March 9, 2005
The Missing Methane on Titan
Arizona's Jonathan Lunine presented a lecture entitled "Titan: A Personal View after Cassini's first six months in Saturn orbit" at a NASA Director's Seminar on January 24, 2005. Lunine discusses the question of missing methane in an edited transcript of Part 2.
Astrobiology Magazine -- "How much methane is really in the surface and atmosphere of Titan today? How much has been there in the history of Titan?
Methane is a carbon atom surrounded by four hydrogen atoms, and it can be broken apart by ultraviolet light at wavelengths of about 1600 Angstroms. The fragments, or radicals, that are produced from this process are very chemically reactive. They're things like CH, CH2, and in some cases CH3.
And then of course there is hydrogen that's been knocked off, because this fragmentation process removes one, two or three hydrogen atoms. In the atmospheres of the giant planets, the hydrogen stays around because of the high gravity of those planets. Their atmospheres are primarily hydrogen anyway, and after the fragmentation occurs, the products sink into the deeper atmosphere, and methane is reconstituted. It's a complete chemical cycle.
On Titan, as far as we understand, that does not occur. Because the gravity is low, hydrogen should escape. The Voyager ultraviolet spectrometer saw a corona of hydrogen around Titan, which is a good indication that hydrogen is escaping. Now, if hydrogen is going away, the products that can be made from the methane are going to have a higher carbon to hydrogen ratio than methane itself, and you're not going to be able to remake methane. So as far as we understand the photochemistry, Titan should be destroying methane and making more carbon-rich products, the simplest of which are acetylene - C2H2 - and ethane - C2H6. These are made directly from methane, although in some models the acetylene is the source of ethane through a secondary photochemical cycle.
Ground-based spectroscopy done since the 1970s has shown us that Titan's atmosphere does contain acetylene and ethane. There is also propane, hydrogen cyanide, methyl acetylene, diacetylene, ethylene, and so on. There is an abundance of organic molecules in the atmosphere of Titan.
The products of the methane photolysis are less volatile than methane. So as this stuff is being made, it's condensing out in the form of aerosols. These aerosols will appear at different altitudes, depending upon what's being made. At the highest altitude, there's a tendency for the chemistry to drive toward the highest carbon to hydrogen ratio, and you get this very orange, high molecular weight material that Carl Sagan first called 'tholin.'
But lower down in the stratosphere there are other aerosols. There must be aerosols of ethane and acetylene, because those are the dominant products of the methane chemistry. So below the upper level orange haze that obscures Titan's surface from our view in the optical, there are other deeper layers of haze that should extend down as far as 50 or 60 kilometers above the surface. In fact, what the DISR camera shows is that they seem to extend even lower down, maybe to 20 kilometers above the surface.
These aerosols sink, and they end up on the surface. At 95 degrees Kelvin, acetylene should be a solid. Ethane has almost the same freezing point of methane, about 91 degrees Kelvin. So methane, the source of the photochemistry, and ethane, one of the primary products, should both be liquid at the surface.
Since this chemistry is irreversible, we can say that products are being made and deposited on the surface. If the chemistry on Titan has gone on in steady-state over the age of the solar system, then we would predict that a layer of ethane 300 to 600 meters thick should be deposited on the surface. That would make it the biggest hydrocarbon reservoir on any of the solid bodies in the solar system.
Yet the remote sensing data prior to Cassini tended to argue against very large areas of the surface being covered with liquid hydrocarbon.
The methane that's contained in the atmosphere today is limited by condensation. The atmosphere is cold enough in the troposphere - from 95 up to 70 degrees Kelvin - that methane condenses out as clouds. There have been quite a lot of Earth-based observations of these clouds. So that limits how much methane the atmosphere can hold, and that amount of methane would sustain this photochemistry for only about 1 percent of the age of the solar system.
So either there are sources of methane that are underneath the surface, or on the surface, or supplied from above. Or the chemistry is an occasional process, and we happen to be seeing Titan today at a time when chemistry is ongoing, where there is enough methane in the atmosphere.
There could be reservoirs of methane under the solid surface of ice. There are some places in the Huygens images where you see these extremely short and truncated channels, and they're associated with some bright channels that seem to cut across the higher areas. One interpretation that's been offered is that those brighter areas represent refrozen, once-liquid water - maybe liquid water ammonia flows, maybe warm ice flows - and those have mobilized methane from beneath the surface, and that's where you see those very short, less well-developed channels.
It's possible that the channels that are better developed are due to methane rain and flow. In the current epoch, it's not easy to get rain on Titan, because there's not a lot of energy available to make weather and storms. But from time to time it might rain, so you can't rule that out. The shorter channels could be from rainfall too, but they're not well developed compared to many drainage systems on the Earth.
So where are the large liquid deposits of methane and ethane? There are hints, based on the radar, that the darker areas are composed of organics. Whether these are areas that could be exposed methane and ethane but are being covered by other kinds of organic crud is an open issue. One kind of organic solid - polyacetylene - has a low enough density that could actually float on liquid. So some areas of Titan may have liquid on the surface, but they're being covered by other components."
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