Titan Look at Saturn

The Hubble Space Telescope captured the clearest image yet of Saturn at maximum tilt, an event viewable only about three times a century when the southern pole points toward Earth. The spectacular view of the southern side of the giant planet’s rings also provides a closer glimpse of its detailed ring structure. But only when the forthcoming Cassini probe releases its landing probe onto the ringed planet’s moon, Titan, will astronomers get what promises to be the most ambitious planetary view yet.

The Hubble series of images of Saturn, as seen at many different wavelengths, were capture when the planet’s rings were at a maximum tilt of 27 degrees toward Earth. Saturn experiences seasonal tilts away from and toward the Sun, much the same way Earth does. This happens over the course of its 29.5-year orbit. This means that approximately every 30 years, Earth observers can catch their best glimpse of Saturn’s South Pole and the southern side of the planet’s rings. Between March and April 2003, researchers took full advantage to study the gas giant at maximum tilt. The detailed images of Saturn’s Southern Hemisphere highlight the southern face of its rings.

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Click image for high-resolution. Hubble image of Saturn rings. Credit: Hubble

The telescope’s Wide Field Planetary Camera 2 used 30 filters to snap these images on March 7, 2003. The filters span a range of wavelengths. "The set of 30 selected filters may be the best spectral coverage of Saturn observations ever obtained," says planetary researcher Erich Karkoschka of the University of Arizona.

Various wavelengths of light allow researchers to see important characteristics of Saturn’s atmosphere. Particles in Saturn’s atmosphere reflect different wavelengths of light in discrete ways, causing some bands of gas in the atmosphere to stand out vividly in an image, while other areas will be very dark or dull. One image cannot stand by itself because one feature may have several interpretations. In fact, only by combining and comparing these different images, in a set such as this one, can researchers interpret the data and better understand the planet.

By examining the hazes and clouds present in these images, researchers can learn about the dynamics of Saturn’s atmosphere. Scientists gain insight into the structure and gaseous composition of Saturn’s clouds via inspection of images such as these taken by the Hubble telescope. Over several wavelength bands, from infrared to ultraviolet, these images reveal the properties and sizes of aerosols in Saturn’s gaseous makeup.

For example, smaller aerosols are visible only in the ultraviolet image, because they do not scatter or absorb visible or infrared light, which have longer wavelengths. By determining the characteristics of the atmosphere’s constituents, researchers can describe the dynamics of cloud formation. At certain visible and infrared wavelengths, light absorption by methane gas blocks all but the uppermost layers of Saturn’s atmosphere, which helps researchers discern clouds at different altitudes. In addition, when compared with images of Saturn from seasons past (1991 and 1995), this view of the planet also offers scientists a better comprehension of Saturn’s seasonal changes.

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Scientists would like to know the origin of the atmospheric patches imaged on Saturn’s moon, Titan, as imaged by Hubble. Image Credit: Hubble Space Telescope/UA Smith

Even a quick look at the set of six images reveals the changing face of Saturn between different spectral regions. In the ultraviolet, Saturn is much brighter than the rings. The opposite is true at some near-infrared wavelengths. Saturn’s bands change dramatically throughout the displayed spectral region.

These spectral changes can be viewed by combining three images into a false-color image by displaying the short, middle, and long-wavelength image in blue, green, and red light, respectively.

The images were taken on March 7, 2003 between 1:43 and 2:27 am EST. The wavelengths for the six images are 0.27, 0.41, 0.59, 0.73, 0.89, and 0.95 micrometers.

Titan in Focus

Saturn’s giant moon Titan, cloaked in a thick nitrogen atmosphere laced with hydrocarbons, could provide a laboratory in the sky for scientists seeking insight into the origins of life. Surface features of the distant moon revealed so far by Earth-bound observatories and by the Hubble space telescope are limited to a resolution of a few hundred kilometers, says planetary scientist Jonathan Lunine. Such resolution, applied to Earth, would register California or Texas as just a few tiny spots. The Cassini-Huygens mission "will be our first chance to really understand, on a global scale, what Titan is all about," says Lunine.

Titan’s haze was breached by the Hubble telescope, scanning in the near infrared wavelengths, in 1994. Surface features included a bright spot the size of Australia. Early in 2005, the ESA‘s Huygens probe will descend through Titan’s atmosphere, settling on the moon’s surface. It will provide researchers with chemical analysis, spectra and images

With the Cassini-Huygens mission, scheduled for a 2004 rendezvous with Saturn and Titan, scientists hope to find evidence for primitive organic chemistry, preserved in the extreme cold of the moon’s icy surface. For while "Titan is not a place where life began or could flourish," says Lunine, it is a good place to look for biomolecules.

Titan is a Mercury-sized world comprised of a 50-50 mix of ices and rock. The chemical composition of its environment resembles that of early Earth but it is far colder and lacks liquid water. Scientists think Titan may have carbon- and nitrogen-containing molecules accumulated on its surface. And these primitive precursors to life might be brought even further towards life’s door if liquid water makes an occasional appearance which Lunine believes it may well do. Studies of Titan so far have indicated enough evidence for both temporal and spatial variability, two signatures required for the presence of organic molecules.

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Huygens landing probe to Saturn’s moon, Titan.
Image Credit: ESA

Given the mix of organic materials known to be available in the moon’s atmosphere, and conditions believed to exist on the surface, Titan could turn out to be a "good reaction crucible" for birthing biomolecules, says Lunine, a professor at the University of Arizona in Tucson and chair of the university’s theoretical astrophysics program. If a transient heat source caused liquid water to become available, the otherwise frozen giant would provide an extensive surface area and long reaction times for these molecules to form. When the water refroze, it would provide excellent preservation for these starting materials for life.

Titan’s thick atmosphere as well as its frigidity should keep safe any organics preserved in this way, Lunine says. Any ultraviolet and cosmic rays that reached Titan’s atmosphere would "tend to break up polymers, and form radicals not good for building up biopolymers." But, he points out, "Titan’s surface is shielded from most UV and cosmic rays, so stuff polymerizing there doesn’t get destroyed."

The Huygens probe is geared primarily towards sampling the atmosphere. "Anything else will be gravy," Lunine says. The probe is equipped to take measurements and record images for up to a half an hour on the surface. But the probe has no legs, so when it sets down on Titan’s surface its orientation will be random. And its landing may not be by a site bearing organics.

Still, the Cassini-Huygens mission should provide a wealth of data to guide a future, more mobile probe to a likely site on Titan for mining samples of biomolecules. "The strategy would be to look for places, like impact craters," Lunine says, that are "good places to look for evolved organic chemistry."


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