Titan: Passport to the Early Earth?

Icy pebbles on Titan. Click image for larger view. Credit: ESA

If there is life on Saturn’s moon Titan, it’s not producing all that methane in the atmosphere. That is one conclusion in a series of Cassini/Huygens mission reports published online this week by the journal Nature.

On Earth, methane (CH4) is produced by life, the degradation of organic material, or geologic processes. On freezing cold Titan, where methane is the second-most abundant gas next to nitrogen, it’s long been a mystery how the methane is produced.

Methane is constantly being broken apart by sunlight and cosmic radiation in Titan’s upper atmosphere, so something must be replenishing it. Measurements of the moon’s atmospheric argon and carbon isotopes have led the Huygens scientists to conclude that the methane most likely comes from geologic activity in the moon’s interior rather than from biological processes.

For instance, the scientists say that the presence of argon-40 in Titan’s atmosphere is suggestive of geological activity. On Earth, argon-40 is produced in volcanic rocks by the radioactive decay of potassium-40. The scientists suspect the same process could be occurring on Titan.

Titan’s changing face as dark and light patches rotate in circulation. Image Credit: JPL/Space Science Institute

The ratio of carbon isotopes on Titan also does not point to life, say the scientists. Methane molecules can have different carbon isotopes, such as carbon-12 (12C) or carbon-13 (13C). Since organisms on Earth prefer the lighter 12C, the methane they generate is enriched in that isotope. However, there was no 12C enrichment in Titan’s methane.

Titan is often described as a passport to the early Earth, showing us what our own planet may have been like before life developed. But the harsh chemistry and frigid temperatures of the moon make many scientists skeptical about the current potential for life on Titan.

"If there is or if there was life on Titan, the best place would be in the interior of the satellite, where you have liquid water," says science team member Francois Raulin of the University of Paris. "You have there, in the interior, what you need to make good prebiotic chemistry. But from what we know [about what is] coming out from the interior of Titan now, it’s more and more unlikely that this prebiotic chemistry was able to produce a replicating system of life."

The Huygens probe parachuted down to Titan’s icy surface on January 14, 2005. Although the moon’s thick atmospheric haze continued all the way down to the surface, the probe still was able to capture images of mountains and dark river channels as it descended. These now-dry channels led scientists to believe that liquid methane must flow on Titan’s surface periodically.

Shorelines may be dry but intermittently defined by drainage channels of methane rain. Click image for larger view. Credit: ESA

Titan’s surface currently seems to be composed of water ice and frozen carbon-and-hydrogen compounds like methane. Some of the water ice is interlaced with an unknown material, and more studies are underway to determine what that material could be.

When the Huygens probe first landed on Titan’s surface, scientists likened the surface texture to Crème Brûlée, with a hard outer crust covering a softer material below. John Zarnecki, principal investigator for the Huygens Science Surface Package, says the Crème Brûlée analogy is now dead. Instead, the probe likely hit one of the many ice pebbles littering the landing area before sinking into the softer ground material. This material, says Zarnecki, is probably texturally similar to wet clay, lightly packed snow, or wet or dry sand.

"According to a local newspaper in my town, Huygens was due to bring back samples of the surface of Titan to the Earth. And I can announce this is the sample that it brought back," Zarnecki said at yesterday’s press conference, smiling as he brandished a glass jar filled with wet sand. After pausing to make sure no one took his joke seriously, he explained how the material in the jar provided the best Earth-world match to the surface texture of Titan.

The Huygens landing site is probably an icy mixture of granular materials and methane. When the heated inlet of the probe’s Gas Chromatograph Mass Spectrometer (GCMS) touched down on the surface, it vaporized some of the surface material. The GCMS immediately detected a 40 percent increase in methane, and this level remained constant for about 50 minutes.

The Huygens probe discovered that atmospheric methane is condensing near the surface, with three times more methane at lower altitudes than in the stratosphere. The noble gases krypton and xenon were not detected, and the scientists say this indicates that Titan’s current atmosphere is not the one it was born with. Instead, the atmosphere was initially composed of ammonia – which was delivered by icy planetesimals during the moon’s formation process — and then the ammonia eventually turned into nitrogen by interaction with sunlight.

Jonathan Lunine of the Lunar and Planetary Laboratory at the University of Arizona
Image Credit: space.com

The probe did detect some electrical discharges in the atmosphere, suggesting that Titan might experience lightning. Huygens also discovered that winds in the upper atmosphere blow west to east, in the direction of Titan’s rotation, with speeds up to 450 kilometers per hour (280 mph). The winds steadily decrease at lower altitudes, until close to the surface they die down to walking speed.

The Cassini spacecraft recently performed RADAR observations over the Huygens landing site, and these images have helped provide the site’s coordinates of within 5 kilometers (1.4 miles) of 192.4 degrees west longitude and minus 10.2 degrees south latitude.

Although organic materials continually rain out of the atmosphere and blanket the surface, it will take a long time for the Huygens probe to become buried, says Jonathan Lunine of the University of Arizona. He notes that it took over 4.5 billion years to build up a layer of organics on the surface that is somewhere between 500 meters and a kilometer thick (1,640 to 3,280 feet).

"The probe is less than a meter thick, so it would take a few million years to get completely buried," says Lunine. "You could go back in 50 years, dust it off, and it should look pretty good."

Related Web Pages

European Space Agency: recent Huygens results
Rendezvous with Titan
Huygens, Phone Home
Saturn– JPL Cassini Main Page
Space Science Institute
Planet Wannabe
Where is Cassini Now?
Did Fluid Once Flow on Titan?
Is Titan’s Bright Spot Hot?
Titan’s Icy Volcanoes Erupting Methane?
Titan versus Earth