Titans methane What high-pressure mineral physics has to tell us. Dominic Fortes, Department of Earth Sciences University College London
Only two more months to Saturn Orbital Insertion – July 1 st. From 30 million km away, Cassini can already see the surface of Titan.
What are clathrates? At room pressure, water forms a molecular cage that can trap methane gas. These crystals have cubic symmetry, and belong to a class of so-called Structure I clathrates (sI). Clathrates are compounds consisting of a molecular cage, and an occluded molecule Clathrates are like a kind of 3D bubble-wrap.
The fate of methane in Titans atmosphere Methane is broken down by various forms of EM and particle radiation. The fragments recombine into heavier hydrocarbons. These rain / snow out onto the surface. Methane is irreversibly destroyed by this process.
The methane in Titans atmosphere will be completely used up in ~ 50 Ma Therefore, there must be a means of replacing lost methane: Surface lakes and / oceans ? Subsurface aquifers ? Cryovolcanism ? Methane producing organisms ?
How did methane get into Titan to begin with? In the nebula around proto-Saturn, methane and water condensed together as clathrates. These were accreted into proto-Titan along with rocky material and other ices. Titan presumably differentiated into a rocky core overlain by an ice-rich mantle 800- 1000km deep.
Once upon a time…… It used to be thought that methane clathrate decomposed under pressure. CH 4 -clathrate CH 4 gas + water ice ( c.f., bursting bubble-wrap) The released gas was then free to escape from the moons interior to form an atmosphere.
Mineral physics shoves an oar in… High-pressure experiments upon CH 4 -clathrate show that it does not break down. New high-pressure structures are observed. Methane is still trapped in the solid crystal.
MH-I structure I (cubic) water : methane = 5.75 : 1 ~ 1 GPa MH-II structure H (hexagonal) water : methane = 3.5 : 1 ~ 2 GPa MH-III stuffed ice (orthorhombic) water : methane = 2 : 1 stable to at least 42 GPa
MH-III is a stuffed form of ordinary ice, which is extremely stable to high pressure. So…..
…if the methane in Titans interior is trapped in clathrates, how on earth did it get into the atmosphere?
Chemistry? Unknown interactions in the CH 4 – NH 3 – H 2 O system. We have no idea what effect ammonia has on clathrate stability, or whether ternary compounds could exist in this system.
Impacts could devolatilise the upper crust through melting. How much clathrate do you need to de-gas in order to generate all of Titans near-surface methane inventory? Answer: about the upper 20km of crust ! Impacts? (part 1)
New research by Kress and McKay  (Icarus 168, 475-483): The reduction of carbon monoxide to methane can be catalysed by metal-rich dust in cometary impacts. This hypothesis is testable, since it should also yield vast surface deposits of solid CO 2. Impacts? (part 2)
Summary High-pressure experiments have shown that the original models for the origin of Titans methane need rethinking. There are two hypotheses for generating Titans atmosphere that involve cometary impacts. The ternary CH 4 – NH 3 – H 2 O is likely to be important inside Titan and urgently needs study.