Magellan - Venus Express = European SpaceAgency, launched in 2005 and is collecting data - Diameter = 12,104 km - Density = 5.24 g/cc - About 108 million km/ 0.72AU from the sun - Orbit about sun takes 225 days - Revolves around sun in a slightly elliptical orbit that is the most circular of any planet - Orbital path of venus = almost same plan as earth, differs just under 3 and half degrees - Rotates in a clockwise/retrograde motion very slowly, one complete rotation in every 243 earth days - Sun rises in the west and sets in the east - Venusian day = 117 earth days - Clouds rotate every 4 days - Covered in thick blanket of clouds (=droplets of liquid/solid sulfur and droplets of sulphuric acid) - Atmosphere composed of carbon dioxide (96.5), nitrogen (3.5), water vapour (0.1-0.4) sulfur dioxide (0.01) and free oxygen (60ppm) - Venus once had abundant water but has since lost it - Atmosphere pressure at surface 90 times greater than on earth - Large volumes of venus’atmosphere escaping into space: stream of electrons from sun collides with particles and upper atmosphere and energizes them and blasts them into space - Carbon dioxide is pretty transparent to in-coming light energy, but fairly opaque to out-going infrared energy - Any greenhouse effect begins with incoming short wavelength light energy from sun warming a planet’s surface o But ends up with a surface or near-surface growth of heat because of the inability of the long wavelength infrared energy to escape a CO2 rich atmosphere o Wavelengths of radiated energy that CO2 does allow to escape are blocked by water and sulfur dioxide, although not alot in the atmosphere of venus, enough to strengthen the greenhouse effect very substantially - Surface temp of 462 degrees C (hotter than mercury) - Likely CO2 can from volcanic emissions like earth - When Ultra-violet radiation strikes a h2o breaks it up into H and OH - Two stable isotopes of H: normal H (mass # = 1), heavy H (deuterium, mass # =2) - Gravity of both earth + venus is such that hydrogen atoms can escape, lighter ones escape more readily - Probe discovered about 150 times more deuterium per atom of light hydrogen in the venus atmosphere than in earths - Also detected OH, only other planet so far to have that in the atmosphere other than earth - Used to have enough water to make a planet-wide ocean up to 25 m deep - w/o an ozone layer to protect venusian atmosphere from uv radiation, atmospheric water was broken up and much of the H was lost to space - as surface of venus warmed, any oceans that did exist evaporated and venus lost ability to cleanse its atmosphere of CO2 o high temp baked even more CO2 out of the surface and the atmosphere became even less transparent to infrared radiation o consequently temp rose further o surface so hot that sulfur, chlorine, and fluorine have baked out of the rock and formed vapours o runaway greenhouse ! o earth avoided this because further from sun and cooler (thus it could form and preserve liquid-water oceans to absorb the carbon dioxide, which left nitrogen atmosphere that was relatively transparent in some parts of the infrared wavelength range) o if all the CO2 in earths rocks was put back into the atmosphere our air would be as dense as venus and we would have severe greenhouse effect - lower atmosphere of venus is clear - winds at surface are relatively gentle (13km/h) - Faster winds exist higher up - Atmospheric circulation is organized as a single planet-wide wind pattern BUT not so at the poles - GEOLOGY - Surface relative to the Moon, mars, or mercury, has relatively few impact craters and very few small impact craters (all medium to huge) - Result of atmosphere, small objects just burn up through friction with it - Venus has fewer craters of any size relative to Moon, mercury, and mars and those we see appear to be young (rarely filled with lava, indicating they formed after a recent major volcanic activity) o Also rough, suggesting they’ve not had time to erode much - Pattern of craters is random - Surface of venus is young (surface = 500 million years old) - Planet is about the same age as earth 4.5 billion years old - Has to have been some sort of global resurfacing about 500 million years ago (planet-wide eruptions) - Surface hugely variable – from clusters of very small volcanic domes sitting on a fairly level lava field, immense volcanoes to immense blocky ridges and valleys bounded by faults - Has a huge dome surrounded by concentric fractures, called coronae o Thought to be formed by mantle plumes (remember term- go back in notes ch7) that bring magma right up under the crust, then partially subside - Deep volcanic craters possibly result of coronae blowing its top - Volcanoes less than 20 km = so many - Many appear as flattened domes called pancake volcanoes o Thought to be formed by quite viscous (high resistance to flow; opposite of fluid) magma, flattened by the high atmospheric pressure o Shield fields = groups of these - Also have lava flow channels - Key to geology of venus appears to be its heat - Radioactive heat keeps earth hot inside and must do the same for venus - On earth, plate tectonics allows heat to escape from the mantle - Venus lacks any signs of plate tectonics o Could build up lots of heat, and periodically boil over into volcanic eruptions that resurface the whole planet - Theory is sometime between 300-500 million years ago, whole planet was resurfaced by basaltic lava flows, erupted for thousands to millions o fyears - Crater0counting technique suggests planet is not older than about 500 million years ago - WEATHERING/EROSION FEATURES - Knife-sharp mountain ridges and cliffs that show no sign eroision - Rocks may be stronger on genus simply because they have no water content ! - On the other hand visual evidence of breakdown of surface rock, has to be done by wind o Problem = spacecrafts that have landed measured surface winds of only about 3.5 m/s o Preferential direction of distribution of dust + debris from crater o Large field of sand dunes - The interior of venus - Interior must be a mix of rock and metal - Volcanism of venus says that the interior is hot - We do not whether venus has an asthenosphere beneath the brittle crust or whether it has a solid metal inner core surrounded by a liquid metal core - Prior to Venus Express, missions to venus found no evidence of a magnetic field, thus no magnetosphere - If Venus’core is completely liquid metal there should not be magnetic field (earth’s is generated by speed difference of an inner solid metal core and outer liquid metal core) - Then Venus Express detected a weak magnetic field o Interaction between solar wind and the atoms/molecules of the upper atmosphere creates a very weak magnetosphere that drapes itself over the planet and stretches out behind the night side in a long tail - Summary - Earth, moon and mercury passed through pretty much the same four stages o Differentiation: separation of material according to density o Cratering: period of violent impacts o Flooding: by lava flows o Slow surface evolution - Not 100% sure on how venus evolved - Venus formed only slightly closer to sun vs Earth, so we might expect it to be a similar planet differentiated into a silicate mantle and a molten iron core - Desnity and size of venus tell us taht it must have adense interior much like earths, but if the core is molten then we might expect the dynamo effect to generate a magnetic field (hasn’t been detected noted above) - Venus and earth have out-gassed about the same amount of carbon dioxide, but earth’s oceans have dissolved it and converted it to sediments such as limestone - Main difference between earth and venus is the lack of water on venus - 70 percent of the heat from earths interior flows outward thru midocean spreading center volcanism o Venus lacks tensional crutal rifts and its numerous volcanoes cannot carry much of the heat of the interior o Venus seems to get rid of its interior heat through large currents of hot magma that rise beneath the crust o Coronae, lava flows, and volcanism - True plate tectonics is not active on venus - Crust = dry and 12 percent less dense than earths - Low-density crust is more byoyant than earth’s crust and resists being push/pulled into the interior o Crust is so hot it is halfway to the melting point of the rock, not stiff, cannot form rigid plates - Evidence that convection currents below the crust are deforming the crust to create coronae and push up mountains - Some mountains are held up by rising currents of magma Chapter 11: mars - Good first map of mars was produced by astronomer G.V. Schiaperelli in 1877 - He drew artificial lines across the so called desert regions calling them ‘canali’meaning channels - Trick of the eye - Mars = half the size of earth, has atmosphere with clouds and ice deposits at its poles (polar caps) - Long ago it was much like earth - ORBITAND ROTATION - Aday on mars is 24 hours and 40 minutes - Year lasts 1.88 earth years - Mars axis = 25 degrees, has seasons - Seasonal changes visible even through a telescope - As spring comes to southern hemisphere, white polar cap shrinks and the grayish surface markings grow darker - MISSIONS TO MARS - Current to 2009 there are 3 spacecrafts orbiting mars (Mars Odeyssey USA, Mars Express ESA, and Mars Reconnaissance Orbiter USA) - 5 space craft still orbiting but no longer functioning - Dozen crashed spacecrafts - Working on ground 2 intrepid explorers (spirit and opportunity USA) also at one point a stationary lander - Spirit and Opportunity, only had a lifetime of 90 days, still operating 7 years later - Scientists able to make statements about Martian geology with considerable certainty - Spirit is stuck in a sand trap - Opportunity – robotic arm motors are partially seized but besides that in good health, heading toward Endeavour Crater - The Mars Science Lab rover launched Nov 25, 11 (“curiosity”) o Designed to travel over rougher surfaces o Will assess whether mars ever had an env’t capable of supporting microbial life o Determining past habitability on mars gives us a better understanding of whether life could have existed, and if it could have existed an idea of where to look for in the future - ATMOSPHEREAND GREENHOUSE - Air on mars is: o 95 % CO2 o 3% Nitrogen o 1% argon o Contains miniscule traces of water vapour and oxygen, and its density at the surface of the planet does not provide enough pressure to prevent liquid water from boiling into vapour - Air is thin, but dense enough to be photographed - Originally mars, would have had a primitive atmosphere of H and a little helium with a bit of argon, neon, methane, and perhaps some ammonia - Leakage of primitive atmosphere would have been rapid (i.e low escape velocities) - Replaced by a secondary atmosphere of volcanically emitted gases o The giant gas + ice planets have gravitational fields strong enough to retain their primitive atmospheres (i.e. high escape velocities) but the atmospheres of the terrestrial planets are secondary (or even, tertiary as for earth) - Appreciation that the atmospheres of venus, earth, and mars were all produced by the same mechanism, together with ou knowledge that all 2 have similar planetary compositions, explains why all 3 developed almost identical secondary atmospheres \ - Earths tertiary atmosphere evolved differently because of the development of life, production of oxygen, role of liquid water, and fixation of carbon dioxide in rock - Volcanism on terrestrial planets typically released CO2 and water vapour plus other gases - Out-gassing occurred early in mar’s history, and being smal
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