Class 13 - Earth, Venus, Mars - 3

Reading:

  1. Atmospheres - Goody & Walker pp23-43
  2. New Solar System - Ch 13.

Photochemistry

Why is ozone limited to a layer?

Not all photons reach the ground - Ozone is a major reason for the UV between 2000 and 3000 A being absorbed.

Ozone source

O + O2 -> O3

 

why is this not a likely reaction?

O + O2 + M -> O3 + M

 

Ozone loss

O + O3 -> O2 + O2

O3 + photon -> O2 + O

Why a layer?

Hint - Ozone density is production limited..... (once made it has a ~fixed lifetime)

What is it about the source reaction that will limit production to a layer?

ANSWER:

(1) In upper atmosphere there are not enough reactions - the densities of O2 and M are too low - for the production reaction to occur frequently

(2) In the lower atmosphere there is not enough O around (too readily recombined via reactions such as the first Ozone loss reaction)

In reality, the atmosphere involves many species and many reactions.

 

 

Venus - first the neutrals

then the reaction rates for ions for low solar activity on the left - high solar activity on the right.

 

and finally the ion distributions... also for low solar activity on the left - high solar activity on the right.

 

Next class - summarize chemistry of Earth vs. Venus vs. Mars - similarities and differences