Lighting Up the Flammable Moon
Titan’s Golden Goo
The region where the Huygens probe landed looks oddly familiar. The same processes of precipitation, erosion, and abrasion that shape the Earth are also occurring on Titan, although the ingredients are very different.
While the rocks of Earth – from the sandy beaches to tallest mountains – are mostly made of silicates, on Titan the rocks are made of water ice. While the Earth’s rain showers, mountain springs, and rivers are composed of liquid water, on Titan they’re all liquid methane.
Titan is a world where bits of muck continually fall out of the smoggy sky, blanketing the frozen surface like dark gooey snow. Squalls of methane rain periodically wash the surface clean, sweeping the organic gunk into rivers. The methane rivers ferry the gunk down through hills of rock-hard ice, and empty into the valley below. But the river delta photographed by Huygens does not lead to an open sea.
"The region we landed in is more typical of arid regions on the Earth," says Martin Tomasco, Principal Investigator for the Descent Imager and Spectral Radiometer, from the University of Arizona in Tucson. "It’s like Arizona, where the riverbeds are dry most of the time. Right after a rain you might have open flowing liquids, but then there are pools that gradually dry out, or liquid sinks down into the surface."
Both Titan and the Earth have an atmosphere mainly composed of nitrogen. But the secondary gas on Earth is oxygen, while on Titan it is methane.
"Whenever you see a planet with a dominant nitrogen atmosphere, you have to ask yourself, Where is the carbon?’ because carbon is more abundant than nitrogen," says Toby Owen, Cassini Interdisciplinary Scientist for the atmospheres of Titan and Saturn, from the Institute for Astronomy in Honolulu, Hawaii.
Owen says that on Earth, the carbon became carbon dioxide (CO2), which then became carbonate rocks. If the CO2 from carbonate rocks were put back into the atmosphere, the Earth’s surface pressure would increase 70 times its present level, making our planet more like Venus.
|Shoreline horizon during descent to Titan. Click image for larger view. Credit: ESA|
"Venus and Mars, which have predominately CO2 atmospheres, represent normal conditions," says Owen. "Earth is kind of odd, because the CO2 is not there."
Titan is odd, too, because its methane has not turned into CO2 over time. The reason for that, says Owen, is because there is no oxygen on Titan to facilitate the transition.
"Water, which is the dominant oxygen reservoir on these planets, is frozen out on the surface," says Owen. "There’s no source of free oxygen available, which is a good thing, or Titan would have exploded a long time ago."
The vast amount of methane on Titan makes the whole moon flammable. But where is all the methane coming from? If methane has been producing the moon’s signature photochemical smog for millions or even billions of years, why hasn’t it been used up by now?
Scientists suspect there are reservoirs of methane just beneath the thin surface crust. When the Huygens probe landed on Titan’s surface, it broke through this crust and settled a few centimeters deep into the icy sand-like surface. Heat from the probe’s instruments thawed the icy ground and produced a burst of methane gas.
Another puzzle about Titan’s atmosphere involves the lack of noble gases like krypton and xenon.
|Do surface features suggest some relic or present tectonic activity? Click image for larger view. Credit: ESA|
"We find these gases in our atmosphere, we’re all breathing them right now," says Owen. "We find them on Venus, on Mars, on Jupiter, but they’re not present on Titan at limits that we can detect with our sensitivity – which is equal to one thousand times less than what we see on the Earth. Surely there’s an interesting clue there as to how Titan formed, which we’re going to be working on over the next few months."
Jean Pierre Lebreton, ESA’s Huygens Project Scientist and Mission Manager, says the next mission to Titan will probably involve some sort of machine that would fly all around the moon to sample the atmosphere, and then land and sample the surface.
"But I just got a message from the Mars rover team," says Lebreton. "I don’t know if Mars is now becoming boring, but they are now dreaming of sending their rovers to the surface of Titan. I think from what we have seen, this is possible."
Listen to sounds from the microphone onboard the Huygens during its descent (wav file format, approx. 600 kB each):