Defogging Titan’s methane mystery

Methane fog hovering above Saturn’s moon Titan has cleared away any doubt that the hydrocarbon cycles between the moon’s surface and its atmosphere, planetary scientists say.

Titan is the only solar system body other than Earth known to have large quantities of liquid — in this case methane and ethane — on its surface. Scientists have speculated that these liquids may serve as a prebiotic brew, offering a snapshot of the chemistry of the early Earth (SN Online: 8/5/09) .

Methane acts on Titan the way water does on Earth, notes Michael Brown of the California Institute of Technology in Pasadena. On Titan, methane can form clouds and is likely to produce rain. But it’s less certain, says Brown, whether methane is truly part of a cycle, in which methane rain “makes it to the surface and pools into ponds or streams that then evaporate back into the atmosphere.” The discovery of fog would settle this question.

Brown and his colleagues searched for fog by examining data collected with an infrared mapping spectrometer on the Cassini spacecraft, which has been orbiting the Saturn system since 2004. Infrared light penetrates through Titan’s thick hydrocarbon haze, revealing emissions from underlying material. Reddish-white patches found in Titan’s southern hemisphere appear to be fog banks of methane, the researchers report online August 28 at arXiv.org.

To determine whether the patches were high-altitude clouds or low-lying fog, the researchers relied on observations at several infrared wavelengths. The patches reflect light at a wavelength of 5 micrometers brilliantly, which clouds and fog both do. But in light at shorter infrared wavelengths, which can only be recorded from structures that lie high above Titan’s surface, the patches can’t be seen.

Those observations therefore restrict the location of the patches, Brown says, suggesting that they either reside on Titan’s surface or directly above it. The patches appear and vanish over time, indicating that they are ephemeral, low-lying banks of fog, rather than permanent structures on the surface.

The conclusions are interesting, but “fog is extremely difficult to observe at the kinds of resolutions available to either the Cassini imaging system or ground-based telescopes,” cautions Jonathan Lunine of the University of Arizona in Tucson.

Lacking direct compositional information, Brown’s team turned to the infrared data to deduce what the fog is made of. To reflect so much light at a wavelength of 5 micrometers, the fog particles must have a diameter greater than that wavelength, Brown says. Such particles must also be “an abundant atmospheric constituent, which can only be methane,” he adds.

The presence of fog suggests that methane cycles between land and atmosphere on the moon, as many researchers had suspected. Fog can only be generated if the surrounding air is saturated with methane, and to saturate the air, liquid must be evaporating from the moon’s surface. Liquid ethane on Titan is too cold to evaporate and form fog, leaving liquid methane as the only other possible source. “Fog means that Titan has a currently active methane hydrological cycle,” Brown asserts. 

That fog doesn’t reside over large dark areas on the surface, assumed to be hydrocarbon lakes. That suggests that most of the lakes are ethane. In contrast, “most of the methane is probably in shallower pools that come and go with the seasons,” Brown says.

The fog is seen only at Titan’s south pole, but Brown says the activity is likely to shift to the north pole in about seven years, when summer arrives there.