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Why some
atmospheres are lost
•Compare
velocity of gas atoms (Vgas) to planet’s escape velocity Vesc
–If any significant # of atoms have escape speed
atmosphere will eventually be lost
–In a gas the atoms have a range of velocities,
with a few atoms having up to about 10 ´ the average velocity,
so we need 10 ´ Vavg gas < Vesc to keep atmosphere for 4.5 billion years.
with a few atoms having up to about 10 ´ the average velocity,
so we need 10 ´ Vavg gas < Vesc to keep atmosphere for 4.5 billion years.
–In above equations R = planet radius, M = planet
mass, T = planet temperature,
m = mass of atom or molecule, k and G are physical constants
m = mass of atom or molecule, k and G are physical constants
•Big
planets have larger Vesc (i.e. larger M/RµR3/R) so hold atmospheres better
–Earth would retain an atmosphere better than Mercury
or the Moon
•Cold
planets have lower Vgas so hold atmospheres better
–Saturn’s moon Titan will hold an atmosphere
better than our moon
•Heavier
gasses have lower Vgas so are retained better than light
ones
–CO2 or O2 retained
better than He, H2, or H
–Even with “heavy” gasses like we H2O we need to worry about
loss of H if solar UV breaks H2O apart. That is what happens on Venus.
loss of H if solar UV breaks H2O apart. That is what happens on Venus.
