14 DAY TRIAL // Before the Cassini-Huygens mission to Saturn and Titan, the largest of Saturn's moons, scientists expected to find large amounts of ethane on this moon. Moreover, they assumed that the entire surface of Titan should be covered by a 300-meter-deep ocean. "We expected to see lots of ethane - vast ethane clouds at all latitudes and extensive seas on the surface of Saturn's giant moon Titan," University of Arizona planetary scientist Caitlin Griffith said.
But none of these predictions materialized. Titan's surface has almost no lakes whatsoever and the atmosphere doesn't contain the expected ethane. Why is that? Researchers thought the atmosphere should contain ethane because this is the main byproduct when methane is broken down by solar ultraviolet light. For the same reason, Titan should have been awash with seas of ethane, scientists theorized. (Scientists have detected thousand-mile-long methane clouds at southern latitudes.)
What's even more mysterious is that Cassini images have revealed that other secondary products of methane breaking down are abundant on the moon's surface forming dunes and covering the craters left by meteorites. So if secondary byproducts are there, where did the ethane go?
Cassini's Visual and Infrared Mapping Spectrometer (VIMS) might have now finally solved this puzzle. It has detected a vast cloud of ethane at last. The cloud surrounds the moon's north pole and might be snowing ethane snowflakes on methane lakes below. VIMS probed Titan's high northern latitudes on Cassini flybys in December 2004, August 2005, and September 2005.
The VIMS instrument is an imaging spectrometer that produces a special data set called an image cube. While an ordinary video camera takes images in three colors (red, green, and blue) and combines them to produce images as seen by the human eye, the VIMS takes images in 352 separate wavelengths! A certain material has a certain color, because it reflects some wavelengths and it absorbs others. Thus, by taking a picture not in three wavelengths, but in more than three hundred, the camera can identify the composition of various things that reflect light - such as a cloud on Titan. In fact, it can identify molecules of any element or compound as each of them has a certain specific reflection "signature" spectra.
"Our observations imply that surface deposits of ethane should be found specifically at the poles, rather than globally distributed across Titan's disk as previously assumed," Griffith said. "That may partially explain the lack of liquid ethane oceans and clouds at Titan's middle and lower latitudes."
"We think that ethane is raining or, if temperatures are cool enough, snowing on the north pole right now. When the seasons switch, we expect ethane to condense at the south pole during its winter," Griffith said.
The reason why the ethane precipitations don't form the vast ocean the scientists have predicted is that the ethane cloud stays at poles and snows there. Because the polar conditions are very cold, the ethane accumulates there as polar ice. If ethane was produced at today's rate over Titan's entire lifetime, a total of two kilometers of ethane would have precipitated over the poles.
But, according to Griffith, that's hard to believe. The probe has imaged Titan's south pole and "the morphology seen in those images doesn't suggest a two kilometer polar ice cap, but the images do show flow features," Griffith said.
"We're going to start making more polar passes in the upcoming months," she added. "By the end of next year Cassini will have recorded the first polar temperature profile of Titan, which will tell us how cold conditions are at the pole."
VIMS detected the cirrus cloud as a bright band at altitudes from between 30 km and 60 km at the edge of Titan's arctic circle. But, as most of the northern polar region is in winter's shadow and won't be fully illuminated until 2010, only part of the cloud was observed.