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)?
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.
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.