Astronomy 311: Lecture 4 - Terrestrial Planetary Atmospheres. An atmosphere is a layer of gas that surrounds a planet.

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1 Astronomy 311: Lecture 4 - Terrestrial Planetary Atmospheres An atmosphere is a layer of gas that surrounds a planet. On Earth, atmosphere is about 10km thick: O 2, N 2, H 2 O, CO 2. Atmospheric pressure decreases as you go up. Principle of hydrostatic equilibrium applies. Ideal gas law: P = ρt constant. Average pressure at sea level is 1.03kg/m 2 which is 1 bar of pressure. Atmospheres create pressure that can determine whether water is in liquid form on the surface. Atmospheres can absorb and scatter light: daytime skies are blue in worlds with atmospheres. Atmospheres create winds and weather. Atmospheres can contain gases which make planetary surfaces warmer than they would be otherwise using the greenhouse effect. Greenhouse effect Visible light from Sun absorbed by the ground. Ground reradiates this radiation in the form of I=infra-red (IR) light because this is the appropriate wavelength for a body of that temperature (Wien s law). Then some gases in lower atmosphere trap this IR radiation for a while and are heated by it. These greenhouse gases are water vapor (H 2 O, CO 2, and CH 4 ). These gases aborb an IR photon coming from the ground and then reradiate it in a random direction. Further collisions occur. Escape of this IR radiation is slowed down but ensuing molecular motion heats the surrounding air. The more greenhouse gases that are present the greater the degree of surface warming. These gases abosrb IT because their molecular structure makes them prone to begin rotating or vibrating when struck by a IR photon. Energy balance is not altered. Without the greenhouse effect, planet s temperature much cooler. 1

2 Planet s temperature without greenhouse affected by distance from Sun (inverse square law) and the planet s reflectivity. Black absorbs more than white. Rflectivity depends on surface composition and color. For example, the temperature of a planet s surface is roughly T = 280K 4 (1 ref) d 2. X rays ionize most gases and dissociate molecules. UV rays dissociate molecules. Visible light: blue light is scattered more than red light. IR photons abosrbed by molecules causing them to rotate and vibrate. Troposphere, Stratosphere, Thermosphere and Exosphere. Sky is blue because blue light is scattered more than red light. Sunsets are red because you are watching the beam rather than the scattered light. Greenhouse effect warms the troposhere and because more is absorbed closer to the ground, temperature drops with altitude in troposhere. Without greenhouse effect not such a difference in temp. between mountain tops and sea level. Convection in troposphere: storms, weather etc. At top of troposphere IR photons no longer collide and go into space. Now only the effect of sunlight on atmosphere. Now primary source of atmospheric heating is UV photons from Sun. This is stronger at higher altitudes so temp increases with altitude in lower stratosphere. Convection cannot occur in stratosphere because of this temp. inversion. No weather and no rain: pollution stays here. 2

3 Planet can only have a stratosphere if its atmosphere contains molecules that are good at absorbing UV photons: Ozone, O 3 plays this role on Earth. Earth is the only terrestrial world with a stratosphere. Nearly all gases are good X-ray absorbers so X rays are absorbed by the first gases they encounter in atmosphere. Desnity in exosphere too low so not much absorption there. Most X rays absorbed in thermosphere. Virtually no Xrays penetrate below thermosphere. Part of thermosphere is ionized: ionosphere. This layer reflects radio waves and hence important for communications. Exosphere is low-density boundary between atmosphere and space. High temperature but low density. Mercury and Moon have contain tiny atmpsheres: very small, no structure, essentially an exosphere. Mars, Earth and Venus have a warm troposhere at the base caused by the greenhouse effect and a warm thermosphere at the top caused by X-ray absorption. Only Earth has a stratosphere. Venus, Earth and Mars have all experienced and are experiencing the greenhouse effect in order of severity, starting with the most severe. Solar wind: outpouring of subatomic charged particles from Sun. Earth s strong magnetic field creates a magentosphere that deflects these charged particles around the planet. Some enter through the poles and reside in the Van-Allen belts encircling Earth. The interaction of these charged particles produces aurora eg. Northern lights. Climate Change Solar brightening due to the Sun brightening somewhat during its main sequence lifetime. 3

4 Tilt of the earth s axis can change due to gravitational tugs from other planets: Earth s axis changes from 22 to 25 degrees. Greater tilt means more severe seasons. These tilt changes are called Milankovitch cycles. Mars experiences more extreme changes in axis tilt than the Earth. Changes in reflectivity of a planet. One possible cause for changes in Earth s reflectivity is human activity, deforestation, smog etc. Smog is a good reflector and so one theory is that increased smog has actually reduced increasing temperatures due to increased greenhous gas emission becuase the smog reflects a lot of sunlight back. Hence efforts to clean up smog pollution may actually accelerate greenhouse warming. Changes in greenhouse gas abundance. Origin of Atmospheres During their births, planets captures some H,He but the planetesimals were too small to retain these primitive atmospheres during the solar T-Tauri phase. It is thought that volcanic outgassing has been the primary source for terrestrial atmospheres: terrestrials made up of metal and rock but impacts of ice-rich planetessimals from beyond the frost line brought in water and gas that became trapped in the interiors. This was further solidified when the crust formed and the mantle started to cool from the outside in. Volcanic outgassing releases H 2 O, CO 2, N 2, H 2 S, SO 2. It is thought that this occurred either during the molten mantle stage or as a consequence of a catastrophic collision for the Earth. Recent work though suggests that argon can stay trapped in the mantle even at very high temperatures, with mantle convection and volcanism. This suggests its is difficult for the Earth to outgass all the argon trapped in the mantle. Argon is also a byproduct of the radioactive deavy of potassium. But the earth s atmosphere contains 1of the crust is the suggestion. Thus argon trapped in the mantle billions of years ago may still be there. Mars and Venus have similar mantle materials so the situation may be similar for them as well. Can also form by evaporation of liquid of sublimation of surface ices into gases. After outgassing, some gases condense to become liquids or ices eg. Earth s oceans and polar caps. There is a continual exchange between surface liquids/ices and the atmosphere. 4

5 Outgassing is the primary origin for atmospheres on Mars, Venus and Earth. Planets can loose atmospheric gas by thermal escape, condensation, solar wind sweeping up particles, chemical reactions and large impacts blasting gases into space. For thermal escape, the average velocity of a particle, mass m is related to the temperature by V thermal = 2kT/m, so of this equals or exceeds the escape velocity of the planet, mass and radius M, R, V esc = 2GM/R, the particle will escape. Here k is Boltzmann s constant. Mars is cold and dry with a very low atmospheric pressure - water is unstable - it can evaporate easily. Because of its orbit, its distance from the Sun varies much more than the Earth and has an effect on its seasons: seasons are more extreme in the southern than northern hemispheres. Martian atmosphere is mainly CO 2, no ozone, so there is considerable UV radiation on the surface. Mars has cooled more than Earth, hence its magnetic field is much less, it lost its magnetosphere and so the solar wind was able to blow away the Martian atmosphere. Venus has a thick atmosphere with a strong greenhouse effect - thats why it is so hot. Rotates slowly, thus a weak Coriolis effect and weak winds: too hot for rain to fall. Little axis tilt implies no seasons. It may have got so hot due to a runaway greenhouse effect. Earth s planetary temperature just right for outgassed water vapour to condense and form oceans. Oceans dissolve CO 2 and moderate the greenhouse effect. The Earth s atmosphere stays stable due to the CO 2 cycle: too cool, less perciptation, rainwater dissolves less CO 2 from the atmosphere, so more CO 2 in the atmosphere which leads to a strengthened greenhouse effect. If its too warm, more precipitation, more CO 2 from atmosphere dissolves in rainwater and into oceans and less CO 2 in the atmosphere etc. 5