Hubble: evidence of oceans on Titan?

Of all the solar system bodies other than Earth, planetary scientists believe just one may harbor oceans or lakes. Researchers have long debated whether Saturn's moon Titan, cloaked in a methane-rich atmosphere, contains hydrocarbon oceans or lakes.

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In penetrating much of Titan's haze, the Hubble Space Telescope has taken a new step in clearing up the mystery. Peering through a "window" that allows near-infrared light to escape through the smong and methane engulfing Titan, Hubble has imaged intriguing bright and dark patches that seem to lie on the moon's surface. One explanation for the dark patches: They represent oceans of hydrocarbons; the brightest patch could be a continent-size chunk of frozen water and ammonia ice.

Peter H. Smith of the University of Arizona in Tucson and his colleagues presented the findings last week in Bethesda, Md., at the annual meeting of the American Astronomical Society's Division for Planetary Sciences.

He cautions that the dark patches could have more mundane explanations. Hydrocarbon tars coating parts of the moon's surface, for example, could also lower Titan's reflectivity and account for the dark features.

Because the near-infrared wave-lengths at which Hubble observed don't penetrate all of Titan's haze, Smith and his colleagues had to subtract the murky atmosphere to produce the final images. Some astronomers express concern that the pictures may show a mix of surface and lower-atmosphere features rather than just the surface.

But Smith notes that the light and dark regions imaged by Hubble match several ground-based observations. Unlike Hubble, telescopes on Earth must contend with Earth's wavering atmosphere and for now can't resolve individual features on Titan. But near-infrared observations from Earth taken over several years reveal that the regions of Titan that reflect the most and least sunlight are the same ones in which Hubble finds the brightest and darkest patches, respectively.

"The real proof of the pudding that these are surface features is that our bright spot and our dark spot line up precisely with the ground-based measurements," Smith says.

Jonathan I. Lunine of the University of Arizona, a proponent of the ocean hypothesis, says the Hubble images suggest that the composition and topography of the moon's surface vary considerably. Both hydrocarbon tars and a small ocean might reside within the same dark patch, he notes.

Researchers proposed Titan's hydrocarbon ocean in order to solve a dilemma. They realized that to sustain the amount of methane found in Titan's atmosphere, the moon must have a reservoir of the hydrocarbon. Without it, sunlight would have long ago destroyed the atmospheric methane.

Given Titan's temperature and surface pressure, scientists suggested that the reservori took the form of methane and ethane oceans and lakes. Radar studies later ruled out an ocean covering the entire moon but allowed for individual lakes or seas.

Several developments may help identify what lies on Titan, even before a space probe parachutes onto the moon in 2004. New techniques to compensate for Earth's blurry atmosphere should enable ground-based infrared telescopes to image individual features on Titan. Similarly, an upgrade of the Arecibo Observatory's radio telescope in Puerto Rico, to be completed in 1996, should provide higher-resolution radar images. In space, NASA plans to install a new infrared camera on Hubble in 1997. This should enable the telescope to get an even clearer view of Titan's surface.