Molecular Monday: Liquid Water vs. Liquid Methane

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Welcome to Molecular Monday! On the first Monday of the month, we take a closer look at the atoms and molecules that make up our physical universe. Today, we’re comparing some of the properties of:

LIQUID WATER AND LIQUID METHANE

So you’re a moon or other planetary body, and you want to get some biochemical action going on. First, you need some organic substances. Titan has set a great example with the tholin haze that forms spontaneously in its atmosphere.

Next, you need a liquid to dissolve that organic material in, in the hopes that the organic material will recombine as amino acids, peptide chains, and ultimately DNA. But which liquid should you choose? Liquid water (as seen on Earth) or liquid methane (as seen on Titan)?

Pick Water!

Water (H2O) makes an excellent solvent for our purposes because it’s a polar molecule. There are two big reasons for water’s polarity.

  • First, oxygen has an extremely high electronegativity, meaning oxygen atoms like to yank electrons away from other atoms. Within a water molecule, oxygen’s electron-hogging tendencies cause it to become negatively charged, while the two hydrogen atoms become positive.
  • Second, you know how water molecules have that Mickey Mouse shape? Because of that shape, with the two hydrogen atoms bent toward each other, the positive charges accumulate on one side of the molecule and the negative charge accumulates on the other.

Thus, water is a polar molecule, and it’ll go around interacting with other polar molecules, like tholins or amino acids.

Don’t Pick Methane

Unlike water, methane (CH4) is a nonpolar molecule. Why?

  • Carbon is slightly more electronegative than hydrogen, but not by much, so the atoms in a methane molecule share electrons almost equally. This minimizes the electric charges that might build up inside the molecule.
  • Methane molecules are symmetrical, with the carbon atom in the center and the four hydrogens evenly spaced around in, like the four corners of an equilateral pyramid.

Sp05 Methane vs Water

Any electrical charges in a methane molecule balance out, due to the molecule’s symmetry. And those charges are fairly weak anyway, due to the similar electronegativities of carbon and hydrogen.

I won’t be so bold as to say life can’t develop in a liquid methane environment, but the idea does seem a bit farfetched in light of the chemistry. Polar molecules like tholins just aren’t likely to dissolve in a methane lake, like the lakes found on Titan.

On the other hand, the universe keeps surprising us, and the giant lake monster I recently met on Titan might dispute my assessment of Titan’s biochemical potential.

P.S.: Titan’s lakes also contain liquid ethane, but that doesn’t really change anything. Ethane is also nonpolar.

9 Responses to Molecular Monday: Liquid Water vs. Liquid Methane

  1. Kirov099 says:

    Any chance you’ll be covering ammonia as a solvent?

    Liked by 1 person

  2. It seems like the more I learn about alternative chemical paradigms for life (BTW, excellent series James!), the more it seems like our type of life (carbon based in liquid water) is the most probable. I used to wonder if astronomers and SETI enthusiasts weren’t being a bit narrow minded with their definition of the habitable zone. But increasingly, it’s starting to look like they’re on the money.

    In a universe with trillions of years of physical processing left, there will be plenty of time for life (or life like processes) to form from alternate chemical substrates, but here in the first 14 billion years, our type may be the only one worth looking for.

    Liked by 1 person

    • James Pailly says:

      Yeah, chemistry sort of knocks the wind out of a lot of really cool science fiction ideas. However, David Grinspoon wrote in one of his books that there could be some sulfur-breathing, silicon-based organism out there that cannot imagine how carbon-based biochemistry could work. Nature sometimes combines chemicals in ways that we would never have thought of.

      So I do think it’s worth keeping an open mind about alternative biochemistries. Although when you’re NASA or ESA or SETI and you only have so much money to throw around, it makes sense to focus on what’s likely according to our current science and skip over the unlikely scenarios.

      Liked by 1 person

      • From what I’ve read, the issue with silicon is that it forms compounds with far less elements than carbon does, apparently drastically reducing the possible molecular patterns it can be involved in. Some form of complex systems that we might be tempted to call life might still be able to evolve with it, but their path is reportedly much narrower than with carbon. In an environment where both carbon and silicon are prevalent, carbon seems more likely to be what gets used.

        Of course, that’s talking about natural evolution. Once an intelligent species takes over its own evolution, silicon might have a lot of advantages. But I’m not particularly optimistic that we will encounter another intelligent species for a long time.

        Totally agree with your second paragraph.

        Liked by 1 person

      • James Pailly says:

        Also, the compounds that silicon does form tend to be rather brittle compared to similar carbon compounds. I find silicon-based life to be highly unlikely, but not impossible.

        Liked by 1 person

  3. […] have evolved on the surface of Titan, if only because liquid methane (Titan’s “water”) is not a good solvent for amino acids. But unlikely is not the same as […]

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