Titan: rains of methane, mountains of ice

A flammable world...

By Lucy Sherriff, 22nd January 2005

The data sent back via NASA's Cassini spacecraft confirm that Titan is a moon with weather. It rains methane on Titan. They don't know how often, but they know that it does. The latest news from the Huygens probe is that "it might have rained yesterday". Of course, "yesterday" is the day before the probe landed, not yesterday, yesterday, so mission scientists are being a little poetic.

Still, mountain ranges of ice are gently eroded by the methane rainfall, which washes dark organic compounds into streams, channel and pools. The researchers think that the landing site is in a pretty arid area of Titan, much like Arizona on Earth.

Martin Tomasko, principal investigator for the Descent Imager and Spectral Radiometer (DISR), said that everywhere the team looks there is evidence of fluid flow, although the area is pretty dry now. "We think the riverbeds are dry most of the time, with liquid only flowing just after the rains," he said.

The exact site of the landing could be a dry riverbed. The probe sent back pictures of rounded rocks, which are most likely ice blocks with their hard edges eroded by flowing rivers of methane. The dark "pools" near the landing site are probably dry places where the rainfall collects, then evaporates, leaving the dark organic material behind.

Jean-Pierre Lebreton, ESA Huygens mission manager, said the research teams working on each of the science packages have spent long hours unravellins the data from the probe. "We have not had much sleep," in the seven days since Huygens landed on Titan, he told the press.

Toby Owen, a specialist from the Cassini team, investigating the atmospheres of both Titan and Saturn, says the data shows the atmosphere is, as expected, mostly made of nitrogen. As the probe descended, the levels of methane detected rose steadily. There are also traces of argon, but no primordial argon, unlike Earth. This absence, he says, is a clue to the formation of the moon.

The data also confirms that the liquid on the surface of Titan is methane, not ethane. This discovery raises the question of where all the CH4 is coming from. The researchers explained that it can't be left over from Titan's primordial atmosphere, or it would all have been used up. Photochemical reactions occurring in the upper atmosphere break the methane down, and other organic compounds form as a result.

The probe touched down at about 4.5 m/s, a whole series of instruments gathered data on the texture of the surface. It is similar to wet sand or clay with a thin solid crust, but is mainly a mix of dirty water ice and hydrocarbon ice. It is also darker than researchers expected.

Owen points to a spike in the amount of methane detected by the instruments, after the probe had landed. This same spike was observed by the surface science package team, headed by John Zarnecki. After the initial impact, the probe sank about 15 cm into the soft surface, Zarnecki said.

The temperature at ground level was about -180 degrees Celsius, but the probe was generating plenty of heat:

"Once it has landed, the probe heats the soil. The CH4 evaporates through the soil, and the mass spectrometer gets a whiff," he said. "It is still early days, but it is a pretty consistent picture. There are some truly remarkable processes on Titan, very, very similar to those on Earth. Similar processes are at work, but the ingredients are different. On Earth, rocks are silicate. on Titan they are ice. There is even dirt on Titan, the dark organic matter."

The researchers are still pretty guarded about what these other organics that might exist on Titan could be. All they will say is that there are some hints in the data, but there is a lot more work needed before they can say anything with any degree of certainty.

"So what have we learned?" Owen asks. "The main thing is the detection of liquid methane. Titan is a flammable world, it is really quite extraordinary." ®

Update

Regarding the question of a flammable world: It is a pretty safe bet that the scientists involved in running space exploration missions are cognisant of the fact that for methane to combust, there must be oxygen present. We suspect Owen was using poetic license.