AST101- Chapter 10..docx

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University of Toronto St. George
Astronomy & Astrophysics
Michael Reid

Planetary Atmospheres [Chapter 10] 10.4 THE ATMOSPHERIC HISTORY OF MARS  Planetary size is the most important influence on gain and losses of gasses; determines the level of volcanism and outgassing  Mars is 40% larger in radius than mercury What is mars like today?  The thin atmosphere and low atmospheric pressure explains why liquid water is unstable on the surface (evaporation and freezing)  Total amount of gas very small, weak greenhouse effect, temperature below freezing, lack of oxygen means there is no ozone layer, and suns damaging ultraviolet radiation passes through to the surface Martian seasons and winds  The elliptical orbit of mars puts it significantly closer to the sun during the southern hemisphere summer and farther from the sun during the southern hemisphere winter o This means it has extreme seasons in its southern hemisphere; shorter hotter and longer summers and colder winters than in its northern hemisphere  Seasonal changes result in the major feature of mars weather which is wind blowing from the summer pole to the winter pole o This results in polar temperatures at the winter pole to drop (where CO2 condenses to dry ice at polar cap) while frozen CO2 at the summer pole sublimates into C02  The atmospheric pressure increases the summer pole and decreases the winter pole; driving strong pole to pole winds  The winds vary depending on the seasons, however can sometimes initiate huge dust storms which can engulf the planet and ultimately would alter the reflectivity (colour of ground would change) Water ice at the Martian poles  Fair amount of frozen water at the polar caps, made of mostly ice plus a thin layer of Cos ice  Belief that there is still liquid water unground where it is kept warm by volcanic heat Colour of the Martian sky  Winds and dust storms leave the air dusty which explains the colour of the sky (combination of dust and wind effects the colour)  Since there is much less air on mars compared to earth, it scatters less light  The dust absorbs the blue light which leaves the sky a brownish pink colour  Colour depends on time of day, the season, and how much dust is in the air  Usually during daytime a yellow brown colour Mars climate and axis tilt  Changes in tilt result in longer term cycles of climate change, other than that the weather does not change  Varies within a wide range due to Jupiter’s gravity (since it is closer than to earth) and the two tiny moons orbiting mars (whereas earth has a large moon to stabilize the gravity)  When the tilt is small the poles are in a perpetual deep freeze and pressures lowers which weakens the greenhouse effect, however when greatly tilted the summer pole becomes warm and allows water to sublime along with CO2 into the atmosphere, this increase in pressure strengthens the greenhouse effect Why did mars change?  Climate change due to the varying axis tilt  Mars used to have a much denser and warmer atmosphere to allow rainfall of any kind in the past  In the past had active volcanoes which indicates the atmosphere was much thicker and had a stronger greenhouse effect  How did mars lose all of its CO2? o Loss weakened the greenhouse effect until the planet was frozen o Unsure; but might be linked to magnetic field which weakened as the small planet cooled and the core convection eventually ceased, leaving the atmosphere vulnerable to solar wind particles  No water due to lack of ultraviolent absorbing stratosphere which means water molecules would have been easily broken apart by the ultraviolet photons  The oxygen rusted the rocks and have the planet the red distinctive tint  Mars changes because of its relatively small size  Big enough for volcanism and outgassing but too small to maintain the internal heat needed to prevent the loss of the water and gasses  Also if the planet were closer to the sun (the polar caps could melt) 10.5 THE ATMOSPHERIC HISTORY OF VENUS  Venus is a contrast to mars, has extreme greenhouse effect due to outgassing released in vast quantities of CO2 What is Venus like today?  Consists almost entirely of CO2; no oxygen  The thick atmosphere scatters all the blue light away which is why the dimly lit sky appears reddish-orange in colour  Has a very weak coriolis effect (rotation is every 243 days) o There has little wind on surface and never any hurricane storms  No rain because all gas evaporates faster than it can reach the ground  Has two large circulation cells o Surface temperature the same every where due to efficient transportation of heat from equator  Has no seasons because it has no axis tilt  Sometimes has sulfuric acid rain which is responsible for the dynamic cloud patterns How did Venus get so hot?  Because of the CO2 levels in the atmosphere The fate of outgassed water and CO2  What happened to all the outgassed water and CO2 on earth? o Water vapor released into our atmosphere condensed into rain, forming our oceans (water is here in liquid form rather than gas) o CO2 is released into our atmosphere in solid form; it dissolves in water where it undergoes chemical reactions to make carbonate rocks (rocks rich in carbon and oxygen) o Because all this CO2 is in our rocks, it is not as hot as Venus  What happened to Venus’s water? o Same processes as mars; ultraviolet light from the sun breaks apart water molecules in the atmosphere o And the hydrogen atoms escape into space o Oxygen from water molecules was lost to a combination of chemical reactions with surface rocks and stripping by solar wind (has solar wind because its atmosphere is vulnerable due t lac of magnetic field) o Belief that it outgassed as much water as earth but lost all of it The runaway greenhouse effect  Why Venus does not have oceans: o If we moved earth to Venus’s orbit there would be more intense sunlight upon it and this would raise surface temperature by about 30 degrees, which in return would increase evaporation and warmer air holds more water vapor, and additional water vapor further strengthens the greenhouse effect o This results in oceans evaporating and carbonate rocks decomposing, releasing of C0
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