Chlorine photosynthesis on exoplanets

[stevebowers.]

http://labs.adsabs.harvard.edu/ui/abs/20.....10..953H
Abstract;
The modern search for life-bearing exoplanets emphasizes the potential detection of O2 and O3 absorption spectra in exoplanetary atmospheres as ideal signatures of biology. However, oxygenic photosynthesis may not arise ubiquitously in exoplanetary biospheres. Alternative evolutionary paths may yield planetary atmospheres tinted with the waste products of other dominant metabolisms, including potentially exotic biochemistries. This paper defines chlorinic photosynthesis (CPS) as biologically mediated photolytic oxidation of aqueous Cl- to form halocarbon or dihalogen products, coupled with CO2 assimilation. This hypothetical metabolism appears to be feasible energetically, physically, and geochemically, and could potentially develop under conditions that approximate the terrestrial Archean. It is hypothesized that an exoplanetary biosphere in which chlorinic photosynthesis dominates primary production would tend to evolve a strongly oxidizing, halogen-enriched atmosphere over geologic time. It is recommended that astronomical observations of exoplanetary outgoing thermal emission spectra consider signs of halogenated chemical species as likely indicators of the presence of a chlorinic biosphere. Planets that favor the evolution of CPS would probably receive equivalent or greater surface UV flux than is produced by the Sun, which would promote stronger abiotic UV photolysis of aqueous halides than occurred during Earth's Archean era and impose stronger evolutionary selection pressures on endemic life to accommodate and utilize halogenated compounds. Ocean-bearing planets of stars with metallicities equivalent to, or greater than, the Sun should especially favor the evolution of chlorinic biospheres because of the higher relative seawater abundances of Cl, Br, and I such planets would tend to host. Directed searches for chlorinic biospheres should probably focus on G0-G2, F, and A spectral class stars that have bulk metallicities of +0.0 Dex or greater.

[stevebowers.]

We've had chlorine exoplanets at OA for years;

http://www.orionsarm.com/eg-article/48fe1a14962ff

[Matterplay1]

Hmm. And there I was thinking that I was stretching things a bit when I made that up (replicating the standard SF 'chlorine-breather' was just too much of a temptation to pass up). Looks like some details of this differ from what I'd outlined though. I'll have to see if I can dig up a copy of the full paper. Thanks for this!

[stevebowers.]

One difference that has come up in discussions elsewhere on the 'Net is that chlorine photosynthesis might prevent the evolution of oxygen photosynthesis, probably by chlorine and oxygen reaction to form sterilising hypochlorite compounds. If life can evolve to tolerate bleach-like compounds then maybe a chlorine/oxygen environment could form, but it would be very far outside of chemical equilibrium.

I note that methane and oxygen were both present in Earth's early atmosphere during the Proterozoic era, but they should react with each other quite rapidly. Life processes replenished both as quickly as they could react with each other. Given the right circumstances chlorine and oxygen could conceivably co-exist, but it seems likely that one or the other would predominate (probably oxygen, since it is the more common element).

[iancampbell]

I can think of one possible reason why chlorine-based ecosystems might be rare. It's simply that chlorine is much rarer than oxygen. In addition, it isn't immediately obvious what such an ecosystem might use as a working solvent; polar chlorine compounds aren't all that common and their interaction with the far more common water might be a problem.

[stevebowers.]

I tend to think that oxygen and chlorine could coexist on some planets, just like methane and chlorine co-existed on Precambrian Earth. Of course the chlorine would be little more than a trace - but if it were continually replenished, it would be significant enough to make the atmosphere toxic to humans.

[radtech497]

I tend to think that oxygen and chlorine could coexist on some planets, just like methane and chlorine co-existed on Precambrian Earth. Of course the chlorine would be little more than a trace - but if it were continually replenished, it would be significant enough to make the atmosphere toxic to humans.

A nominally Earth-like world could host an algae-like organism that learned to incorporate chlorine ions into its biomolecules in order to present a less-tasty aspect to potential predators. Over time, as predators became more able to tolerate the chlorine, the prey (algae) might develop an ability to produce chlorine gas from those ions, again as a defense against predators. This chlorine gas could accumulate over aeons, eventually reaching perhaps a few percent of the total atmosphere in composition. The atmosphere will likely consist of nitrogen, oxygen, carbon dioxide, chlorine, various chlorinated hydrocarbon vapors, and, probably, more than a hint of phosgene gas. Since any life on the planet breathes oxygen and carbon dioxide (but can tolerate relatively high concentrations of chlorine), the biochemistry is at least vaguely similar; noticeably, carbon compounds will be somewhat chlorinated and will likely resemble plastics (an analogue of cellulose, for example, will be something like polyvinylchloride (PVC)).
All this chlorine in the atmosphere will react with any water on the surface to produce an aqueous solution of hydrochloric acid and hypochlorous acid. All bodies of "water," then, will be composed of a dilute acid and bleach solution. If chlorine gas comprises one percent of the atmosphere, this solution will have a pH of about 4.50 (similar to undiluted vinegar).
So, yes, chlorine and oxygen can coexist on a Gaian world, as long as the chlorine gas did not dominate the atmosphere.

Radtech497

[Matterplay1]

[snip]
Since any life on the planet breathes oxygen and carbon dioxide (but can tolerate relatively high concentrations of chlorine), the biochemistry is at least vaguely similar; noticeably, carbon compounds will be somewhat chlorinated and will likely resemble plastics (an analogue of cellulose, for example, will be something like polyvinylchloride (PVC)).
All this chlorine in the atmosphere will react with any water on the surface to produce an aqueous solution of hydrochloric acid and hypochlorous acid. All bodies of "water," then, will be composed of a dilute acid and bleach solution. If chlorine gas comprises one percent of the atmosphere, this solution will have a pH of about 4.50 (similar to undiluted vinegar).
So, yes, chlorine and oxygen can coexist on a Gaian world, as long as the chlorine gas did not dominate the atmosphere.
Radtech497

Many of these issues got consideration in our existing Chlorine Worlds articles. Of course a difference from the scenario presented by the authors Steve quoted at the start of this thread is that the photosynthetic process I'd imagined is just a source of hydrogen ions & high-energy electrons, just as with known real-world photosynthetic processes that use something other than water as a hydrogen donor. Also, I'd not envisioned chlorocarbons as anything more than an incidental product. Another difference from the scenario the authors of the paper is that I'd imagined that a world with chlorine in the atmosphere would be more likely if the local star gives out less UV, not more. Halogens are very hard on the ozone layer, so if you want life to be able to survive on land or even in the upper photic zone of the oceans you might be better off with a star that is a bit redder than ours. A final factor which I didn't consider at first when I wrote the OA articles and which the authors Steve quotes might not have considered either is that chlorine is good enough at blocking visible light that even a small percentage of it in the atmosphere would make for a very dark surface. When I found that out I dialed down the percentages I mentioned, but honestly I don't know that I took it as far that direction as I ought. At some point I'll have to take the time to visit one of the local university libraries and read the full article. Has anybody else here seen more than the abstract?

[radtech497]

[snip]

Many of these issues got consideration in our existing Chlorine Worlds articles. Of course a difference from the scenario presented by the authors Steve quoted at the start of this thread is that the photosynthetic process I'd imagined is just a source of hydrogen ions & high-energy electrons, just as with known real-world photosynthetic processes that use something other than water as a hydrogen donor. Also, I'd not envisioned chlorocarbons as anything more than an incidental product. Another difference from the scenario the authors of the paper is that I'd imagined that a world with chlorine in the atmosphere would be more likely if the local star gives out less UV, not more. Halogens are very hard on the ozone layer, so if you want life to be able to survive on land or even in the upper photic zone of the oceans you might be better off with a star that is a bit redder than ours. A final factor which I didn't consider at first when I wrote the OA articles and which the authors Steve quotes might not have considered either is that chlorine is good enough at blocking visible light that even a small percentage of it in the atmosphere would make for a very dark surface. When I found that out I dialed down the percentages I mentioned, but honestly I don't know that I took it as far that direction as I ought. At some point I'll have to take the time to visit one of the local university libraries and read the full article. Has anybody else here seen more than the abstract?

While it is very hard on atmospheric ozone, chlorine absorbs light with wavelengths shorter, IIRC, than about 420 nanometers; this would, depending on the abundance of chlorine in the atmosphere, offset the lack of ozone with regard to UV absorption. A redder star, producing less UV than Sol, might not provide a noticeable advantage in this regard, while restricting the range of useful planetary orbits.
The chlorine abundance of an atmosphere with a photosynthetic flora as described above is assumed to be about one percent (by volume); the amount of light blocked by this amount mightn't be very great, though I haven't done the calculations to check this.

Radtech497

[Matterplay1]

[snip]
While it is very hard on atmospheric ozone, chlorine absorbs light with wavelengths shorter, IIRC, than about 420 nanometers; this would, depending on the abundance of chlorine in the atmosphere, offset the lack of ozone with regard to UV absorption. A redder star, producing less UV than Sol, might not provide a noticeable advantage in this regard, while restricting the range of useful planetary orbits.
The chlorine abundance of an atmosphere with a photosynthetic flora as described above is assumed to be about one percent (by volume); the amount of light blocked by this amount mightn't be very great, though I haven't done the calculations to check this.

Radtech497

You could be right. I've not done the calculation myself, either and frankly don't know how to do it without some research. According to this source even tiny amounts of chlorine in the atmosphere would be too much and the surface would be pitch black. He references an article by Gregory Benford all the way back in a 1971 issue of Natural History. Benford's a pretty well informed individual on such things, to perhaps he is right. Another item for me to check if I can dig up the article itself (looks more and more like I'll be making that trip to the library...). If all this is right then perhaps the old science fiction trope of chlorine-breathing aliens can't be maintained in OA's 'hard SF' setting, despite all the effort I put into making it work. Hmm.