# Can a planet have more than one ozone layer?

Can a planet possess more than one ozone layer? Can multiple ozone layers cause more harm? If one ozone layer had a hole, would the other(s) act like a backup shield to harmful rays from a star? Assume the star is similar to the Sun and the planet is habitable.

Thank you!

• Let's put it to test first list of ozone generations: 1. Corona discharge(need only high voltage) 2. UV (need specific range of wavelength only) 3. Electrolysis (need to pass electric current in liquid) 4. Radiochemical (need radiation) so which shall we test first? – user6760 Sep 20 '15 at 5:51

Yes, in fact we have an extra one right here on Earth. Mostly found in large cities, we call this layer smog. It's pretty harmful, because it's not at the right altitude. At sea level, it's just a pollutant. Here's a nice picture showing what we have:

Here's the NASA website I got it from: http://spaceplace.nasa.gov/greenhouse/en/

Note: there is no such thing as an ozone layer. What is called ozone layer, as if it were a sort of crystal shell high in the atmosphere, is actually a volume of increased ozone density, so that the density function with height behaves a little like this:

%
|                 #####
|                #     ###
|             ###         ###
| ############               ############
+------------------------------------------------------> outer space


But ozone density is never zero anywhere. As per @Alpha3031's answer, you can have more or less temporary conditions in which you get ozone near the ground from photochemical smog, or near the border of the ecosphere, or anywhere in between.

So "more than one ozone layer" would imply a density function more or less like this:

%                      Second layer
|       First layer      ######
|          ####         #      ##
|         #    ##      #         ##
|       ##       ######            #####
| ######                                #########
+------------------------------------------------------> outer space


# Question 1: "More than one ozone layer".

Yes, but it's tricky

The ozone layer is actually a dynamic phenomenon taking place in a volume of space where oxygen density, ultraviolet light from the Sun and (photo)catalytic processes from several substances (e.g. chlorine from chlorofluorocarbon compounds) interact in a narrow range of ways.

So the greatest problem is having a high enough atmospheric column where the oxygen concentration is just right. This requires a shallow gravitational potential (the shallower, the best), which means a large, not so dense planet, and this has its drawbacks (technology as we know it is hugely based on iron and elements heavier than iron). Also, atmospheric pressure at sea level depends on the height of the atmospheric column.

Once we have enough oxygen in the air to work, we can pursue two different avenues. With reference to the ASCII graphics above, you either raise a second "hump" far enough from the existing one, or you spread the one hump you have, and drive a "wedge" in the middle to split it into two.

• two different ozone sources. One is the Sun; the other "hump" would need to be a source at the right wavelength originating in the opposite direction, from the surface, and could not obviously be reflected from the Sun (UV at that wavelength are blocked by the ozone layer, and that is what creates the ozone layer in the first place). One possibility could be secondary ionization from shorter wavelength UVs, that do not get absorbed by the ozone layer (I'd need to check the transmittance curve of ozone - if it's flat at the key frequency and below, we're out of luck: there would be no "shorter, unabsorbed wavelength"). These "UV-D" rays would penetrate a bit more, hit some replenishable atmospheric constituent - perhaps nitrogen - that's denser than ozone, and release their energy as less energetic UV rays that would get refracted and, if these newborn UVs are the right frequency, generate more ozone "some way down".

This phenomenon already happens in the atmosphere with normal ozone, which is one factor, together with atmosphere mixing, that makes the ozone layer "thick": it's actually several intertwining layers that scatter and amass. You'd need to do it with a different substance so that the separation between the two "humps" is larger, or you'd end up with a very thick, but single, ozone layer.

• one creation, two destruction mechanisms (standard spontaneous recombination being one). The ozone is generated in a thicker layer thanks to a shallower gravitational potential or a larger UV input from the Sun, then this large hump is split into two by e.g. some gas that deposits in the atmosphere at a specific height (we're doing wild assumptions on atmospheric diffusion and upper-atmosphere winds...). The gas is a sort of super-halogen, a super-chlorine or super-bromine, and catalyses the 2O3->3O2 ozone-killing reaction with such efficiency that where the gas is, no ozone at all can abide and you have a deep "wedge" separating two "humps". As a result, to all intents and effects you've now got two ozone layers, and only need to explain how the catalytic gas layer is kept together.

You could have some unknown process (high energy collisions, but with what?) generate unumseptium atoms in the upper atmosphere. Its reactivity against ozone ought to be frightfully high (update: it actually isn't, see Wikipedia), even with a half-life of milliseconds, and the small half-life could help explain the localization of the layer - Uup atoms don't wander far from their birthplace before alpha decaying.

Another more promising possibility, and one which already happens, is you have a volatile molecule where a sufficiently evil halogen is bound to something keeping it quiet. Then the molecule diffuses in the high atmosphere, and almost the same UV rays that produce ozone destroy it, thus freeing the halogen atom, which proceeds to destroy ozone before combining with water vapour or hydrogen or something else and precipitating away. This is what happens now with chlorofluorocarbons.

You might also imagine a large fleet of aircraft with some ozone-harmful chemical in the exhaust fumes (this also happens in reality).

# Question 2: "Can multiple ozone layers cause more harm?"

Ozone layers do not cause harm per se. What they do is intercept radiation and either absorb it or reflect it back at the same wavelength or at a longer one ("greenhouse effect").

In the appropriate circumstances both phenomena might be either harmful or beneficial: think some organism that needs radiation at a specific wavelength, or a planet that would be too cold unless some of its thermal emission was bounced back to heat it a little more. The same layer would be harmful to the former and beneficial to the latter.

# Question 3

If one ozone layer had a hole, would the other(s) act like a backup shield to harmful rays from a star?

Not entirely. What would happen is that the depletion of one layer would increase radiation on the surface, but it would depend on the amount of the ozone shielding left. The layers are not "redundant", each one is there because of a specific cause.

Also, depletion of one layer would increase radiation in the volume of air where the second layer is, and might then lead to the thickening of the second layer, leading to a null net effect on the surface (as far as radiations go). The thickening of the second layer might also have other effects, some beneficial and some harmful.

A second ozone layer could form IF a second UV source can be invented

NASA describes how ozone is made in a rather excellent paper on atmospheric chemistry. In summary, UV light hits an O2 molecule forming oxygen ions that bond with other O2 molecules to form O3, or ozone. Since it takes UV light to form ozone and Earth's ozone layer absorbs 97-99%, creating a second ozone layer would require a second UV light source that doesn't come from the sun. If a second major UV source could be invented then a second ozone layer could form.

From the article:

Stratospheric ozone is formed naturally by chemical reactions involving solar ultraviolet radiation (sunlight) and oxygen molecules, which make up 21% of the atmosphere. In the first step, solar ultraviolet radiation breaks apart one oxygen molecule (O2) to produce two oxygen atoms (2 O) (see Figure Q2-1). In the second step, each of these highly reactive atoms combines with an oxygen molecule to produce an ozone molecule (O3). These reactions occur continually whenever solar ultraviolet radiation is present in the stratosphere. As a result, the largest ozone production occurs in the tropical stratosphere.

• If there were two UV sources, won't their aggregate impact on the planet be like one UV source of multiple magnitude (resulting in one, more thick ozone layer)? Not trying to argue for argument sake, just raising a question. – Youstay Igo Sep 20 '15 at 4:18

As far our current knowledge of atmosphere and oxygen bonding types stands, no, multiple layers of ozone are not possible. But that shouldn't stop you. You could employ other ozone like layers for the same purpose. For example, try something like nitrogen ozone layer or carboxy ozone layer. These don't exist naturally yet but hey, fiction is fiction.

However if they were possible and present, then yes, the damage of one layer would be covered up by the layers underneath it.

• Do you have any references for the assertion that multiple layers of ozone aren't possible? – Green Sep 20 '15 at 4:03
• We haven't found any on any planet anywhere no matter what their atmospheric composition. So the burden of proof (that they are possible) lays on the claimant. Plus I did begin my sentence with "as far as our current knowledge ... stands" which implies that I am talking practically here, not theoretically. – Youstay Igo Sep 20 '15 at 4:06
• I would have thought that Ozone, being a gas, would all gather around a specific altitude depending on its density relative to the rest of the atmosphere. How could a higher layer not sink down through less dense atmosphere, or a lower layer not rise? – colmde Sep 22 '15 at 10:00