THE stars and planets have fascinated human beings since time immemorial. Using the crude telescopes available at the time, the 17th century French astronomer Giovanni Domenico Cassini discovered the division in Saturn's rings and four of Saturn's moons-among them Titan, the largest.|
Today, Titan holds particular fascination for astronomers. It is the only moon in the solar system with a thick nitrogen-dominated atmosphere, similar to Earth's, surrounding it. Also as on Earth, Titan's organic chemistry is driven by sunlight. Titan is several hundreds of degrees colder than Earth and has a methane-rich atmosphere, but its chemistry seems in some ways to be like that of Earth before life appeared.
Although imaging methods have improved enormously since Cassini's day, Titan is still difficult to see. Ultraviolet light changes atmospheric methane gases into a thick, smoglike haze that sits in the upper atmosphere. Imaging techniques that use visible light cannot penetrate this haze. When the Voyager spacecraft flew by Titan and took photographs with a telescope that used visible light, the resulting photos showed only a bright blob.
Infrared light can partially penetrate the smog. But Titan is so far away that conventional infrared telescopes on Earth also see only a blob because the image is blurred by Earth's atmosphere. The Hubble Space Telescope uses infrared light, but it lacks sufficient resolution to see much detail. Even with these deficiencies, both Hubble and ground-based studies have shown that Titan has a complex surface.
Hungry for more information about Titan and other celestial bodies, Livermore scientists adapted speckle interferometry, an imaging technique developed during the 1980s, for astronomical use. Until recently, speckle interferometry at Hawaii's 10-meter Keck I telescope gave the world the best look at this mysterious planetary moon.
Scientists have suspected for some time that Titan may have liquid seas formed by ethane that has "rained out" of the atmosphere to produce reservoirs of liquid hydrocarbons. Livermore astrophysicists Claire Max and Bruce Macintosh believe that the extraordinarily dark, unreflective area in the lower left corner of the image below could well be an oily, black ocean of hydrocarbon. Brighter, more reflective patches appear to be continents of ice and rock.
"If Titan does have a sea, it is the only other one in the solar system besides those on Earth, and we would like to know what is going on there," says Macintosh. "Titan seems to be similar to Earth 4 billion years ago, before life formed. Although Titan is too cold for life as we know it, it could be a laboratory for the processes that occurred here on our own planet."
Reflecting the Surface Only|
Traditional astronomical imaging uses long exposures to gather as much light as possible into a single image. Because of Earth's atmosphere, that method often results in a fuzzy image for objects that are small or far away. In speckle imaging, several hundred pictures with short exposures are taken to freeze Earth's atmospheric turbulence. The pictures of Titan are taken using specific infrared wavelengths that are transparent "windows" through the methane spectrum, or haze. At wavelengths of 1.9 to 2.1 micrometers, scientists can more easily observe photons reflecting from Titan's surface, and thus the pictures have more contrast than would be possible at other wavelengths.
Each exposure of about 100 milliseconds produces a specklegram, which is the pattern caused by the interference of light rays as they travel through Earth's atmosphere. As shown above, a complex computer algorithm combines the interference patterns from 100 specklegrams taken over a 90-second period into a single final image.
These composite images can then be used to measure the reflectance, or albedo, of Titan's surface. Livermore astrophysicist Seran Gibbard adapted a radiative transfer model to separate reflectance data of Titan's atmosphere from those of its surface so scientists can map surface features only. Albedo measurements range from 0 to 1, with 0 being black and totally unreflective and 1 being white. The dark area on Titan that scientists believe may be a hydrocarbon sea has an albedo of nearly zero. The brightest, ice- or rock-like continental area has an albedo of 0.15.
Using speckle imaging, the Livermore team has mapped both of Titan's hemispheres, one of which is shown in the figure above.
For several years, speckle imaging has been the best way to view small, distant celestial objects such as Titan. But better ways have been developed. Constructing a final image using speckle imaging takes considerable time on both the telescope and the computer. A faster method that produces almost immediate results is adaptive optics, which allows telescope mirrors to compensate directly for the distortions generated by Earth's atmosphere. An adaptive optics system has been installed at the Keck observatory and has recently produced even clearer images of Titan. (See The Laboratory in the News of this issue and S&TR, July/August 1999, A New View of the Universe.) Spectroscopic data obtained using adaptive optics will also help improve models of Titan's atmosphere.
In late 2004, the most detailed information yet about Titan will begin to arrive on Earth from another source altogether. The spacecraft Cassini, which blasted off in October 1997, will begin to orbit Saturn to learn more about its famous rings, its magnetosphere, and its moons, Titan in particular. The primary contributors to the Cassini program are NASA, the European Space Agency, and the Italian Space Agency.
In November 2004, Cassini will drop a probe called Huygens (named for a Dutch physicist and astronomer) into Titan's upper atmosphere. As it breaks through the cloud deck, a camera will capture pictures of the Titan panorama. Other instruments will measure the organic chemistry of Titan's atmosphere as the probe descends to Titan's surface on the rubber duck-shaped continent visible in the first figure above. Designed by the European Space Agency, Huygens can both bounce and float, so it is prepared for whatever surface it finds. But Huygens will only send information for a few hours because it must operate on batteries. Titan's haze is too thick for solar power. Cassini will continue to orbit Saturn and Titan for years, sending data back to information-hungry, Earth-bound scientists.
Key Words: adaptive optics, Saturn, speckle imaging, Titan.
For further information contact Bruce Macintosh (925) 423-8129 (firstname.lastname@example.org).
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