Sciency Words A to Z: Tholin

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, T is for:

THOLIN

Have you ever been stuck trying to say something, but you just don’t have the right words to say it?  In the 1970’s, planetary scientists Carl Sagan and Bishun Khare had that problem.

They’d conducted a series of experiments using gaseous chemicals that were known to be common in outer space, chemcials like ammonia, methane, water, hydrogen sulfide… they mixed all these chemicals together and zapped them with either an electric spark or ultraviolet light.  Then they studied the orangey-brown gunk that formed as a result.

Initially, this gooey gunk was thought to be a polymer, but as reported in this 1979 paper, Sagan and Khare soon determined that wasn’t what it was.

It is clearly not a polymer—a repetition of the same monomeric unit—and some other term is needed.

Sagan and Khare propose the word “tholin,” which is sort of a pun.  It’s taken from two Greek words that are spelled the same, except for an accent mark that’s shifted from one vowel to another.  One word means “muddy,” the other means “dome” or “vault,” as in the great dome or vault of the sky.  Sagan and Khare go on to mention that they were “tempted by the phrase ‘star-tar.’”

Tholin may be present on some asteroids and comets, and tholin or tholin-like material has been observed on several moons in the outer Solar System, most notably Titan.  We may have even found tholin on Pluto, and several other red-hued dwarf planets could have it too.

So what specifically is this stuff?  Well, I can’t really say.  Tholin is not a specific substance but rather a general category of organic matter.  As planetary scientist Sarah Hörst explains in this article:

The best analogy I have been able to come up with is “salad.”  Salad, like tholin, is a mixture of a number of different compounds and spans a fairly broad range of materials.  Most of us would agree on a case by case basis whether or not something is a salad, but the definition is not at all specific and the material itself depends on the starting materials, temperature, etc.

So there are many different tholins out there.  The tholin we might find inside a comet is probably different from the tholin we find on Pluto, which is different from the tholin we find on Titan.  What all these tholins have in common is that they’re the kind of yucky gunk you’d expect life to make, except life didn’t make it.

However, while life doesn’t make tholin, tholin could, in theory, be used to make life.  Or at least, once life gets started, tholin can serve as a source of food for primitive microorganisms.

Titan has long been the poster child for tholin chemistry, simply because Titan has so much of this stuff.  More than enough, you’d think, for some sort of biological activity to get started—assuming it hasn’t already!  However, with all that tholin lying around, sending astronauts to explore Titan properly may prove to be a sticky proposition.

Next time on Sciency Words A to Z, there’s no way we’ll find life on Venus… right?