The blurring effects of Titan's aerosol are obvious in this image, where the orange moon peeks from behind two of Saturn's rings. Small, battered Epimetheus, another of Saturn's 62 moons, appears just above the rings
Credit: NASA/JPL/Space Science Institute
Every day is a bad-air day on Saturn's largest moon, Titan. Blanketed by haze far worse than any smog belched out in Los Angeles, Beijing or even Sherlock Holmes's London, the moon looks like a dirty orange ball.
Described once as crude oil without the sulphur, the haze is made of tiny droplets of hydrocarbons with other, more noxious chemicals mixed in. Gunk.
Gross as it may sound, Titan is really the rarest of gems: the only moon in our Solar System with an atmosphere worthy of a planet.
This atmosphere comes complete with lightning, drizzle and occasionally a big, summer-downpour style of cloud made of methane or ethane - hydrocarbons that are best known for their role in natural gas.
Now, thin, wispy clouds of ice particles, similar to Earth's cirrus clouds, are being reported by Carrie Anderson and Robert Samuelson at NASA's Goddard Space Flight Centre in Greenbelt, Maryland.
The findings, published on 1 February 2011 in Icarus, were made using the Composite Infrared Spectrometer (CIRS) on NASA's Cassini spacecraft.
Unlike Titan's brownish haze, the ice clouds have the pearly white appearance of freshly fallen snow.
Their existence is the latest clue to the workings of Titan's intriguing atmosphere and its one-way ‘cycle’ that delivers hydrocarbons and other organic compounds to the ground as precipitation.
Those compounds don't evaporate to replenish the atmosphere, but somehow the supply has not run out (yet?).
"This is the first time we have been able to get details about these clouds," says Samuelson, an emeritus scientist at Goddard and the co-author of the paper.
"Previously, we had a lot of information about the gases in Titan's atmosphere but not much about the [high-altitude] clouds."
Puffy methane and ethane clouds had been found before by ground-based observers and in images taken by Cassini's imaging science subsystem and visual and infrared mapping spectrometer.
Compared to those clouds, these are much thinner and located higher in the atmosphere.
"They are very tenuous and very easy to miss," says Anderson, the paper's lead author. "The only earlier hints that they existed were faint glimpses that NASA's Voyager 1 spacecraft caught as it flew by Titan in 1980."
Even before Voyager 1 reached Titan, scientists knew the moon was wrapped in a thick atmosphere that probably contained hydrocarbons. Part of that atmosphere, Voyager found, is a haze so smothering that it hides every bit of the moon's surface.
Only a small amount of visible light penetrates this haze, or aerosol, so studies rely on instruments that operate at wavelengths beyond human sight. This is how Voyager learned that Titan's atmosphere is made mostly of nitrogen, as is Earth's.
Unlike Earth's atmosphere, though, Titan's has neither oxygen nor water to speak of. Instead, it contains small amounts of organic materials, including members of the hydrocarbon family such as methane, ethane and propane.
Voyager also picked up indications that Titan's stratosphere, the second-lowest layer of its atmosphere, harboured "ices made from some exotic organic compounds," Samuelson says. "At the time, that was about all we could tell."
