Alternative Biochemistry

JoshNH4H · 2018-01-12 15:42:45

A fun game I like to play sometimes is "Alternative Biochemistry". Basically, using chemistry and physics as a guide you create a world (or other environment) and design the biochemistry of an ecosystem that could plausibly live there. I'll start!

The Outlines:

The setting is an ice world about 280 AU (4.2 x 10^13 m) from the Sun. This is about 7 times farther than Pluto. The planet's radius is about 7,500 km, and is composed primarily of Methane, water, and ammonia ices with a rocky core. Net density is 2,000 kg/m^3, giving a surface gravity of 4.2 m/s^2. The surface is mostly covered with an ocean of liquid Hydrogen, under a Helium atmosphere at 2.5 bars. Mean planetary temperature is 17 K, and insolation from the Sun is 5 milliwatts per square meter.

The World:

Per scenario given, the world is frigid but large, with a surface temperature around 17 K. The world’s surface is mostly covered by oceans of liquid Hydrogen. There is an atmosphere composed mostly of Helium at 2.5 bar, with Hydrogen at saturation vapor pressure. There is also a layer of insulating clouds made from Hydrogen droplets and Hydrogen ice.

This world is a rogue planet located far from any star. The hydrogen clouds reflect away most starlight. Energy comes from a small system of orbiting moons (1-3 moons), which generate limited amounts of heat and tides by their gravity.

The world’s solid surface is made from ices. The composition is mostly methane, but also with substantial amounts of ammonia and water ice and smaller amounts of Hydrogen Sulfide, Carbon Monoxide, Carbon Dioxide, and other chemicals. These ices behave in an analogous way to metal oxides on Earth, as the material palette for geology. There was plate tectonics and cryovolcanism at some point in the past and these have had a substantial effect on the terrain of the planet, which features extreme vertical relief. The planet is now mostly frozen solid and these geological processes have terminated.

Basic Biochemistry:

At 17 K, this world is nearly 20 times colder than the temperatures at which Terran life thrives. This means that the available energies are much lower and the reaction rate is much lower. Therefore, instead of a biochemistry based on covalent bonds between Carbon atoms, this biochemistry is based on hydrogen bonds between water molecules. At these temperatures and pressures, the equilibrium form of water ice is Ice XI, which consists of stacked layers of intersecting hexagons.

The analogue to a paraffin in this biochemistry is a stack of these hexagons, “capped” at potential hydrogen bonding points by other molecules such as ammonia, Hydrogen Sulfide, Carbon Monoxide, perhaps Methanol, etc. These molecules may form a kind of “jacket” around the backbone that protects it from undesired crystallization with environmental water molecules.

These macromolecules will also have the property (in common with hydrocarbons in the Terran biochemistry) that they will undergo a reaction to release energy. For hydrocarbons this reaction is combustion. For these molecules it would be recrystallization, where all the component molecules of the macromolecule crystallize in their most thermodynamically stable form and release (small amounts of) energy.

Without getting too deeply speculative on the chemistry, it is possible to imagine that from this basis, over time, molecular patterns could become self-replicating and evolve complex structures to generate energy from the environment and do other things that living creatures do. These structures might or might not be analogous to those found in cells on Earth.

Implications:

Energy primarily reaches this world through the flexing of the crust and mantle. Therefore the greatest amount of energy will come from below and not from above. I expect that organisms on this planet will generate energy in two primary ways: Firstly, by proximity to vents below the surface comparable to black smokers in the deep oceans on Earth, and secondly through biological mechanisms that take advantage of waves, tides, and wind to generate energy.

Organisms on this planet are unlikely to develop any kind of quick motion as kinetic energy far exceeds what is available from phase change in water ice. Instead, they will likely either anchor themselves in place or float freely through the oceans and in the wind.

All of the ices mentioned previously are much more dense than liquid Hydrogen. Therefore even small organisms (on a cellular scale?) may contain some kind of gas bladder to help them keep a neutral buoyancy with respect to the oceans.

Due to the larger size of their basic units and in order to prevent from developing an excessive density, these organisms will likely be much larger than a Terran organism of comparable complexity, perhaps two orders of magnitude larger.

What do you guys think? Anyone else want to take a whack at it, either on the world I've proposed or a different one?

IanM · 2018-01-12 17:58:35

Pretty much all life on Earth generates energy via redox reactions, which have the two advantages of being energetically favorable (in being both plausible and generating more energy) and being kinetically slow (so that energy doesn't get transferred/depleted too fast). I can't think of any other reaction type off the top of my head that also have both of these advantages (acid-base reactions don't give enough energy, any sort of nuclear process would give off energy too fast to be meaningfully used, etc.), so I'll assume that it'd be the same on this world.

Hydrogen can be used as an oxidant, as can Oxygen if also present. Oxygen is really good for redox reactions, being second only to the comparatively scarce Fluorine in electronegativity (tendency to gain electrons). The problem with Oxygen is that Oxygen and Hydrogen in the absence of any intervening force will almost certainly come together to form water, which freezes at temperatures well above 17K. I see two potential ways to overcome that barrier. The first would be to somehow "supercool" water, in which case it remains liquid below its freezing point. This can be done with the introduction of salt, but that doesn't work for temperatures below 0F (which is by definition the freezing point of brine). It can also be done in the absence of any seed that would initiate crystallization, but even that only works down to 220K or so. A more plausible way would be to use electrolysis to keep Oxygen and Hydrogen separate, like in a classroom experiment. A redox cycle to generate electrons, and then to use those electrons to keep Oxygen and Hydrogen separate, while using Oxygen to oxidize that which is desired, and so on, is somewhat feasible. However, Oxygen (according to Wikipedia) freezes at around 54 K, as do all of the elements other than Hydrogen and the noble gases, so Oxygen would have to be brought to that temperature by some means. Perhaps deep down closer to the heated mantle?

JoshNH4H · 2018-01-13 15:18:20

Hey Ian,

I like it! It's carbon-based, then?

IanM · 2018-01-13 16:04:16

Perhaps, depending on how warm the "deep sea"/mantle is. Methane melts at below 100K, so methane-using "photosynthesis" (albeit without light) could be feasible at such temperatures similar to how Terran organisms in the deep ocean use it. Perhaps the initial life on the planet evolved in such a fashion, and throughout the eons it evolved to be more accommodated to the even harsher surface of the planet similar to how you described it.

However, there is an important caveat. Most of the initial heat of planets, at least in the solar system, originates from the decay of 26Al, which has a half-life of 730k years, becoming "extinct" (i.e., below detection/ability to heat the planet) within the first millions of years of the planet's existence. However, you say that there has been tectonic activity after that point, which would stall the cooling off of the planet and provide heat necessary for life. You say that such activity is extinct on the planet, but perhaps the organisms on it have been able to take enough advantage of it while it lasted to perpetuate themselves.

JoshNH4H · 2018-01-17 14:09:03

When you get deep down into the planet the pressure can get quite high and I would not be surprised if there were a layer (or layers) of liquid volatile slush depending on the pressure/temperature profile. Seems feasible to me though

SpaceNut · 2018-01-18 16:49:20

The plot thickens as Mystery Solved! Discovery of Organic Molecules Explains Universe's Infrared Glow

AAuNVdH.img?h=374&w=728&m=6&q=60&o=f&l=f&x=921&y=485

organic molecule called benzonitrile, which seems to permeate every part of the known universe

https://en.wikipedia.org/wiki/Benzonitrile
Benzonitrile is the chemical compound with the formula C 6 H 5 CN, + 3 H2O which is real interesting.....

RobertDyck · 2018-01-18 20:47:12

IanM wrote:

Pretty much all life on Earth generates energy via redox reactions

Anaerobic respiration. Microbes used it for billions of years before cyanobacteria evolved. Earth's atmosphere was ~95% CO2, just like Mars and Venus. I suspect most other planets with life will not have oxygen. And since anaerobic respiration does not provide as much energy, multi-cellular life is difficult. But life finds a way. Other planets may have other things that ours does not.

elderflower · 2018-01-19 03:17:30

There are huge amounts of water and CO2 in the earths mantle. You might expect that the CO2 would have been largely degassed by now, leading to an atmosphere like that of Venus. It is probably down to subduction of carbonate rocks and organic materials that it isn't.

RobertDyck · 2018-01-19 05:32:32

elderflower wrote:

There are huge amounts of water and CO2 in the earths mantle. You might expect that the CO2 would have been largely degassed by now, leading to an atmosphere like that of Venus. It is probably down to subduction of carbonate rocks and organic materials that it isn't.

Actually, scientific belief is that primordial Earth did have a thick CO2 atmosphere like Venus. But when the first hot oceans formed, dissolved CO2 in rain and ocean water turned it to carbonic acid. That's a weak acid, but heat and acid accelerated the rate rivers dissolved igneous rock. That washed dissolved calcium and magnesium into the ocean. Carbonic acid combined with calcium to form calcium carbonate aka calcite, which precipitated out. Both calcium and magnesium combined with carbonic acid to form dolomite, which also precipitated out. Calcite and dolomite form limestone. This is called chemical sedimentary limestone, as opposed to limestone formed by living organisms eg coral. Chemical sedimentary limestone predate the first life on Earth. It converted CO2 from the atmosphere into rock, sequestering CO2.

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#1 2018-01-12 15:42:45

Alternative Biochemistry

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