JakobVirgil wrote:
There is also no reason why life has to be water + organic chemical based.
Well, again we are dealing with the distinction between possibilities and probabilities. I would agree that non-carbon based biochemistries are possible--but I cannot readily conceive of an environment in which the would naturally occur. That doesn't make them impossible, but vastly less likely that carbon-based.
First of all, let's consider that "life" is a stable system of chemical reactions that extract energy from the environment, and use that energy to create more complex chemical structures which are capable of replication. If we are dealing with non-chemical life or anit-matter life, then we really have no basis on which to speculate. We probably wouldn't be able to recognize each other as "life" in any event.
Within that framework (baryonic matter based, chemical life), the first issue is that of elemental abundance. About 98% of the universe's mass is Hydrogen and Helium. Helium is largely non-reactive, so it doesn't contribute much to the party. Next on the abundance list are oxygen (1%), carbon (0.5%), neon, iron and nitrogen (0.1%), silicon, magnesium and sulfur (0.05%). All the rest of the elements
combined are less abundant than nitrogen. So, if you are going to have a non-carbon based biochemistry, you have to have it in an environment in which all of the elements that you require are present in sufficient abundance.
Now the next problem is efficiency. The carbon-carbon bond is incredibly effective at creating biochemical compounds because it is short, strong, it allow for rotation, and can create aromatic rings (ring formations where the atoms share electrons). The carbon-hydrogen bond is even shorter, stronger and thus can release considerable energy when it is broken. Structurally, silicon should be able to do many of the things that carbon does--but it suffers from two great faults. First, silicon is bigger than carbon, which means that all of its bonds are weaker, and it has difficulty forming the double bonds that lie at the root of many organic molecules. Second, it lacks the reactivity with the wide range of other atoms that form part of organic chemistry. A silicon based biochemistry would have to replace this vast diversity of structures, but silicon simply doesn't behave in this way. Silane (SiH4) is the cognate molecule to methane (CH4), while carbon and hydrogen have comparable electronegativity, making methane bonds generally non-polar, the bonds in Silane are polarized, creating a different physical chemistry. Silicon does not create flat rings like carbon, it creates chair shaped rings, which are only weakly aromatic. At the end of the day, hydrocarbons are vastly more efficient than hydrosilanes, and in the presence of carbon, hydrocarbons will proliferate in preference to hydrosilanes.
Now, of course, silicon is not the only basis for alternative biochemistries, to be sure. Boron is a very interesting atom from the perspective of creating complex compounds. But boron is incredibly rare because it is not a product of stellar fusion, and can only be created through the gamma ray bombardment of heavier elements. Sulfur can chain as carbon does, but like silicon, it lacks carbon's abundance and its reactivity. Even more esoteric systems have been postulated, but generally they require such extraorinarily high energy states for stability, that they are unlikely to create stable compounds that will sustain a process of replication.
Even within organic chemisty as we know it, there is room for variation. Water is the essential solvent for life as we understand it, but ammonia has similar properties. But, again, abundance gets in the way. Oxygen is vastly more abundant than Nitrogen (which is another element that is not created in stellar fusion), and water is a much more stable molecule than ammonia. If an environment can be conceived in which nitrogen is superabundant to water,
and oxygen can be dispensed with in biochemical processes, then some potential may exist for different biochemistry. But can such an evironment even be conceived? How would such an evironment naturally occur without being at such a high energy state as to prevent the formation of liquid ammonia?
Now I don't say that all these problems might not be overcome somewhere in a universe as big as ours. But I think the reality is that we will discover water and hydrocarbons everywhere in the universe--meaning that organic chemistry as we understand it will likely exist almost everywhere in the universe.
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--James
21 Aug 2012, 9:40 am
I'm sure other life forms would beg to differ if they used human language.
