# Plausibility/mechanisms for biogenic silica on a terrestrial planet with a reducing atmosphere

### Situation:

• a large terrestrial planet (a 'super earth', around 9 earth masses with a surface gravity roughly double earth's) with a mostly hydrogen atmosphere and ammonia oceans (pressure is high enough that ammonia is liquid up to around 50°C)
• life uses ammonia as a solvent
• plants convert methane to biomass and release hydrogen (and animals obviously inhale hydrogen and exhale methane)

Question: Is it plausible for (the carbon based) life to use biogenic silica (= hydrated silica, SiO$$_2$$.$$n$$H$$_2$$O) as a structural material?

Ignore strength issues please (According to this research paper, silica is significantly stronger and less dense than bone, so that should not be an issue) - I'm interested in the chemistry of the situation.

As a terrestrial planet, water and CO$$_2$$ will be released by volcanic outgassing, and they have to go somewhere. Is it plausible that SiO$$_2$$ dissolves in the ammonia rivers/seas to be taken up by plants and animals, which then form hydrated silica?

I'm unsure about how oxides would behave/exist in a reducing atmosphere.

EDIT: (More details to make the question more specific)

On earth, silica forms (as quartz) from magma in volcanic rocks. That should be similar on this planet too. On earth it dissolves in water by hydration to form silicic acid, which is absorbed by the diatoms and other creatures which use silica as a structural material. On this planet, the seas are ammonia, not water.

That seems to leave two questions:

• Can silica dissolve in ammonia?
The abstract of this paper says that it forms a SiO$$_2$$-NH$$_3$$ complex when exposed to gaseous ammonia and then dissociates to the molecule H$$_2$$NSiOOH under the influence of (UV to visible) light.
Would this (or something else) work on the planet in question as a source of silica in the ammonia seas?
Edit: (thanks to Kingledion) the full paper says that both reactions are exothermic, and that the formation of the initial complex has negligible activation energy. (Edit edit: I intended that edit to be a "yes, it would work" answer to the question)
• Can the organisms hydrate the silica internally?
We produce ammonia and (a massive number) of other things as biochemical reaction intermediates and products - is it implausible that organisms on this planet can do the the same with water to produce biogenic hydrated silica?
• Mar 24, 2017 at 0:54
• @kingledion I was just editing info from the abstract of that article into the question as you were posting your comment. I hadn't found the full text as you had though - great find! Thanks Mar 24, 2017 at 1:06
• Much of the abundance of various materials on earth is due to geological processes. You need to figure out plausible compounds not only for life, but also for rocks. May want to ask a similar question on the geology group (is there one?) and see if they have an exoplanetary tag. Mar 29, 2017 at 2:44
• @SherwoodBotsford As a terrestrial planet, the rocks are mostly silicates, similar in composition to earth's crust (the ammonia can be handwavy-explained by comets or something). Volcanic processes should be basically the same as earth; erosion and ocean sedimentation will be different, but silica should still be in solution and available. (I just checked and it seems like this would be off-topic on the Earth Sciences SE) Mar 29, 2017 at 2:58

My own answer to the question:

Water from volcanic outgassing and biological waste will dissolve in the ammonia seas - much as CO$_2$ dissolves in the seas on earth. This paper says that silica dissolves easily in ammonia, so we can assume that silica will also be available in the rivers and oceans, as it is on earth.

In analogy to earth's carbon cycle, I suggest a water cycle - plankton-like microorganisms absorb dissolved water and silica to form hydrated silica which they use to form their shells/skeletons (similar to earth's diatoms). This de-acidifies the oceans (in ammonia, water behaves as an acid). These are eaten - passing silica into the food chain - or sink to the bottom of the sea, returning silica and water to the crust as sedimentary rocks.

The use of silica as a structural material in these ancestral early organisms provides the basis for its use in higher organisms. Being stronger and less dense than bone and comparing favorably to other structural materials, evolution did not find an alternative (silica was 'good enough').

"The paper says that both reactions are exothermic, and that the formation of the initial complex has negligible activation energy." This answers your first question with a resounding yes.

On to the second: We produce ammonia and (a massive number) of other things as biochemical reaction intermediates and products - is it implausible that organisms on this planet can do the the same with water to produce biogenic hydrated silica?

Given water and the necessary components, no it isn't implausible, as life on earth has developed the mechanisms to produce a stunning array of compounds, and the necessary reactions don't take much catalysis to get going. The difficulties will be getting rid of waste products, but humans produce many waste products and have systems to deal with them without much fuss, so this shouldn't be a limiting factor.

My main concern is that you said we are dealing with a planet "with a mostly hydrogen atmosphere and ammonia oceans", where "life uses ammonia as a solvent". You did mention that water is released by volcanic outgassing, but is that in sufficient quantities to satisfy the requirements for hydrating silica for use in organic structures? If not, it's not worth it for life to develop that mechanism. If so, how does this water collect, and once collected, how is it gathered by the lifeforms being considered? If these questions are answerable, then it's plausible that organisms on your planet can and would produce biogenic hydrated silica.

I hope this helps!

• In "We produce ammonia ... - is it implausible that organisms on this planet can do the same with water...?" - my point was about internal generation of water as a metabolic waste, not collection. The earlier outgassing point was background in case anyone could use it to construct an answer - although I suppose this water would dissolve in the ammonia (forming a dilute acid), and sea organisms would need to deal with that - perhaps diatom-like, sequestering the water in silica shells, which gave rise to the use of silica. (I knew about the 'resounding yes' - edited to make it clearer) Mar 29, 2017 at 1:37
• So, to clarify, is your question about the feasibility of the internal production of hydrated silica from silica (prob. in an ammonia solution) and water, or the internal production of water given the nature of the planet? Mar 29, 2017 at 1:55
• 1 - internally hydrating the silica after absorbing it from ammonia solution (since the plausibility of ammonia solution was solved), 2 - metabolically producing water to do this given the overall; 3 - general plausibility of its use - is it going to melt or explode or do something else unhelpful in ammonia? A bonus 4 - have I asked the wrong question with hydrated silica, and maybe something else (ammoniated silica?) would be more suitable? All of these to the background of the planet's chemistry and biochemistry Mar 29, 2017 at 2:04

In water and CO2 the oxygen is already bound to other atomic species. Unless you have some mechanism to massively release the oxygen from those oxides (atmospheric electric discharge may dissociate water, but not sure how long the free oxygen would last before recombining with the hydrogen or other species), you are not going to have that much SiO2.

• I don't think that's true. See my recent edit to the question. On earth, SiO2 forms from magma as crystals in volcanic rocks, and then dissolves in water to be taken up by organisms. My point about the water and CO2 released by volcanic outgassing was that they react in the crust/in the ammonia in the past to form minerals - i.e. that oxygen is plausibly present in minerals and things that can react. SiO2 doesn't have to form spontaneously from them in the way you suggest Mar 24, 2017 at 1:04