Lovely little article at JPL about Titan’s seas Punga, Ligeia and Kraken reveals that they are pure methane, down to the sludge of complex HCs and other stuff that darkens their floors.
Liquid farts.
But in the article, it says that sunlight acting on the methane in the atmosphere breaks it down to form ethane. Which confuses me, because methane is CH[sub]4[/sub] but ethane is C[sub]2[/sub]H[sub]6[/sub]. How the hell does the simplest HC “break down” to form a more complex HC? Does the enormous amount of nitrogen in the atmosphere somehow act as a catalyst?
(apologies to the mods for putting this mundane & pointless post in GQ, but I wanted an answer)
“Before Cassini, we expected to find that Ligeia Mare would be mostly made up of ethane, which is produced in abundance in the atmosphere when sunlight breaks methane molecules apart.”
Maybe not the best wording but it doesn’t say the production of ethane is the direct result of sunlight breaking apart methane molecules. It does happen as a result of that breakdown so a little better wording could have been used to clarify that.
“Breaking down” in this case means abiotic chemical decomposition. However, that leads to less complex, not more complex molecules. I think it’s more accurate to just say it is “converted” to ethane, the way Wikipedia describes the conversion of methane to acetylene in the upper atmosphere of Titan.
I am frankly amazed that scientists were able to use the Cassini radar to measure the “water” depth on Titan. That just amazes me. Oceanography in space!
I myself am awestruck at all that exploration. Titan is an incredible place – the only other spot in our little system known to have puddles on the surface (which, oddly, seem to be only in the northern half). I hope we can see more of it in my lifetime.
It’s simplified. Most reactions that we see happening are greatly simplified from the form that they actually take. There are plenty of intermediate species involved in practically any reaction; those that are extremely reactive with other intermediate species that are produced in the area tend to be used up rather quickly, while some of them at the edges might get away from the other reactants and thus not have the reaction go to stoichiometric perfection. It’s extremely rare for the entirety of both reactants to be used perfectly unless the reaction is literally a one-step process.
So in this case, we have the opposite situation. We’re told about the net products of sunlight reacting with methane, and are told that it happens through methane being broken down by sunlight, which gives an image of the resultant molecule being smaller. But we’re really talking about the net reaction, which is that two molecules of methane are broken up by sunlight and combine to form ethane. Since the CH3 molecule is extremely reactive and is being constantly produced, it effectively immediately finds a partner (or manages to grab back a hydrogen ion to replace the one that it lost), and very little of it exists as free CH3. One thing tangentially related to this is that pure water is an extremely weak conductor of electricity, as the concentration of charged ions by which electricity would be conducted is very small* - all you have are hydrogen ions and hydroxide ions forming from random splitting of water, and almost immediately recombining with another one in the general area. In this case we have the splitting and recombining as different molecules.
At least, that’s how I understand it.
*Water is generally thought to be a good conductor of electricity because in practice there are always other dissolved ions. But if you get freshly made deionized water, it conducts very weakly.